Quinn Smith Thesis (PDF 6MB)

Queensland University of Technology School of Natural Resource Sciences

Volcanic Stratigraphy, Alteration Zoning, and

Vein Paragenesis of the Sascha-Pelligrini Low-

Sulphidation Epithermal System, Santa Cruz,

Argentina

By

Quinn Eric Smith B. App. Sc. (QUT)

2009

Supervisor: Assoc. Prof David Gust

A Thesis submitted for the degree of Master of Applied Science (Queensland University of Technology)

I

KEYWORDS Epithermal, Low-sulphidation, Gold-Silver, Vein Paragenesis, Deseado Massif, Chon Aike, Santa Cruz, Argentina

II

Abstract The Sascha-Pelligrini low-sulphidation epithermal system is located on the

western edge of the Deseado Massif, Santa Cruz Province, Argentina.

Outcrop sampling has returned values of up to 160g/t gold and 796g/t silver,

with Mirasol Resources and Coeur D‟Alene Mines currently exploring the

property.

Detailed mapping of the volcanic stratigraphy has defined three units that

comprise the middle Jurassic Chon Aike Formation and two units that

comprise the upper Jurassic La Matilde Formation. The Chon Aike Formation

consists of rhyodacite ignimbrites and tuffs, with the La Matilde Formation

including rhyolite ash and lithic tuffs. The volcanic sequence is intruded by a

large flow-banded rhyolite dome, with small, spatially restricted granodiorite

dykes and sills cropping out across the study area.

ASTER multispectral mineral mapping, combined with PIMA (Portable Infra-

red Mineral Analyser) and XRD (X-ray diffraction) analysis defines an

alteration pattern that zones from laumontite-montmorillonite, to illite-pyrite-

chlorite, followed by a quartz-illite-smectite-pyrite-adularia vein selvage.

Supergene kaolinite and steam-heated acid-sulphate kaolinite-alunite-opal

alteration horizons crop out along the Sascha Vein trend and Pelligrini

respectively.

Paragenetically, epithermal veining varies from chalcedonic to saccharoidal

with minor bladed textures, colloform/crustiform-banded with visible electrum

and acanthite, crustiform-banded grey chalcedonic to jasperoidal with fine

pyrite, and crystalline comb quartz. Geothermometry of mineralised veins

constrains formation temperatures from 174.8 to 205.1°C and correlates with

the stability field for the interstratified illite-smectite vein selvage.

Vein morphology, mineralogy and associated alteration are controlled by host

rock rheology, permeability, and depth of the palaeo-water table.

Mineralisation within ginguro banded veins resulted from fluctuating fluid pH

III

associated with selenide-rich magmatic pulses, pressure release boiling and

wall-rock silicate buffering.

The study of the Sascha-Pelligrini epithermal system will form the basis for a

deposit-specific model helping to clarify the current understanding of

epithermal deposits, and may serve as a template for exploration of similar

epithermal deposits throughout Santa Cruz.

IV

CONTENTS

INTRODUCTION ........................................................................................... 1

REGIONAL GEOLOGICAL SETTING ............................................................................................ 3 SASCHA-PELLIGRINI EPITHERMAL SYSTEM ............................................................................... 7

METHODS ..................................................................................................... 9

FIELD INVESTIGATIONS ........................................................................................................... 9 SAMPLE PREPARATION AND ANALYTICAL TECHNIQUES ............................................................ 10

Short Wave Infrared (SWIR) spectrometry ................................................................... 10 X-ray Diffraction ............................................................................................................. 11 Microscopy .................................................................................................................... 12 Geochemistry ................................................................................................................ 13 Remote sensing ............................................................................................................. 14

RESULTS .................................................................................................... 16

STRATIGRAPHY .................................................................................................................... 16 PETROGRAPHY .................................................................................................................... 21

Chon Aike Formation ..................................................................................................... 21 La Matilde Formation ..................................................................................................... 22 Other units ..................................................................................................................... 23

GEOCHEMISTRY ................................................................................................................... 26 STRUCTURAL SETTING .......................................................................................................... 33 EPITHERMAL VEINING ........................................................................................................... 36

Quartz textures .............................................................................................................. 36 VEIN GEOCHEMISTRY AND MINERALOGY ................................................................................ 39 GEOTHERMOMETRY ............................................................................................................. 42 ALTERATION ........................................................................................................................ 43

Regional Alteration – Multispectral Mineral Mapping .................................................... 43 Prospect Alteration – PIMA and XRD............................................................................ 46

ALTERATION GEOCHEMISTRY ................................................................................................ 55

DISCUSSION ............................................................................................... 61

VOLCANOLOGY .................................................................................................................... 61 Depositional setting ....................................................................................................... 61 Eruption styles ............................................................................................................... 63 Magma Petrogenesis..................................................................................................... 68

HOST ROCK CONTROL AND STRUCTURAL MODEL .................................................................. 70 ALTERATION ZONING ............................................................................................................ 74 VEIN PARAGENESIS .............................................................................................................. 82 SUPERGENE OVERPRINT ...................................................................................................... 88 SUMMARY ............................................................................................................................ 89

CONCLUSION ............................................................................................. 93

REFERENCES ............................................................................................ 96

APPENDIX 1.............................................................................................. 109

APPENDIX 2.............................................................................................. 115

APPENDIX 3.............................................................................................. 119

APPENDIX 4 (MAP) .................................................................................. 121

APPENDIX 5 (MAP) .................................................................................. 122

V

List of Figures Figure 1. Conceptual models and genetic classifications of epithermal deposits .................... 3

Figure 2. Location of the Sascha-Pelligrini study area. ............................................................ 6

Figure 3. Sample and prospect locations ................................................................................. 7

Figure 4. Comparison of accepted results for rock standard SEQG566 with returned

analysis. ...................................................................................................................14

Figure 4. Stratigraphy for the Sascha-Pelligrini study area. ...................................................19

Figure 5. Interpretive geology, structure and mapped veining for the Sascha-Pelligrini

study area. ...............................................................................................................20

Figure 7. XRF major element results for Sascha-Pelligrini volcanic samples ........................29

Figure 8. ICP-MS trace element results for Sascha-Pelligrini volcanic samples. ..................30

Figure 9. REE spider diagrams for Sascha-Pelligrini volcanic samples. ...............................31

Figure 10. REE geochemistry normalised to upper crust, Bajo Pobre and lower crust

xenoliths ................................................................................................................32

Figure 11. Vein trace orientation in Sascha Main indicating dextral oblique-slip

movement..............................................................................................................33

Figure 12. Sascha Main saccharoidal and chalcedonic vein phases ....................................35

Figure 13. Sascha Main ginguro vein phases ........................................................................35

Figure 14. Sascha Main pyritic-chalcedonic vein phases ......................................................35

Figure 15. Backscattered SEM images of characteristic vein mineral assemblages.............41

Figure 16. Backscattered SEM images of coexisting electrum-sphalerite grains ..................42

Figure 17. Selected end-member spectra compared to known library spectra .....................44

Figure 18. Aster mineral mapping results, simplified geology and gold geochemistry ..........45

Figure 19. Selected end-member spectra used for mineral mapping and spectral

unmixing ................................................................................................................47

Figure 20. PIMA and XRD profiles of individual vein phases from the Sascha Main

vein zone ...............................................................................................................50

Figure 21. PIMA and XRD profiles of individual vein phases from the Sascha Ginguro

vein zone ...............................................................................................................51

Figure 22. PIMA and XRD profiles of individual vein phases from the pyrite-chalcedony

vein zone ...............................................................................................................52

Figure 23. PIMA and XRD profiles of individual vein phases from the Sascha Sur

vein zone ...............................................................................................................53

Figure 24. ESEM images of alteration mineral morphologies ................................................54

Figure 25. Selected immobile elements and geochemical mass-changes for rhyolite

crystal ash tuff alteration within Sascha Main. ......................................................57

Figure 26. Selected immobile elements and geochemical mass-changes for rhyodacite

ignimbrite alteration within Sascha Sur. ................................................................58

Figure 27. Selected immobile elements and geochemical mass-changes for rhyolite

ash tuff alteration within Pelligrini. .........................................................................59

VI

Figure 28. Bar graph comparing net mass-changes for immobile elements Dy and Zr ........60

Figure 29. Styles of explosive eruptions ................................................................................67

Figure 30. Structural model for the Sascha-Pelligrini study area ...........................................72

Figure 31. Riedel shear model for the Sascha – Pelligrini and Huevos Verdes systems ......72

Figure 32. Alteration zoning and mineral assemblage model ................................................75

Figure 33. Alteration zoning and mineral assemblages of the Hishikari epithermal

system ...................................................................................................................78

Figure 34. Vein paragenetic relationships for the Sascha-Pelligrini epithermal system ........82

Figure 35. Vein mineral paragenetic relationships for the Sascha-Pelligrini epithermal

system ...................................................................................................................83

Figure 36. Conceptual epithermal model for the Sascha-Pelligrini epithermal system..........92

List of Tables Table 1. End-member alteration mineral spectra locations ....................................................14

Table 2. Representative whole-rock geochemical analysis. ...................................................25

Table 3. Summary geochemical signatures of Sascha Main vein phases. ............................39

Table 4. Calculated electrum-sphalerite formation temperatures...........................................42

Table 5. PIMA and XRD results of characteristic alteration assemblages .............................47

Table 6. Tabulated correlation coefficient values for immobile elements Dy, Sm and Y. ......60

List of Appendices Appendix 1. Petrography .....................................................................................................110

Appendix 2. Quartz textures. ...............................................................................................116

Appendix 3. Digital Dataset. .................................................................................... CD Pocket

Appendix 4. Sascha-Pelligrini Fact Geology Map. ................................................ Map Pocket

Appendix 5. Sascha-Pelligrini Interpretive Geology Map. ..................................... Map Pocket

VII

Statement of Original Authorship The work contained in this Thesis has not been previously submitted for a degree of diploma at any other higher education institution. To the best of my knowledge, this contains no material previously published or written by another person except where due reference is made. Signed: ___________________________ Date: ___________________________

VIII

Acknowledgements There have been numerous people who have provided invaluable assistance

throughout the duration of my project who I would like to thank. Firstly, I

would like to sincerely thank my supervisor Associate Professor David Gust

for his time, work and commitment over the duration of the project.

I would also like to acknowledge the support and technical advice provided

by Stephen Nano, Daryl Nunn and Mirasol Resources during field visits to

Argentina. The project would not have been able to succeed without them.

Thank you to the QUT technical staff, in particular Luke Nothdurft, Loc Duong

and Bill Kwiecien for their assistance and time on all the machines at QUT.

Thanks to Peter Cole from the UQ rock prep lab for helping to process all the

thin sections and samples.

Finally, special thanks must go to my family and friends for their support and

encouragement along the way.

Quinn Smith Master of Applied Science Thesis

1

Introduction

Epithermal deposits provide significant gold for world reserves. Individual

deposits can exceed 40 million ounces of contained gold (Lihir, Papua New

Guinea), produce over 1 million ounces of gold per annum (Porgera, Papua

New Guinea), and contain spectacular gold grades in excess of 100 ounces

per ton (Midas, Nevada; Hishikari, Japan). Epithermal deposits vary

significantly in size and form. They are often characterised by relatively

small, banded quartz-chalcedony veins with spectacular visible gold, or large

scale disseminated mineralisation associated with residual „vuggy‟ silica.

Their importance provides impetus for understanding how they form.

Epithermal deposits are defined as being „formed by ascending hot waters

near the surface in or near effusive rocks at relatively low temperature and

pressure‟ (Lindgren, 1922) and are analogous to modern geothermal

systems, with formation conditions of less than 200°c and less than 100 bars

(Lindgren, 1933). Recent fluid inclusion studies suggest temperatures of ore

deposition are less than 300°C, stable isotope analysis are consistent with a

meteoric source of water and a magmatic volatile source for sulphur and

carbon (Cooke and Simmons, 2000).

Initial classifications of epithermal deposits were based on geologic studies

that summarise common features and formulate schematic or conceptual

models. Conceptual models portray the anatomy of an epithermal deposit,

showing the vertical and horizontal mineral and alteration zoning typically

observed in epithermal districts (Buchanan, 1981). End-member models

were formulated to incorporate deposit variability, and lead to classification

based on depositional settings (Berger and Eimon, 1982). Detailed

paragenetic studies outline variations in mineralogy, and indicate that

epithermal deposits can be classified by observed mineral assemblages

irrespective of depositional setting (Bonham, 1986; Heald et al, 1987; Berger

and Henly, 1988).


2

Studies on phase relationships of observed mineral assemblages suggest

that the variability of epithermal deposits is due principally to variations in

fluid chemistry. Genetic classifications incorporate this observation and relate

the deposit type to the oxidation state of the mineralising fluid (White and

Hedenquist, 1990; Sillitoe, 1993; White and Hedenquist, 1995). Low-

sulphidation (LS) epithermal mineralisation forms from reduced, near-neutral

pH conditions, with H2S(aq) the predominant sulphur species. Temperatures of

ore deposition are less than 300°C and salinities are usually less than 3.5

weight percent NaCl equivalent (Cooke and Simmons, 2000). High-

sulphidation (HS) epithermal mineralisation forms from oxidised, acidic

conditions, with SO2(g) formed from the disproportionation of magmatic gases.

Temperatures of ore deposition vary from greater than 400°C to 100°C, with

salinities generally less than 5 weight percent NaCl equivalent (Cooke and

Simmons, 2000).

Current research in epithermal mineralisation indicates that HS and LS

deposits are end-members of a transitional environment, with the existence

of deposits characterised by mineral assemblages intermediate between HS

and LS deposits (Hedenquist and Arribas, 2000). Genetic classifications of

epithermal deposits are subject to continual debate. Corbett (2002, 2004, and

2005) suggests epithermal mineralisation forms largely from the same fluid

source, with end-member deposits due to differing tectonic regime, host

rocks, depth of formation, relation to intrusive bodies and dominance of

circulating meteoric fluids (Figure 1).

Specific deposits often differ from the general model, with conceptual and

genetic classifications of epithermal deposits continually evolving.

Examination of individual deposits both test and refine the general model.

This thesis aims to develop a deposit specific model for the geology, zoning

of vein mineral textures, mineral assemblages and associated geochemistry,

and alteration assemblages for the Sascha – Pelligrini LS epithermal system

in Santa Cruz, Argentina. The deposit specific model will help to clarify the

current understanding of epithermal deposits by providing a test of the

genetic and conceptual classifications. The deposit specific model may also


3

serve as a template for exploration of similar epithermal deposits throughout

Santa Cruz.

Regional geological setting

The Sascha-Pelligrini epithermal system falls within the Deseado Massif - a

large region of subdued upland physiography that is flanked by the Austral

Basin to the south, the San Jorge Basin to the north, the Andean cordillera to

the west and the Atlantic Ocean to the east. The Deseado Massif is host to

thick sequences of Permo-Triassic rift sediments emplaced in north- to

northwest-trending basins in Precambrian and Lower Palaeozoic rocks

(Echavarria et al, 2005). Precambrian and Lower Palaeozoic upper

greenschist to amphibolite facies metamorphic basement crops out within or

near the valley of the Rio Deseado. Sedimentation occurred at the onset of

widespread extensional tectonics that eventually resulted in the

Gondwanaland break-up. The Middle Triassic El Tranquilo Formation

represents the last traces of this phase of sedimentation as uplift commenced

Figure 1. Conceptual models and genetic classifications of epithermal deposits showing end-member high- and low-sulphidation styles to be part of a broader range

of hydrothermal systems. (Corbett, 2005).


4

in the Lower Jurassic. Calc-alkalic granitic stocks and dikes of the La Leona

Formation, which are found in rare localities to the east, (Sanders, 2000)

were emplaced during this uplift.

After the lower Jurassic uplift, widespread subsidence and deposition of a

range of continental-fluvial sedimentary rocks occurred in the Deseado

Massif.

Volcanic activity commenced in the Lower-Middle Jurassic. This activity was

characterised by the extrusion of flood basalts and the intrusion of mafic

dikes and sills that, together with minor clastic sedimentation, compose the

Bajo Pobre Formation. The thickness of this sequence ranges from 200 to

1,000 meters and is controlled by northeast-trending half-graben structures of

which the southern sides show the thickest accumulations (Sanders, 2000).

These structures were created by a new kinematic regime that persisted

through to the Neocomian that produced a structural fabric that is

approximately normal to the Permo-Triassic graben trends.

Unconformably overlying the Bajo Pobre Formation is the Bahia Laura

Group, which is comprised of the Middle Jurassic Chon Aike Formation and

the Middle to Upper Jurassic La Matilde & Bajo Grande Formations. These

rocks form a large ignimbritic plateau composed of lava flows, pyroclastic

rocks, ash-flow tuffs and re-worked volcanic and non-volcanic epiclastic

sequences. The volcanic rocks range in composition from basaltic-andesite

lavas and rhyodacite to rhyolitic ash-flow tuffs. The sequence represents a

marked increase in the volume and areal extent of volcanic deposition as

magmatic activity migrated westward towards the Andean continental margin

(Gust et al, 1985).

The Deseado Massif epithermal veins are contemporaneous with the waning

stages of the volcanism represented by the Bahia Laura Group rocks. Dating

of various mineral deposits (Echavarria et al, 2005) indicates that the

volcanic host rocks are only several millions of years older than the

hydrothermal systems responsible for the economic mineralisation.


5

Circular caldera structures, irregular partial-collapse features and linear

fissures are identified within the Chon Aike Formation and are locally

associated with mineralised hydrothermal alteration systems. A late-stage

resurgent dome activity has emplaced the rhyolitic and dacitic intrusive rocks

and ash-flow tuffs of the La Matilde and Bajo Grande Formations (Sanders,

2000). This period of hypabyssal volcanic activity is closely related to

economic mineralisation throughout the Massif.

A series of rock formations have been deposited within the Deseado Massif

subsequent to the Late Jurassic. These form cover sequences to the

mineralisation and represent the end of subsidence and the establishment of

„cratonic‟ stability across the Massif. These cover sequences include the

continental sediments and pyroclastic rocks of the Middle Cretaceous

Baqueró Formation and a series of back-arc, olivine basalt flows that inter-

finger with continental and shallow marine volcano-clastic sediments of

Upper Cretaceous to Upper Tertiary transgression-regression cycles (Gorring

et al, 1997). The Andean stage of Late Cainozoic uplift resulted in further

widespread eruption of olivine basalt flows and tuffs that form the dissected

plateaus of the modern landscape (Panza and Franchi, 2002). The Deseado

Massif is covered in the north and south by Late Pliocene to Recent coarse

gravels known as the “Rodados Patagónicos”.


6

Figure 2. Location of the Sascha-Pelligrini study area, distribution of Chon Aike

volcanics, operating mines and advanced exploration projects of Santa Cruz, Argentina.


7

Figure 3. Locations of samples used for whole-rock geochemistry, and alteration analysis. Also shown are the prospect locations across the study area. Sascha Main, Sascha Central and Sascha Sur comprise the Sascha Vein Zone (SVZ). Map projection WGS84 SUTM19.


8

Sascha-Pelligrini epithermal system

The Sascha-Pelligrini epithermal system comprises an area of approximately

70 square kilometers on the western edge of the Deseado Massif, north-

central Santa Cruz Province, Argentina (Figure 2). The Sascha-Pelligrini

epithermal system is expressed as intermittent outcropping epithermal veins

and pervasive silicification, and consists of the Sascha Vein Zone (SVZ),

Marcellina and Pelligrini prospects (Figure 3).

The SVZ was initially discovered during reconnaissance exploration for

Orvana in 1997. Mirasol subsequently visited the area and staked the

property in October 2003 during the inception of its Santa Cruz exploration

program (Smith et al, 2006).

The SVZ is centered on a 4.4 kilometer long vein trend and encompasses the

Sascha Main, Sascha Central and Sascha Sur zones. Sascha Main is a 1.7

kilometer-long, northwest-trending zone of intermittently outcropping, sub-

parallel veins and structural splays that collectively define a corridor reaching

300m in width. Exposed veins are up to 2 meters wide, and display classic

low-sulphidation crustiform-colloform quartz textures. Assay results from 50

vein samples average 14.23g/t gold and 89.8g/t silver, with values of up to

160g/t gold and 796 g/t silver.

Sascha Central is a continuation of the SVZ, and is expressed as small

discontinuous veinlets and un-mineralised goethite-rich shears. Sascha

Central encompasses a 1.2 kilometer long un-mineralised corridor between

the Sascha Main to the north and Sascha Sur to the south.

Sascha Sur is a 1.2 kilometer long zone of semi-continuous multi-directional

veinlets. Individual veinlet zones are up to 40 meters wide. Individual

veinlets are typically 1 to 30 centimeters wide with rare veining up to 1.5

meters wide. Assay results from 120 composite veinlet samples average 0.2

g/t gold and 3.4 g/t silver with values of up to 1.6 g/t gold and 158 g/t silver.


9

The Marcellina prospect occurs on a parallel structure to the SVZ and crops

out as a small veinlet and vein breccia zone. The veinlet zone reaches 20

meters in width, with individual veinlets typically being 1 to 20 centimetres

wide. Vein sampling has returned assays of up to 0.25 g/t gold and 1.16 g/t

silver.

The Pelligrini prospect forms a predominant topographic high within the study

area and is manifest as a large zone of intense silica replacement,

brecciation and minor veining. Assayed samples contain up to 1.47 g/t gold

and 11 g/t silver from minor quartz veins located stratigraphically below the

silica replacement horizon.

Methods

Field investigations

Geologic mapping has been completed across the study area including

detailed outcrop and vein facies mapping at 1:1000 and 1:2500 scales over

the vein zones (Appendix 3). Prospect scale geologic mapping was

completed at 1:5000 with regional mapping completed at 1:25,000 (Appendix

4 & 5). Detailed mapping was constrained by GPS-surveyed tape and

compass grids, with prospect and regional mapping constrained with rectified

air photo images and GPS. Spatial positioning of the air photo was achieved

through the use of ER Mapper, using a cubic polynomial rectification

constrained to ASTER satellite imagery with approximately 120 control

points. Original detailed outcrop mapping was subsequently repositioned with

differential GPS (DGPS) control.

Trench sections were mapped at 1:200 scale, with vein windows mapped at

1:50 scale. Trench start and end points were DGPS-positioned, with trench

length, orientation and topography measured with tape, compass and

clinometer. Survey data for individual sections were plotted on graph paper

for field mapping. Vein windows were mapped on a measured tape grid, with


10

control at 50cm intervals. Trench floor and wall geology, veining, structure

and alteration were mapped with tape and compass control along the trench

floor. 39 trenches were mapped over 1485 meters (35 at Sascha Main, 4 at

Sascha Sur)(Appendix 3). All trenches were photographed on 1 to 2 meter

intervals, with photos combined into composite images for individual

trenches.

