Sukhoi Log and the Victorian Goldfields: An Alternative ... · Slide 1 Prof. Ross Large: AusIMM Technical Meeting, September 2008 Sukhoi Log and the Victorian Goldfields: An Alternative

Slide 1

Prof. Ross Large: AusIMM Technical Meeting, September 2008

Sukhoi Log and the Victorian Goldfields: An Alternative View on the Genesis of

Orogenic Gold Deposits

Ross LargeCODES UTAS

ARC Centre of Excellence in Ore Deposits University of TasmaniaHOBART Australia

Slide 2


OR…….Could a Giant Sukhoi Log Deposit

Exist in Victoria ?

Ross LargeCODES UTAS

ARC Centre of Excellence in Ore Deposits University of TasmaniaHOBART Australia

Slide 3


Acknowledgements

• CODES TEAM• Rob Scott• Stuart Bull• Helen Thomas• Leonid Danyushevsky• Sebastien Meffre• Ron Berry• Sarah Gilbert

• COLLABORATORS• Valery Maslennikov• Paul Emsbo• Richard Buerger• Grant Garven

Slide 4


This Talk Challenges Three Current Beliefs Related to Orogenic

Gold Deposits

Belief 1: “Gold is coming from some deep source or from

crustal granites”

No……..

Gold is Already Present in the Sedimentary Basin

Slide 5


Belief 2: “Organic-Rich Sediments are Good Trap Rocks for Gold”

Yes, But……..

Organic-rich Sediments are Ideal Source Rocks for Au & As

plus Zn, Mo, Ni, Se, Te, V, PGE…….

Slide 6


Belief 3: “Gold is Introduced Late; i.e. Syn-tectonic or Post-tectonic”

No……..

Gold is Introduced Early; i.e. Pre-tectonic and Moved

Around Late During Tectonism

Slide 7


Location of Sukhoi Log

Slide 8


Major Gold Deposits around Siberian Craton

Lena GoldProvince

Slide 9


Lena Gold Province

Sukhoi Log

Rifted continentalMargin Sequence

Host Rocks:

NeoproterozoicKhomolkho Fm:

Black shales, siltstones and limestones

Slide 10


Sukhoi Log: Resource• Sukhoi Log is a giant low grade sediment-hosted gold

deposit

• Resource: 686 Mt @ 2.8 g/t Au

• Black shale-hosted, disseminated pyrite and minor pyrite-quartz veinlets

Slide 11


Geological plan and

Cross Section

Wood and Popov (2005)

Slide 12


Exploration Grid- Sukhoi Log

Slide 13


Regional Setting

Sukhoi Log

Slide 14


Core Layout

Slide 15


Sulfide Mineralisation

1 cm

1 cm

1 cm1 cm

1 cm

1 cm

1 cm

Slide 16


Quartz Veins & Alteration

1 cm1 cm

1 cm

Slide 17


Underground Photos of Folded Veins

from Francois Robert

Slide 18


Current & Previous Models

• Orogenic deposit: gold introduced and concentrated during metamorphism (Goldfarb, et al., 2005)

• Intrusion related gold deposit (Distler et al., 2004)

• Syngenetic gold deposit with remobilisation into anticlinalcore during metamorphism (Buryak, 1982)

• Recent detrital zircon dating in our lab (Meffre et al., 2008) shows the host sediments have a maximum age of ~600 Ma

• The main metamorphic event has been dated at 520 Ma (Laverov et al., 2000), and emplacement age of local granitoidsis around 360 Ma (Rundquist el al., 1992)

Slide 19


Aims of this Research

• To determine the sulfide paragenesis at Sukhoi Log

• To determine the gold and trace element contents and associations for the various stages of pyrite

• Was gold introduced during sedimentation & diagenesis, or later during metamorphism and/or granite intrusion, or at several times?

