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MINISTÉRIO DA EDUCAÇÃO
UNIVERSIDADE FEDERAL DE MATO GROSSO
FACULDADE DE GEOCIÊNCIAS
PROGRAMA DE PÓS-GRADUAÇÃO EM GEOCIÊNCIAS
Neper Klein Condori Gutierrez
The Ag-Au Pallancata mine: a low-sulphidation epithermal
system in southern Peru.
Jayme Alfredo Dexheimer Leite
Orientador(a)
Co-orientador(a)
CUIABÁ
2016
ii
UNIVERSIDADE FEDERAL DE MATO GROSSO
REITORIA
Reitora
Profª. Drª. Maria Lucia Cavalli Neder
Vice-Reitor
Prof. Dr. Francisco José Dutra Souto
PRÓ-REITORIA DE PÓS-GRADUAÇÃO
Pró-Reitora
Profª. Drª. Leny Caselli Anzai
FACULDADE DE GEOCIÊNCIAS
Diretor
Prof. Dr. Paulo César Corrêa da Costa
Diretor Adjunto
Prof. Dr. Carlos Humberto da Silva
PROGRAMA DE PÓS-GRADUAÇÃO EM GEOCIÊNCIAS
Coordenador
Prof. Dr. Ronaldo Pierosan
Vice-Coordenador
Prof. Dr. Jayme Alfredo Dexheimer Leite
iii
DISSERTAÇÃO DE MESTRADO
N° 79
The Ag-Au Pallancata mine: a low-sulphidation epithermal
system in southern Peru.
Neper Klein Condori Gutierrez
Jayme Alfredo Dexheimer Leite
Orientador(a)
CUIABÁ
2016
Dissertação apresentada ao Programa de Pós-
Graduação em Geociências da Faculdade de
Geociências da Universidade Federal de Mato
Grosso como requisito parcial para a obtenção
do Título de Mestre em Geociências.
Dados Internacionais de Catalogação na Fonte.
Ficha catalográfica elaborada automaticamente de acordo com os dados fornecidos pelo(a) autor(a).
Permitida a reprodução parcial ou total, desde que citada a fonte.
G984t Gutierrez, Neper Klein Condori.The Ag-rich Pallancata epithermal district is located in the Huaylilllas magmatic
arc along the Neogene belt of southern Peru, which historically produced more than58 Moz of Ag. The mineralization occurs in quartz veins hosted by 16.58 ±016 Madacitic ignimbrites. Ore minerals include proustite-pirargirite, polibasite-pearceite andargentite with sphalerite, galena and chalcopyrite to a lesser extent; quartz, pyrite,adularia and sericite represent the main gangue minerals. Hydrothermal alterationevolved from regional propylitic to proximal quartz-illite +pyrite and quartz-adularia+pyrite ±illite. The mineralized veins fill open spaces in NW-trending, high-angle,subparallel, transtensional faults with sinistral movement and in related extensionfractures. Veins are 1 to 30 m wide and up to 3 Km in length with a recognized depthof 350 m. The Pallancata evolution is complex and multiepidsodic and recorded bygangue mineral textures, which include crustiform-, colloform-, cockade- and comb-textured quartz, / Neper Klein Condori Gutierrez. -- 2016
38 f. : il. color. ; 30 cm.
Orientador: Jayme Alfredo Dexheimer Leite.Dissertação (mestrado) – Universidade Federal de Mato Grosso, Instituto de
Ciências Exatas e da Terra, Programa de Pós-Graduação em Geociências, Cuiabá,
The Ag-Au Pallancata mine: a low-sulphidation epithermal system in
southern Peru.
BANCA EXAMINADORA
_______________________________________
Jayme Alfredo Dexheimer Leite
Orientador(a)
_______________________________________
Francisco Egídio Cavalcante Pinho
Examinador(a) Interno (UFMT)
_______________________________________
Antônio João Paes de Barros
Examinador(a) Externo
Dedicatória
Agradecimentos
Sumário
Resumo
O distrito argentífero epitermal de Pallancata está localizado no arco magmático de
Huaylillas ao longo do cinturão neogênico do Sul do Perú que historicamente produziu
mais de 58 milhões de onças de Prata. A mineralização ocorre em veios de quartzo
hospedados em dacitos ignimbriticos de 16.5±0.1 Ma. Os minerais de minério incluem:
proustita-pirargirita, polibasita-pearceita e argentita com esfalerita, galena e calcopirita
subordinadas. Quartzo, pirita, adularia e sericita representam os principais minerais da
ganga. A evolução hidrotermal varia de alteração propilítica regional para quartzo-
ilita±pirita e quartzo-adularia±pirita±ilita proximais. Os veios mineralizados preenchem
espaços vazios em falhas transtensionais de alto ângulo e de direção NW com
movimentação sinistral e também em fraturas relacionadas. Os veios mostram
espessuras variáveis entre 1 e 30 metros, comprimentos de até 3 Km e uma
profundidade reconhecida de 350 metros. A evolução do depósito de Pallancata é
complexa e multiepisódica sendo registrada pelas texturas dos minerais de ganga que
incluem quartzo com crustiforme, colofome, cockade e comb, adularia rômbica e calcita
substituída por quartzo; as duas últimas variedades texturais sugerem a ebulição como
mecanismo principal de deposição da ganga. A evolução do veio Pallanca registra sete
estágios de deposição de ganga e minério sendo que a mineralização mais rica, estágio
de bonanza, está relacionada ao estágio 5. A petrografia e microtermometria de
inclusões fluidas em quartzo depositado no estágio 5 revelou que a assembleia de
inclusão fluída é dominada por inclusões aquosas bifásicas com temperaturas de
homogeneização em torno de 207°C e salinidades médias de 1.77%NaCl equivalente.
Estudos de espectrometria Raman mostram SO2 como fase vapor subordinada e
confirmam a ausência de CO2. As razões isotópicas de Oxigenio em quartzo e adularia
varia entre -2 e 6 ‰ enquanto que o fluido em equilíbrio com este minerais apresenta
razões δ18O em torno de -6 ‰ compatível com uma fonte meteórica para o fluído.