Sample preparation and analytical techniques

Short Wave Infrared (SWIR) spectrometry

Alteration sampling was conducted across the study area for analysis by a

Portable Infrared Mineral Analyser (PIMA). Regional traverses perpendicular

to the vein trend at 250 meter sample spacing were undertaken to determine

the extent of the alteration halo and define background geological response.

Detailed traverse lines perpendicular to the vein at 5 and 1 meter sample

intervals were conducted to identify zonation within the alteration system.

Vein wall rock and vein clay samples were collected to define individual vein

phase assemblages. A total of 199 hand samples were collected across the

study area for PIMA analysis (Appendix 3). PIMA hand samples were air

dried for 48 hours, with PIMA sample surfaces cleaned of loose material prior

to analysis.

Alteration samples were analysed using a PIMA II, operated by Integrated

Spectronics control software version 3.4.0, at IAMGOLD in Mendoza,

Argentina. The PIMA II cycle count was left at the standard 0.2409, with

controller version of 1.58. An internal calibration was conducted on start-up,

and then after every 10-20 samples. Calibration was performed on samples

SP223, SP210, SP189, SP179, SP159, SP149, SP139, SP125, SP109,

SP079, SP059, and SP039. Samples were held to the sight window for an

analysis of approximately 30 seconds, with the internal reference sample

following for another 30 seconds. The PIMA II operating temperature was

maintained below 38ºC.


11

PIMA spectra were analysed with „The Spectral Geologist‟ (TSG) computer

program. Raw PIMA II spectra were interpreted through „The Spectral

Assistant‟ (TSA) within TSG. Output information included mineral 1, weight 1,

mineral 2, weight 2, TSA Error, AlOH 2200nm Absorption Wavelength, and

ALOH 2200nm Absorption Depth (Appendix 3).

X-ray Diffraction

Samples were prepared for chemical analysis and x-ray diffraction at the

University of Queensland (UQ) sample preparation laboratory. Samples were

dried for three days at 60ºC, and then crushed using a hardened steel jaw

crusher and disc mill. Rock chips were pulverised using a hardened steel

swing mill, with approximately 100 grams of material pulverised for 45

seconds to obtain an ideal particle size of 100 microns.

Samples of vein and wall rock alteration were prepared for clay and mineral

assemblage identification. Orientated clay samples were prepared by

ultrasonic dispersion of approximately 2g of pulverized material in ten times

its volume of distilled water. Material left in suspension after 5 minutes was

separated by pipette, and spread over a glass slide. The samples were left to

dry on top of a warm surface until the water had evaporated, leaving a

gravimetrically separated clay fraction.

Randomly orientated powder samples for quantitative XRD analysis were

prepared by micronisation. Approximately 3g of pulverized material and 12ml

of alcohol were placed into the micronisation mill using agate cylinders and

milled for 5 minutes. The slurry obtained is homogenous and the particle size

is ideally 1-5 microns. The slurry was placed into pre-labeled glass beakers

and left to dry in an oven at 60°c. Once dried, about 1.5-2g of sample was re-

mixed and lightly packed into circular aluminum sample holders.

58 PIMA and hand samples of vein and wall rock alteration were analysed for

clay and mineral assemblages by X-Ray Diffraction (XRD) at the X-Ray

Analysis Facility (XAF) Queensland University of Technology (QUT). The


12

XRD analyses were carried out on a Philips wide-angle PW 1050/25 vertical

goniometer using Co Kα radiation. The samples were measured with steps of

0.02° 2θ and a scan speed of 1.00° per minute from 3 to 75° 2θ. Spectra

were analysed with the software packages TRACES and SIROQUANT for

mineral identification (Appendix 3).

Microscopy

Twenty-four cover slipped sections were prepared for petrographic analysis.

Nineteen polished sections were made of vein samples for petrographic and

microprobe analysis. Twelve alteration samples were prepared by breaking

rock fragments to expose fresh surfaces, and mounting on aluminium stubs

with carbon tape. Polished sections and mounted rock fragments were

carbon coated prior to microprobe analysis.

Microprobe analyses were undertaken at the QUT Analytical Electron

Microscopy Facility using a JEOL-JXA-840A Scanning Electron Microprobe

with an Energy-Dispersive Spectrometry (EDS) detector. Operating

conditions for the quantitative determination of mineral chemistry were: 20kV

accelerating voltage, beam current of approximately 1.7nA, count time of 100

seconds, 38mm working distance, 40° take off angle for the EDS detector

and a focused beam of <10μm in diameter. EDS spectra were collected and

interpreted through Moran Scientific quantitative EDS software. Vein minerals

were probed between 2 and 8 times from core to rim, with a total of 381

spectra collected from 8 sections (Appendix 3).

Clay morphology analyses were undertaken at the QUT Analytical Electron

Microscopy Facility using a FEI Quanta Environmental Scanning Electron

Microscope with an Energy-Dispersive Spectrometry (EDS) detector

Samples were analysed in high vacuum with operating voltage between 15

and 20kV. Working distance was set to 10mm with a spot size of 3 to 4

angstroms (Appendix 3).


13

Geothermometry

Formation temperatures are calculated from compositional relationships

between coexisting electrum and sphalerite mineral grains. The equation

derived by Shikazono (1985) uses the mole fraction of silver in electrum and

the mole fraction of FeS in sphalerite to calculate a pressure independent

temperature and is expressed as:

T = (28,765 + 22,600 x (1- NAg)2 – 6,400 x (1- NAg)

3) /(49.008 – 9.152 log XFeS + 18.2961 log

NAg + 5.5 x (1- NAg)2),

Where NAg, XFeS, and T denote mole fraction of silver in electrum, mole

fraction of FeS in sphalerite and absolute temperature in degree Kelvin (+/-

20º) respectively.

Geochemistry

Twenty-four whole rock samples, including 2 quartz blanks, 1 standard and 1

duplicate were analysed for major, trace and rare-earth geochemistry. Rock

sample SEQG566 (Moultrie, 1995), with known major and trace element

geochemistry, was used as an internal standard for comparison (Figure 4)

(Appendix 3). Major elements (Si, Al, Ca, Fe, K, Mg, Mn, Na, P, Ti, S, and

Cr) were determined by X-ray fluorescence (XRF) Silicate Fusion at Ultra

Trace Analytical Laboratories in Perth, Western Australia. Glass beads were

prepared using a sample/flux ratio of 12:22.

Trace and rare earth elements (Ag, As, Ba, Ce, Co, Cs, Cu, Dy, Er, Eu, Ga,

Gd, Hf, La, Li, Nb, Nd, Rb, Sb, Sm, Sr, Th, Y,Yb, and Zr) were determined by

Inductively Coupled Mass Spectrometry (ICP-MS) at Ultra Trace Analytical

Laboratories.

Samples were digested in hydrofluoric, nitric, hydrochloric and perchloric

acids allowing a total digestion in most samples. Loss on ignition (LOI) was

determined with samples heated between 105 and 1000 degrees Celsius.

LOI results were determined gravimetrically and reported on a dry sample

basis.


14

Remote sensing

Image acquisition

ASTER Level 1B data was acquired through NASA‟s data acquisition request

(DAR) process. A formal research proposal was submitted to NASA for

ASTER data acquisition. The proposal was submitted through;

http://asterweb.jpl.nasa.gov/gettingdata/authorization/proposal.asp

The proposal was accepted by NASA, with Michael Abrams, ASTER Science

Team Leader Jet propulsion laboratory, uploading two level 1B scenes to the

asterweb.jpl.nasa.gov FTP site for download. The scenes were downloaded

and contained the following image identification numbers;

AST_L1B_003_03122003143108_03272003165841.hdf

And

AST_L1B_00301192005143600_01312005113946.hdf

y = 1.0618x

R2 = 0.9987

0

50

100

150

200

250

300

350

0 50 100 150 200 250 300 350

Trace Element Analysis (ppm)

Tra

ce

Ele

me

nt

Sta

nd

ard

(p

pm

)

Figure 4. Comparison of accepted results for rock standard SEQG566 with returned analysis performed by Ultra Trace Laboratories, Perth.

http://asterweb.jpl.nasa.gov/gettingdata/authorization/proposal.asp


15

Image processing

ASTER scene „AST_L1B_00301192005143600_01312005113946.hdf‟ was

cloud-free over the entire study area and was subsequently chosen for

multispectral image processing.

The level 1B ASTER scene was atmospherically corrected through the use of

the computer software „FLASH‟, utilising a modified transform 5 process. The

atmospherically corrected ASTER scene was processed in ENVI4.2 and

registered from the ephemeral satellite information. Mineral spectra were

identified through band rationing outlining the presence of kaolinite, illite and

alunite. The most spectrally pure pixels were determined through the use of

the „Pixel Purity Index‟ (Broadman et al, 1995), highlighting end-members of

kaolinite, illite and alunite. End-member spectra were chosen from the image

for comparison with the USGS spectral library and subsequently used as a

standard for further processing.

The locations for each of the end-member mineral spectra are as follows;

Mineral Spectra Easting Northing Datum/Projection

Alunite 415106E 4713855N WGS84/SUTM19

Illite 410096E 4705185N WGS84/SUTM19

Kaolinite 412646E 4705425N WGS84/SUTM19

Table 1. End-member mineral spectra locations used for ASTER image processing.

The ASTER scene was subset to the study area and transformed to

„minimum noise fraction‟ (MNF) space to reduce background noise and

spectral scatter (Broadman et al, 1995). Natural surfaces are rarely

composed of a single uniform material and spectral mixture modelling is

necessary to identify areas of mixed spectral signatures (Kruse and

Hintington, 1996). The subset MNF image was processed with Global Ore

Discovery‟s proprietary „mixture tuned matched filter‟ (MTMF) analysis to

identify pixels containing variable amounts and mixtures of kaolinite, illite and

alunite. The MTMF analysis produced information related to abundance, and

infeasibility for the mineral within a given pixel. Pixels containing a low


16

infeasibility and high abundance for a given mineral were subjectively chosen

by evaluating populations from a scatter plot of infeasibility versus

abundance. The MTMF data was smoothed with a convolution median filter

on a 3x3 pixel matrix.

Abundance grids for kaolinite and illite were exported as high resolution

geotiff's and subsequently opened in MapInfo. The alunite shape files were

opened in MapInfo and converted to vector data. The ASTER VNIR 231

image was compressed and exported to ecw format (Appendix 3).

Results

Stratigraphy

The volcanic stratigraphy of the Sascha-Pelligrini area is divided into five

units; of these, three units comprise the Chon Aike Formation and two

comprise the La Matilde Formation (Figure 5). The Chon Aike Formation

consists of a basal massive, biotite rhyodacite welded ignimbrite, a middle

pumiceous, biotite rhyodacite welded tuff, and an upper clast-rich, welded

rhyodacite crystal ash tuff. The La Matilde Formation comprises a basal lithic

ash rhyolite tuff which grades into a crystal ash rhyolite tuff, and is overlain by

a unit of finely laminated rhyolite ash tuffs with spherulitic and accretionary

lapilli horizons. Unit thickness is highly variable and the thicknesses reported

are representative of maximum exposed thickness. The Chon Aike Formation

is 678 meters thick and the La Matilde Formation is 130 meters thick in the

Sascha-Pelligrini area. Reported Chon Aike Formation and La Matilde

thicknesses range from 300 to >900 meters and 15 to >175 meters

respectively (Echavarria et al, 2005; Sanders, 2000).

The lower unit of the Chon Aike Formation crops out throughout the study

area and is relatively homogenous in appearance (Figure 6; Appendix 4 & 5).

Internal variations include variably welded horizons and the inclusion of small

clasts of rare mica schist. Welding within the rhyodacite ignimbrite varies

both vertically and horizontally, and forms composite welding horizons


17

preserved as topographic highs across the study area. The estimated

thickness of the unit is 570 meters. The overlying unit is similar in

composition, but contains pumice and occasional hematite-chlorite altered

juvenile lava clasts. It is also welded with compaction ratios of fiamme of up

to 10:1. The estimated maximum thickness of this unit is 100 meters. The

top of the Chon Aike Formation is composed of a welded rhyodacite crystal

ash tuff that contains juvenile and accidental clasts comprised of fine grained

granite, hematite-chlorite, and mica schist. The unit also contains abundant

large angular spherulitic devitrified volcanic glass and pumice fragments. The

estimated thickness of the horizon is 8 meters.

The La Matilde Formation exhibits paraconformable contacts with the

underlying sequence. The basal unit of the La Matilde Formation is a rhyolitic

tuff that varies from a sparsely distributed lower lithic –rich horizon (~10 to 20

meters thickness) to a more geographically widespread upper ash-flow tuff

(~75 meters thickness). The lithic-rich horizon contains accidental clasts of

angular and rounded metamorphic rocks. Clast size and angularity increase

towards the northwest with the unit becoming pumiceous towards the

southeast. The upper ash-flow is a rhyolite tuff with occasional devitrification

textures. Finely laminated rhyolite ash tuffs with locally developed spherulites

and accretionary lapilli horizons crop out within the Pelligrini prospect (Figure

6; Appendix 4 & 5) and overlie the ash-flow tuff. The laminated ash tuff

mantles the topography of the underlying unit, and attains a maximum

exposed thickness of 55m. The La Matilde Formation exhibits extreme

vertical and lateral variation.

A large flow-banded to spherulitic rhyolite dome with auto-brecciated margins

crops out within the Pelligrini prospect and intrudes into the upper-most unit

of the La Matilde Formation (Figure 5 & 6; Appendix 4 & 5). The volcanic tuff

sequences are intruded by small, spatially restricted, biotite-albite porphyritic

granodiorite dykes and sills.

The Jurassic volcanic sequence is unconformably overlain by a 15 meter

thick Oligocene feldsarenite to sparry grainstone that is preserved in


18

topographic depressions. The unit has a conglomeritic base comprised of

rounded tuff fragments to 10 centimeters in diameter, grading upward to a

sandy feldsarenite intercalated with bivalve, gastropod and bryzoan

fragments. Epiclastics and fine laminated rhyolitic ash surges with prominent

cross stratification and carbonized plant fragments are locally developed at

the top of the unit (Appendix 1, plate 12). The unit is best exposed to the

west of the SVZ and intermittently crops out under overlying cover

sequences (Figure 6; Appendix 4 & 5).

Pliocene olivine tholeiite basalt forms a large plateau that runs through the

middle of the study area and also intermittently crops out as remnant plugs

and dykes. Pleistocene gravels and recent sediments cover most of the low-

lying areas, with gravels being best preserved as plateau caps to the

Oligocene feldsarenite (Figure 6; Appendix 4 & 5).


19

Figure 5. Stratigraphy of the Sascha-Pelligrini study area showing correlated unit

age, diagrammatic relationship of rock units, unit thicknesses and grain size.


20

Figure 6. Interpretive geology, structure and mapped veining of the Sascha-Pelligrini study area. Unit colours are the same as figure 5.


21

Petrography

Least altered samples from each lithology within the study area were

selected for petrographic examination.

Chon Aike Formation

The lower ignimbrite of Chon Aike Formation is crystal rich with subordinate

lithic clasts. The phenocryst assemblage comprises sodic plagioclase (30%),

sanidine (20%), quartz (30%), biotite (10%), muscovite (5%), ilmenite and

magnetite (5%). Plagioclase and sanidine phenocrysts occur as moderately

altered, subhedral to euhedral, broken fragments ranging up to ~4mm in

length. Quartz phenocrysts are large, ranging up to ~5mm in size, and are

significantly embayed. Biotite and rare muscovite occur as small plates and

books ranging up to ~2mm in size, and are usually altered to chlorite and

sericite respectively. Ilmenite and magnetite constitute minor phenocryst

phases but are generally abundant in the less altered groundmass. Very

small accessory phases including zircon and apatite rarely occur in the

samples. Devitrified glass shards (<1mm) comprise the majority of the

groundmass. The glass shards are intensely welded with no original texture

preserved (Appendix 1, Plate 1).

The middle welded tuff of the Chon Aike Formation is pumice and ash rich

with rare juvenile lava clasts. Strongly welded and deformed devitrified glass

shards (<1mm) comprise the majority of the unit with no original x, y or

cuspate shapes preserved. Pumice glass is totally altered, with rare axiolitic

devitrification developed on pumice margins and spherulites developed within

pumice interiors. Pumice clasts are strongly flattened, range in size of up to

12cm in length and define a strong eutaxitic texture. The phenocryst

assemblage of the middle unit comprises albite (20%), sanidine (15%),

quartz (30%), biotite (10%), pumice (20%), ilmenite and magnetite (5%).

Albite and sanidine phenocrysts are subhedral, strongly sericitised, and

range up to ~2mm in length. Biotite occurs as subhedral to euhedral plates

and minor books ranging up to ~2mm in length, and is also strongly


22

sericitised. Quartz occurs as angular subhedral fragments ranging up to

~4mm in size (Appendix 1, Plate 2).

The upper crystal ash tuff of the Chon Aike Formation is crystal and ash rich

with abundant accidental and juvenile lava clasts. The unit is comprised of

abundant moderately welded and strongly deformed devitrified glass shards

(<1mm). The horizon is unique with rare cuspate shapes preserved within the

glass shards. Strong mantling and welded textures occur around larger lithic

and juvenile clasts. Phenocryst are comprised of albite (20%), sanidine

(25%), quartz (35%), biotite (15%), ilmenite and magnetite (5%). Albite and

sanidine phenocrysts occur as moderately altered subhedral to euhedral

broken fragments ranging up to ~3mm in length. Quartz phenocrysts range

up to ~4mm in size, and are moderately embayed. Biotite and rare muscovite

occur as small plates and books ranging up to ~2mm in size, and are usually

altered to chlorite and sericite respectively. Ilmenite and magnetite constitute

minor phenocryst phases and are generally abundant only in the lesser

altered groundmass. Lithic clasts distinct to the horizon are comprised of rare

mica schist and strongly altered fine-grained granitic fragments. Abundant

juvenile lava clasts range up to 15mm in diameter and are strongly altered to

chlorite and hematite. Angular volcanic glass fragments contain abundant

spherulitic devitrification textures and range up to 10mm in size. Silicification

replaces pumice glass with fine grained quartz, preserving devitrification

textures (Appendix 1, Plate 3).

La Matilde Formation

The basal unit of the La Matilde Formation is crystal rich with abundant lithic

clasts. The phenocrysts comprises feldspar (35%), quartz (45%), muscovite

(15%), ilmenite and magnetite (5%). Feldspars are subhedral broken

fragments, completely sericitised, and range up to 5mm in size. Quartz

phenocrysts are subhedral broken fragments and range up to 2mm in size.

Muscovite occurs as small plates and is strongly sericitised. Weakly-welded

devitrified glass shards (<1mm) comprise the groundmass, with no original

shard textures preserved. Lithic clasts are comprised of distinct rounded

muscovite schist and range up to 10cm in size (Appendix 1, Plate 4).


23

The lithic basal unit of the La Matilde Formation grades into an ash-rich

crystal rhyolite tuff. The phenocryst assemblage of the crystal rhyolite tuff

comprises sanidine (30%), quartz (50%), and muscovite (20%). Sanidine

phenocrysts occur as large subhedral to euhedral broken fragments and

range up to 3mm in size. Quartz phenocrysts occur as strongly embayed

subhedral broken fragments and range up to 3mm in size. Muscovite

phenocrysts range up to 1mm in length and occur as small plates. Poorly

welded devitrified glass shards comprise the majority of the groundmass,

with rare cuspate textures preserved. Rare axiolitic to bow-tie devitrification

textures form around phenocrysts (Appendix 1, Plate 5).

The upper unit of the La Matilde Formation is comprised of abundant ash

with minor phenocrysts. Broken, subhedral to euhedral sanidine crystals are

the only phenocrysts phase in the unit. Intensely altered glass shards

comprise the majority of the tuff, with no original shard textures preserved.

Silicification is strong within the unit and replaces volcanic glass, preserving

spherulitic devitrification and accretionary lapilli textures (Appendix 1, Plate

6).

Other units

The flow-banded rhyolite contains euhedral phenocrysts within a finely

crystalline groundmass. Euhedral sanidine crystals are the only phenocryst

phase present. The strongly sericitised groundmass is comprised of very fine

feldspar and quartz crystals. Flow-banded textures are distinguished by

alternating quartz-rich and feldspar-rich layers. Large spherulitic

devitrification textures (<5mm) develop within the unit in areas that are glass-

rich and phenocryst-poor. Silicification is strong within the unit preserving

flow-banded and spherulitic textures (Appendix 1, Plates 8 & 9). Auto-breccia

is locally developed around the margins of the flow-banded rhyolite. Large

angular clasts of flow-banded and spherulitic rhyolite up to 2 meters in

diameter are hosted within a finely crystalline groundmass composed of

euhedral sanidine and quartz crystals (Appendix 1, Plate 7).


24

Granodiorite dykes and sills intrude the tuff sequence and contain large

euhedral phenocrysts within a crystalline groundmass. The phenocryst

assemblage is composed of sodic plagioclase (50%), hornblende (20%),

biotite (15%), quartz (10%), ilmenite and magnetite (5%). Plagioclase

phenocrysts are moderately altered to sericite, occur as euhedral crystals,

rarely show concentric zoning, and range up to ~4mm in size. Hornblende

and biotite phenocrysts are strongly altered to chlorite, sericite and minor

calcite and range up to ~3mm in size. Quartz occurs as small euhedral

phenocrysts and range up to ~1mm in size. Ilmenite and magnetite constitute

minor phenocryst phases and range up to ~0.5mm in size. Phenocrysts

occur in a feldspar lath groundmass (Appendix 1, Plate 10).

The feldsarenite to sparry grainstone is well sorted with variably rounded

clasts and high porosity. The clasts comprise feldspar (20%), quartz (35%),

lithics (5%), echinoderm, gastropod and mollusc fragments (40%). Clasts are

variably altered, mostly matrix supported, and cemented with sparry calcite.

Quartz fragments range from euhedral crystals to well rounded and embayed

clasts up to ~2mm in size. Lithic fragments are comprised of altered tuffs and

angular chalcedonic quartz fragments (Appendix 1, Plates 11 & 12).

Basalts occur as remnant dykes, plugs and flows, and are aphanitic to

slightly porphyritic. Basalt flows are vesicular, and comprised of fine grained-

olivine and feldspar. Feldspars range in composition from oligoclase to

labradorite, with minor zeolites infilling vesicles. Basalt dykes and plugs are

slightly porphyritic and weakly chloritised. Euhedral feldspars are weakly

albitised, range from oligoclase to labradorite, and are in a feldspar lath

groundmass (Appendix 1, Plates 13 & 14).