Slide 20


Pyrite HistorySedimentary-diagenetic

Py1Py1 Early-diagenetic

Py2Py2

Late-diagenetic

Py3Py3

Metamorphic

Py4Py4

Py5Py5

Late-metamorphic

Slide 21


Pyrite 1• Fine grained stratiform py• Three types -

– micron sized pyrite clusters– Framboidal pyrite– “sooty” pyrite

• Interpretation: syn-sed. to early diagenetic

Slide 22


• Microtexture of the stratiform py1;

• Clusters of fine 1-2 micron py

Slide 23


Py1 Chemistry

• Py1 is rich in a wide range of trace elements• Gold content varies from 0.44 to 12.10 ppm; mean = 3.32

ppm Au• Arsenic varies 180 to 14,000 ppm ; mean = 1900 ppm As• Elements which show a positive correlation with Au in Py1;

Cu, Ag, Pb, As, Sb, Te

Slide 24


Py1 Chemistry

1

10

100

1000

10000

100000

1000000

10000000

1 7 13 19 25 31 37 43 49 55 61 67 73 79 85 91 97 103 109 115 121 127 133 139

Fe57

Co59

Ni60

Cu65

As75

Ag107

Sb121

Pt195

Au197

Pb208

Bi209

Fe standard

288 ppm Pb170 ppm Co650 ppm As84 ppm Sb175 ppm Ni5 ppm Bi6 ppm Ag

0.75 ppmAu

Laser analysis: Fe09A17

Slide 25


0.001 0.01 0.1 1 10 100 1000 10 4 1050

5

10

15

20

25

pyrite trace elements in ppm

AsNi

MnPbCoTi

BaCu

ZnSbCr

VMo

SeAg

BiAu

TeU

SnTl

Pyrite 1 (Py1)

Slide 26


• py2 euhedraovergrow and replace py1

• interpretation:early diagenetic

Py2

Py2

Py1Py1

Py1Py1

Pyrite 2

Slide 27


Py1 overgrown by Py2 overgrown by arsenopyrite

Py1Py1

AspyAspy

Py2Py2

Slide 28


Py2 Chemistry

• Py2 is depleted in most trace elements compared to Py1; Au, Ag, Cu, Pb, Te, Zn, Sb

• As, Ni and Se remain at similar levels to Py1• Gold content varies from 0.02 to 13.00 ppm;

mean = 1.02 ppm Au• Arsenic varies 2 to 18,550 ppm ; mean = 4260

ppm As

Slide 29


Pb and Au are strongly depleted in py2 compared to py1

0.1

1

10

100

1000

104

0.01 0.1 1 10 100

Py1

Py2

Pb p

pm

Au ppm

Slide 30


Pyrite 3• Inclusion rich• Irregular outlines• Fluffy or porous

texture• No obvious

structural fabric• Aligned along

bedding• Confined to

sandstone layers• Interpretation:

diagenetic or metamorphic??

Slide 31


Pyrite 3

1 cm

Slide 32


Pyrite 3: internal fabric revealed after etching indicates py3 most likely has a late diagenetic to early metamorphic timing

overprinting a weak D1 sandstone fabric

Slide 33


250 microns250 microns

Py1Py1

Py2Py2

Py3Py3

Slide 34


Py3 in Sample 156/135.5

Po

Framboidal Py1 in Py2

Py2 in Py3

Framboidal Py1

Py2 clusters

Cpy

Cpy

Sphalerite

Weak fabric

Slide 35


Pyrite 3, also has Au inclusions

Slide 36


Gold Content py3 < py2 < py1

Slide 37


Py1Py1

Py3Py3 Pyrite LaserICPMS Maps

Slide 38


Pyrite 4Isolated large euhedra

Slide 39


Pyrite 4• Internal fabric in

Py4 shows the main cleavage

• Abundant aligned micro-inclusions

• Interpretation:pyrite replaces all sedimentcomponents and inherits the structural fabric of sediments: Late metamorphic