Todas as características acima apresentadas sugerem uma mineralização do Epitermal
de baixa-sulfetação para o depósito de Pallancata.
Palavras chave: epitermal, inclusão fluída, Espectrometria Raman, isótopos estáveis.
Abstract
The Ag-rich Pallancata epithermal district is located in the Huaylilllas magmatic arc
along the Neogene belt of southern Peru, which historically produced more than 58 Moz
of Ag. The mineralization occurs in quartz veins hosted by 16.58±016 Ma dacitic
ignimbrites. Ore minerals include proustite-pirargirite, polibasite-pearceite and argentite
with sphalerite, galena and chalcopyrite to a lesser extent; quartz, pyrite, adularia and
sericite represent the main gangue minerals. Hydrothermal alteration evolved from
regional propylitic to proximal quartz-illite+pyrite and quartz-adularia+pyrite±illite.
The mineralized veins fill open spaces in NW-trending, high-angle, subparallel,
transtensional faults with sinistral movement and in related extension fractures. Veins
are 1 to 30 m wide and up to 3 Km in length with a recognized depth of 350 m. The
Pallancata evolution is complex and multiepidsodic and recorded by gangue mineral
textures, which include crustiform-, colloform-, cockade- and comb-textured quartz,
rhombic adularia and bladed calcite replaced by quartz; the latter two suggests boiling
as a precipitation mechanism. The vein evolution records seven stages of gangue and
ore deposition with the main mineralization event related to the stage 5, the bonanza
stage. The fluid inclusion petrography and microthermometry in quartz from bonanza
stage revealed FIA’s mostly consisting of biphasic inclusions (LH2O-VH2O).
Homogenization temperatures (Th) and calculated salinities are low with a mean,
respectively, around 1.77 % NaCl eq. and 207°C. Raman spectroscopic studies showed
SO2 as a minor additional vapor phase and confirmed the lack of CO2. Oxygen isotopes
ratios in quartz and adularia and the resulting mean isotopic δ18O composition for
water in equilibrium with both minerals is around -7.0 ‰ suggesting a meteoric source
for the mineralizing fluid. All the above presented characteristics suggests the
Pallancata mineralization as a low-sulphidation epithermal system.
Keyword: epithermal; fluid inclusion; Raman Spectrometry; stable isotopes
The Ag-Au Pallancata mine: a low-sulphidation epithermal system in southern Peru.
Neper K. Condori1, Jayme A. D. Leite2,3
1. Programa de Pós-Graduação em Geociências. Universidade Federal de Mato Grosso. Av. Fernando
Correia da Costa 2367. Cuiabá. Brasil. [email protected]
2. Faculdade de Geociências Universidade Federal de Mato Grosso. Cuiabá. Brasil.
3. Programa de Pós-Graduação em Geociências. Universidade Federal de Mato Grosso, Cuiabá, Brasil
Abstract
The Ag-rich Pallancata epithermal district is located in the Huaylilllas magmatic arc along the
Neogene belt of southern Peru, which historically produced more than 58 Moz of Ag. The
mineralization occurs in quartz veins hosted by 16.58 ±016 Ma dacitic ignimbrites. Ore
minerals include proustite-pirargirite, polibasite-pearceite and argentite with sphalerite, galena
and chalcopyrite to a lesser extent; quartz, pyrite, adularia and sericite represent the main
gangue minerals. Hydrothermal alteration evolved from regional propylitic to proximal quartz-
illite +pyrite and quartz-adularia +pyrite ±illite. The mineralized veins fill open spaces in NW-
trending, high-angle, subparallel, transtensional faults with sinistral movement and in related
extension fractures. Veins are 1 to 30 m wide and up to 3 Km in length with a recognized depth
of 350 m. The Pallancata evolution is complex and multiepidsodic and recorded by gangue
mineral textures, which include crustiform-, colloform-, cockade- and comb-textured quartz,
rhombic adularia and bladed calcite replaced by quartz; the latter two suggests boiling as a
precipitation mechanism. The vein evolution records seven stages of gangue and ore deposition
with the main mineralization event related to the stage 5, the bonanza stage. The fluid inclusion
petrography and microthermometry in quartz from bonanza stage revealed FIA’s mostly
consisting of biphasic inclusions (LH20-VH2O). Homogenization temperatures (Th) and calculated
salinities are low with a mean, respectively, around 1.77 % NaCl eq. and 207°C. Raman
spectroscopic studies showed SO2 as a minor additional vapor phase and confirmed the lack of
CO2. Oxygen isotopes ratios in quartz and adularia and the resulting mean isotopic δ18O
composition for water in equilibrium with both minerals is around -7.0 ‰ suggesting a meteoric
source for the mineralizing fluid. All the above presented characteristics suggests the Pallancata
mineralization as a low-sulphidation epithermal system.
Key words: epithermal, low-sulphidation, fluid inclusion, Raman spectroscopy, stable isotope
Introduction
The Occidental Cordillera of Central Andes hosts several mines and ore deposits (Noble
et al. 1989, 1999; Noble and Vidal, 1994; Kay et al. 1999; Bissig et al. 2008; Bissig and Tosdal
2009; Cerpa et al. 2013) which build up the Metallogenetic Front of Au-Ag-rich epithermal
deposits of the Mio-Pliocene (Acosta et al. 2009). In Peru, the main Ag-Au epithermal deposits
spread across the Arequipa and Ayacucho Departments. The first encompasses Caylloma,
(Echavarria et al. 2006) Orcopampa, (Gibson et al. 1990, 1995), Arcata (Candiotti et al. 1990),
Ares, (Candiotti and Guerrero 2002) and Shila deposits (Cassard et al. 2000) wheareas the latter
hosts Selene, (Palacios 2006) and Pallancata mine (this contribution). The Ag- and base metals-
rich Pallancata epithermal district is located 355 Km East of Nazca at approximately 4,400 m
above the sea level (Fig. 1).