25

Rhyodacite ignimbrite

Pumice Rhyodacite

Tuff

Pumice Ash Rhyodacite

tuff

Crystal Lithic

Rhyolite Tuff

Crystal Ash

Rhyolite Tuff

Rhyolite Ash Tuff

Flow-banded Rhyolite

Sample QR25 QR05-27 QR1 QR23 QR05-26 QR05-21 QR05-37

SiO2 63.2 74.5 70.3 84.9 76.1 78.3 77.1

TiO2 0.6 0.28 0.23 0.17 0.07 0.18 0.15

Al2O3 15.4 14.8 13.6 8.28 13.1 10.6 12.6

Fe2O3 5.14 1.16 2.39 1.46 1.55 1.46 0.39

MnO 0.13 BDL 0.06 BDL 0.02 0.03 0.02

MgO 1.52 0.27 0.46 0.23 0.14 0.26 0.2

CaO 4.41 0.14 1.79 0.43 0.21 0.14 0.09

Na2O 2.63 1.38 2.9 0.1 0.22 1.5 0.99

K2O 3.23 4.36 4.45 0.94 5.41 3.24 4.9

P2O5 0.172 0.019 0.068 0.032 0.03 0.049 0.035

SO3 BDL 0.02 0.29 0.54 0.03 1.06 0.12

LOI 3.54 2.18 2.38 3.43 2.67 3.13 1.55

Total 99.97 99.11 98.92 100.51 99.55 99.95 98.15

Ag BDL BDL BDL BDL BDL BDL 2

As 1 2 14 127 248 3 4

Co 32 8 22 BDL 6 2 12

Cu 9 2 2 13 3 4 4

Mo 8.5 3 7 6.5 3.5 1.5 3.5

Ga 16.4 15.2 14.4 9.4 11.8 13.8 18.6

Sb 0.6 1 1 14.6 2.4 0.4 2

Rb 115 182 159 74.6 220 117 191

Sr 315 82.5 215 23 24.5 137 137

Y 20.9 15.6 19.3 11.3 18.6 6.7 11.7

Zr 60 67 70 43 87 100 86

Nb 6.5 4.5 7 5 10 9 10

Cs 4.9 8.6 8.2 4.1 5.6 2.4 4.3

Ba 902 943 996 81 889 830 1350

La 29.9 29.3 39.2 20.3 29.8 25.2 31.1

Ce 61.1 57.1 69.7 39.7 60.2 41.2 64.4

Nd 25.1 20.9 25.4 14.8 26.1 14.4 23.8

Sm 5.05 4 4.65 3 5.5 2.6 4.5

Eu 1.2 0.8 0.85 0.55 0.5 0.4 0.7

Gd 4.2 3 3.6 2.4 3.8 2 3

Dy 3.75 2.7 3.25 2.05 3.8 1.55 2.45

Er 2.2 1.65 1.95 1.25 2.35 0.8 1.35

Yb 2.15 1.9 2.1 1.3 2.8 0.85 1.5

Hf 1.8 2.4 2.6 1.4 3.6 4.8 3

Th 12.5 17.3 18.8 9.7 21.5 6.6 21.1

Table 2. Representative whole-rock geochemical analyses for Jurassic volcanic units of the Sascha-Pelligrini study area.


26

Geochemistry

Least altered representative samples of each of the volcanic tuffs were

analysed for both major and trace elements (Table 2). REE values for Bajo

Pobre andesite (Pankhurst and Rapela, 1995) were used to test for fractional

crystalisation trends within the volcanic suite, with REE values for the Sierra

Los Chacays xenoliths (Pankhurst and Rapela, 1995) used to test for partial

melting of the upper crust.

The volcanic rocks of the Sascha-Pelligrini area are high-K rhyodacites and

rhyolites with K2O contents ranging from 3.2 to 5.4 weight percent. SiO2

contents vary from 63 to 78 weight percent, with most of the samples being

greater than 70 weight percent SiO2. Na2O concentrations are low, and show

and inverse correlation with K2O concentration (Figure 7). The Sascha-

Pelligrini volcanic tuffs have a broad range in Al2O3 content (8.28 to 15.4

weight percent), Fe2O3 content (1.16 to 5.14 weight percent) and CaO

content (0.14 to 4.41 weight percent).

Groups identified on the basis of stratigraphy and petrography are easily

discernable on most major and trace element graphs (Figures 7 and 8). Chon

Aike Formation rhyodacites are distinguished primarily by their relatively low

SiO2 contents (63 to 74 weight percent). Abundances of all major elements

except for K2O have correlations with SiO2, however Al2O3, TiO2, NaO and

K2O show decrease in concentrations above 70 weight percent SiO2. Trace

elements Sr and Yb decrease with increasing SiO2, with Rb, Cs and Zr

showing correlation with SiO2. Trace elements Ba, Ce, Hf, Nb and Th

increase in concentration to 70 weight percent SiO2, with a marked decrease

in concentration above 70 weight percent SiO2.

La Matilde Formation rhyolites are distinguished by their very high SiO2

contents. The rhyolites have a limited SiO2 content ranging from 76.1 to 78.3

weight percent. Abundances of Al2O3, Fe2O3, CaO and K2O decrease with

increasing SiO2 content, while TiO2, MgO, NaO and P2O5 correlate with SiO2.

Trace elements Sr, Zr, and Hf correlate with SiO2, with Ba, Cs, Rb, Ce, Yb,

Nb and Th contents decreasing with increasing SiO2.


27

Chondrite-normalised REE patterns of the Sascha-Pelligrini samples are

light-REE enriched (Figure 9). LREE concentrations range between 85 and

165 times chondritic levels, with Yb concentrations approximately 5.2 to 17.4

times chondritic levels. La/Yb ratios vary from 7.2 to 20.2. REE patterns

shallow towards the heavy-REE, with La/Gd values from 5.9 to 10.6 and

Gd/Yb values from 1.09 and 1.90. Chondrite-normalised REE patterns for the

Chon Aike Formation are smooth and near parallel (Figure 9), with minor

negative Eu anomalies. Chondrite-normalised REE patterns for the La

Matilde Formation have a distinct negative Eu anomaly. La Matilde rhyolites

diverge towards the heavy-REE, with Dy/Yb values between 0.88 and 1.19.

Chondrite-normalised REE values for the flow-banded rhyolite are similar to

the La Matilde rhyolite tuffs, and show a negative Eu anomaly.

The Rhyodacite suite REE pattern is relatively flat normalised against crustal

abundances, with slight negative Nd and Y, and positive Yb anomalies

(Figure 10). REE patterns show a decrease in total REE concentrations with

increasing SiO2. The Rhyodacite suite of samples is slightly enriched relative

to upper crust and shows a small positive Eu anomaly. La concentrations

range from 0.9 to 1.3 times upper crust. Ce/Yb values range from 0.85 to

1.14. Increasing SiO2 and K2O in the rhyolite suite leads to a slight depletion

in REE relative to upper crust, with Eu inverting to a small negative anomaly.

La concentrations in high SiO2 rhyolites range from 0.6 to 1.0 times upper

crust, with Ce/Yb values from 0.73 to 1.66.

REEs for the Rhyodacite suite are slightly enriched, with small negative Eu

and Y anomalies (Figure 10). La concentrations range between 2.0 and 2.7

times Bajo Pobre. Ce/Yb values range from 1.27 to 1.49. REE patterns show

a progressive depletion in heavy REE‟s with increasing SiO2, with negative

Eu, Y and positive Yb anomalies becoming more pronounced. Highest SiO2

rhyolites are enriched in light, and depleted in heavy REE‟s relative to Bajo

Pobre andesite. High SiO2 rhyolites have La concentrations which range from

1.7 to 2.1 times Bajo Pobre. Ce/Yb values range from 0.96 to 2.18.


28

Sierra Los Chacays xenolith normalised REE patterns are light-REE enriched

(Figure 10). The Rhyodacite suite show slight negative Eu, Y and positive Yb

anomalies. La concentrations range between 13.7 and 18.3 times xenolith

levels. Ce/Yb values range from 2.78 to 3.25. REE patterns shallow towards

the heavy-REE, with Ce/Sm values from 2.53 to 3.14 and Sm/Yb values from

0.98 to 1.10. REE patterns show a progressive depletion in heavy REE‟s with

increasing SiO2, with negative Eu, Y and positive Yb anomalies becoming

more pronounced. High SiO2 rhyolites have La concentrations which range

from 11.81 to 14.57 times Sierra Los Chacays xenoliths. Ce/Yb values range

from 2.10 to 4.75. REE patterns of high SiO2 rhyolites shallow towards the

heavy-REE, with Ce/Sm values from 2.29 to 3.32 and Sm/Yb values from

0.91 to 1.43.


29

Figure 7. XRF major element results for Sascha-Pelligrini volcanic samples in weight percent plotted against SiO2. Least altered samples for each of the stratigraphic units

are shown in red.

TiO2

MgO

NaO

P2O5

SiO2

0

0.2

0.4

0.6

0.8

60 65 70 75 80 85 90

0

0.5

1

1.5

2

60 65 70 75 80 85 90

0

1

2

3

4

60 65 70 75 80 85 90

0

0.05

0.1

0.15

0.2

60 65 70 75 80 85 90

Chon Aike

La Matilde

Flow Banded Rhyolite

Chon Aike Least Altered

La Matilde Least Altered

Al2O3

Fe2O3

(total)

CaO

K2O

SiO2

6

8

10

12

14

16

18

60 65 70 75 80 85 90

0

1

2

3

4

5

6

60 65 70 75 80 85 90

0

1

2

3

4

5

60 65 70 75 80 85 90

0

1

2

3

4

5

6

60 65 70 75 80 85 90


30

Figure 8. ICP-MS trace element results (Sr, Ba, Cs, Rb, Ce, Yb, Zr, Hf, Nb & Th) for

Sascha-Pelligrini volcanic samples in parts per million (ppm) plotted against SiO2.

Cs

Ce

Zr

Nb

2

4

6

8

10

12

60 65 70 75 80 85 90

0

100

200

300

60 65 70 75 80 85 90

0

2

4

6

8

10

12

60 65 70 75 80 85 90

0

10

20

30

40

50

60

70

80

90

100

60 65 70 75 80 85 90

40

60

80

100

120

60 65 70 75 80 85 90

0

200

400

600

800

1000

1200

1400

1600

1800

60 65 70 75 80 85 90

0

50

100

150

200

250

300

60 65 70 75 80 85 90

0

1

2

3

60 65 70 75 80 85 90

1

2

3

4

5

6

60 65 70 75 80 85 90

5

10

15

20

25

60 65 70 75 80 85 90

Chon Aike

La Matilde

Flow Banded Rhyolite

Chon Aike Least Altered

La Matilde Least Altered

SiO2 SiO2

Sr Ba

Rb

Yb

Hf

Th

Cs

Ce

Zr

Nb


31

Rhyolite Tuff (La Matilde)

1

10

100

1000

La Ce Nd Sm Eu Gd Dy Y Yb

Rhyolite Ash Tuff (QR05-21)

Crystal Ash Rhyolite Tuff (QR05-26)

Lithic Rhyolite Tuff (QR23)

Rhyodacite Ignimbrite (Chon Aike)

1

10

100

1000


Rhyodacite Ash Tuff (QR1)

Pumaceous Rhyodacite Tuff (QR05-27)

Crystal Ash Rhyodacite Ignimbrite (QR25)

Rhyolite Intrusive (La Matilde)

1

10

100

1000


Flow Banded Rhyolite (QR05-37)

Figure 9. Chondrite-normalised REE diagrams for least altered Sascha-Pelligrini volcanic stratigraphic units. Normalised to C1 chondrite of McDonough and Sun (1995).


32

Figure 10. REE geochemistry for least altered stratigraphic units from the Sascha-Pelligrini study area normalised to upper crust, Bajo Pobre andesite, and Sierra Los

Chacays xenoliths with increasing primitive lower crustal signatures respectively.

Upper Crust Normalised REE (Rudnick & Gao, 2003)

Bajo Pobre Normalised REE (Pankhurst & Rapela, 1995)

Sierra Los Chacays Xenolith Normalised REE

(Pankhurst & Rapela, 1995)

Rh

yo

da

cit

e Ig

nim

bri

te (

Ch

on

Aik

e)

0.11

10

La

Ce

Nd

Sm

Eu

Gd

Dy

YY

b

Pu

mic

e A

sh

Rh

yo

dacit

e T

uff

(S

022)

Pu

maceo

us R

hyo

dacit

e T

uff

(S

023)

Cry

sta

l A

sh

Rh

yo

dacit

e I

gn

imb

rite

(S

024)

0.11

10

La

Ce

Nd

Sm

Eu

Gd

Dy

YY

b

0.11

10

10

0

La

Ce

Nd

Sm

Eu

Gd

Dy

YY

b

Rh

yo

lite

Flo

w D

om

e (

La M

ati

lde)

La

Ce

Nd

Sm

Eu

Gd

Dy

YY

b

Flo

w B

an

ded

Rh

yo

lite

(S

007)

La

Ce

Nd

Sm

Eu

Gd

Dy

YY

b

La

Ce

Nd

Sm

Eu

Gd

Dy

YY

b

Rh

yo

lite

Tu

ff (

La M

ati

lde)

La

Ce

Nd

Sm

Eu

Gd

Dy

YY

b

Lam

inate

d R

hyo

lite

Ash

Tu

ff

Cry

sta

l A

sh

Rh

yo

lite

Tu

ff (

S008)

Cry

sta

l L

ith

ic R

hyo

lite

Tu

ff (

S018)

La

Ce

Nd

Sm

Eu

Gd

Dy

YY

b

La

Ce

Nd

Sm

Eu

Gd

Dy

YY

b


33

Structural setting

The SVZ is hosted on a right-lateral, oblique-slip fault system termed the

Sascha Fault (Figure 11). Sascha Main is hosted within a normal fault-

bounded graben trending approximately 315°. Mineralised veins are

developed at right-stepping structural splays along the southwesterly dipping

315° trend. North-trending left-lateral and east-trending right-lateral structures

within the normal faulted block are often unmineralised and offset the vein

trend. The graben is comprised of La Matilde rhyolite tuffs which are inferred

to thicken to the southeast and thin towards the northwest. Structural studies,

based on airphoto and satellite image interpretation, indicate the block is

bound by regional northeast-trending left-lateral basement transfer

structures. The 315° trending Sascha fault rotates through to a north-trending

360° orientation in Sascha Central. The orientation change of the Sascha

fault is controlled by, and bounded by, northeast-trending left-lateral transfer

structures. Epithermal veining is limited to small, unmineralised quartz

veinlets, within the north-trending Sascha fault.

Figure 11. Mapped surface vein trace orientation in Sascha Main indicating dextral oblique-slip movement with mineralised veins forming on right-stepping structural

splays to the main 315° trending Sascha fault.


34

The Sascha fault changes orientation from north-trending within the transfer

structure corridor, back to 315°on the northern margin of Sascha Sur.

Epithermal veining is hosted within the hanging-wall of a half-graben. The

half-graben is bounded to the west by the northeast dipping, oblique-slip,

normal Sascha fault. The strike extent of the veinlet zone is controlled by

northeast-trending left-lateral transfer structures that offset the Sascha fault.

North-trending left-lateral, and east-trending right-lateral structures offset the

vein trend with the half-graben block.

The Marcellina veinlet zone parallels Sascha Sur, and is hosted within a 315º

trending normal fault. Epithermal veining is hosted within the hanging wall of

a half graben structure. The normal fault dips towards the northeast,

extending under basalt cover to the north, and recent sediment to the south.

Pelligrini is hosted within a similar structural setting to Sascha Sur and

Marcellina. Epithermal veining is hosted within the hanging wall blocks of

315º trending half grabens. Veining is controlled by northeast dipping

oblique-slip normal structures, bounded to the north and south by northeast-

trending right-lateral transfer structures.


35

1 cm

A B

1 cm

A B D C E F G H

0.1 mm

I

A. Wall rock silicification B. Initial crustiform gold-silver ginguro band

(Ginguro Stage I) C. Pseudoacicular fine bladed quartz D. Colloform/crustiform chalcedony E. Colloform/crustiform chalcedony-ginguro bands

with minor adularia (Ginguro Stage II) F. Secondary kaolinite G. Coarse lattice bladed quartz H. Saccharoidal and vuggy quartz I. Photomicrograph of gold and acanthite within

chalcedony-ginguro band (Reflected PPL x40)

0.25 mm

1 cm

A

B

C

A. Saccharoidal and vuggy quartz B. Crustiform banded chalcedonic quartz

with disseminated pyrite infilling cavity within saccharoidal vein phase

C. Photomicrograph of fine disseminated pyrite within chalcedonic quartz (Reflected PPL x10)

Figure 12. Sascha Main saccharoidal vein phase (B) overprinting the earlier

chalcedonic (A) vein phase.

Figure 13. Sascha Main ginguro vein phase.

Figure 14. Pyritic chalcedonic vein phase overprinting saccharoidal vein phase.


36

Epithermal veining

Quartz textures

Veins in the Sascha Main prospect are subdivided into a series of vein-types

with different textural and geochemical signatures. These veins vary from

chalcedonic to crystalline comb quartz.

Chalcedonic veins are white-grey to brown, massive to finely-banded chaotic

veins and veinlets; they generally occur on the western side of the vein trend

(Figure 12). Angular leached clasts of argilllised wall rock that include

strongly colloform-banded, fine saccharoidal silica define zones of single

pulse tectonic breccia. These zones generally occur as large deflation

surfaces with no outcrop expression and are considered equivalent to the

chalcedonic phase (Appendix 2, Plate 1).

Saccharoidal veins with minor bladed textures are white and consist of

medium- to coarse-grained saccharoidal and crystalline silica with prominent

bladed carbonate pseudomorphs (Appendix 2, Plates 3 & 4). The

saccharoidal veins consist of massive to weakly crustiform-banded quartz

exhibiting local ghosted breccia textures. These veins overprint the

chalcedonic veins (Figure 12), occur through the center of the vein trend, and

are considered to be equivalent to the ginguro mineralization event (see

below).

Fine-grained, strong colloform / crustiform-banded vein and vein breccia with

bands of adularia, clay and bladed quartz textures supersede the

saccharoidal veins. Dark grey to black, sulphide-rich bands with native gold

and silver sulphides occur with colloform-banded chalcedonic silica. The

sulphide bands are several millimeters in width and are similar in character to

the „ginguro‟ bands described by Izawa et al (1990). This phase typically

forms as an initial pulse of several repeated bands on the margin of vuggy

saccharoidal and bladed quartz. Colloform / crustiform-banded chalcedonic

quartz and ginguro deposition occurs prior to the formation of bladed quartz

textures (Figure 13). Wall rock breccias proximal to the colloform / crustiform


37

veins contain ginguro-like bands encrusting wall rock fragments, followed by

saccharoidal to crystalline silica fill (Appendix 2, Plate 2).

Crustiform-banded grey chalcedonic veins with fine pyrite and hematite

typically occur as chaotic veins and breccia zones on vein margins (Appendix

2, Plate 5). Rare saccharoidal and cockade-textured quartz is present.

Outcropping veins have strong iron oxide gossanous zones (Appendix 2,

Plate 6). This phase is associated with pervasive kaolinite alteration and

silica-pyrite flooding of the lithic tuff on the eastern side of the vein trend. The

chalcedonic veins with fine pyrite and hematite are observed overprinting the

saccharoidal with minor bladed texture quartz vein phase (Figure 14).

Jasperoidal veins are massive to moderately banded, contain disseminated

pyrite and are distinguished by their unique cryptocrystalline matrix amongst

vuggy quartz cavities (Appendix 2, Plate 7). The jasperoidal veins crop out

along strike from the chalcedonic veins with fine pyrite.

Late stage veins of euhedral, axiolitic, clear to milky quartz crystals (comb

quartz) are typically less than 5 cm wide and grow perpendicular to vein

margin. The comb quartz veins overprint all other vein phases (Appendix 2,

Plates 8 & 9). Silica-poor, limonite-rich, tectonic breccias and veins

commonly occur within structural intersections associated with major faulting.

Limonite-stained structures are observed cross-cutting veining.

Sascha Sur exhibits multiple quartz textures expressed as multiphase

veinlets, veins and vein breccias. Grey-white chalcedonic quartz with patchy

disseminated pyrite is associated with anomalous gold-silver values and

occurs as a late infill to paragenetically earlier saccharoidal to crystalline

quartz veins (Appendix 2, Plate 10). Amethystine quartz and euhedral

axiolitic comb quartz veinlets cross-cut both chalcedonic with disseminated

pyrite and saccharoidal vein phases (Appendix 2, Plates 8 & 9).

The Marcellina veinlet zone is expressed as multiphase veinlets which

collectively define a stockwork. The veinlets form with an initial


38

crustiform/colloform phase, followed by axiolitic comb quartz growth, and are

infilled with ladder-banded chalcedonic quartz with banding perpendicular to

vein margin (Appendix 2, Plate 11).

The Pelligrini prospect forms a predominant topographic high within the study

area, and is a large zone of intense silica replacement, brecciation and minor

veining. A stratigraphically controlled zone of pervasive silica replacement

forms within the rhyolite ash tuff. Silicification is texturally destructive, with

minor rock textures locally preserved (Appendix 2, Plate 17). Massive

chalcedonic to weakly banded chalcedonic veins and associated wall-rock

breccias occur within the same stratigraphic level as the pervasive

silicification. The chalcedonic veins have a distinctive red-black colour due to

inclusions of hydrothermal hematite and pyrolusite (Appendix 2, Plate 13).

Weakly banded chalcedonic to saccharoidal quartz veins with prominent

bladed textures occur stratigraphically below the pervasive silicification and

contain anomalous gold and silver values (Appendix 2, Plate 12). Multiple-

pulse, milled-clast hydrothermal breccia veins occur directly below the

pervasive silicification. Breccia veins contain rock, vein and earlier breccia

clasts within a chalcedonic quartz matrix (Appendix 2, Plates 13 to 16).

Zones of clast-supported, silica-flooded jigsaw breccias occur within all

stratigraphic levels, and contain angular rock fragments with a hydrothermal

quartz matrix (Appendix 2, Plate 16).

Vein morphology along the SVZ is found to be primarily controlled by host

rock rheology. The least competent lithic rhyolite tuffs consistently host

discontinuous, chaotic veining. Crystal-ash rhyolite tuffs host upward

terminating veins with high grade ginguro bands that encompass the vein

margins. Pumiceous rhyodacite tuffs at Sascha Sur and Marcellina host

broad zones of discontinuous multi-directional veinlets, with ash tuffs at

Pelligrini being pervasively silicified and brecciated.