Slide 40


Pyrite 4

Growth zoning

Internal deformation fabric revealed by acid etch

Slide 41


Py4 has low gold; 0.02 to 1 ppm

Slide 42


Py5 overgrows Py4

Py 5

Py 4

Slide 43


Pyrite is Progressively Depleted in Gold from Py1 to Py5

Slide 44


Au-As Relations in SL Pyrite

10 100 1000 104 105 1060.01

0.1

1

10

100

1000

104

Py1Py2Py3Py4Py5

As ppm

Au

ppm

Au saturation line

Slide 45


Au-As Relations in Carlin Pyrite

10 100 1000 104 105 1060.01

0.1

1

10

100

1000

104

Carlin pyrite

As ppm

Au

ppm

Au saturation line

Sukhoi Log pyrites

Slide 46


Free Gold in Py3/4

Slide 47


Pyrite and Gold Paragenesis

Pyrite

golddissolved in As-rich pyrite

Au-Te-Pb-Bi-Agassociation

free gold

arsenopyrite

py-qtz veins

Sedimentation-diagenesis Metamorphism Post-peak metam.

Py1

Py2

Py3

Py4Py5 Py6

Slide 48


Underground Photos of Veins


Slide 49


Pyrite in py-qtz veins is zoned

py3py4

py5

Slide 50


Pyrite in py-qtz veins

Py5

aspyPy3

Slide 51


SL 524-68.5-C2

Slide 52


SL 524-68.5-C2

Slide 53


Free Gold, Galena and Au-Tellurides in Early Py3 Cores

Py3

Py5

Py5

Au aspy

Py3

Py3Au

Slide 54


Au-As-Te in Early Cores of Pyrite infolded py-quartz veinlets


Slide 55


Quartz Growth in Pressure Shadows


Are these veins??

Slide 56


Pyrite and Gold Paragenesis

Pyrite

golddissolved in As-rich pyrite

Au-Te-Pb-Bi-Agassociation

free gold

arsenopyrite

py-qtz veins

Sedimentation-diagenesis Metamorphism Post-peak metam.

Py1

Py2

Py3

Py4Py5

Py6

py-qtz veinlets

Slide 57


Stage 1: Exhaltion of Au-As-Rich Basinal Fluids: deposition of gold-arsenic bearing py1: syn-diagenetic growth of py2 and py3

Imnyakh Formation carbonates

Au-As-rich reduced basinal fluids

Khomolkho Formation

Regional enrichment of gold and arsenic in black shales of Khomolkho Formation

Slide 58


500 m

Stage 2: Deformation; Normal faults Reactivated as Thrust Faults

Slide 59


500 m

Stage 2: Fluids focused into anticlinal cores below impermeable carbonates; leach gold in py1 from fold limbs

Slide 60


500 m

Stage 2: Deformation; gold remobalisation into bedding parallel py3-py4-py5-quartz veinlets in anticlinal cores

Slide 61


Gold concentration in anticlinal core

Gold Remobilisationfrom 15 km3 @ 30 ppb Au

Gold Deposit

10 km

Slide 62


Is Sukhoi Log Unique?

• Many other black-shale-hosted gold deposits may have a similar origin to Sukhoi Log

• E.g. Muruntau, Zarmitan, Natalka, Macraes

• Early concentration of syngenetic/diagenetic gold in sedimentary arsenian pyrite may be the key to the formation of these deposits

Slide 63


Victorian Goldfield

Slide 64


Au-Reef Structures

Willman & Wilkinson (1992)

Slide 65


Recent Paper in AJES (2007):

“Syngenetic Gold in Western Victoria”

BL Wood and RR Large

Slide 66


Sedimentary Pyrite Contains Gold

Area4 10x

Area3 6.25x

09

06

08

2.18 ppm Au

0.31 ppm

0.41 ppm Au

Slide 67


Indicator Beds - Gold in Pyrite

0

2

4

6

8

10

12

14

0 0.5 1 1.5 2 2.5 3

Au197

Cou

nt

Au ppm

n = 75

Slide 68


Black Shales at Bendigo also Show Early Gold Enrichment

Slide 69


Examples of Organic-Rich Shales

Slide 70


Two Types of Pyrite in Shales• Diagenetic pyrite after marcasite

• Metamorphic-Hydrothermal pyrite

Slide 71


Composite Pyrite in Black Shalediagenetic core with metamorphic overgrowth

Slide 72


Laser Mapping of Pyriteshows gold in both diagenetic core and outermost metamorphic rim