During the Hispanic colonial period, 1626 to 1824, the shallower parts of main veins,
like Central Pallancata, Rina, San Javier and San Cayetano, were intensely mined; the
Pallancata district has produced 7,6 Mt of ore and more than 58 Moz of Ag. Several studies
have reported the geologic and metallogenetic evolution of some of these veins, especially in
Orcopampa, Caylloma and Arcata areas (Noble et al. 1974, 1984, 1989, 1999; Fornari and Vilca
Neyra, 1979; McKee and Noble, 1982, 1989; Peterson et al. 1983; Clark et al. 1990; Sébrier and
Soler, 1991; Mercier et al. 1992; Noble and Vidal, 1994; Sandeman et al. 1995; Candiotti et al.
1999; Echevarria et al. 2006; Sarmiento et al. 2010). On the other hand, few contributions are
available for Pallancata mine, despite the fact of being one of the few mines still active in
southern Peru. Gamarra et al. (2008, 2013) studied the mineralogy and hydrothermal alteration
pattern of the Pallancata vein while Palacios (2006) dealt with the hydrothermal alteration and
mineralization of Selene mine which is 15 Km north of Pallancata. Gamarra et al. (2013)
defined the hydrothermal alteration evolution through a sequence of seven main paragenetic
stages of which stage 5 hosts the main mineralization phase of Pallancata mine, known as the
bonanza stage.
This paper present new date regarding veins textures, fluid inclusions petrography and
microthermometry and oxygen isotopes for the main ore stage of Pallancata mine along a
SHRIMP U-Pb age which, altogether allow a discussion over its genetic classification as
belonging to the Ag-rich low-sulphidation epithermal deposit.
Regional geologic setting and structural framework
The Pallancata district lay down over rocks of the Huaylillas arc of the Occidental
Cordillera of Central Andes in southern Peru. This arc resulted from the convergence between
the Nazca and South American plates during Mid-Devonian to late Miocene (Fig. 1; Thorpe et
al. 1984; Trumbull et al. 1999; Mamani et al. 2010; Galas, 2014). As a whole, the arc comprises
voluminous and extensive ignimbrite flows followed by shield volcanos of andesitic
composition, respectively developed between 26 and 16 Ma and 18 and 10 Ma (Wörner et al.
2000, 2002; Farías et al. 2005; Charrier et al. 2007). Intense hydrothermal alteration occurred
during interspersed volcanic quiescence periods that accounts for the development of several ore
deposits that are included into the Puquio-Caylloma epithermal belt (Acosta et al. 2009; Carlotto
et al. 2010).
Figure 1. Location map of the studied area (small insert) and the distribution of the Huaylillas arc in southern Peru,
including the location of the Pallancata district.
The Pallancata District
Local geologic
The geology of the Pallancata district consists of four main units, which from the base
to the top are, Alpabamba, Anisio, and Saycata formations and Intrusive subvolcanic and domes
units (Fig. 2).
The basal unit, Alpabamba formation, was only recognized in depth through diamond
drill holes. It comprises a finely stratified volcano-sedimentary succession made up of brown,
fine-grained andesitic flows interlayered with sandstones and argillites followed by andesitic
flows and breccias, which in turn are overlain by greenish grey quartz-rich lapilli tuffs.
The Aniso formation unconformable overlies the basal unit and occurs mostly in the
central part of the district with a lesser extent to NW. The unit comprises greyish green
porphyritic, pumice and crystal-rich dacitic to rhyolitic ignimbrites (Condori et al. 2015).
Phenocrysts mineralogy include broken by-pyramidal quartz, corroded plagioclase and euhedral
chloritized-biotite, which are set in an aphanitic groundmass.
The upper unit, Saycata formation, comprises extensive, greyish white, flow-banded
porphyritic andesites. Chloritized hornblende and sericitized plagioclase occurs as phenocrysts
that are set in a groundmass of fine-grained plagioclase, quartz, chlorite and iron oxides.
Porphyritic subvolcanic rocks intrude Aniso and Saycata formations at central
Pallancata zone and at Tucsa; the latter situated along the possible northwest extension of the
Pallancata fault. These subvolcanic rocks vary in composition from dacite to rhyodacite, are
porphyritic with altered plagioclase, and chloritized hornblende and biotite phenocrysts set in
medium to fine-grained groundmass of quartz, plagioclase, sericite, chlorite and iron oxides.
Intrusive rhyolitic domes occur in mid-north and southeast of the district, the Ranichico
and Sarnahuire domes respectively. These flow-banded domes show circular to elliptic forms,
diameters between 200 and 1500 m and an essential composition of espherulitic alkaline
feldspar and quartz set in a fine-grained groundmass of volcanic glass, chlorite, sericite and iron
oxides.
Figure 2. Geologic map of Pallancata district including location of main faults and veins.
Structural framework
The structural framework at the Pallancata district record a NNW-SSE-striking corridor
limited by regional inferred faults, likely related to the convergence between Nazca and South
America plates, during the formation of the Huaylillas arc between 24-10 Ma (Miriam et al.
2010). In this corridor, three main fault systems are present, namely: WNW, E-W and NE-SW
(Fig. 3A).
Figure 3. Structural framework of the Pallancata District. A) Map of faults, fractures and veins distribution at
Pallancta district; note that the regional NW-SE corridor is defined by inferred regional faults. B) Longitudinal NW-
SE section of the district showing the distribution of the veins through a negative flower structure as well as the
mineralized horizon. C) Application of Rieldel’s model to the Pallancata structural framework.
The WNW system strikes 120° and includes high-angle, subparallel, oblique-slip faults
with sinistral sense of movement. This system hosts the Pallancata and Cimoid veins and was
reactivated in both pre- and post-mineralization times. The E-W system strikes between 080°
and 105° and consists of south-dipping, low-angle (45° to 65°) normal faults which hosts the
Luisa, Rina and Rina1 veins. The first shows an intimate correlation between depth and dipping
so that at depths deeper than 4,300 m the dipping decreases down to 45° suggesting a behavior
similar to that found in listric faults. The NE-SW system strikes between 220° and 225°,
comprises high-angle (70° to 80°), NW-dipping normal faults and hosts the San Javier, Paola,
Virgen del Carmen, Vanesa, Mercedes, Alexandra and Farallon veins.