39

Vein geochemistry and mineralogy

The vein phases across the Sascha-Pelligrini area have distinct geochemical

signatures (Table 3). Early phase comb and chalcedonic quartz veins are

anomalous in gold and silver, and have elevated concentrations of arsenic,

barium and manganese. The early phase comb and chalcedonic quartz veins

average 0.52g/t gold and 3.2g/t silver. Colloform/crustiform and ginguro-

banded veins exhibits high gold and silver values associated with low

concentrations of arsenic, barium and antimony. Colloform/crustiform and

ginguro-banded veins average 14.23g/t gold and 89.8g/t silver. The

colloform/crustiform-banded vein phase is hosted on the margins of the more

voluminous saccharoidal and bladed vein phase. The saccharoidal and

bladed vein phase is anomalous in gold and silver, averaging 0.19g/t gold

and 0.5g/t silver. The chalcedonic with disseminated pyrite phase is

anomalous in gold and silver, averaging 0.76g/t gold and 9.5g/t silver, and

also has high concentrations of antimony and arsenic. The jasperoidal phase

is anomalous in silver with high concentrations of barium and moderate

concentrations of arsenic. The flow-banded rhyolite and auto-breccia at

Pelligrini is anomalous in silver, with vein samples characterised by local

mercury values of up to 633ppb. On the basis of 639 rock chip samples

(Appendix 3) collected from the SVZ, the generalized multi-element signature

is strongly elevated in gold, silver and arsenic, moderately elevated in

antimony and barium, weakly elevated in copper, lead and zinc, and locally

anomalous in mercury.

Vein Phase Au Ag As Ba Cu Hg Mn Pb Sb Zn Au/Ag Ratio

As/Au Ratio

No. of Samples

Comb & chalcedonic

0.52 3.2 409 201 11 1 236 56 8 28 6 786 408

Ginguro 14.23 89.8 186 107 12 1 133 20 7 17 6 2 50

saccharoidal & bladed

0.19 0.5 511 94 12 1 132 38 7 25 3 1022 123

chalcedonic & pyrite

0.76 9.5 1124 97 17 1 135 30 41 21 13 118 44

Jasperoidal & pyrite

0.06 1.7 465 486 13 1 170 24 4 29 27 273 14

Table 3. Summary geochemical signatures of Sascha Main vein phases with key

epithermal elements. Elements are reported as ppm.


40

Element ratios of exploration rockchip geochemistry indicate that the textural

variants of the different veins are geochemically distinct. While their

differences are evident in a range of elements, it is most pronounced in the

arsenic:gold ratio (Table 3). The arsenic:gold ratio clearly differentiates the

early phase chalcedonic, the colloform/crustiform ginguro-banded phase, and

the jasperoidal and chalcedonic with disseminated pyrite phases. The

arsenic:gold ratio shows a separation of three orders of magnitude between

the ginguro and jasperoidal and chalcedonic with disseminated pyrite

phases.

Individual vein phases are also characterised by unique mineral

assemblages. The colloform/crustiform ginguro-banded veins are comprised

of the ore minerals, acanthite and selenium-rich acanthite, electrum, silver

halides, uytenbogaardtite, petrovskaite and jalpaite, with gangue minerals

jamesonite, hematite, calcite, sphalerite, galena, chalcopyrite, dufrenoysite,

barite, adularia and muscovite (Figure 15). The chalcedonic veins with

disseminated pyrite contain ore minerals acanthite, selenium-rich acanthite

and jalpaite, and gangue minerals, pyrite, arsenopyrite, hematite, barite,

gypsum, muscovite and jarosite.

Electron microprobe analysis of individual mineral grains from samples from

across the Sascha-Pelligrini area show geochemical zoning. Acanthite

incorporates selenium contents ranging from 1.65 to 6.21 weight percent and

has compositional ranges of Ag2S0.96Se0.04 to Ag2S0.83Se0.17. Iron content of

sphalerite ranges from 0.71 to 2.75 weight percent with calculated

compositional ranges from Zn0.95Fe0.05S to Zn0.99Fe0.01S . Sphalerite also

incorporates manganese (<1.86 weight percent Mn) and copper (<3.08

weight percent Cu); calculated compositions range from Zn0.91Fe0.05Cu0.04S to

Zn0.93Fe0.04Mn0.03S. Silver content of electrum varies over the entire ginguro

event, and ranges from 6.49 to 87.17 weight percent; electrum varies from

Ag0.05Au0.95 to Ag0.88Au0.12.


41

E

1

2

3

A

1

2

3 4

5

B

1

2

3

4

G

1

2

3

4

5

F

1

2

3

4

H

1

2

3

4

C D 1

2

2

1

3

A. Ginguro Stage I (QR05-4A) 1-Se rich Acanthite, 2-Jamesonite, 3-Acanthite, 4-Barite, 5-Quartz B. Ginguro Stage I (QR05-4A) 1-Se Acanthite, 2-Iodoembolite, 3-Barite, 4-Quartz C. Ginguro Stage I (QR05-4A) 1-Ag rich electrum, 2-acanthite and uytenbogaardtite intergrowth D. Ginguro Stage II (QR05-4B) 1-Acanthite, 2-Iodoargyrite, 3-Barite E. Ginguro Stage II (QR05-4B) 1-Iodoargyrite, 2-Gold, 3-Quartz F. Ginguro Stage II (QR05-4B) 1-Iodoembolite, 2-Hematite, 3-Calcite, 4-Quartz G. Pyrite Chalcedony (QR05-6) 1-Pyrite, 2-Hematite, 3-Hematite & Gypsum, 4-Acanthite, Se Acanthite &

Jalpaite, 5-Quartz H. Pyrite Chalcedony (QR05-6) 1-Pyrite (As zoning), 2-Hematite & Gypsum, 3-Acanthite, 4-Quartz

Figure 15. Backscattered SEM images of characteristic vein mineral assemblages.


42

Geothermometry

Shikazono (1985) proposed that formation temperatures can be calculated

from coexisting electrum and sphalerite by combining the thermodynamic

equations for electrum in equilibrium with argentite (Barton and Toulmin

1964), and equilibrium between FeS in sphalerite and pyrite (Barton and

Skinner 1979).

In order to obtain reliable formation temperatures from coexisting electrum

and sphalerite grains, Shikazono (1985) suggests that the grains must be in

direct contact with each other without mutual replacement textures; that the

FeS content of sphalerite and the Ag content of electrum have not changed

during the post depositional period; and that trace element impurities in

electrum and sphalerite are of low concentration.

Table 4. Silver content of electrum, iron content of sphalerite, and calculated

electrum-sphalerite formation temperatures from the Sascha ginguro vein phase.

Sample Grain Site Ag content of

electrum Fe content of

sphalerite NAg XFes

Electrum-sphalerite geothermometer

Electrum-Sphalerite

(atm fraction) (atm fraction) (mol fraction) (mol fraction) (Shikazono eq. 6,

1985) (°C)

4B-18 Core-Core 81.12 2.25 0.887 0.041 204.8

4B-18 Rim-Rim 79.45 2.54 0.876 0.040 205.1

4B-19 Core-Core 72.64 0.79 0.829 0.012 177.6

4B-19 Core-Rim 72.64 0.71 0.829 0.011 174.8

4B-20 Core-Core 87.15 3.05 0.925 0.047 204.2

4B-20 Rim-Rim 86.67 1.91 0.922 0.030 190.3

E

E E

E

S

S S

J

Figure 16. Backscattered SEM images of coexisting electrum-sphalerite grains used for

geothermometry calculations. E-electrum, S-sphalerite, J-jalpaite.


43

Formation temperatures are estimated from compositional relationships

between coexisting electrum and sphalerite mineral grains. Mineral grains

considered to be in equilibrium are presented in figure 16, with calculated

temperatures of formation presented in table 4. Coexisting electrum and

sphalerite were found within the Sascha ginguro stage II mineralising phase.

Electrum in equilibrium with sphalerite has a silver content which ranges from

72.64 to 87.15 weight percent, with calculated composition varying from

Ag0.829Au0.171 to Ag0.925Au0.075. The iron content of sphalerite in equilibrium

with electrum ranges from 0.71 to 3.05 weight percent, with calculated

composition varying from Zn0.953Fe0.047S to Zn0.998Fe0.018S. Calculated

electrum-sphalerite formation temperatures range from 174.8°C to 205.1°C

+/- 20°C, and vary less than 13.2°C from core to rim.

Alteration

Regional Alteration – Multispectral Mineral Mapping

Using the spectral information recorded by the ASTER sensor, mineral maps

for the remotely sensed alteration minerals kaolinite, illite and alunite are

created for the entire study area (Figure 18). Surface cover across the study

area includes localised woody shrubs and grasses, rock outcrop, gravels and

recent alluvium, allowing good exposure of the alteration system. End-

member spectra of minerals used for mineral mapping and spectral un-

mixing correlate well with known library spectra (Figure 17). Areas of

mineralogically homogenous and mixed pixels are abundant over the

prospect areas. ASTER-derived mineral maps readily define the regional

extent of alteration over the known prospects.

Sascha Main is characterised by abundant, moderate intensity kaolinite, with

small areas of mixed, moderate intensity illite and kaolinite (Figure 18). Pixels

containing pure illite and alunite are located in small areas to the north and

west of the area respectively. Vein distribution and associated anomalous

gold values correlate with areas of mixed kaolinite and illite.


44

ASTER Selected Endmember Spectra - Library Spectra Comparison

1.7 2.2 2.2 2.3 2.3 2.4

Wavelength μm

Refl

ecta

nce

Illite (This Study) Kaolinite (This Study) Alunite (This Study)

Illite IL101 Kaolinite CM92 Alunite GDS84

Sascha Sur is characterised by a broad zone of mixed illite and kaolinite,

which is zoned outwardly to pure illite (Figure 18). Illite and kaolinite intensity

is strong over the veinlet zone and is dominated by illite. Similar to Sascha

Main, vein distribution and associated anomalous gold values correlate with

areas of mixed kaolinite and illite.

The Marcellina veinlet zone is characterised by abundant kaolinite, with

zones of mixed kaolinite and illite associated with veining and weakly

anomalous gold values (Figure 18). The alteration at Pelligrini is a large area

of mixed kaolinite and illite, in combination with zones of pure kaolinite or illite

(Figure 18). The pervasive silicification correlates to areas of high abundance

kaolinite, mixed kaolinite and illite, and scattered alunite. Mineral mapping

has highlighted a zone of intense kaolinite and illite that corresponds to an

outcropping chalcedonic vein and vein-breccia. In the western part of the

Pelligrini prospect, and stratigraphically below the zone of pervasive

silicification, the alteration assemblage is dominated by illite.

Figure 17. Selected end-member spectra compared to known library spectra.


45

Figure 18. ASTER mineral mapping results showing kaolinite, illite and alunite intensities and illite-kaolinite ratios. Simplified geology and gold assays are also presented from the

Sascha-Pelligrini study area.

Quinn Smith Master of Applied Science Thesis 46

Prospect Alteration – PIMA and XRD

In combination with the regional scale ASTER multi-spectral mineral

mapping, detailed analysis of individual prospects reveal a variety of

hydrothermal alteration minerals occur within the Sascha-Pelligrini epithermal

system. PIMA spectral analysis across the study area identifies spectrally

pure end-members of kaolinite, illite, muscovite and alunite (Figure 19).

Mixtures of end-member spectra are common, and characterise distinct

alteration assemblages at each prospect location. XRD analysis confirms

PIMA results and identifies mixed-layered illite-smectite vein selvage

assemblages (Table 5). Hydrothermal alteration across the Sascha-Pelligrini

area is strong to intense. Hydrothermal minerals completely replace primary

phenocrysts and glass in intensely altered wall-rocks, except for primary

quartz and zircon. Strongly altered rocks contain relict plagioclase and mica,

with primary rock textures preserved.

Montmorillonite and iron-chlorite alteration of the host ignimbrite unit can be

detected several kilometers away from the outcropping veins at Sascha

Main. Veins are hosted within a strong pervasive kaolinite +/- iron oxide

alteration halo of several hundred meters. Individual vein phases have

distinct alteration selvages at the vein/wall-rock margin, extending

centimeters to meters into the host sequence. Silicification of the wall-rock

occurs within the alteration selvage and becomes more pervasive and

texturally destructive within 30 centimeters of the main vein.


1.3

1.4

1.5

1.6

1.7

1.8

1.9

2.0

2.1

2.2

2.3

2.4

2.5

Wavelength (μm)

Re

fle

cta

nc

e

Illite

K-Alunite

Kaolinite

Muscovite

Sample PIMA XRD SP196 Kaolinite Quartz (37.3%), Kaolinite (44.8%), Illite (17.9%) SP197 Illite, Muscovite Quartz (59.9%), Illite (+illite-smectite mixed layer)(40.1%) SP198 Illite, Phengite Quartz (71.4%), Illite (+illite-smectite mixed layer)(25.6%),

Kaolinite (2.9%) SP199 Muscovite Quartz (71.5%), Illite (28.5%) SP200 Kaolinite Quartz (40.8%), Kaolinite (44.5%), Illite

(+illite-smectite mixed layer)(14.8%)

SP054 Kaolinite Quartz (51.5%), Kaolinite (48.5%) SP216 Kaolinite, Halloysite Quartz (74.8%), Kaolinite (25.2%) SP053 Kaolinite Quartz (68.9%), Kaolinite (31.1%)

SP103 Kaolinite Quartz (48.8%), Kaolinite (31.3%), Illite (19.9%) SP201 Kaolinite, Halloysite Quartz (40.7%), Kaolinite (44.1%), Illite (15.2%) SP104 Kaolinite Quartz (56.1%), Kaolinite (43.9%) SP202 Kaolinite Quartz (46.7%), Kaolinite (40.7%), Illite (12.6%) SP105 Kaolinite Quartz (44%), Kaolinite (52.7%), Illite (2.4%), Calcite (0.9%)

SP160 K Alunite, Opal Quartz (74.3%), Alunite (16.5%), Jarosite (9.3%) SP159 Illite, NH Alunite Quartz (57.3%), Mica (24.9%), Orthoclase (12.6%), Albite

(1.1%), Illite (2.6%), Jarosite (1.4%) SP158 Illite, NH Alunite Quartz (46.6%), Mica (24.5%), Orthoclase (12.3%), Albite

(13.7%), Illite (1.7%), Jarosite (1.3%) SP157 Dickite, Nacrite Quartz (49.1%), Kaolinite (44.5%), Hematite (6.3%), Anatase

(0.2%)

QR05-23 NA Quartz (28.5%), Albite (26.8%), Orthoclase (17.2%), Mica (9.5%), Chlorite (8.8%), Calcite (4.6%), Sanidine (3%), Illite (1.7%)

QR11 NA Quartz (27.9%), Albite (29.5%), Mica (14.9%), Orthoclase (13.6%), Chlorite (10.6%), Calcite (1.8%), Illite (1.7%)

SP021 Illite, Kaolinite Quartz (56.7%), Mica (23.5%), Kaolinite (7.4%), Orthoclase (3.9%), Albite (3.9%), Illite (2.3%), Jarosite (2.3%)

SP023 NA Quartz (56.8%), Mica (23.6%), Kaolinite (6.4%), Orthoclase (6.3%), Albite (3.3%), Illite (2%), Jarosite (1.2%), Calcite (0.3%)

Table 5. PIMA results compared to quantitative XRD results for samples containing characteristic alteration assemblages from individual prospect areas. Sample locations are shown on figures 3, 20, 21, 22 and 23.

Figure 19. End-member PIMA mineral spectra foriIllite, k-alunite, kaolinite and muscovite from the Sascha-Pelligrini area. Spectra presented across the short wave infrared (SWIR) band width of 1.3μm to 2.5μm.


At Sascha Main, PIMA and XRD analysis of alteration associated with the

chalcedonic vein phase indicates the vein phase is hosted within the

background kaolinite-dominated alteration halo. Alteration outside the zone of

intense silicification on the margin of the saccharoidal to bladed and

colloform / crustiform ginguro-banded vein phase is characterised by illite

overprinted by kaolinite (Table 5, Figures 20, 21 & 24). Wall-rock alteration

within the zone of intense silicification is characterised by illite-smectite

mixed-layered clays, with the vein assemblage comprised of muscovite and

Illite, with secondary kaolinite infilling vugs. The chalcedonic and jasperoidal

vein phases are hosted within pervasive kaolinite, illite and halloysite

alteration. Overprinting crystalline to comb quartz veins are associated with

pervasive, texturally preserving halloysite (Table 5, Figures 22 & 24).

Sascha Sur is hosted within a background laumontite alteration halo of

several kilometers. The alteration halo over the veinlet zone is characterised

by illite (+/- mixed-layer illite-smectite) > kaolinite +/- laumontite, alunite and

lesser gypsum. PIMA and XRD analysis of alteration assemblages of better

developed veins indicates alteration outside the zone of intense silicification

is characterised by illite, patchy laumontite and minor chlorite overprinted by

kaolinite. Wall-rock alteration within the zone of intense silicification is

characterised by illite>kaolinite/halloysite>gypsum. Vein assemblages are

dominated by illite, with kaolinite overprinting illite and infilling vugs (Table 5,

Figures 23 & 24).

A background alteration halo of kaolinite that overprints laumontite around

the periphery of the pervasive silicification is characteristic of Pelligrini. The

alteration assemblage proximal to the pervasive silicification is characterised

by kaolinite, minor jarosite and alunite overprinting illite. Pervasive hematite

alteration forms a small halo on the northern periphery of the kaolinite-

jarosite-alunite alteration zone. Minor veins and veinlets cross-cutting the

pervasive silicification are associated with potassium alunite, natroalunite,

opal and dickite. (Table 5, Figure 24).


Detailed PIMA analysis of individual vein selvages along the SVZ indicate

each alteration assemblage is associated with distinct Al-OH absorption

wavelengths and absorption depths within the short wave infrared (SWIR)

spectrum. PIMA results from samples taken along a traverse perpendicular to

the saccharoidal and bladed vein phase shows the Al-OH spectra absorption

wavelength increases towards the vein, with decreases in the Al-OH

absorption depth in samples of silicified wall-rock (Figure 20). PIMA results

from adjacent to the ginguro-banded vein shows the Al-OH spectra

absorption wavelength and absorption depth increases on the vein selvage,

and decreases in samples of silicified wall-rock (Figure 21). The Al-OH

spectra absorption wavelength and absorption depth decrease in samples of

silicified wall-rock for the pyrite-chalcedony PIMA traverse (Figure 22). PIMA

sampling across gossanous chalcedonic veins at Sascha Sur shows the Al-

OH spectra absorption depth increases towards the vein selvage, and the Al-

OH spectra absorption wavelength decreases in samples of silicified wall-

rock adjacent to the vein (Figure 23).


00.0

5

0.1

0.1

5

0.2

0.2

5

0.3

0.3

5

22

06

22

07

22

08

22

09

22

10

22

11

22

12

22

13

SP

19

6S

P1

97

SP

19

8S

P1

99

SP

20

0S

P11

3

Absorption Depth

Absorption Wavelength

Sa

mp

le

Sa

sc

ha M

ain

PIM

A-X

RD

Pro

file

AL

(OH

) A

bso

rptio

n

Wa

ve

len

gth

AL

(OH

) A

bso

rptio

n D

ep

th

Saccharo

idal quart

z v

ein

w

ith b

laded t

extu

res

PIM

A s

am

ple

location

Com

b q

uart

z v

ein

Wall-

rock s

ilicifi

cation

Rhyolit

e T

uff

LE

GE

ND

LE

GE

ND

FeO

x v

ein

Vein

Analy

sis

0.8

m @

22

.5g

/t A

u

0.8

m @

22

.5g

/t A

u

0.8

m @

22

.5g

/t A

u

0.8

m @

22

.5g

/t A

u

0.8

m @

22

.5g

/t A

u

0.8

m @

22

.5g

/t A

u

0.8

m @

22

.5g

/t A

u

0.8

m @

22

.5g

/t A

u

0.8

m @

22

.5g

/t A

u

50

cm

50

cm

50

cm

50

cm

50

cm

50

cm

50

cm

50

cm

50

cm

SP

113

SP

113

SP

113

SP

113

SP

113

SP

113

SP

113

SP

113

SP

113

SP

200

SP

200

SP

200

SP

200

SP

200

SP

200

SP

200

SP

200

SP

200

SP

199

SP

199

SP

199

SP

199

SP

199

SP

199

SP

199

SP

199

SP

199

SP

198

SP

198

SP

198

SP

198

SP

198

SP

198

SP

198

SP

198

SP

198

SP

196

SP

196

SP

196

SP

196

SP

196

SP

196

SP

196

SP

196

SP

196

SP

197

SP

197

SP

197

SP

197

SP

197

SP

197

SP

197

SP

197

SP

197

Figure 20. PIMA and XRD profiles of individual vein phases from the Sascha Main vein zone showing sample location, geology and veining, gold/silver assays, and

SWIR absorption wavelength and absorption depth.