Slide 73


Au MapLA-ICPMS pyrite map and spot analyses

500 m

0.91

2.73

6.08

0.10

0.i3

0.27

0.76 0.28

0.07

0.14

0.31

0.70

0.26

2.43

Slide 74


Arsenic Map

As distribution (colour drape, non-linear stretch)

500 m

2880

6050

8500

4.9

690

2140

630 1080

700

7.4

820

1870

450

5240

Slide 75


Timing of Gold

• The pyrite mapping in the black shale host rocks demonstrates:– An early gold event during sedimentation /

diagenesis

– A later gold concentration by hydrothermal fluids during metamorphism and deformation

Slide 76


Source of Victorian Gold?

Ordovician Mud Pile?

or Cambian Basic Volcanics?

Willman (2007)

Slide 77


Source of Victorian Gold?

or Cambian Basic Volcanics?

Willman (2007)

• Assume OMP averages 5 ppb Au

• Leach 20% of Au• Produces 150 Bendigo’s

Slide 78


Importance of LQ veins

• Help in the targeting of reefs

• Bedding parallel laminated veins folded with the sediments

(Willman, 2007)

Slide 79


Bedding Parallel LQ Veins in Shales

Laminated Quartz (LQ) Vein

Slide 80


Bedding Parallel LQ Veins

Slide 81


Saddle Reef Formation

Sandstone: permeable and SiO2 bearing

Shale: impermeable and organics + Au-As bearing pyrite

Bedding Parallel LQ Veins form in Shales due to fluid over pressure during folding

Slide 82


• Folding and FluidFlow; maximum dilation in fold hinges ---> Saddle Reefs

• Connate / metamorphicfluids migrate to fold hinges

sandstone

shale

Slide 83


SiO2, Au & As Transported from Sediments to Reef via LQ Veins

Si

SiSi

Si

Si

SiSi

Au, As

Au, As

Au, As

Au, As

Organic-rich shales are source for Au & As; sandstones are source for SiO2

Slide 84


Fold Lock-Up and initiation of Reverse Faults

Linking adjacent anticlines ----> Neck Reefs:

Tapping Fluids and Au from deeper in the

Mud Pile

(Willman, 2007), Buerger (pers. comm.)

Slide 85


Back of Envelope Calculation:

Shales only need to contain 25 ppb Au

to be a viable source for gold

reefs which average

5 ppm Au

400 m

20 m

Shale: 25 ppb Au

Reef: 5 ppm Au

Slide 86


Conclusions

• Early gold enrichment is the KEY to world class gold provinces

• Gold is trapped in organics and diageneticarsenian pyrite in sea floor muds ---> black shales

• Later deformation leads to remobilisation and up-grading of free gold released from organics and arsenian pyrite in shales

New Research Proposal:

Application of New Technology to Gold Exploration & Processing

CODES, UTAS

• PROVINCES SUGGESTED FOR RESEARCH

• Carlin, Nevada

• Victorian Goldfields

• Yilgarn, WA

• Eastern Asia

• Others proposed by Sponsors

Slide 88


Where do we Find a Giant SukhoiLog Deposit in Victoria?

• Organic-rich pyritic shale and siltstone sequence

• Syn-sedimentary rift faults to enable basin fluid exhalation

• Above background sedimentary As (V, Zn, Mo…)

• Overturned folds and thrusts

• No major quartz veining or pyrrhotite

Documents

Sukhoi Log and the Victorian Goldfields: An Alternative ... · Slide 1 Prof. Ross Large: AusIMM Technical Meeting, September 2008 Sukhoi Log and the Victorian Goldfields: An Alternative