An integration of the regional structural data allows the understanding the structural
framework of Pallancata district as a negative flower structure (Fig. 3B), likely developed
during the collapse of part of Huaylillas arc. On the other hand, when the angular relationships
among the diverse fault systems are compared with the Riedel’s model (Fig. 3C), the structural
framework of Pallancata district can be modeled as a transtensional system with sinistral sense
of movement. In this model, R1-type fractures host the Pallancata vein, tensional (T) fractures
host Luisa, and R2-type normal fractures with dextral movement host Rina and San Javier veins.
Hydrothermal alteration
Gamarra et al. (2008) depicted the regional hydrothermal evolution at Pallancata district
in terms of three main alteration patterns (Fig. 4), namely; propylitic, argillic and silicic.
Propylitic alteration is distal to veins, predates the ore deposition and replaces original
mineralogy of ignimbrites and dacites to a lesser extent into an assemblage of
chlorite+calcite±epidote. Argillic alteration shows a proximal relationship to veins and replaces
both original ignimbrites and propylitic alteration into an assemblage of quartz+illlite±pyrite.
Silicification only occurs close to Farallon vein, where it develops a 200 m wide halo, and to a
lesser extent at the Yurika vein.
Figure 4. Regional hydrothermal alteration pattern at Pallancata district, modified from Gamarra et al. (2008).
Samples and analytical techniques
Samples from the Pallancata vein were collected from cores of three diamond drill holes
PAC-22, PAC-23 and DLPL-A461, respectively in upper (4,350-4,250 m a.s.l.), intermediate
(4,250-4,150 m a.s.l.) and lower (4,150-4,100 m a.s.l.) levels (Fig. 5).
Gangue mineral textures were determined from 23 thin sections by transmitted and
reflected light in a BX560 model Olympus microscopy. Six doubled-polished sections allowed
the investigation of the petrographic aspects of Fluid Inclusion Assemblages (FIA’s, Goldstein
and Reynolds, 1984), from the bonanza stage 5 at the Pallancata intermediate level.
Microthermometric data were obtained with a LINKHAM TH600 heating/freezing
stage adapted on an OLYMPUS BX56 transmitted light microscope from the Mineral Resources
Department (Universidade Federal de Mato Grosso, Cuiaba, Brazil).
Raman Spectroscopy analyzes made use of a Horiba/JY LabRam HR 800 spectrometer
at Physic Institute (Universidade Federal de Mato Grosso, Cuiaba, Brazil) with Laser He-Ne
excitation source @532 nm.
Stable isotope studies were performed at LABISE (Stable Isotope Laboratory,
Universidade Federal de Pernambuco, Recife, Brazil) facilities. Oxygen was liberated in a high
vacuum line through the reaction with BrF5, having a CO2 laser as a heat source and converted
to CO2 by reaction with graphite at 750°C. Obtained CO2 gas was analyzed with a
THERMOFINNING Delta V Advance mass spectrometer.
The Ag-Au Pallancata Vein
The Pallancata vein is hosted by the high-angle, normal, NW-striking homonymous
fault which is 3,1 km long, 2 m up to, locally, 30 m wide with a recognized vertical extension of
400 m. The vein is banded and brecciated and suggests that the Pallancata fault was episodically
active.
Hydrothermal alteration follows a general pattern of early and regional pre-
mineralization propylitic alteration to a proximal sin-mineralization argillic and adularia+quartz
alteration. The propylitc alteration occurs in regions of low water:rock ratios, i.e., outside
conduit zones, and replaces original mineralogy of ignimbrites and dacites to an assemblage of
calcite+epidote+quartz. Adularia+quartz and argillic alteration are restricted and closely
associated to vein and vein to host boundaries, respectively. Argillic alteration develops a halo
of up to 2 m wide around the vein and comprises an assemblage of ilite+quartz±pyrite whereas
adularia-quartz+pyrite±ilite alteration is restricted to the vein (Fig. 5).
Figure 5. Cross section of Pallancata vein showing its vertical distribution, the general hydrothermal alteration pattern
and location of drill holes used in this study. Note the half-cymoid structure at 4,200 m level
The economic mineralization at Pallancata mine is heterogeneously distributed along
several ore-shoots. In the west and central sectors, the ore–shoots are more continuous and may
reached up to 400 m of horizontal extent, plunge horizontally and show the higher and more
homogenous distribution of Ag grade (Fig. 6). On the other hand, in the east sector the ore
shoots are more discontinuous, have higher plunge angle, heterogeneous distribution and lower
Ag grades (Fig. 6)
Figure 6. Longitudinal NW-SE section of Pallancata vein showing ore-shoot configuration and Ag distribution along
its main sectors: west, central and east.
Late, barren, reverse, steeply dipping NE-striking faults cut and displace some of the
ore-shoots resulting in half-cymoids structures (Fig. 6) which are similar to what Echavarria et
al. (2006) described in the Caylloma district.
Gangue mineral textures
The Pallancata vein shows a wide variety of textures, which are indicative of complex,
episodic open-filling spaces precipitation during boiling and or slow cooling, recrystallization
during brecciation and replacement of earlier precipitated minerals (Fig. 7A-D).
Crustiform-textured quartz (Fig. 7A) is a primary precipitation feature (Bodnar et al.
1985; Dong et al. 1995) and intimate associated to Ag-bonanza grades. Banding is often
symmetric in relation to vein walls and results from rapid and episodic fluctuation of
temperature and or pressure during a boiling event (Moncada et al. 2012).
Comb-textured quartz (Fig. 7A) is also a primary depositional feature and present at
Pallancata vein. The texture shows coarse, imperfect and euhedral crystals growing
perpendicular to vein walls (Adams 1920).