00.0

5

0.1

0.1

5

0.2

0.2

5

0.3

22

05

22

06

22

07

22

08

22

09

22

10

22

11

22

12

22

13

SP

10

5S

P2

02

SP

10

4S

P2

01

SP

10

3

Absorption Depth


Sa

mp

le

Sa

sc

ha G

ing

uro

PIM

A-X

RD

Pro

file

AL

(OH

) A

bso

rptio

n

Wa

ve

len

gth

AL

(OH

) A

bso

rptio

n

De

pth

50cm

50cm

50cm

50cm

50cm

50cm

50cm

50cm

50cm

0.5

m @

0

.5m

@

0.5

m @

0

.5m

@

0.5

m @

0

.5m

@

0.5

m @

0

.5m

@

0.5

m @

4

8.5

1g

/t A

u /

79

6g

/t A

g4

8.5

1g

/t A

u /

79

6g

/t A

g4

8.5

1g

/t A

u /

79

6g

/t A

g4

8.5

1g

/t A

u /

79

6g

/t A

g4

8.5

1g

/t A

u /

79

6g

/t A

g4

8.5

1g

/t A

u /

79

6g

/t A

g4

8.5

1g

/t A

u /

79

6g

/t A

g4

8.5

1g

/t A

u /

79

6g

/t A

g4

8.5

1g

/t A

u /

79

6g

/t A

g

0.3

m @

0

.3m

@

0.3

m @

0

.3m

@

0.3

m @

0

.3m

@

0.3

m @

0

.3m

@

0.3

m @

1

7.7

9g

/t A

u /

10

4.5

g/t

Ag

17

.79

g/t

Au

/ 1

04

.5g

/t A

g1

7.7

9g

/t A

u /

10

4.5

g/t

Ag

17

.79

g/t

Au

/ 1

04

.5g

/t A

g1

7.7

9g

/t A

u /

10

4.5

g/t

Ag

17

.79

g/t

Au

/ 1

04

.5g

/t A

g1

7.7

9g

/t A

u /

10

4.5

g/t

Ag

17

.79

g/t

Au

/ 1

04

.5g

/t A

g1

7.7

9g

/t A

u /

10

4.5

g/t

Ag

1m

@

1m

@

1m

@

1m

@

1m

@

1m

@

1m

@

1m

@

1m

@

3.8

8g

/t A

u /

29

.5g

/t A

g3

.88

g/t

Au

/ 2

9.5

g/t

Ag

3.8

8g

/t A

u /

29

.5g

/t A

g3

.88

g/t

Au

/ 2

9.5

g/t

Ag

3.8

8g

/t A

u /

29

.5g

/t A

g3

.88

g/t

Au

/ 2

9.5

g/t

Ag

3.8

8g

/t A

u /

29

.5g

/t A

g3

.88

g/t

Au

/ 2

9.5

g/t

Ag

3.8

8g

/t A

u /

29

.5g

/t A

g

SP

104

SP

104

SP

104

SP

104

SP

104

SP

104

SP

104

SP

104

SP

104

SP

201

SP

201

SP

201

SP

201

SP

201

SP

201

SP

201

SP

201

SP

201

SP

103

SP

103

SP

103

SP

103

SP

103

SP

103

SP

103

SP

103

SP

103

SP

202

SP

202

SP

202

SP

202

SP

202

SP

202

SP

202

SP

202

SP

202

SP

105

SP

105

SP

105

SP

105

SP

105

SP

105

SP

105

SP

105

SP

105

Au-A

g B

anded Q

uart

z-G

inguro

Rhyolit

e C

rysta

l A

sh T

uff

Saccharo

idal &

Bla

ded Q

uart

z

Wall-

rock s

ilicifi

cation

PIM

A s

am

ple

location

Vein

Analy

sis

LE

GE

ND

LE

GE

ND C

om

b Q

uart

z

0.1

6

0.1

65

0.1

7

0.1

75

0.1

8

0.1

85

0.1

9

0.1

95

0.2

0.2

05

22

06

.5

22

07

22

07

.5

22

08

22

08

.5

22

09

22

09

.5

SP

02

2S

P0

21

SP

02

0

Absorption Depth


Sa

mp

le

Sa

sc

ha S

ur P

IMA

-XR

D P

rofi

le

AL

(OH

) A

bso

rptio

n

Wa

ve

len

gth

AL

(OH

) A

bso

rptio

n D

epth

70

70

7070

7070

70

70

70

1m

@ 0

.23g/t

Au /

1.8

g/t

Ag

1m

@ 0

.23g/t

Au /

1.8

g/t

Ag

1m

@ 0

.23g/t

Au /

1.8

g/t

Ag

1m

@ 0

.23g/t

Au /

1.8

g/t

Ag

1m

@ 0

.23g/t

Au /

1.8

g/t

Ag

1m

@ 0

.23g/t

Au /

1.8

g/t

Ag

1m

@ 0

.23g/t

Au /

1.8

g/t

Ag

1m

@ 0

.23g/t

Au /

1.8

g/t

Ag

1m

@ 0

.23g/t

Au /

1.8

g/t

Ag

2m

2m

2m2m

2m2m

2m

2m

2m

SP

021

SP

021

SP

021

SP

021

SP

021

SP

021

SP

021

SP

021

SP

021

SP

022

SP

022

SP

022

SP

022

SP

022

SP

022

SP

022

SP

022

SP

022

SP

020

SP

020

SP

020

SP

020

SP

020

SP

020

SP

020

SP

020

SP

020

Rhy

od

acite

Pum

ice

Tuff

Ve

in A

na

lysis

PIM

A s

am

ple

loca

tio

n

Co

mb

qua

rtz

vein

LE

GE

ND

LE

GE

ND F

eO

x st

ain

ed

cha

lce

do

nic

vein

s w

ith

dis

se

min

ate

d p

yrite

Wa

ll-ro

ck s

ilicific

atio

n

Rhy

od

acite

Cry

sta

l Tuff

Figure 21. PIMA and XRD profiles of individual vein phases from the Sascha ginguro vein zone showing sample location, geology and veining, gold/silver assays, and

SWIR absorption wavelength and absorption depth.


0.2

0.2

2

0.2

4

0.2

6

0.2

8

0.3

0.3

2

0.3

4

0.3

6

0.3

8

22

05

22

06

22

07

22

08

22

09

22

10

22

11

22

12

SP

05

4S

P2

16

SP

05

3

Absorption Depth


Sa

mp

le

Sa

sc

ha P

yri

te-C

halc

ed

on

y P

IMA

-XR

D P

rofi

le

AL

(OH

) A

bso

rptio

n

Wa

ve

len

gth

AL

(OH

) A

bso

rptio

n

De

pth

1m

@ 1

.37g/t

Au /

12.4

g/t

Ag

1m

@ 1

.37g/t

Au /

12.4

g/t

Ag

1m

@ 1

.37g/t

Au /

12.4

g/t

Ag

1m

@ 1

.37g/t

Au /

12.4

g/t

Ag

1m

@ 1

.37g/t

Au /

12.4

g/t

Ag

1m

@ 1

.37g/t

Au /

12.4

g/t

Ag

1m

@ 1

.37g/t

Au /

12.4

g/t

Ag

1m

@ 1

.37g/t

Au /

12.4

g/t

Ag

1m

@ 1

.37g/t

Au /

12.4

g/t

Ag

50cm

50cm

50cm

50cm

50cm

50cm

50cm

50cm

50cm

SP

054

SP

054

SP

054

SP

054

SP

054

SP

054

SP

054

SP

054

SP

054

SP

216

SP

216

SP

216

SP

216

SP

216

SP

216

SP

216

SP

216

SP

216

SP

053

SP

053

SP

053

SP

053

SP

053

SP

053

SP

053

SP

053

SP

053

FeO

x s

tain

ed c

halc

edonic

to

opalin

e s

tockw

ork

vein

s

with d

issem

inate

d p

yrite

Lithic

Rhyolit

e T

uff

Wall-

rock s

ilicifi

cation

Cry

sta

l A

sh R

hyolit

e T

uff

Vein

Analy

sis

PIM

A s

am

ple

location

Com

b q

uart

z v

ein

LE

GE

ND

LE

GE

ND C

om

b q

uart

z v

ein

0.1

6

0.1

65

0.1

7

0.1

75

0.1

8

0.1

85

0.1

9

0.1

95

0.2

0.2

05

22

06

.5

22

07

22

07

.5

22

08

22

08

.5

22

09

22

09

.5

SP

02

2S

P0

21

SP

02

0

Absorption Depth


Sa

mp

le

Sa

sc

ha S

ur P

IMA

-XR

D P

rofi

le

AL

(OH

) A

bso

rptio

n

Wa

ve

len

gth

AL

(OH

) A

bso

rptio

n D

epth

70

70

7070

7070

70

70

70

1m

@ 0

.23g/t

Au /

1.8

g/t

Ag

1m

@ 0

.23g/t

Au /

1.8

g/t

Ag

1m

@ 0

.23g/t

Au /

1.8

g/t

Ag

1m

@ 0

.23g/t

Au /

1.8

g/t

Ag

1m

@ 0

.23g/t

Au /

1.8

g/t

Ag

1m

@ 0

.23g/t

Au /

1.8

g/t

Ag

1m

@ 0

.23g/t

Au /

1.8

g/t

Ag

1m

@ 0

.23g/t

Au /

1.8

g/t

Ag

1m

@ 0

.23g/t

Au /

1.8

g/t

Ag

2m

2m

2m2m

2m2m

2m

2m

2m

SP

021

SP

021

SP

021

SP

021

SP

021

SP

021

SP

021

SP

021

SP

021

SP

022

SP

022

SP

022

SP

022

SP

022

SP

022

SP

022

SP

022

SP

022

SP

020

SP

020

SP

020

SP

020

SP

020

SP

020

SP

020

SP

020

SP

020

Rhy

od

acite

Pum

ice

Tuff

Ve

in A

na

lysis

PIM

A s

am

ple

loca

tio

n

Co

mb

qua

rtz

vein

LE

GE

ND

LE

GE

ND F

eO

x st

ain

ed

cha

lce

do

nic

vein

s w

ith

dis

se

min

ate

d p

yrite

Wa

ll-ro

ck s

ilicific

atio

n

Rhy

od

acite

Cry

sta

l Tuff

Figure 22. PIMA and XRD profiles of individual vein phases from the Sascha pyrite-chalcedony vein zone showing sample location, geology and veining, gold/silver

assays, and SWIR absorption wavelength and absorption depth.


Figure 23. PIMA and XRD profiles of individual vein phases from the Sascha Sur vein zone showing sample location, geology and veining, gold/silver assays, and SWIR

absorption wavelength and absorption depth.

0.1

6

0.1

65

0.1

7

0.1

75

0.1

8

0.1

85

0.1

9

0.1

95

0.2

0.2

05

22

06

.5

22

07

22

07

.5

22

08

22

08

.5

22

09

22

09

.5

SP

02

2S

P0

21

SP

02

0

Absorption Depth


Sa

mp

le

Sa

sc

ha S

ur P

IMA

-XR

D P

rofi

le

AL

(OH

) A

bso

rptio

n

Wa

ve

len

gth

AL

(OH

) A

bso

rptio

n D

epth

70

70

7070

7070

70

70

70

1m

@ 0

.23g/t

Au /

1.8

g/t

Ag

1m

@ 0

.23g/t

Au /

1.8

g/t

Ag

1m

@ 0

.23g/t

Au /

1.8

g/t

Ag

1m

@ 0

.23g/t

Au /

1.8

g/t

Ag

1m

@ 0

.23g/t

Au /

1.8

g/t

Ag

1m

@ 0

.23g/t

Au /

1.8

g/t

Ag

1m

@ 0

.23g/t

Au /

1.8

g/t

Ag

1m

@ 0

.23g/t

Au /

1.8

g/t

Ag

1m

@ 0

.23g/t

Au /

1.8

g/t

Ag

2m

2m

2m2m

2m2m

2m

2m

2m

SP

021

SP

021

SP

021

SP

021

SP

021

SP

021

SP

021

SP

021

SP

021

SP

022

SP

022

SP

022

SP

022

SP

022

SP

022

SP

022

SP

022

SP

022

SP

020

SP

020

SP

020

SP

020

SP

020

SP

020

SP

020

SP

020

SP

020

Rhy

od

acite

Pum

ice

Tuff

Ve

in A

na

lysis

PIM

A s

am

ple

loca

tio

n

Co

mb

qua

rtz

vein

LE

GE

ND

LE

GE

ND F

eO

x st

ain

ed

cha

lce

do

nic

vein

s w

ith

dis

se

min

ate

d p

yrite

Wa

ll-ro

ck s

ilicific

atio

n

Rhy

od

acite

Cry

sta

l Tuff

0.1

6

0.1

65

0.1

7

0.1

75

0.1

8

0.1

85

0.1

9

0.1

95

0.2

0.2

05

22

06

.5

22

07

22

07

.5

22

08

22

08

.5

22

09

22

09

.5

SP

02

2S

P0

21

SP

02

0

Absorption Depth


Sa

mp

le

Sa

sc

ha S

ur P

IMA

-XR

D P

rofi

le

AL

(OH

) A

bso

rptio

n

Wa

ve

len

gth

AL

(OH

) A

bso

rptio

n D

epth

70

70

7070

7070

70

70

70

1m

@ 0

.23g/t

Au /

1.8

g/t

Ag

1m

@ 0

.23g/t

Au /

1.8

g/t

Ag

1m

@ 0

.23g/t

Au /

1.8

g/t

Ag

1m

@ 0

.23g/t

Au /

1.8

g/t

Ag

1m

@ 0

.23g/t

Au /

1.8

g/t

Ag

1m

@ 0

.23g/t

Au /

1.8

g/t

Ag

1m

@ 0

.23g/t

Au /

1.8

g/t

Ag

1m

@ 0

.23g/t

Au /

1.8

g/t

Ag

1m

@ 0

.23g/t

Au /

1.8

g/t

Ag

2m

2m

2m2m

2m2m

2m

2m

2m

SP

021

SP

021

SP

021

SP

021

SP

021

SP

021

SP

021

SP

021

SP

021

SP

022

SP

022

SP

022

SP

022

SP

022

SP

022

SP

022

SP

022

SP

022

SP

020

SP

020

SP

020

SP

020

SP

020

SP

020

SP

020

SP

020

SP

020

Rhy

od

acite

Pum

ice

Tuff

Ve

in A

na

lysis

PIM

A s

am

ple

loca

tio

n

Co

mb

qua

rtz

vein

LE

GE

ND

LE

GE

ND F

eO

x st

ain

ed

cha

lce

do

nic

vein

s w

ith

dis

se

min

ate

d p

yrite

Wa

ll-ro

ck s

ilicific

atio

n

Rhy

od

acite

Cry

sta

l Tuff


A. Pelligrini (SP157) 1-Laumontite, 2-Kaolinite B. Pelligrini (SP158) 1-Illite, 2-Quartz C. Pelligrini (SP160) 1-Alunite, 2-Opal D. Sascha Main (SP196) Kaolinite E. Sascha Main (SP197) Illite F. Sascha Main (SP199) 1-Muscovite, 2-Illite, 3-Quartz G. Sascha Sur (SP021) Laumontite H. Sascha Sur (SP023) 1-Illite, 2-Gypsum

H

1

2

10 μm

G

5 μm

F

1

2

3

20 μm

E

E

5 μm

D

5 μm

C 1

2

10 μm

B

1

2

10 μm

A

1

2

5 μm

Figure 24. ESEM images of alteration mineral morphologies from individual

alteration systems within the study area.


Alteration geochemistry

Mass balance alteration geochemistry is calculated for individual alteration

zones, within stratigraphically homogeneous units using the methods of

MacLean and Barret (1993). Least altered samples for each of the

stratigraphic host units were chosen through petrographic and XRD

examination. Least altered precursor samples for the rhyolite crystal ash tuff,

rhyodacite ignimbrite, and rhyolite ash tuff are QR05-26, QR25 and QR05-21

respectively. Sample locations are shown in figure 3.

The Sascha Main alteration profile shows a general mass gain with

increasing alteration intensity proximal to veining. Immobile element

concentrations decrease with increasing alteration intensity and mass gain

(Figure 25 A). Sample SP196 is located 160cm from the vein margin (Figure

19), and is characterised by moderate mass gains in SiO2 (16.5%), Al2O3

(14.5%), small mass gains in TiO2 (1%) and Fe2O3 (4.6%), and a small mass

loss in K2O (-2.2%) (Figure 25 B). SP197 and SP198 are located 90cm and

25cm from the vein respectively, and show the largest mass gains in the

profile. SP197 shows large mass gains in SiO2 (235.9%), Al2O3 (42.6%),

moderate mass gains in Fe2O3 (14.4%), K2O (10.2%), and small mass gains

in MgO (3.9%), TiO2 (2.1%), Na2O (0.9%) (Figure 25 B).

Similar to Sascha Main, the Sascha Sur alteration profile shows mass gains

adjacent to epithermal veins. Increasing alteration intensity decreases

immobile element concentrations associated with mass gains (Figure 26 A).

Sample QR05-23 is located approximately 2.3km from the Sascha Sur

veinlet zone (Figure 3), and is characterised by small mass gains in Al2O3

(2.5%), Fe2O3 (2.6%), CaO (3.8%), Na2O (2.9%), and MgO (0.6%), with

small mass losses in SiO2 (-5.2%) and K2O (-1.5%) (Figure 26 B). The

largest mass gains in the profile are observed in samples located directly on

the vein margin. SP023 shows large mass gains in SiO2 (163.3%), Al2O3

(22%), a moderate mass gain in K2O (6.2%), and small mass gains in Fe2O3

(1.6%), MgO (1.1%), and TiO2 (0.6%) (Figure 26 B).

Table XX. Selected PIMA and XRD results


The Pelligrini alteration profile is unique, with a mass loss in the kaolinite-illite

peripheral alteration zone, and a mass gain in the zone of pervasive

silicification with alunite and opal. Increasing alteration within the host

lithology at Pelligrini causes immobile element concentrations to increase

with mass losses, and decrease with mass gains (Figure 27 A). The

alteration of the rhyolite ash tuff around the periphery of the silicified-

alunite/opal zone is characterised by a large mass loss in SiO2 (-45.9%),

small mass losses in Al2O3 (-3.7%), K2O (-3.2%), and Na2O (-1.4%), with a

small mass gain in Fe2O3 (4.5%) (Figure 27 B). SP160 is located directly

within the silicified-alunite/opal zone and shows the largest mass gain in the

profile. SP160 shows large mass gains in SiO2 (63.3%), moderate mass

gains in SO3 (16.2%), small mass gains in Al2O3 (5.4%), Fe2O3 (3.4%), and

K2O (1.3%), with a small mass loss in Na2O (0.6%) (Figure 27 B).


Rhyolite Crystal Ash Tuff Immobile Elements

y = 1.4372x

R2 = 0.9792

0

1

2

3

4

5

6

0 0.5 1 1.5 2 2.5 3 3.5 4

Dy ppm

Sm

pp

mSm vs Dy

Linear (Sm vs Dy)

A

QR05-26

SP196

SP197

SP199

SP198

-50

0

50

100

SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 SO3 NET

Ma

ss

Ch

an

ge

(g

/10

0g

)

Sascha Main Alteration

SP196 SP199 SP198 SP197235.92

311.05101B

Figure 25. Selected immobile elements and geochemical mass-changes for

rhyolite crystal ash tuff alteration within Sascha Main.

Selected immobile elements showing single precursor, highlighted by red circle, and altered samples along linear alteration trends for homogenous stratigraphic horizons. Bar graphs showing mass-changes of major elements in samples representing increasing alteration intensity for individual alteration systems.

A. Immobile elements Sm and Dy within the rhyolite crystal ash tuff which hosts Sascha Main.

B. Mass changes for major elements within the Sascha Main alteration system based on the single precursor QR05-26


Figure 26. Selected immobile elements and geochemical mass-changes for

rhyodacite ignimbrite alteration within Sascha Sur.


A. Immobile elements Dy and Y within the rhyodacite ignimbrite which hosts Sascha Sur

B. Mass changes for major elements within the Sascha Sur alteration system based on the single precursor QR25

Sascha Sur Alteration

-50

0

50

100


Ma

ss

Ch

an

ge

(g

/10

0g

)

QR05-23 QR11 SP021 SP023163.3 196.16

D

Rhyodacite Ignimbrite Immobile Elements

y = 0.1788x

R2 = 0.9951

0

0.5

1

1.5

2

2.5

3

3.5

4

0 5 10 15 20 25

Y ppm

Dy

pp

m

Dy vs Y

Linear (Dy vs Y)

CQR25

QR05-23

QR11

SP021

SP023

A

B



A. Immobile elements Dy and Y within the rhyolite ash tuff which hosts Pelligrini

B. Mass changes for major elements within the Pelligrini alteration system based on the single precursor QR05-21

Figure 27. Selected immobile elements and geochemical mass-changes for rhyolite ash tuff alteration within Pelligrini.

Rhyolite Ash Tuff Immobile Elements

y = 0.193x

R2 = 0.9703

0

0.5

1

1.5

2

2.5

3

3.5

0 2 4 6 8 10 12 14 16 18

Y ppm

Dy

pp

m

Dy vs Y

Linear (Dy vs Y)

E

QR05-21

SP160

SP157

SP158

SP159

A

B

-50

0

50

100


Ma

ss

Ch

an

ge

(g

/10

0g

)

Pelligrini Alteration

SP160 SP158 SP159 SP160

-50.87

F


Dy vs Zr net mass change comparison

-60

-40

-20

0

20

40

60

80

100

120

SP198 SP199 QR05-23 QR11 SP158 SP160

Ma

ss

Ch

an

ge

(g

/10

0g

)

Zr net mass-change

Dy net mass-change

The trace element Zr is generally considered as immobile in many alteration

systems (Maclean and Kranidiots, 1987; Cail and Cline, 2001, Grant, 1986;

Grant 2005). Comparisons of possible immobile elements Dy, Sm and Y with

Zr show good correlations (Table 6). Mass change calculations using Zr

show similar trends to mass-changes calculated from Dy, however absolute

values differ by up to 58% (Figure 28).

Samples Outliers Dy vs Zr R

2

Sm vs Zr R

2

Y vs Zr R

2

Rhyolite Crystal Ash Tuff

QR05-26, SP198, SP199

SP196, SP197 0.94 0.96 0.84

Rhyodacite ignimbrite

QR25, QR05-23, QR11

SP021, SP023 0.92 0.91 0.92

Rhyolite Ash Tuff

QR05-21, SP158, SP160

SP157, SP159 0.83 0.34 0.98

Table 6. Tabulated correlation coefficient values for immobile elements Dy, Sm, and Y plotted against Zr.

Figure 28. Bar graph comparing net mass changes for immobile elements Dy and Zr.


Discussion

Epithermal deposits are characteristically hosted within complex geological

environments subject to multiple episodes of deformation (Begbie et al, 2007)

and overprinting alteration (Mauk and Simpson 2007; Gemmel 2007; Warren

et al,2007; Ducart et al, 2006). Host sequences exhibit strong controls on the

type and nature of alteration and the morphology of veining. Understanding

the local geology within the study area is vital in generating a realistic and

accurate generic model of the epithermal system.

Volcanology

Depositional setting

Large silicic volcanic environments have complex and diverse facies

associations and stratigraphic relationships. Facies associations within the

Sascha-Pelligrini area comprise two compositionally distinct volcanic

sequences of rhyodacite and rhyolite ash-flows and minor air-fall deposits

(Chon Aike and La Matilde Formations). The rhyolite sequence is deposited

on a minor unconformity on the rhyodacite sequence and represents

deposition of significantly less material. The rhyodacite sequence is very

thick (~670m) strongly welded, and laterally continuous across the study

area. The upper rhyolite sequence is relatively thin (~130m), poorly welded

and spatially restricted within the study area.

Classification into broad facies models is achieved by the unique

characteristics within the volcanic stratigraphy. Stratigraphic associations, or

facies associations, can be used to reconstruct palaeogeological

environments and depositional settings aiding in the interpretation of eruption

styles. Continental silicic provinces are composed of rhyolite domes and

flows of subdued topography rising above large ignimbrite shields (Cas and

Wright, 1987). The focal element of a silicic province is the caldera, which

may contain multiple eruption points and a basin for accumulation of volcanic

material. The critical facies association within silicic volcanic terrains is the


recognition of volcanics formed in intracaldera or extracaldera environments

(Cas and Wright, 1987). Intracaldera deposits are comprised of lavas and

domes, thick crystal-rich ignimbrites and associated near vent co-ignimbrite

breccias and intercalated epiclastics. Extracaldera successions are

dominated by thin sheet-like outflow ignimbrites interspersed with pyroclastic

fall and abundant epiclastic deposits.

The deposition of the homogenous and strongly welded rhyodacite ignimbrite

of the Chon Aike formation within the study area requires the pre-existence of

a large topographic depression proximal to an eruption source (Cas and

Wright, 1987). The lithic concentration zone, or lag deposit, within the La

Matilde rhyolite ash tuff would suggest eruption proximal to source vents and

characteristically shows a decrease in maximum clast size away from the

source (Sheriden, 1979). Resurgent rhyolitic flow domes, such as at

Pelligrini, form within or at the margins of the caldera structure, however they

may also be erupted outside the caldera margin (Cas and Wright, 1987). On

the basis of facies models (Cas and Wright, 1987), the thick, homogenous

and strongly welded nature of the rhyodacite ignimbrite, lithic concentration

zones within the rhyolite sequence, and the resurgent flow dome at Pelligrini

is consistent with an intracaldera depositional setting.

Although facies associations within the study area suggest an intra-caldera

setting, Guido et al (2004) proposes that the distribution of the Jurassic

volcanics of the Chon Aike and La Matilde formations are associated with

intersections of regional east-northeast and west-northwest basement

fractures. Regional east-northeast and west-northwest basement transfer

structures traverse the study area, with spatially restricted rhyolite tuffs

confined within grabens and half-grabens bound by the structures. Maximum

size lithic clast vectors from the base of the rhyolite sequence suggest a

source close to the intersection of the basement structures in the north-west

of the study area. Echeveste et al (1999) and Guido et al (2004) propose the

large volume of volcanic material that comprises the Chon Aike and La

Matilde formations were produced through a complex system of Jurassic

extensional graben-forming structures with only minor caldera formation.


Therefore the facies associations of the volcanics within the study area may

closely represent an intra-caldera setting, with volcanics erupted from a

complex series of graben-forming extensional fractures instead of a classic

caldera margin.

Eruption styles

Eruption styles depend strongly on the physical properties of magma. Magma

properties are related to temperature, melt composition, proportion of

crystals, amount of dissolved volatiles and the abundance of gas bubbles

(Sparks et al, 1997). Explosive eruptions of dacitic-rhyolitic material usually

occur through degassing of dissolved volatiles with decreasing lithostatic load

(Sparks et al, 1997). Highly explosive dacitic-rhyolitic eruptions can produce

tremendous amounts of pyroclastic material. Collapse of the giant La Garita

caldera in the San Juan Volcanic Field erupted as much as 5000km3 of

material in individual eruptions, producing the well studied Fish Canyon Tuff

(Lipman, 2000).

Pyroclastic deposits can be classified into genetic groups of fall, flow and

surge according to their mode of transport and deposition (Sparks and

Walker, 1973) (Figure 29). Pyroclastic fall deposits are formed from material

that has been explosively ejected from a vent (Sheriden, 1979) and usually

mantle topography, with the geometry and size of the deposit controlled by

eruption column height and wind conditions (Cas and Wright, 1987).

Pyroclastic flows are formed by the collapse of a convective ash cloud, with a

lateral deposition of pyroclastic material away from the eruptive centre

(Sheriden, 1979). Pyroclastic flows are controlled by topography with material

filling valleys and depressions; flows emplaced at extremely high velocities

may mantle topography (Cas and Wright, 1987). Pyroclastic surges are

deposited from directed blasts caused by plug explosions, phreatomagmatic

eruptions or changing vent geometry (Sheriden, 1979).

The volcanic units of the Sascha-Pelligrini area record several distinct

eruption styles. The rhyodacite crystal ignimbrite forms a thick, homogeneous


composite welded package with no clear lithological separation between

welded horizons. Lithic clasts are rare and occur as small discrete fragments

within a much finer-grained crystal-rich matrix. The rhyodacite unit may

represent a composite flow formed by eruption column collapse from multiple

vents or fissures. Alternatively, this unit may have been produced by multiple

rapid eruptive events from a single vent or fissure. Both models imply a

relatively synchronous and rapid deposition.

Welding within the rhyodacite ignimbrite varies both vertically and

horizontally, and forms composite welded horizons preserved as topographic

highs across the study area. Complex welding patterns develop in pyroclastic

flows due to varying temperature and load stress and vary irregularly from

proximal to distal vent facies (Christiansen, 1979; Wilson and Hildreth, 1997).

The composite welding within the lower rhyodacite ignimbrite of the Chon

Aike Formation suggests deposition originated from a succession of small

eruptions that quickly deposited material at relatively hot temperatures. The

lack of air-fall horizons within the ignimbrite package suggests that the

pyroclastic flows were associated with low eruption columns (Guido et al,

2004), or alternatively, the volcanic ash was elutriated from the eruption

column.

An increase in lithic and juvenile lava clasts towards the top of the rhyodacite

ignimbrite coupled with the transition into a massive strongly welded pumice

tuff represents a change in the vent geometry producing a more energetic

eruption sequence. Thin air-fall ash horizons at the top of the pumiceous unit

may indicate that buoyant lift-off occurred within the pyroclastic flow,

producing a co-ignimbrite plume. Buoyant lift-off is thought to be the main

mechanism for producing co-ignimbrite plumes, elutriating fine ash and

preferentially enriching pyroclastic flows in crystal material (Sparks et al,

1997). Dilute gravity flows can form on the sides of an eruption column, and

deposit thin ash layers proximal to eruption centres in a similar manner to co-

ignimbrite plumes (Sparks et al, 1997).


The transition at the top of the Chon Aike from a massive welded pumice tuff

to welded crystal ash rhyodacite tuff with abundant juvenile chlorite-hematite,

granitic and lithic clasts represents renewed vent activity. Lithic clasts signal

a possible change in vent geometry or vent clearing. Abundant deep-origin

chlorite-hematite and granitic clasts represent a tapping of deeper parts of an

exhausted magma chamber. The welded crystal ash rhyodacite tuff is the last

eruptive unit of the Chon Aike sequence, and subsequently has the least

amount of lithostatic load. Strong welding with abundant undeformed clasts

and minimal lithostatic load suggests the crystal ash rhyodacite package was

deposited at high temperatures proximal to source. Current exposure of

highly welded ignimbrites and disconformable relationships with the overlying

un-welded rhyolite ash package, represents a period of erosion and volcanic

quiescence.

The spatially restricted La Matilde rhyolite ash tuffs show disconformable

relationships with the underlying Chon Aike Formation, suggesting they were

deposited after a brief period of volcanic quiescence. Stratigraphic

relationships between the La Matilde and Chon Aike formations are

debatable, with significant lateral and vertical facies changes within the units

obscuring depositional contacts. The laminated tuffs of the La Matilde

Formation often interdigit with the ignimbrites of the Chon Aike Formation,

leading Sanders (2000) to suggest that the two units were deposited

concurrently. Epiclastic deposits of the La Matilde show disconformable

contacts with the underlying Chon Aike, however they do not represent a

significant hiatus in volcanic activity, but rather reworking of pyroclastic

material between eruptions (Pakhurst et al, 1998; Guido et al, 2004).

The massive rhyolite package of the La Matilde Formation formed from a

single, violent eruption filling a pre-existing valley. The lithic rhyolite tuff at the

base of the La Matilde represents a proximal lag breccia, sourced from a

single vent following a transition from rhyodacite to rhyolite volcanism.

Metamorphic muscovite schist clasts compose >95% of the total lithics and

represent a deep vent source of pre-Jurassic basement. Lithic fragments in

pyroclastic deposits result from conduit and vent erosion during explosive


eruptions, recording changes in mechanics and timing of caldera collapse

(Suzuki-Kamata et al, 1993: Browne and Gardner, 2003). The lithic unit has

extreme vertical and lateral geometry variations, with clast size vectors

identifying a single vent in the north-west of the study area. The lithic lag

breccia is contemporaneous with the massive valley-filling un-welded rhyolite

crystal ash tuff.

Eruption column collapse after the initial explosion that deposited the lithic

lag breccia produced a relatively cool pyroclastic flow that travelled along a

pre-existing valley towards the southeast. The flow deposited the un-welded

rhyolite crystal ash tuff, the most extensive unit of the La Matilde Formation.

The overlying rhyolite ash unit at the top of the La Matilde sequence mantles

topography and contains zones of accretionary lapilli. Mantled topography

and accretionary lapilli indicate the rhyolite ash is an air-fall sequence

deposited during a rain event. The planar-bedded rhyolite ash is

contemporaneous with the air-fall ash, and represents the settling of the

convecting cloud from the initial transitional eruption that produced both the

lag breccia and the un-welded crystal ash rhyolite tuff.

Several distinct eruption styles are recorded within the volcanic sequence.

The rhyodacite crystal ignimbrite exposed at the base of the sequence was

rapidly deposited by eruption column collapse from multiple vents or fissures

creating a composite welded unit. An increase in lithic and juvenile lava

clasts towards the top of the unit, and the transition into a massive strongly

welded pumice tuff represents a change to a more energetic eruption

sequence. The transition at the top of the Chon Aike to welded crystal ash

rhyodacite tuff with abundant undeformed juvenile and lithic clasts represents

renewed vent activity, with deposition under minimal lithostatic load proximal

to source. The spatially restricted La Matilde rhyolite sequence was

deposited from a single violent eruption depositing the lag breccia proximal to

the vent. The pyroclastic flow produced by the eruption column collapse

deposited the un-welded rhyolite crystal ash tuff, with settling of the

convective cloud depositing the planar-bedded rhyolite ash tuff.


A. Convective – Development of Plinian eruption column, fine grained pyroclastic fallout covers wide area.

B. Transitional – Development of convective plume with some pyroclastic fallout, oscillating fountain feeds some pyroclastic flows with associated co-ignimbrite plumes.

C. Collapsing – Sustained fountain of pyroclastic material feeds continual massive pyroclastic flows that develop large co-ignimbrite plumes. Small convecting cloud above collapsing fountain.

(Modified from Purdy 2003, after Neri et al 2002)

Figure 29. Styles of explosive eruptions with vent development restricted to half-graben

and graben forming regional basement structures.


Magma Petrogenesis

Two petrographically and geochemically distinct groups (rhyodacite and

rhyolite) compose the Sascha-Pelligrini suite of the study area. Hydrothermal

alteration affects rocks throughout the majority of the study area and

precludes quantitative petrochemical modeling the volcanic rock suite.

Sorting, welding, post emplacement crystallisation and devitrification, as well

as erosion prior to hydrothermal alteration contribute to the complexity of the

suite‟s genesis.

The pyroclastic rocks of the study area belong to the Jurassic Chon Aike and

La Matilde Formations which form a silicic large igneous province (LIP) with

an estimated volume of 235,000km3 (Pankhurst et al, 1998). The generation

of large volumes of silicic magma reflects large-scale crustal melting

controlled by the water content and composition of the crust, and a large

thermal (+/- mass) input from the mantle (Bryan, 2007). Jurassic rhyolites of

the Chon Aike Formation were produced through anatexis of sedimentary

source material, with silicic melts generated from heat input of mantle-derived

basaltic melts ponding at the base of the crust (Gust et al, 1985). The Chon

Aike silicic LIP is characterised by the absence of mantle-derived rocks

(Pankhurst el at, 1998), with basaltic andesite of the Bajo Pobre Formation

being the least evolved and restricted to only a few localities (Pankhurst and

Rapela, 1995; Pankhurst et al, 1998; Sanders, 2000; Sharpe et al, 2002).

Trace element similarities between the basaltic andesite of the Bajo Pobre

and the rhyolites of the Chon Aike suggest the rhyolite formed from partial

melting of the basaltic andesite (Storey and Alabaster, 1991; Pankhurst and

Rapela, 1995; Pankhurst et al, 1998).

Pankhurst and Rapela (1995) demonstrate that the sequence from basaltic

andesite (Bajo Pobre) to dacite/rhyodacite to rhyolite (Chon Aike) can be

modeled by a combination of partial melting and fractional crystallisation.

Batch partial melting of a depleted Bajo Pobre andesite produces Chon Aike

equivalent dacite through 20% melt extraction, with a residual composition

composed of orthopyroxene and plagioclase. Fractional crystallisation of

dacite to rhyodacite is accomplished with the removal of 50% crystal


assemblage composed of 10% amphibole and 90% plagioclase. High-silica

rhyolite is produced from fractional crystallisation of the rhyodacite, with the

removal of 40% crystal assemblage composed of approximately 58%

plagioclase, 40% hornblende, 1% apatite and trace allanite.

The exposed rhyodacite sequence of the Chon Aike Formation exhibits some

variation in REE abundances. The last ash-flow unit of the sequence, the

pumice ash welded rhyodacite tuff, is enriched in the LREE, depleted in the

middle REE with a pronounced negative Eu anomaly relative to the first ash-

flow rhyodacite. These REE differences are consistent with the proposed

fractionation trends of Pankhurst and Rapela (1995).

The rhyolitic tuff sequence of the La Matilde Formation shows a general

decrease in REEs from the massive crystal ash tuff to the planar bedded air-

fall ash tuff. The REE pattern for the lag breccia lithic tuff deviates from the

general rhyolite sequence patterns and may represent contamination from a

variety of components. The lag breccia is considered to be deposited through

the initial explosive eruption of the rhyolitic sequence, contains vent derived

material, and outcrops within the intense alteration zone of Sascha Main. The

discrepancies observed for the lag breccia REE trend may be a complex

product of obscuring alteration and vent contamination. Similar to the

rhyodacite package, the rhyolite sequence REE patterns show a negative Eu

anomaly, decrease in REE concentrations with continued eruptive units, and

depressed HREEs. The negative Eu anomaly may be controlled by the

fractionation of sanidine, with the depletion of REEs controlled by the

fractionation of hornblende. Compositional zoning observed in sequence

suggests eruption of a zoned magma chamber.

Eruptive sequences commonly show an orderly progression from most

evolved to least evolved magmatic ejecta, representing eruption from

compositionally zoned magma chambers (Hildreth, 1979). Individual ash-flow

sheets of the rhyodacite sequence within the study area have a limited

compositional range. Large volume, phenocryst-rich ignimbrites such as the

rhyodacite sequence rarely develop from zoned magma chambers, with the


fractionation process prematurely aborted by venting of the dominant magma

volume (Hildreth, 1979). This may be the case for the rhyodacite sequence,

however the volcanic stratigraphy below the ignimbrite is not observed, and a

more evolved silicic unit may exist at the base of section below the

rhyodacite.

The rhyodacite and rhyolite units form two distinct groups within the study

area. Strong hydrothermal alteration, sorting, welding, post emplacement

crystallisation and devitrification, as well as erosion throughout the majority of

the study area precludes quantitative petrochemical modeling the volcanic

rock suite. Variation in REE abundances across the rhyodacite and rhyolite

sequences is consistent with the proposed fractionation trends of Pankhurst

and Rapela (1995). Pankhurst and Rapela (1995) demonstrate that the

sequence from basaltic andesite (Bajo Pobre) to dacite/rhyodacite to rhyolite

(Chon Aike) can be modeled by a combination of partial melting and

fractional crystallisation.

Host Rock Control and Structural Model

Host rock rheology controls vein distribution and morphology within the

Sascha-Pelligrini system and is a common control in many epithermal

deposits (Brathwaite et al, 2001; Christie et al, 2007; Izawa et al, 1990).

Outcrop distribution of ginguro-banded veins at Sascha Main is limited, with

quartz veins and lenses hosted within poorly welded crystal ash rhyolite tuff.

The heavily altered and relatively friable tuff restricts coherent vein

development within the current stratigraphic exposure. Outcrop morphology

of discontinuous quartz veins within the rhyolite tuff suggests that veins are

upward terminating within a friable tuff that does not host continuous brittle

fractures. The lithic rhyolite tuff hosts discontinuous and chaotic veining, with

friability and high porosity favouring multiple discontinuous fractures and

veinlets. Pumiceous rhyodacite tuffs at Sascha Sur and Marcellina host

broad zones of discontinuous multi-directional veinlets, while the ash tuffs at

Pelligrini are pervasively silicified and brecciated.


Lithological controls are well documented at numerous epithermal deposits,

with coherent 2 to 3 meter wide veins in andesite at the Hauraki goldfield

changing to stockwork veining in rhyolite with individual veins less than 10

centimeters in width (Christie et al, 2007). Well developed coherent

epithermal veins are developed within crystalline and brittle Bajo Pobre

andesite that underlies the Chon Aike Formation (Dietrich et al, 2008) as well

as felsic intrusives within the Chon Aike Formation (Wallier and Tosdal,

2008). The rheologically brittle massive crystalline rhyodacite ignimbrite that

stratigraphically underlies the current outcrop exposure at Sascha Main may

host larger epithermal veins in more continuous fractures. The rhyodacite

ignimbrite at Sascha-Pelligrini is analogous to the massive quartz-feldspar

porphyritic, densely welded, pumiceous ignimbrite (Granosa) within the Chon

Aike Formation that hosts the majority of veining within the Cerro Vanguardia

epithermal deposit (Sharpe et al, 2002).

Epithermal veining at Sascha-Pelligrini was emplaced during a period of

extensional tectonics that formed northwest-trending grabens. The

extensional event was contemporaneous with the last stages of the Chon

Aike (Echavarria et al, 2005) with the stratigraphically high La Matilde rhyolite

filling the grabens and half-grabens. Maximum dilation and associated fluid

flow formed at right-stepping structural spays along the right-lateral, oblique-

slip, 315° trending normal Sascha Fault (Figure 30). The tension axis

trended toward the northeast quadrant, producing left-lateral movement in

north- and northeast-trending fault and shear zones, and right-lateral

movement in structures oriented between 330° and 250°.


Figure 30. Structural model for the Sascha – Pelligrini study area showing vein

styles and kinematic indicators for observed structural orientations.

Figure 31. Riedel shear model for the San Jose district applied to the Sascha –

Pelligrini study area. Mineralised veins striking at 315° parallel to 1 represent extension fracture (T) in the model. Veins striking >315° are observed with a sinistral strike-slip component (San Jose), whereas veins striking <315° are observed to have

a dextral strike-slip component (SVZ). (After Dietrich et al 2008).

Sascha

Vein Zone

San Jose

District


Contrary to recent structural interpretations of the Desseado Massif that

suggest right-lateral structures develop only narrow discontinuous and en

echelon veins of no economic significance (Echavarria et al, 2005), detailed

mapping of the SVZ indicates mineralised veins are hosted by right-lateral

fault structures. A recent structural study of the Huevos Verdes veins within

the San Jose district also supports the concept that economic significant

epithermal veins are hosted on right-lateral fault structures. The structural

model of the SVZ correlates well with the structural model of the San Jose

district for the northwestern edge of the Desseado Massif. This structural

model suggests right-lateral west-northwest-trending faults (280°) and left-

lateral north-northwest-trending faults (350°) act as a conjugate shear pair.

The orientation of 1 is modelled as the acute bisector of the conjugate shear

pair and is orientated at 315°. The majority of the veins at Huevos Verdes

within the San Jose district are modelled as purely extensional, and similar to

the SVZ, are orientated parallel to 1 at 315°. The trend of the Huevos

Verdes and Sascha systems varies from 300° to 340° with an overall left-

lateral strike-slip component observed for the Huevos Verdes veins striking at

325°(Dietrich et al, 2008), and an overall right-lateral strike-slip component

observed for the Sascha veins striking at 315°. The Huevos Verdes model

correlates with the observed structural pattern of the SVZ and fits well with

the Riedel shear model (Figure 31). Left-lateral north-northwest-trending

lineaments represent the main shear plane and north-northwest and west-

northwest structures represent R and R‟ shears respectively (Dietrich et al,

2008).

Host rock rheology controls vein morphology and alteration patterns across

the study area. The current exposure along the SVZ indicates the friable

rhyolite tuffs of the La Matilde Formation do not host continuous brittle

fractures or veins, and instead host upward terminating veins, stockwork vein

zones, and pervasive silicification. Brittle fractures and more massive veining

may be developed within the crystalline rhyodacite ignimbrite that

stratigraphically lies below the current vein exposure. Structural analysis of

the SVZ indicates the tension axis trended toward the northeast quadrant,

producing right-lateral movement along the 315° trend. The structural pattern


of the SVZ conforms to the Riedel shear model, with 1 orientated at 315°

and modelled as the acute bisector of the conjugate shear pair.

Alteration zoning

Hydrothermal alteration within active geothermal fields is well documented

(eg. Browne, 1978; White, 1981; Hedenquist and Henley, 1985; Spycher and

Reed, 1989; Reyes, 1990; Fulignati et al, 1997; Cox and Browne, 1998;

Ruggieri et al, 1999; Simmons and Browne, 2000; Patrier et al, 2003; and

Bignall et al, 2004), with observed alteration assemblages and mineral

zoning forming the basis of epithermal alteration models (Berger and Eimon,

1982; Heald et al, 1987; Berger and Henley, 1988 etc).

Hydrothermal alteration of the SVZ is characterised by quartz, adularia, illite,

pyrite and minor calcite, and is overprinted by a late stage kaolinite-

dominated assemblage. Throughout the Sascha-Pelligrini area, the common

hydrothermal minerals define distinct alteration zones around epithermal

veins (Figure 32). Each zone is characterised by a unique mineral

assemblage and can be grouped into two major alteration types according to

fluid chemistry. Clay abundance progressively increases above and towards

the vein margins, with formation of alteration assemblages driven by reduced

neutral or oxidized acid fluids.


Figure 32. Schematic cross section of alteration zoning and mineral assemblage

observed for the Sascha-Pelligrini epithermal system.


The primary alteration assemblage along the SVZ formed from a near neutral

to weakly alkaline chloride water, and is characteristic of the alteration found

in many active geothermal systems (Simmons and Browne, 2000). Primary

vein calcite, quartz pseudomorphs after bladed calcite and adularia occur

within the saccharoidal vein phase at Sascha Main and form from gas loss

and cooling associated with boiling (Browne and Ellis, 1970). Platy calcite

scales, analogous to bladed calcite within epithermal veins, form in

geothermal wells 100 to 300 meters above the point where the geothermal

water first flashes to steam (Simmons and Browne, 2000). The weakly

alkaline chloride-rich geothermal waters at Broadlands-Ohaaki contain

approximately 1,300 mg/kg chloride and 1,900 mg/kg CO2 (Hedenquist and

Henley, 1985), and produce a characteristic alteration assemblage

dominated by quartz, adularia, illite, calcite, chlorite and pyrite (Simmons and

Browne, 2000).