Massive-textured quartz (Fig. 7A) refers to quartz vein in which quartz is homogeneous
and presents no banding or deformation characteristics. Massive quartz represents a primary
feature developed in open spaces under slow precipitation rates; this texture is unrelated to
boiling.
Colloform-textured quartz (Fig. 7B) is closely associated to Ag-bonanza grades and
consists of rounded or botryoidal silica in continuous bands. This primary precipitation texture
is interpreted to represent rapid deposition of quartz in open spaces of shallow epithermal
systems (Bodnar et al. 1985); cyclic deposition of quartz due to recurrent sealing and cracking
events leads to rhythmic banding (Roedder 1984). Henley and Hughes (2000) suggested this
texture to result from rapid decrease of pressure associate to boiling or flashing.
Plumose-textured quartz (Fig. 7B) is also a common texture found throughout the
Pallancata vein. It consists of fibrous aggregates of chalcedony with rounded borders as result of
silica gel recrystallization (Dong et al. 1995). Precipitation of silica gel results from rapid
decrease of pressure and or temperature (Henley and Hughes 2000) and is considered as an
intermediate product in the silica precipitation cycle, between early amorphous silica and late
crystalline quartz (Camprubí and Albinson 2007).
Jigsaw-textured quartz (Fig. 7B) is the most common texture found along the known
vertical extent of Pallancata vein. According to Dong et al. (1995), this texture represents the
recrystallization of earlier precipitated chalcedony or amorphous silica at temperatures higher
than 180°C, which is approximately the upper limit of chalcedony stability (Saunders 1994).
Lattice-bladed calcite develops due to rapid calcite growth due to loss of CO2 to vapor
phase during a boiling event (Simmons and Christenson 1994) and, its replacement by quartz
(Fig. 7C) occurs as system cools.
Cockade texture (Fig. 7D) is also ubiquitous at Pallancata and forms where concentric
crusts of quartz surround fragments of brecciated host rocks and earlier veins.
Figure 7. Epithermal textures in Pallancata vein. A) crustiform, comb and massive quartz, B) jigsaw, plumose and
colloform textures, C) lattice bladed calcite replaced by quartz and, D) cockade texture around earlier vein and host
rock fragments. All photos taken under crossed polarizers.
Paragenetic stages
Based on field relationships among different outcrops, drilling core interpretation and
underground workings, nine paragenetic stages were identified, namely: S0 to S8. The initial
stage, S0, corresponds to the regional propylitic alteration, stages S1 to S7 record the vein
evolution and stage 8 relates to supergene alteration (Fig. 8). The evolutionary stages 1 to 7 are
symmetric in relation to the vein walls so that earlier stages are partially or totally overprinted
by the latter ones.
Stages 1 to 3 corresponds mainly to the framework preparation for ore deposition with
the development of veining, brecciation, re-brecciation and weak depositon of galena,
chalcopyrite and sphalerite (Fig. 8A-C) with a correponding gangue of pseudo-rhombic
adularia, sericite and quartz.
Stage 4 records the brecciation of earlier veins, breccias and host rock fragments (S3) and the
deposition of the economic ore assemblage, proustite-pyrargirite-sphalerite-galena (Fig. 8D)
followed by chalcopyrite-pearceite-polybasite-electrum-argentite intergrown with quartz and
rhombic adularia.
Stage 5 records the higher Ag grades in Pallacanta vein. The mineralization consists of grey
silica irregular bands-rich sulphurets interspersed with white quartz-adularia bands (Figs. 8E
and F) in a crustiform-textured mosaic. Intense argentite deposition after proustite-pyrargirite,
pearceite, polybasite, galena and sphalerite occurs filing open-spaces in earlier quartz and
lattice-bladed calcite replaced by quartz. Mineralization follows with stephanite deposition, and
sulphurets in a lesser extent and ends with electrum inclusion in coarse-grained pyrite.
Stages 6 and 7 correspond to late and barren injections of thin quartz and chalcedony veinlets
with subordinate pyrite (Fig 8G and H). Stage 8 corresponds to the supergene alteration of most
of the ore by coveline.
Figure 8. Composite schematic section of Pallancata vein at – 50 level (4,220 m), in the center, and illustration of the
different alteration stages, A to F. S0 - regional propylitic alteration, S1 – stockwork development (A), S2 -
deposition of bladed calcite, S3- hydrothermal brecciation of earlier stages, S4 – re-brecciation and weak deposition
of galena-sphalerite-chalcopyrite (C and D), S-5, main mineralization stage with the deposition irregular bands of
grey silica, Ag-sulfosalts and adularia (E and F), S6 – late veins of white crystalline quartz (G) and S7 chalcedony
filling open spaces.
Fluid Inclusions
Fluid inclusion assemblages were studied in quartz crystals from the Stage 5, the known
bonanza stage, in the Pallancata intermediate level (4,250-4,150 m a.s.l). Microthermometric
date was collected from six samples, all related to the Pallancata bonanza level and; results are
presented in Table 1.
Table 1. Microthermometric data from Pallancata vein, intermediate level. Te= eutetic temperature; Tm-ice= ice
melting temperature; Th= homogenization temperature.
Limited Size (µm) Vapor (vol.%) Te (˚C) Tm-ice (˚C) Th (˚C) Salinity (wt. %NaCl eq.) dbulk (g/cm3) Presure (Mpa)
Intermediate zone
PAC23-12
Min. 4.0 10.0 -28.0 -1.7 162.0 0.88 0.82 0.61
Max. 15.0 30.0 -3.0 -0.5 295.0 2.90 0.95 7.62
PAC23-8
Min. 3.0 15.0 -15.8 -1.1 185.0 0.88 0.77 1.08
Max. 5.0 30.0 -12.7 -0.5 294.0 1.91 0.92 7.68
PAC23-7
Min. 4.0 10.0 -25.0 -1.8 178.2 1.91 0.89 0.92
Max. 25.0 25.0 -4.1 -1.1 235.4 3.06 0.95 2.94
PAC23-18
Min. 4.0 15.0 -23.0 -1.1 196.0 0.53 0.63 1.38
Max. 25.0 30.0 -5.1 -0.3 356.0 1.91 0.91 17.42
Petrography
Based on the petrographic and compositional characteristics, three types (T1 to T3) of
fluid inclusions assemblages were identified (Fig. 9). Type 1 corresponds to one-phase liquid
inclusions (Fig. 9); type 2 is the most abundant inclusion type and represented by two-phase
aqueous (LH2O+VH2O) inclusions and type 3 which are restricted consist of aqueous multiphase
solid inclusions (S+LH2O+VH2O).
Type1 inclusions occur along trails restricted to a single grain and vary in the longest
dimension between 1 and 10 µm. They present weak to moderate relief, dark rims and several
different shapes like rounded, elongate, tabular, irregular and negative crystals.
Type 2 inclusions occur in small clouds and trails along crustiform-textured quartz
growing planes. These inclusions are irregular-shaped with lengths up to 20µm with some of
them showing effects of necking down. The liquid phase is clear, presents low to moderate
relief and the degree of filling reaches up to 95% of inclusion volume; the vapor phase consists
of a clear bubble with dark rims.
Inclusions of type 3 have moderate to high relief, diameters between 10 and 30µm and
varied shapes like sub rounded, prismatic and elongate. The liquid phase is clear and the degree
of filling is in the range of 40 and 80% depending on the volume occupied by hosted solid
phases. The vapor phase records clear to dark bubbles with dark boundaries and higher relief
than the liquid phase; its degree of filling varies between 5 and 20%. This type of fluid inclusion
hosts normally one solid phase, which is clear in appearance with high relief and cubic habit,
confirmed as Halite by Raman spectroscopy. Exceptionally we found inclusions hosting three
solid phases as shown (Fig. 9F).
Figure 9. Fluid inclusions in quartz from Pallancata intermediate level. A) Stretched biphasic inclusions, B)
monophasic inclusions, C) biphasic inclusion with 40% of filling, D) Biphasic inclusion with 30% of filling, E)
Multiphase inclusion with one solid phase and, F) multiphase inclusion with three solid phases.
Microthermometry
The microthermometric study developed in quartz from veins associated to the bonanza
stage 5. The following temperatures for fluid inclusions were recorded: homogenization
temperature (Th), eutetic temperature (Te) and ice-melting temperature (Tm). With these data,
we obtained apparent salinity, fluid composition and an evaluation of pressure and depth under
which the gangue and possibly the ore deposited. Inclusions of Type 1 were not analyzed and
the few inclusions of Type 3 did not return consistent date, likely as necking down effects.
Worth to note is that CO2 was not detected in any run, which was also confirmed by Raman
spectroscopy.
Inclusions of Type 2 presented a wide range of homogenization temperatures T(h),
between 162-356 °C, (Fig. 10A); the majority of data lies in the range of 180°C and 220°C with
a mean of 207°C. The final ice- melting temperatures range from -0.5°C to -1.7°C, and
correspond to apparent salinities between 0.53 and 3.06% NaCl Eq. (Fig. 10B); measured
eutectic temperatures are higher than -24°C suggesting a fluid composition of H2O-KCl and
H2O-NaCl (Fig. 10C).
Figure 10. Microthermometric data for Fluid inclusions. A) Distribution of the homogenization temperature (Th), B)
distribution of ice-melting temperature, C) distribution of eutectic temperature and correlation with the system
composition and D) correlation between homogenization temperature and calculated salinity. Contour for epithermal
waters is from Lattanzi, 1991.
The correlation between the homogenization temperature and calculated salinities (Fig.
10D) does not clearly show the expected trend for boiling. However, evidences favoring its
occurrence like the presence of rhombic adularia, bladed calcite replace by quartz and primary
quartz textures were clearly shown.
Assuming that in epithermal systems the homogenization temperatures are closely
similar to trapping temperatures and no pressure corrections is necessary (Bodnar 1984), boiling
should have occurred at depths ~210 m bellow the paleo water table (Fig. 11)
Figure 11. Boiling-point curves for H2O liquid (0 NaCl wt %) and for brine of constant compositions (Haas 1971).
Boiling depth of ~201 m for Pallancata vein taken from the mean salinity of 1.77% NaCl eq., and mean
homogenization temperature of 207°C.
Raman Spectroscopy
Raman spectroscopy allowed the identification of main volatile and solid phases found
in fluid inclusions of the Pallancata bonanza stage.
Inclusions of Type 2 have vapor and liquid phases dominated by H2O with Raman
peaks between 2900-3300 cm-1and 3250 a 3300 cm-1 peaks (Figs. 12 A and B). Additional
phases include: SO2 (v) with a Raman peak of 1151 cm-1 (Herzberg 1945) and NO-3 (l) with a
low frequency peak of 690 cm-1 (Ross 1972).
In Type 3 inclusions, H2O is the main liquid and vapor phases. Vapor phase also
includes low contents of H2(v) recorded by its 354 cm-1 peak (Dubessy et al. 1988). The
dominant solid phase in Type 3 inclusions is NaCl with its 358 cm-1peak and KCl to a lesser
extent which was identified through its 213 cm-1 peak (Fig. 12C and D). (Raman Spectra
Database, Siena Geofluids Lab (http://www.dst.unisi.it/geofluids/raman/spectrum_frame.htm).
Figure 12. Raman spectra for fluid inclusions of Pallancata vein at the bonanza stage. A) Raman spectra for vapor
phases of biphasic inclusions, B) Raman spectra for liquid phases of biphasic inclusions, C and D) Raman spectra for
solid phases for multiphase inclusions.
Correlations with other mines
Additional to Pallancata, the metallogentic front of low and intermediate-sulphidation
epithermal deposits in southern Peru includes several mines and results of fluid inclusions are
available for Julcani, Arcata, Chipmo, Shila-Paula and Caylloma mines. Available data is
presented below and aims to help the interpretation of this study.