Abundant kaolinite, alunite and native sulphur are observed at many

epithermal deposits, and occur overlying or towards the periphery of inferred

fluid up-flow zones (Schoen et al, 1974; Love et al, 1998; Simpson et al,

2001). Steam-heated, acid-sulphate waters commonly occur in the vadose

zone above boiling up-flow points, with alteration formed by leaching of rocks

from fluids concentrated in H2SO4. Fluids concentrated in H2SO4 can be

produced by atmospheric oxidation of sulphides, oxidation at the water table

by H2S release via boiling, and the condensation of magmatic vapor (Rye et

al, 1992). The acid-sulphate water reacts with the host-rock to produce

kaolinite, cristobalite, alunite, pyrite and native sulfur (Scheon et al, 1974).

Stable isotope studies of alunite from acid-sulphate alteration zones suggest

precipitation from dominantly meteoric waters with magmatic SO2 (Rye et al,

1992; Love et al, 1998; Mykietiuk et al, 2004).

Calcium-rich zeolites that characterise the peripheral alteration halo of the

Sascha-Pelligrini system form within the shallow and peripheral zones of

geothermal systems (Simmons and Browne, 2000; Steiner, 1977).

Condensation of CO2 gas and absorption into cool ground waters produces


these zeolites as well as low-temperature clays and carbonates (Simpson et

al, 2001). Country rock exposed to residual water after steam separation has

a high H2S/H2 ratio and also favors pyrite formation (Browne and Ellis, 1970).

The distal laumontite-montmorillonite alteration assemblage of the Sascha-

Pelligrini system passes to illite-dominant alteration proximal to epithermal

veins. The H2O content of calcium zeolites progressively decreases with

increasing temperature, zoning from laumontite to wairakite (Steiner, 1977).

Laumontite forms at temperatures above 110°C, passing to wairakite at

150°C, with illite forming above 200°C (Reed, 1994). The replacement of Ca-

zeolites (laumontite) with calcite and illite proximal to epithermal veins within

the Sascha-Pelligrini system indicates an increase in temperature and

dissolved CO2 content towards fluid up flow points (Browne and Ellis, 1970;

Cox and Browne, 1998).

The alteration assemblage of quartz, adularia, illite, calcite, chlorite and pyrite

along the SVZ is in equilibrium with deep chloride-rich waters (Simmons and

Browne, 2000). This alteration assemblage results from the recrystallisation

of the original rock with uptake of variable amounts of H2O, CO2 and H2S.

Mass balance geochemistry across the study areas shows hydrothermal

fluids proximal to veins introduced variable amounts of SiO2, Al2O3, Fe2O3,

K2O, MgO, TiO2 and Na2O, and correlates well to the chemistry of the

observed alteration assemblages.

PIMA alteration profiles adjacent to the SVZ show a decrease in the SWIR

AlOH clay absorption feature towards the silicified vein selvages representing

a decrease in total clay abundance (Pontual et al, 1997; Herrmann et al,

2001). Although total clay abundance decreases towards the vein, the

increase in the absorption wavelength position corresponds to an increase in

the amount of Fe and Mg in illite attributable to increasing temperature (Post

and Noble, 1993). Compositional variations of white micas studied in various

geothermal systems generally show an increase in K, Si, Fe and Mg and a

decrease in Al with increasing temperature (Bishop and Bird, 1987;

Cathelineau and Izquierdo, 1988; McDowell and Elders, 1983). Interstratified

illite-smectite vein selvages at Sascha Main confine fluid temperature to


below 220°C (KRTA, 1990) with illite-smectite alteration located above

mineralised veins at Golden Cross (Simpson et al, 1998) and Hishikari

(Izawa et al, 1990; Ibaraki and Suzuki, 1993) (Figure 33). Interstratified illite-

smectite usually occurs at the top of the gold mineralised interval

(Hedenquist and White, 2005) and suggests the current exposure at Sascha

Main is at a high level within the epithermal system.

The disassociation of aqueous CO2 during pressure release boiling within the

SVZ provides H+ into solution. The subsequently acidic hydrothermal

environment hydrolyzes feldspars within the host rock adjacent to the

epithermal system, forming the dominant clay-illite alteration halo. Pressure

release boiling, disassociation of aqueous CO2, and acidity buffering from

wall-rock feldspars releases Ca+ into solution to form bladed calcite within the

vein system. Calcite in equilibrium with adularia, illite and muscovite at

Sascha Main results from the reduced acidity buffering of host-rock K-

feldspar (sanidine) and K-mica (muscovite) with deep chloride waters

(Browne and Ellis, 1970; Browne, 1978; Simmons and Browne, 2000).

Figure 33. Alteration zones and mineral assemblages of the Hishikari epithermal system showing alteration zoning from sericite and or kaolinite through to illite-smectite mixed layer clays above the gold-silver veins. The presence of chlorite-sericite-adularia and minor mixed layer clays marks the alteration within the gold mineralised interval.


The Sascha Main and Sur systems are characterised by abundant kaolinite

associated with halloysite, hematite, goethite and very minor alunite and

gypsum. This acid-sulphate alteration overprints illite assemblages and is

spatially related to weathered pyritic veins and wall-rocks. Altered rocks with

acid-sulphate assemblages contain abundant cubic voids after pyrite with

mass-balance geochemistry indicating no introduction of sulphate. SWIR

analysis of alteration adjacent to pyritic veins show a systematic increase in

both absorption wavelength and absorption depth, attributed to an increase in

overprinting kaolinite (Pontual et al, 1997). The Sascha Main acid-sulphate

blanket represents a supergene alteration assemblage formed by oxidation of

wall-rock pyrite above the palaeo-water table. During weathering of pyrite,

iron released by carbonic acid-bearing rainwater precipitates almost

immediately as ferric hydroxide due to the low pH and Eh (Schoen et al,

1974). Bright yellow-orange amorphous iron hydroxides, hematite and

goethite characteristic of weathered pyrite are abundant throughout the

kaolinite blanket over Sascha Main.

The Pelligrini acid-sulphate alteration characterised by abundant kaolinite,

with lesser alunite, natroalunite and opal, forms a broad circular alteration

halo around pervasive silicification. The acid-sulphate alteration passes

vertically to illite-pyrite alteration, and represents overprinted alteration

assemblages. The Pelligrini alteration system is similar to that formed above

blind orebodies within the Fresnillo district (Simmons, 1991) and is most

likely associated with H2S oxidation in a steam-heated environment. Base

cation leaching and mass loss within the pervasively altered ash tuff at

Pelligrini is indicative of steam-heated acid-sulphate leaching (Reyes, 1990;

Rye et al, 1992), with large mass gains in sulphate corresponding to the

introduction of alunite. The intense pervasive silicification is associated with

potassium enrichment and is characteristic of ore-related hydrothermal

silicification within the Desseado Massif (Echavarria et al, 2005). Intense

silicification occurs within stratigraphically high La Matilde and Chon Aike

units above epithermal veining at Manantial Espejo in Argentina (Wallier and

Tosdal, 2008). Silicification within these units is interpreted to form at the


palaeo-water table that channeled lateral flow of the hydrothermal system

and led to silica precipitation (Wallier and Tosdal, 2008). The stratigraphically

high silicification associated with acid-sulphate alteration forming by oxidation

of H2S within a steam-heated environment places the current exposure at

Pelligrini above the water table with formation temperatures less than 100°C

(Rye et al, 1992).

Hematite-rich rocks developed along the northern periphery of the kaolinite-

alunite-jarosite alteration zone at Pelligrini suggest horizontal permeability

with products of hydrolysis migrating laterally from the source. Steamboat

Springs provides a modern analogue to the alteration at Pelligrini, with

hematite-rich zones developed at the transition from acid-sulphate to

montmorillonite alteration (Schoen et al, 1974). High temperature dickite in

outcropping polymict vein breccias associated with pervasive illite-dominant

alteration adjacent to low-temperature alunite-opal at Pelligrini suggest the

acid-sulphate assemblage has encroached downward into hotter parts of the

epithermal system. The complex alteration at Pelligrini represents a well

preserved high-level epithermal alteration system at the palaeo-water table.

Hematite-rich rocks along the northern periphery of the alteration system

represent the location of the palaeo-water table, and the boundary between

the downward migrating low-temperature alunite-opal assemblage with the

higher temperature illite-dominant assemblage. The overprinting alteration

assemblages may represent a descending water table in a waning

geothermal system with low-temperature alunite-opal alteration forming in

illite altered rocks. Oxidation of H2S vapors produce dilute, acid SO4 waters

and acid-sulphate alteration above fluid up flow points (Pelligrini), with

outflow of neutral Cl waters (Sascha) discharged at a considerable distance

from up flow points (Giggenbach, 1992).

Hydrothermal minerals define distinct alteration zones across the study area.

Formation of alteration assemblages is driven by fluid chemistry, and can be

characterised on the basis of an assemblage in equilibrium with reduced

neutral or oxidized acid fluids. The alteration system around the SVZ is

characterised by a broad distal laumontite-montmorillonite alteration halo that


passes to illite-dominant alteration proximal to epithermal veins. Vein

selvages are characterised by smectite and interstratified illite-smectite, with

the alteration assemblage indicating a reduced neutral environment. The

initially acidic hydrothermal fluid is buffered by wall-rock feldspars, forming

the dominant clay-illite alteration halo. The Sascha Main acid-sulphate

blanket represents a supergene alteration assemblage formed by oxidation of

wall-rock pyrite above the palaeo-water table. The acid-sulphate alteration

passes vertically to illite-pyrite alteration, and represents overprinted

alteration assemblages. Base cation leaching and mass loss within the

pervasively altered ash tuff at Pelligrini is indicative of steam-heated acid-

sulphate leaching, with large mass gains in sulphate corresponding to the

introduction of alunite. The overprinting alteration assemblages across the

study area indicate a lowering of the water table during the waning stages of

the epithermal system.

The study of alteration minerals through the use of field portable PIMA

equipment has provided a qualitative estimate of alteration mineralogies.

Based on the methodology employed through the use of PIMA equipment,

potential errors can occur in mineral identification. PIMA alteration sampling

is a useful field based tool for rapid identification of alteration mineralogies

sufficient for a mineral exploration program. Where possible, ground truthing

of PIMA data should be validated by quantitative XRD analysis of selected

individual alteration assemblages. Depending on the use and need of

alteration data, PIMA can be effectively used a quick field base tool for

exploration programs dealing with strong clay alteration of host rocks. The

comparison between PIMA and XRD data shows detailed clay mineralogies

and mixed layer clay assemblages can only be identified through the use of

XRD analysis. Detailed alteration studies must employ lab based XRD

sampling to quantitatively outline the alteration mineralogies.


Vein Paragenesis

Detailed trench and outcrop mapping identifies six temporally and texturally

distinct vein phases along the SVZ. The paragenesis of the vein phases

along the SVZ form six main stages: stage one, chalcedonic; stage two,

colloform-crustiform +/- ginguro; stage three, saccharoidal; stage four,

chalcedonic with disseminated pyrite; stage five, jasperoidal; and, stage six,

crystalline and comb veins (Figure 34). The colloform- crustiform-banding of

stage two forms as an initial mineralising phase on the margins of stage three

saccharoidal veins. The main mineralising events are stage two, associated

with two colloform-crustiform ginguro bands, and stages four and five,

associated with chalcedonic and jasperoidal veins with disseminated pyrite.

Individual mineralising events are comprised of complex mineral paragenetic

sequences with mineral relationships giving insight into system evolution

(Figure 35).

Vein Phase

Chalcedonic

Saccharoidal with minor bladed textures

Colloform/Crustiform +/- ginguro

Chalcedonic with patchy disseminated pyrite

Jasperoidal

Crystalline and Comb

> TIME >

Figure 34. Vein phases plotted against time outlining the vein paragenetic relationships for the Sascha-Pelligrini epithermal system.

Vein

Phase


Min

era

lF

orm

ula

Gin

gu

ro

Gin

gu

ro

Pyri

te C

ha

lce

do

ny

Sta

ge

IS

up

erg

en

eS

tag

e I

IS

up

erg

en

eS

up

erg

en

e

Se

Aca

nth

ite

Ag

4S

eS

Ele

ctr

um

AgA

u

Ja

me

so

nite

Pb

(Ag) 4

Fe

Sb

6A

sS

14

He

ma

tite

Fe

2O

3

Uyte

nb

oga

ard

tite

Ag

3A

uS

2

Go

ldA

u

Aca

nth

ite

Ag

2S

Ca

lcite

Ca

CO

3

Ch

alc

op

yrite

Cu

Fe

S2

Ga

len

aP

bS

Sp

ha

lerite

Zn

S

Pe

tro

vska

ite

AgA

u(S

eS

)

Du

fre

no

ysite

Pb

2A

s2

S5

Ba

rite

Ba

SO

4

Ad

ula

ria

KA

lSi3

O8

Mu

sco

vite

KA

l 2(S

i 3A

l)O

10

(OH

) 2

Iod

oe

mb

olit

eA

g(C

l,B

r,I)

Em

bo

lite

Ag(C

l,B

r)

Iod

oa

rgyrite

AgI

Va

rla

mo

ffite

(Sn

,Fe

)(O

,OH

) 2

Pyrite

Fe

S2

Ars

en

op

yrite

AsF

eS

2

Gyp

su

mC

aS

O4

Ja

lpa

ite

Ag3

Cu

S2

Ja

rosite

KF

e3+(S

O4)2

(OH

)6

Figure 35. Vein mineral paragenetic relationships for the Sascha-Pelligrini epithermal system.


Ginguro stage I mineralisation is characterised by the initial precipitation of

selenium-rich acanthite. Mineral assemblages observed in selenide-bearing

epithermal deposits suggests oxygen fugacities were below or very close to

the hematite-magnetite buffer, ƒSe2(g)/ƒS2(g)

ratios were lower than unity, and

temperatures of formation were between 150° to 210°C (Simon et al, 1997).

Under these conditions, selenium cannot be separated from sulphur; the

early substitution of selenium in sulphide minerals prevents its concentration

in hydrothermal fluids, and limits precipitation to silver selenides (Simon et al,

1997). Selenide-bearing minerals are often associated with gold and silver

mineralisation (Simon et al, 1997) and commonly occurr in many epithermal

deposits throughout Indonesia (Kieft and Oen, 1973), Japan (Shikazono,

1978), Kunashir Island (So et al, 1995), Nevada (Saunder et al, 1988),

Mexico (Petruk and Owens, 1974), and New Zealand (Main et al, 1972).

Selenium-rich acanthite is associated with gold and silver mineralisation

across the SVZ, with silver selenide restricted to a single stage that predates,

or is contemporaneous with gold and silver deposition. The presence of silver

selenide in the ginguro stage I ore marks an acidic-oxidised magmatic-

sourced fluid that rapidly hydrolyses feldspars.

Following the introduction of an acidic-oxidised magmatic-sourced fluid, the

fluid pH increases during equilibration with wall-rocks associated with silicate-

buffered dilution (Reed and Palandri, 2006). The solution pH increases as H+

ions are consumed in breaking down the primary rock silicates, resulting in

an exchange of aqueous H+ for cation from the rock. The overall reaction

changes the initially acidic aqueous phase (magmatic composition) to a

composition in equilibrium with a propylitic assemblage. The H+ is exchanged

for base cations, and sulphate is reduced to sulphide. The molar ratios of

sulphate to sulphide change from 24.5 to 0.00018 and the concentration of

aqueous H2 increases 3 orders of magnitude. Sulphate concentration

decreases due to the combined effects of fractionation of early-formed

sulphate minerals (barite, jarosite, gypsum, and alunite) and the reduction of

sulphate to sulphide with reaction of ferrous iron to form hematite, magnetite

or epidote. The high concentration of SO2 in magmatic gases renders them

much more oxidising than equilibrium with ferrous iron in wall rocks allows


(Reed, 1994). The outward traverse of such fluids inevitably yields reduced

fluids and a relatively more oxidised wall rock (Giggenbach, 1992; Reed,

1994).

The concentration of Ba2+ in the hydrothermal fluid increases as sulphate is

reduced to sulphide through acid neutralisation from wall-rock buffering

(Reed, 1994). The precipitation of barite within ginguro stage I indicates a

reduced fluid that has mixed with shallow acid-sulphate waters above the

water table. Following the increase in pH, hematite replaces pyrite (Spycher

and Reed, 1989), with gold and silver precipitated as electrum (Reed and

Palandri, 2006). Silver-rich electrum coexisting with intergrown

uytenbogaardtite and acanthite represents disequilibrium cooling of a high-

termperature gold-bearing argentite and electrum assemblage (Figure 14 A,

B) (Barton, 1980). This disequilibrium assemblage may be associated with

conduit sealing and system quiescence.

Vein filling associated with ginguro stage I mineralisation sealed the fluid

conduit allowing fluid pressure to increase. Incremental structural dilation and

associated fracturing resulted in rapid pressure release with isoenthalpic

boiling of the hydrothermal fluid depositing fine pseudo-accicular bladed

calcite prior to deposition of ginguro stage II (Figure 11). Calcite is a common

soluble phase in epithermal veins (Dong et al, 1995), with the precipitation of

vein calcite driven by the loss of CO2 due to boiling and the subsequent

generation of CO32- ions from the dissociation of HCO3 (Henley, 1985).

Primary pseudo-acicular bladed calcite is replaced by quartz, preserving the

original crystal morphology.

Mineralisation during Sascha ginguro stage II is characterised by the initial

precipitation of hematite in association with barite. Electrum and acanthite

are the first ore minerals to be depositied, followed by precipitation of gangue

calcite (Figure 14 C, D). Gold and uytenbogaardtite are precipitated after

calcite, followed by selenium rich acanthite. Base metal sulphides,

chalcopyrite, sphalerite and galena, are deposited after uytenbogaardtite,

with the lead sulphosalt dufrenoysite marking the end of ginguro stage II


mineralisation. Gangue minerals adularia and muscovite characterise the

final stages of mineralisation. Quartz and barite are deposited throughout

most of the ginguro mineralising event.

Ginguro stage II mineralisation followed isoenthalpic boiling from 280°C,

decreasing pH and favoured the precipitation of acanthite instead of arsenic

and antimony sulphosalts (Drummond and Ohmoto, 1985; Spycher and

Reed, 1989). Continued wall-rock silicate buffering of the low pH fluid

increases Ba2+ in solution as sulphate is reduced to sulphide through acid

neutralisation (Reed, 1994). The occurrence of gangue barite throughout

ginguro stage II indicates the continuation of fluid mixing between the

reduced hydrothermal fluid and shallow acid-sulphate waters above the water

table. Following an increase in pH, hematite replaces pyrite (Spycher and

Reed, 1989), with gold and silver precipitating as electrum (Reed and

Palandri, 2006). Calculated electrum-sphalerite formation temperatures

range from 174°C to 205°C and indicate a decrease in fluid temperature from

isoenthalpic boiling at 280°C associated with increasing pH. Chalcopyrite

precipitates at pH 3.5-3.8, consuming bornite and pyrite, with sphalerite

precipitating at pH 5.4, and galena precipitating at pH 5.7 (Reed and

Palandri, 2006; Reed, 1994). The rise in pH associated with wall-rock

buffering and acid neutralisation decreases metal concentrations by many

orders of magnitudes as the fluid approaches neutral pH (Reed and Palandri,

2006). The sequential precipitation of chalcopyrite-sphalerite-galena in

ginguro stage II is characteristic of an increase in fluid pH as wall-rock

feldspars neutralise acidic metal-bearing fluids (Reed and Palandri, 2006).

Neutral fluid is marked by the presence of adularia and muscovite

precipitated as the last phases within the ginguro stage II event. Following

fluid neutralisation and precipitation of ginguro bands, the fluid conduit may

have sealed allowing fluid pressure to increase once again.

Ginguro stage II mineralisation is followed by coarse lattice bladed calcite

pseudomorphs and saccharoidal quartz. Rapid pressure release boiling

associated with structural dilation and/or hydrothermal eruptions deposited

coarse bladed and crystalline calcite after the ginguro stage II event. The


current outcrop exposure suggests the epithermal system was sealed after

deposition of coarse bladed and crystalline calcite.

Overprinting pyritic chalcedonic quartz veins represents renewed

hydrothermal activity focused along parallel fractures to the preceding

ginguro event. The presence of pyrite associated with chalcedonic and

jasperoidal silica indicates meteoric incursion within the upper level of the

epithermal system. Chaotic pyritic chalcedonic and jasperoidal quartz veins

are hosted within silicified lithic rhyolite tuff in Sascha Main. Spatially

restricted and laterally discontinuous lithic tuff concentration zones may have

acted as a confined aquifer between underlying strongly welded tuff and

overlying rhyolite crystal ash tuffs, introducing cool, oxygenated meteoric

water at depth below the palaeosurface. Simultaneous dilution and cooling by

cold water mixing yields substantial pyrite in association with acanthite below

temperatures of 177°C (Reed and Palandri, 2006). Copper-rich acanthite

inverts to jalpaite at temperatures below 117°C, and exists as a 2 phase

region with acanthite limiting temperatures of formation to below 106°C

(Skinner, 1966). Strong arsenic zoning in pyrite represents local

disequilibrium during growth, representing local fluctuations in the S2/As2

ratio (Kretchmar and Scott, 1976).

Individual mineralising events of ginguro-banded and pyritic chalcedonic and

jasperoidal veins are comprised of complex mineral paragenetic sequences

that give an insight into systems geochemical evolution. Ginguro stage I ore

is characterized by an initially acidic-oxidised magmatic-sourced fluid. The

solution pH increases with wall-rock buffering, driving the fluid to equilibrium

with a propylitic assemblage. The precipitation of barite within ginguro stage I

marks the point at which the now reduced fluid has mixed with shallow acid-

sulphate waters above the water table. The deposition of fine pseudo-

accicular bladed calcite after ginguro stage I, and prior to ginguro stage II,

suggests isoenthalpic boiling lowering the fluid pH once again. The

occurrence of gangue barite throughout ginguro stage II indicates the

continuation of fluid mixing between the reduced hydrothermal fluid and

shallow acid-sulphate waters above the water table. The presence of lead


sulfosalts Jamesonite and Dufrenoysite indicate a transgression from the

typically low sulphur activity during the systems evolution, indicating brief

intervals of intermediate sulphidation. Electrum-sphalerite formation

temperatures indicate a decrease in fluid temperature associated with

increasing pH. The sequential precipitation of chalcopyrite-sphalerite-galena

in ginguro stage II indicates an increase in fluid pH, with adularia and

muscovite marking the return to neutral conditions in equilibrium with a

propylitic assemblage. The presence of pyrite associated with chalcedonic

and jasperoidal silica indicates simultaneous dilution and cooling by cold

water mixing above the water-table.The progression from deeper level

veining exposed adjacent to shallow level veining correlates with the

overprinting alteration at both Sascha and Pelligrini and represents the

downward migration of the water table across the study area over the

systems evolution.