The Julcani mine is located 65 km to southeast of Huancavelica department (Petersen et
al. 1977), and consists of an Ag-Cu-Pb-W-Bi-Au polymetallic deposit which fills a fracture
system hosted in dacitic-rhyolitic domes. Primary fluid inclusions in quartz yielded
homogenization temperatures (Th) between 150-325 °C and salinities in the range of 3 to 14 %
NaCl Eq., Wolframite-included FI’s yielded homogenization temperatures (Th) between 317-
326 °C and salinities in the range of 15.5-17.5 % NaCl Eq., whereas fluid inclusion hosted in
energites resulted in homogenization temperatures (Th) in the range of 240-253 °C and salinities
between 18-19 % NaCl Eq. (Fig. 13A). Boiling is the suggested mechanism for ore deposition
and fluid should have evolved by mixing of magmatic fluids and low-salinity meteoric waters
(Deen et al. 1994).
Arcata mine is situated 180 Km to north-northwest of Arequipa and comprises an Ag-
Au fracture-fault filling system hosted by andesite flows. Primary fluid inclusions in quartz
yielded homogenization temperatures (Th) between 165-270 °C and salinities in the range of 1-
4 % NaCl Eq. Pyrargirite-included FI’s yielded homogenization temperatures (Th) in the range
of 190-200 °C and salinities around 2.5 % NaCl Eq., whereas fluid inclusions hosted in
sphalerite yielded homogenization temperatures (Th) in the range of 230-250 °C. The latter is
somewhat higher than those found in pyrargirite, with salinities around 3.2 % NaCl Eq. (Fig.
13A). Candiotti et al. (1990) suggests the fluid origin for Arcata mine to be result from the
mixing of low-salinity meteoric and connate waters and boiling as the ore deposition process.
Chipmo mine is located in Orcopampa, Arequipa Department, and comprises an Ag-Au
vein system hosted by dextral faults in dacitic to andesitic flows. Primary fluid inclusions in
quartz yielded homogenization temperatures (Th) between 230-295 °C and salinities around 4.9
% NaCl Eq. Fluid inclusions in quartz from porphyry-type exotic clasts yielded homogenization
temperatures (Th) in the range of 230-251 °C and salinities between 17-22.5 % NaCl Eq. (Fig.
13A). According to Sarmiento, et al. (2010) these brines are associated to magmatic fluids that
preceded the mineralization stage.
The Shila-Paula Au-Ag epithermal vein system is located 35 Km south of Chipmo
district, Arequipa department; the mineralization is of fracture-filling type hosted by dacitic to
andesitic flows and volcanoclastic (Cassard et al. 2000). Primary fluid inclusions in quartz
yielded homogenization temperatures (Th) in the range of 424°C to 321°C and salinities
between 4-10 % NaCl Eq. For sphalerite-included fluid inclusions the same parameters varies
respectively, between 203-315°C and 6-15 % NaCl Eq., whereas calcite-included fluid
inclusions yielded homogenization temperatures (Th) between 278-235 °C and salinities in the
range of 0.2-2.9 % NaCl Eq. (Fig. 13A). Ore precipitation occurred through boiling from fluids
derived from the interaction of surficial waters of low to intermediate salinities (Chauvet et al.
2006).
Caylloma mine is situated 150 Km north of Arequipa and consists of Ag-Au half-
cymmoids fracture-filling system hosted in volcanics and volcanoclastics of andesitic
composition. Primary fluid inclusions in quartz yielded homogenization temperatures (Th) in
the range of 225-310 °C and salinities between 0.5-10.5 % NaCl Eq., whereas sphalerite-
included fluid inclusions yielded homogenization temperatures (Th) between 235-290 °C and
salinities around 23.5 % NaCl Eq. (Fig. 13A). Based on quartz textual variations and chemical
banding along the vein, Echevarria et al. (2006) suggested that boiling is the process responsible
for ore deposition.
From the above-presented data and besides other relevant characteristics, Caylloma,
Chipmo and Julcani mines show a wide range in salinities up to 25% NaCl eq., suggesting that
magmatic fluids should have played an important role on metal transport. Therefore, these
deposits are classified as epithermal systems of intermediate-sulphidation. On the other hand,
Chipmo and Arcata show lower salinity ranges and are classified as low-sulphidation epithermal
systems. By comparison, Pallancata presents the lowest salinity range of all mines and as such
belongs to the latter class. However, some involvement of magmatic contribution during the
lifetime of the hydrothermal system cannot be ruled out as evidenced by the presence of few
multiphase fluid inclusions.
The fluid inclusion signatures of the studied mines (Fig. 13B) straddle the epithermal
field of Wilkinson (2001) and as such might suggest the Peruvian epithermal systems to have a
proper signature.
Figure 13. Fluid inclusion data for Julcani, Pallancta, Arcata, Chipo, Shila-Paula and Caylloma mines. A) Dashed
lines- data for quartz, solid lines – data for ore minerals, B) Fluid inclusion data and mineral deposits, modified from
Wilkinson (2001).
Oxygen isotopes
Stable Oxygen isotopes were performed in three quartz samples and in one adularia
sample from the bonanza stage 5. Table 2 summarizes the details of the analyzed samples. The
resulting δ18O in quartz and adularia are in the range of -2.2‰ and 6.85‰ relative to VSMOW.
By taking the average homogenization temperature for bonanza stage 5 as 207.9°C, the oxygen
fractionation of the water in equilibrium with quartz and adularia, was calculated. The
fractionation of oxygen in the hydrothermal fluid yielded a range of δ18OH2O= -4,34 to -10,08‰,
which is well below 5,7‰ that is proposed for magmatic water and indicates the domination of
meteoric source for the water. (Fig. 14).
Table 2. Oxygen isotopic data for quartz and adularia from the Pallancata bonanza stage.
SampleDeph
(m)Stage Mineral
δ18O mineral
(‰ )
δ18OH2O
(‰ ) at 207
DLPL-A37 4375 S-5 quartz 5.24 -5.95
DLPL-A37 4375 S-5 adularia -2.22 -10.08
DLPL-A03-3 4250 S-5 quartz 6.85 -4.34
DLPL-A23-18 4200 S-5 quartz 3.74 -7.45
Note: δ18Oquartz-H2O calculated from the equation of Clayton et al. (1972) and δ18Oadularia-H2O from the equation of
Matsushita et al. (1979).
Figure 14. Oxygen isotopes. A) Range of δ18O for quartz and adularia from bonanza stage, Pallancata vein. B)
δ18OH2O for calculated water at Pallancata and for different water sources.
SHRIMP U-Pb zircon dating
Ignimbrites of dacitic composition from Aniso Formation hosts the Pallacanta mine and,
its U-Pb age establishes the maximum age for the deposit formation. One representative sample
of this unit, sample T02, was chosen for SHRIMP U-Pb zircon dating. Petrographic and BSE
imaging helped to characterize zircon internal structures, which provides reliable information
about its origin.
Zircon population from sample T02 vary in length between 50 e 100 μm, and consists of
long to short prisms with pyramidal terminations, broken pieces and few rounded crystals (Fig.
15A). The majority of zircons present internal euhedral zoning which is typical of magmatic
origin according to criteria presented by Vavra et al. (1996). Results are presented in the Table 3
and plotted in the figure 15b. Results were only obtained in three crystals due to a break in the
equipment by the time analyzes were performed. These three crystals yielded an age of 16.58
±016 Ma and, with the necessary caution, should represent the magmatic age of the sample.
Table 3. SHRIMP U-PB zircon data for sample T-02.
Note: atomic ratios corrected for common lead fractionation.
Figure 15. A) BSE imaging of zircons from sample T-02, red circles – dated areas. B) Concordia diagram.
Discussions and Conclusions
The Ag-Au-rich Pallancata vein presents a complex evolution history, which is recorded
by the relationships among ore and gangue minerals, epithermal quartz textures and fluid
inclusion and oxygen isotope results. The evolution of Pallancata vein represents a mineralized
low-sulphidation epithermal system between 4,150 and 4,250 m a.s.l. It shares several
similarities with worldwide known epithermal systems as shown by Hedenquist et al. (2000);
Sillitoe and Hedenquist, (2003) and Simmons et al. (2005) and with some Peruvian low-
sulphidation deposits as Arcata (Candiotti and Guerrero 2002), Shila (Cassard et al. 2000) and
Selene, (Palacios 2006).
Tectonic environment and structural model
The epithermal veins of Pallancata district are hosted in 16.58 ±016 Ma dacitic
ignimbrites developed along the Huaylillas magmatic arc in the sense of Mamani et al. (2010).
The Huaylillas volcanic episode relates to increasing of convergence rate and changing of
dipping angle of Nazca Plate under the Sulamerican Plate (Pilger 1983, 1984; Pardo Casas and
Molnar 1987; Sébrier and Soler 1991; Stern 1991; Benavides-Cáceres 1999). The volcanic
episode coincided with a crustal shortening period and later development of NW-trending
normal faults (Sébrier and Soler 1991). In the Pallancata district the ore is hosted by these
normal faults systems in which negative flower structures formed. Late tectonic reactivation
during the Quechua phase 2, between 12-6 Ma, (Farrar and Noble 1976; McKee and Noble
1989; Swanson et al. 1993; Mégard et al. 1984) displaced ore zones up to 20 m.
Deposit zoning and deposition mechanism during the main hydrothermal phase
The mineralization at Pallancata vein records both horizontal and vertical zoning in
terms of mineral composition (prostite-pyrargirate e polibasite-pearceite), metal content (Ag-
Au) and gangue minerals, which decreases to east and downwards whereas sulfurets and base
metal increases with depth and to west. The shallower level of the hydrothermal system, a
chalcedony blanket, is exposed to the north of Pallancata vein close to Farallon zone suggesting
that there the whole hydrothermal system may still be fossilized.
The main hydrothermal phase of the Pallancata mine, bonanza stage 5, developed the
mineralization along with crustiform-massive and cockade textures in quartz suggesting cyclic
depositon in microbands. Each cycle has varied widths from 1 cm up to 1 m and repetition of
several cycles may reach up to 30 m wide; microbands consists of sulphides+grey amorphous
silica+adularia. Amorphous silica deposits through rapid cooling due to decompression boiling
(Drummond and Ohmoto, 1985; Fournier 1985; Saunders and Schoenly 1995); the common
presence of adularia and bladed calcite replaced by quartz also indicates boiling which causes
pH of fluid to raise due to CO2 loss to vapor phase (Browne and Ellis 1970).
Deposit model
In the Pallancata vein, protracted interaction of meteoric fluids with wall rocks followed
by recurrent fluid injection and boiling is responsible for ore deposition. Boiling may have
cyclically occurred during the lifetime of the hydrothermal system and is closely related to Ag
mineralization, between 5 and 15 ounces of Ag. The cyclic nature of mineralization along with
abrupt change in textures and mineral composition suggests recurrent reactivation of the
hydrothermal system (Simmons 1991). This model involves a complex history of ore
deposition, fluid exsolution and recurrent brittle deformation which relates to structural
permeability (Sibson et al.1975; Sibson 1996). The presence of adularia, ilite, crustiform-,
colloform- and lattice-bladed quartz, low-salinity fluid and light Oxygen isotopic ratios suggest
the Pallancata mine as a class of low-sulphidation epithermal deposit.
Acknowledgments
This article contains the results of the M.Sc. Neper Condori dissertation, Federal University
of Mato Grosso (UFMT), Brazil. The research was conducted with support of the Graduate
Program in Geosciences UFMT and the Company Minera Ares SAC, a branch of Hochschild
Mining PLC and managed by Mr. Oscar Garcia. Thanks are extended to and all geologists of
Pallancata Mine. I thank Dr. Jayme Alfredo Dexheimer Leite for supervising this work. To all
teachers, students and staff at UFMT, that somehow contributed to this work and to CAPES-
High-Level Personnel Improvement Coordination. To Marina Canaza and Madeleine Condori
by the time management suggestions.
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