Supergene Overprint

Destabilisation of wall-rock pyrite by oxygenated ground water produces acid

sulphate water associated with kaolinite. Acid sulphate groundwater

destabilises pyrite within chalcedonic quartz mineralisation, with pyrite being

replaced by hematite. Selenium-rich acanthite, acanthite and jalpaite persist

as inclusions within supergene hematite and gypsum. The silver halide

assemblage within outcropping veins at Sascha is uncharacteristic of silver

chloride-rich supergene zones within many silver-rich deposits. Embolite,

iodoembolite and iodoargyrite variably overprint acanthite, and in conjunction

with free gold comprise the supergene assemblage of the Sascha Ginguro

veins. Silver halides are common secondary minerals in supergene zones of

silver rich mineral deposits, zoning from silver chloride to silver iodide with

depth (Gammons and Yu, 1997). Silver iodides are uncommon in

outcropping supergene-enriched veins with iodide rapidly oxidizing to iodate

(Gammons and Yu, 1997), however silver iodide persists within the

supergene zone at Sascha Main. Preservation of silver iodide within

outcropping veins across the Sascha-Pelligrini area can be attributed to the

dry, cold and arid climate of Patagonia.


Summary

Facies associations of the volcanics within the study area closely represent

an intra-caldera setting, with volcancis erupted from a complex series of

graben-forming extensional fractures instead of within a classic caldera

margin. Several distinct eruption styles are recorded within the volcanic

sequence. The rhyodacite sequence was rapidly deposited by eruption

column collapse from multiple vents or fissures creating a composite welded

unit. The spatially restricted La Matilde rhyolite sequence was deposited from

a single violent eruption, and associated column collapse, with settling of the

convective cloud.

Strong hydrothermal alteration, sorting, welding, post emplacement

crystallisation and devitrification, as well as erosion throughout the majority of

the study area precludes quantitative petrochemical modeling the volcanic

rock suite. Variation in REE abundances across the rhyodacite and rhyolite

sequences is consistent with the proposed fractionation trends of Pankhurst

and Rapela (1995). Pankhurst and Rapela (1995) demonstrate that the

sequence from basaltic andesite (Bajo Pobre) to dacite/rhyodacite to rhyolite

(Chon Aike) can be modeled by a combination of partial melting and

fractional crystalisation.

Host rock rheology controls vein morphology and alteration patterns across

the study area. The current exposure along the SVZ indicates the friable

rhyolite tuffs of the La Matilde Formation host upward terminating veins,

stockwork vein zones, and pervasive silicification. Brittle fractures and more

massive veining may be developed within the crystalline rhyodacite

ignimbrite that stratigraphically lies below the current vein exposure.

Structural analysis of the SVZ indicates the tension axis trended toward the

northeast quadrant, producing right-lateral movement along the 315° trend.

The structural pattern of the SVZ conforms to the Riedel shear model, with 1

orientated at 315° and modelled as the acute bisector of the conjugate shear

pair.


Hydrothermal minerals define distinct alteration zones across the study area.

The alteration system around the SVZ is characterised by a broad distal

laumontite-montmorillonite alteration halo that passes to illite-dominant

alteration proximal to epithermal veins. Vein selvages are characterised by

smectite and interstratified illite-smectite, with the alteration assemblage

indicating a reduced neutral environment. The initially acidic hydrothermal

fluid is buffered by wall-rock feldspars, forming the dominant clay-illite

alteration halo. The Sascha Main acid-sulphate blanket represents a

supergene alteration assemblage formed by oxidation of wall-rock pyrite

above the palaeo-water table. Alteration assemblages at Pelligrini are

indicative of steam-heated acid-sulphate leaching.

Individual mineralising events of ginguro-banded and pyritic chalcedonic and

jasperoidal veins are comprised of complex mineral paragenetic sequences

that give an insight into systems geochemical evolution. Ginguro stage I and

II is characterised by an initially acidic fluid, with wall-rock buffering driving

the fluid to equilibrium with a propylitic assemblage. The precipitation of

barite indicates fluid mixing with shallow acid-sulphate waters above the

water table. Electrum-sphalerite formation temperatures indicate a decrease

in fluid temperature associated with increasing pH. The sequential

precipitation of chalcopyrite-sphalerite-galena in ginguro stage II is

characteristic of an increase in fluid pH, with adularia and muscovite marking

the return to neutral conditions in equilibrium with a propylitic assemblage.

The presence of pyrite associated with chalcedonic and jasperoidal silica

indicates simultaneous dilution and cooling by cold water mixing above the

water-table.The progression from deeper level veining exposed adjacent to

shallow level veining correlates with the overprinting alteration at both

Sascha and Pelligrini and represents the downward migration of the water

table across the study area over the systems evolution. The unique

supergene, silver iodide assemblage, preserved within outcropping veins

across the Sascha-Pelligrini area can be attributed to the dry, cold and arid

climate of Patagonia.


The Sascha-Pelligrini study area consists of a well preserved, high-level,

Jurassic low-sulphidation epithermal system hosted within the Chon Aike

Formation. The presence of lead sulfosalts Jamesonite and Dufrenoysite

indicate intermittent intervals of higher sulfur fugacities, with the epithermal

system progressing from typical low-sulphidation to intermediate-sulphidation

assemblages. The transgression between sulphidation states supports the

idea that end-member deposits form as part of a continuum between the two.

The Sascha-Pelligrini epithermal system was produced by a complex

interaction of pyroclastic volcanics, host-rock rheology and geochemistry,

structural setting, paleo-water table and hydrothermal fluid evolution. The

conceptual model for the Sascha-Pelligrini study area is presented in figure

36, with epithermal veining forming on graben and half-graben structures

below the palaeo-water table. The model depicts the association between

host rock, alteration zoning including vein selvages and depth of palaeo-

water table. Each individual aspect plays an important role in the nature and

occurrence of the systems evolution, of which each is essential in combining

to form an epithermal deposit.


Fig

ure

36. C

on

cep

tual ep

ith

erm

al m

od

el fo

r th

e S

as

ch

a –

Pell

igri

ni

stu

dy a

rea.

Mode

l d

ep

icts

altera

tion a

nd v

ein

selv

ag

e z

on

ing, p

ala

eo w

ate

r ta

ble

, str

uctu

ral a

nd s

tratigra

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rela

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ips, rh

eo

log

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ol, v

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dis

trib

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urr

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l surf

ace.


Conclusions

The Sascha-Pelligrini low-sulphidation epithermal system is located on the

western edge of the Deseado Massif, Santa Cruz Province, Argentina.

Outcrop sampling has returned values of up to 160g/t gold and 790g/t silver.

Detailed mapping of the volcanic stratigraphy has defined three units that

comprise the middle Jurassic Chon Aike Formation and two units that

comprise the upper Jurassic La Matilde Formation. The Chon Aike Formation

consists of rhyodacite ignimbrites and tuffs, with the La Matilde Formation

including rhyolite ash and lithic tuffs. The volcanic sequence is intruded by a

large flow-banded rhyolite dome, with small, spatially restricted granodiorite

dykes and sills cropping out across the study area.

The Chon Aike rhyodacite sequence shows an enrichment of LREE through

the fractionation of hornblende, allanite and apatite, with the Eu anomaly and

drepssion of the middle REE controlled by fractionation of plagioclase and

hornblende respectively. Decreasing REE trends in the La Matilde rhyolite

might be due to the eruption of a zoned magma chamber. Similar to the

rhyodacite package, the rhyolite sequence REE patterns show a negative Eu

anomaly, decrease in REE concentrations with continued eruptive units, and

depressed HREE‟s. The negative Eu anomaly may be controlled by the

fractionation of sanidine, with the depletion of REE‟s controlled by the

fractionation of hornblende. High-silica rhyolite can be produced from

fractional crystalisation of rhyodacite, with the removal of 40% crystal

assemblage composed of 58.05% plagioclase, 40% hornblende, 1% apatite

and 0.05 allanite (Pankhurst and Rapela, 1995).

XRD analysis in combination with PIMA and ASTER spectral analysis defines

an alteration pattern that zones from laumontite-montmorillonite, to illite-

pyrite-chlorite, followed by a quartz-illite-smectite-pyrite-adularia vein

selvage. The alteration assemblage of quartz, K-feldspar, illite, calcite,

chlorite and pyrite across the Sascha-Pelligrini area is commonly observed in

geothermal systems and is in equilibrium with near neutral, deep chloride


waters (Simmons and Browne, 2000). Condensation of CO2 gas and

absorption into cool ground waters within the periphery of the epithermal

system has produces low-temperature clays, carbonates and calcium

zeolites (Simpson et al, 2001).

The supergene kaolinite blanket over Sascha Main is charcterised by

hematite, goethite and bright yellow-orange amorphous iron hydroxides, with

the acid-sulphate assemblage formed through the weathering of pyrite. Base

cation leaching and mass loss within the pervasively altered ash tuff at

Pelligrini is indicative of steam-heated acid-sulphate leaching (Reyes, 1990;

Rye et al, 1992), with large mass gains in sulphate corresponding to the

introduction of alunite. The oxidation of H2S within a stream heated

environment takes place above the water table at temperatures not

exceeding 100°C (Rye et al, 1992), with opal at Pelligrini confining

temperature of formation to less than 120°C (KRTA, 1990).

ASTER mineral mapping is an invaluable tool in rapidly identifying alteration

systems across large mineral districts. PIMA spectral analysis enables rapid

field identification of alteration minerals, however cannot identify

interstratified clays. XRD analysis readily identifies and quantifies alteration

minerals, however does not highlight end-member mineral compositions.

Characterisation of epithermal alteration systems should incorporate ASTER,

PIMA and subsequent XRD analysis. Individual methods can readily identify

alteration minerals, however the realisation of the complete alteration

assemblage and spatial distribution can effectively be compiled with all three

tools.

Paragenetically, epithermal veining varies from chalcedonic to saccharoidal

with minor bladed textures, colloform- crustiform-banded with visible electrum

and acanthite, crustiform-banded grey chalcedonic to jasperoidal with fine

pyrite, and crystalline comb quartz. Mineralisation during the ginguro events

is controlled by fluctuating pH, driven by a combination of magmatic gases,

pressure release boiling and silicate wall-rock buffering. Pyrite-rich

chalcedonic veins formed through simultaneous dilution and cooling by cold


water mixing, yielding substantial pyrite in association with acanthite below

temperatures of 177°C (Reed and Palandri, 2006). Electrum-sphalerite

geothermometry of ginguro mineralised veins constrains formation

temperatures from 174.8 to 205.1°C and correlates with the stability field for

the interstratified illite-smectite vein selvage.

Vein morphology, mineralogy and associated alteration are controlled by host

rock rheology, permeability, and depth of the palaeo-water table.

Mineralisation within ginguro banded veins resulted from fluctuating fluid pH

associated with selenide-rich magmatic pulses, pressure release boiling and

wall-rock silicate buffering. The Sascha-Pelligrini epithermal system provides

a deposit specific model helping to clarify the current understanding of

epithermal deposits, and may serve as a template for exploration of similar

epithermal deposits throughout Santa Cruz.


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Appendix 1

Petrography


Plate 1. Rhyodacite crystal ignimbrite with large albite, sanidine, biotite (chlorite after biotite) and embayed quartz phenocrysts in a quartz-feldspar-glass ash matrix. Photo A XPL x50. Photo B PPL x50. Field of view approximately 2mm.

A B

A B

Plate 2. Rhyodacite pumice ash tuff with sericitised albite, sanidine and biotite phenocrysts. Large, devitrified flattened pumice clasts and embayed quartz phenocrysts in a quartz-feldspar-glass ash matrix. Strong compaction textures. Photo A XPL x50. Photo B PPL x50. Field of view approximately 2mm.

A B

Plate 3. Rhyodacite crystal ash tuff with albite, sanidine, biotite (chlorite after biotite) and embayed quartz phenocrysts in a quartz-feldspar-glass ash matrix. Large fine-grained quartz-feldspar-biotite juvenile clasts prominent. Photo A XPL x50. Photo B PPL x50. Field of view approximately 2mm.


A B

Plate 4. Lithic rhyolite tuff with sericitised sanidine, minor muscovite and embayed quartz phenocrysts in a quartz-feldspar-glass ash matrix. Large mica schist lithic clasts prominent. Photo A XPL x50. Photo B PPL x50. Field of view approximately 2mm.

A B

Plate 5. Rhyolite crystal ash tuff with sericitised sanidine, minor muscovite and embayed quartz phenocrysts in a quartz-feldspar-glass ash matrix. Rare devitrification textures in glassy matrix. Photo A XPL x50. Photo B PPL x50. Field of view approximately 2mm.

Plate 6. Rhyolite ash tuff with fine sanidine laths, quartz and glass shards. Rare spherulitic and accretionary lapilli layers. Photo A XPL x50. Photo B PPL x50. Field of view approximately 2mm.

A B A B


A B

Plate 7. Flow-dome auto-breccia fine sanidine-quartz crystalline matrix, with large clasts of flow-banded and spherulitic rhyolite to 2m in diameter. Photo A XPL x100. Photo B PPL x100. Field of view approximately 1mm.

A B

Plate 8. Flow-banded rhyolite with euhedral sanidine phenocrysts in a crystalline quartz-feldspar matrix. Photo A XPL x100. Photo B PPL x100. Field of view approximately 1mm.

A B

Plate 9. Spherulitic rhyolite with spherulites to 5mm in a quatz-feldspar-glass matrix. Photo A XPL x50. Photo B PPL x50. Field of view approximately 2mm.


A B

Plate 10. Porphyritic granodiorite dyke with sanidine and sodic plagioclase phenocrysts. Photo A XPL x50. Photo B PPL x50. Field of view approximately 2mm.

A B

A B

Plate 11. Marine fossiliferous feldsarenite with bivalve, gastropod and bryzoan fragments with a micritic cement. Photo A XPL x50. Photo B PPL x50. Field of view approximately 2mm.

Plate 12. Epiclastic with mica, quartz, feldspar and calcite clasts. Photo A XPL x50. Photo B PPL x50. Field of view approximately 2mm.


Plate 13. Vesicular olivine tholeiite plateau basalt. Photo A XPL x50. Photo B PPL x50. Field of view approximately 2mm.

Plate 14. Basaltic dyke with microcline and oscilitory zoned anorthite phenocrysts in a feldspar lath ground-mass. Photo A XPL x50. Photo B PPL x50. Field of view approximately 2mm.

A B

A B


Appendix 2

Quartz Textures


Plate 1 Chalcedonic single pulse tectonic breccia. Angular leached clasts of argilllized wall rock, including strongly colliform banded fine saccharoidal to chalcedonic silica. Scale bar equals 10mm.

Plate 3 White, fine-grained saccharoidal silica with fine, prismatic-bladed carbonate pseudomorphs. Scale bar equals 10mm. A – Quartz-clay wall rock alteration B – Fine bladed pseudomorphs

Plate 2 Single phase wall rock breccia proximal to main veins contain sulphide bands in matrix to wall rock fragments. Sulphide bands occur on clast edges, grading to late crystalline quartz lining inter clast voids. Scale bar equals 10mm. A – Wall rock clast B – Sulphide band

A

Plate 5 Chalcedonic with disseminated pyrite vein/vein breccia with supergene iron oxide gossanous zones. Scale bar equals 10mm.

Plate 4 White, fine-grained saccharoidal silica with vuggy cavities. Scale bar equals 10mm.

Plate 6 Banded quartz-iron oxide gossanous crust on chalcedonic with disseminated pyrite vein. Scale bar equals 10mm.

A B

A

B


Plate 7 Massive to banded jasperoidal silica with disseminated sulphides and a saccharoidal, moderately banded pyrite matrix. Scale bar equals 10mm. A – Jasperoidal silica B – Pyritic crystalline to saccharoidal quartz

Plate 9 Euhedral axiolitic clear to milky comb quartz crystals growing perpendicular to vein margins. Scale bar equals 10mm.

Plate 8 Amythestine to milky, euhedral axiolitic comb quartz crystals growing perpendicular to vein margins. Scale bar equals 10mm. A – Silicified wall rock B – Amythestine quartz C – Large euhedral zoned quartz crustals

Plate 10 Grey-white fluidised vein breccia with crustiform banded chalcedonic to crystalline quartz. Scale bar equals 10mm.

Plate 11 Chalcedonic vein breccia cross-cut by crustiform chalcedony, followed by milky comb quartz with iron oxide staining, and infilled with „ladder‟ banded cream chalcedonic quartz. Cream chalcedonic quartz preserves meniscus like texture. Horizontal vein slice in formation orientation with top of photo towards palaeosurface. Scale bar equals 10mm.

A B

A

B

C


Plate 17 Intensely silicified tuff with relict rock textures preserved. Scale bar equals 10mm.

Plate 12 Crustiform chalcedonic, bladed and colloform banded quartz vein overprinted by axiolitic comb quartz. Scale bar equals 10mm.

Plate 16 Monomict chalcedonic quartz jigsaw breccia containing wall rock clasts Scale bar equals 10mm.

Plate 13 Chalcedonic fluid streaming breccia with crystalline quartz overprint. Scale bar equals 10mm.

Plate 14 Matrix supported vein breccia with massive to banded chalcedonic vein fragments and silicified tuff fragments within chalcedonic silica. Scale bar equals 10mm.

Plate 15 Monomict vein breccia containing clasts of pervasively silicified wall-rock in chalcedonic to jasperoidal silica matrix. Scale bar equals 10mm.


Appendix 3

Digital Dataset


DVD-ROM includes:

ASTER raw scenes and processed images and alteration maps

Geothermometry calculations and excel spreadsheet

Mapinfo dataset including

o Exploration geochemistry

o Geological mapping

o PIMA and XRD dataset

o Remotesensing including rectified airphoto and ASTER

products

o Workspace folders of maps presented in the thesis

Microprobe data including Jeol840 mineral probe data and Quanta

ESEM alteration probe images

PIMA data files and processed excel spreadsheet

Scanned trench maps

Whole rock data including tables and spreadsheets included within the

thesis

XRD raw data and processed excel spreadsheet


Appendix 4

Sascha-Pelligrini Fact Geology Map 1:10,000 WGS82 SUTM19

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412,000 mE

412,000 mE410,000 mE408,000 mE

408,000 mE 410,000 mE406,000 mE

406,000 mE

404,000 mE

402,000 mE

402,000 mE

404,000 mE4,7

06,0

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04,0

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02,0

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Epiclastics and juvenile clast

ash rhyodacite tuff

Ch

on

Aik

e (

Lo

we

r Ju

rassic

)

Biotite rich rhyodacite ignimbrite

Biotite, pumice/juvenile

clast rhyodacite tuff

Dacite-Andesite dykes and sills

Lower Miocene feldsarenite

La M

atild

e (

Upper

Jura

ssic

)

Flow-banded/spherulitic

lavas (Flow-Dome Complex)

Sperulitic ash tuff with

accretionary lapilli

Crystal ash rhyolite tuff

Flow-banded/spherulitic auto-

breccia (Flow-Dome Complex)

Pliocene olivine tholeite

Pliestocene gravel

Recent alluvium

STRATIGRAPHYSTRATIGRAPHY

Playa

Minor Airphoto Linearment

Major Airphoto Linearment

Flat lying stratigraphy

Statigraphic/Structural Dip

STRUCTURESTRUCTURE

Normal Fault

Mapped Structure

%

!15

Colloform/crustiform +/- ginguro

Chalcedonic quartz and single pulse

banded cockade breccia

HYDROTHERMAL SILICA CLASSIFICATIONHYDROTHERMAL SILICA CLASSIFICATION

Silica matrix phreatic

jigsaw breccia

Pervasive silicification

Silica fill, banded

hydrothermal breccia

Structurally controlled

silicification


breccia with metamorphic clasts

Chalcedonic to opaline

+ diss. pyrite

Crystalline and Comb quartz

Jasperoidal + diss. pyrite

VEIN CLASSIFICATIONVEIN CLASSIFICATION

Weakly banded and bladded

saccharoidal quartz

TIM

E

0000000000000000000000000000000000000000000000000 0.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.5 1111111111111111111111111111111111111111111111111

kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers *

Sascha-Pelligrini Fact Geology1:10,000

WGS84 SUTM19Quinn Smith 2006

SASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGYSASCHA-PELLIGRINI FACT GEOLOGY


Appendix 5

Sascha-Pelligrini Interpretive Geology Map 1:10,000 WGS82 SUTM19

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412,000 mE

412,000 mE410,000 mE408,000 mE

408,000 mE 410,000 mE406,000 mE

406,000 mE

404,000 mE

402,000 mE

402,000 mE

404,000 mE

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06,0

00 m

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04,0

00 m

N4,7

02,0

00 m

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00,0

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Epiclastics and juvenile clast

ash rhyodacite tuff

Ch

on

Aik

e (

Lo

we

r Ju

rassic

)

Biotite rich rhyodacite ignimbrite

Biotite, pumice/juvenile

clast rhyodacite tuff

Dacite-Andesite dykes and sills

Lower Miocene feldsarenite

La M

atild

e (

Upper

Jura

ssic

)

Flow-banded/spherulitic

lavas (Flow-Dome Complex)

Sperulitic ash tuff with

accretionary lapilli

Crystal ash rhyolite tuff

Flow-banded/spherulitic auto-

breccia (Flow-Dome Complex)

Pliocene olivine tholeite

Pliestocene gravel

Recent alluvium

STRATIGRAPHYSTRATIGRAPHY

Playa

Minor Airphoto Linearment

Major Airphoto Linearment

Flat lying stratigraphy

Statigraphic/Structural Dip

STRUCTURESTRUCTURE

Normal Fault

Mapped Structure

%

!15

Colloform/crustiform +/- ginguro

Chalcedonic quartz and single pulse

banded cockade breccia

HYDROTHERMAL SILICA CLASSIFICATIONHYDROTHERMAL SILICA CLASSIFICATION


jigsaw breccia

Pervasive silicification

Silica fill, banded

hydrothermal breccia

Structurally controlled

silicification


breccia with metamorphic clasts

Chalcedonic to opaline

+ diss. pyrite

Crystalline and Comb quartz

Jasperoidal + diss. pyrite

VEIN CLASSIFICATIONVEIN CLASSIFICATION

Weakly banded and bladded

saccharoidal quartz

TIM

E

0000000000000000000000000000000000000000000000000 0.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.5 1111111111111111111111111111111111111111111111111

kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers kilometers * Sascha-Pelligrini Interpretive Geology

1:10,000WGS84 SUTM19Quinn Smith 2006

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