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Palmarejo carbonate : base metal epithermal Ag-Au district, Chihuahua, Mexico

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Galvan Gutierrez, VH (2014) Palmarejo carbonate : base metal epithermal Ag-Au district, Chihuahua, Mexico. PhD thesis, University of Tasmania.

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Abstract

The Palmarejo epithermal Ag-Au district, located in the province of the Sierra Madre
Occidental in north-western Mexico, is part of a larger Tertiary metallogenic belt in westem
Mexico characterized by epithermal deposits. The Palmarejo district contains the Palmarejo,
Guadalupe and La Patria epithermal veins, and has proven and probable resources
of71.8 MozAg and 0.870 MozAu.
The epithermal veins are hosted by volcaniclastic and volcanic rocks of the Cretaceous-
Eocene Lower Volcanic Complex. The basal Cretaceous volcanosedimentary sequence
of interbedded mudstones, sandstones and breccias is overlain by calc-alkaline
amygdaloidal basalts and plagioclase-phyric andesites, which are in part intercalated with,
and overlain by, voluminous Eocene pyroclastic units. Cretaceous to Eocene magmatism
occurred in back-arc to continental arc settings. The regional tectonic setting comprises a
NW system of master faults linked by second order NNE- and ENE-oriented splays. The
dominant NW-trending fault segments have normal displacements (predominantly to the
SE) with sinistral (left lateral) strike slip separation. The volcanic sequence is tectonically
juxtaposed against a Late Cretaceous granodiorite (U/Pbzircon age: 84.6 ± 1.1 Ma) and was
intruded by an Early Miocene rhyolitic dome at Guadalupe (U/Pbzircon age: 24.2 ± 0.4 Ma)
and a rhyolitic dike at La Patria (U/Pb. age: 23.03 ± 0.51 Ma). Mutual cross-cutting rela- ztrcon
tionships between the rhyolites and epithermal veins at La Patria and Guadalupe indicate a
Late Oligocene- Early Miocene age for epithermal mineralisation in the Palmarejo district.
At the Palmarejo deposit, Ag-Au mineralisation was localized by two main structures:
1) La Prieta fault, which trends NW, and 2) La Blanca fault, which trends NWW.
These faults dip 50° to 70°SW. Ore shoots, comprising stockwork zones up to 30 m wide,
occur at the intersection of La Blanca and La Prieta faults and plunge moderately SE. A
sub-vertically-plunging strongly mineralised structural jog is located at an inflection along
La Blanca fault. The Guadalupe vein consists of several extensional fault segments that
predominantly trend NW/55°NE, which have been cut and displaced by the Early Miocene rhyolite dome. The La Patria vein is associated with a corridor ofNW-trending structures
that dip ~40°NE. These structures predate, and were subsequently reactivated by, the Early
Miocene rhyolite dikes and mineralised veins.
Three major stages of vein- and breccia-hosted mineralization have been recognized
at Palmarejo, Guadalupe and La Patria. Stage 1 is composed of grey quartz + chalcedony
+ chalcopyrite + sphalerite + galena + pyrite + acanthite ± molybdenite. Stage 2 comprises
four substages: Stage 2a white quartz + chalcopyrite + galena + sphalerite + bornite +
stromeyerite; Stage 2b white quartz ± calcite + chalcopyrite + pyrite + galena + digenite
+ bornite + sphalerite + tetrahedrite-tennantite ± barite; Stage 2c white quartz ± calcite
± adularia± green sphalerite + galena+ proustite-pyrargyrite ± tetrahedrite-tennantite ±
acanthite ± electrum; and Stage 2d fine grained quartz+ chalcedony + hematite± sphalerite
± pyrite ± galena. Stage 3 is characterized by unmineralised veins and breccias that have
been filled with calcite + chalcedony + rhodochrosite.
The andesitic and basaltic host rocks have undergone background chlorite - pyrite
alteration throughout the Palmarejo district. Proximity to the epithermal is marked by the
development of more intense propylitic alteration halos of epidote+ chlorite+ pyrite+ calcite.
Siliceous and potassic (adularia) alteration halos only occur in the volcaniclastic rocks
proximal to the major veins. An argillic alteration assemblage (kaolinite+ dickite) has been
identified locally in structural jogs within the Palmarejo vein, and as surficial clay blooms
above major mineralised structures.
Fluid inclusions in Stage 1 quartz and sphalerite have homogenisation temperatures
from 19r to 357°C and salinities from 0.5 to 7.8 wt percent NaCl equiv. Calculated o180H2o
values for Stage 1 quartz range from -0.3 to 9.0 %o. These results are consistent with mixing
of magmatic-hydrothermal and meteoric waters during Stage 1. Trace element analyses of
Stage 1 pyrite has revealed only moderate correlation of Ag and Au, but strong correlations
of Ag with Pb or Te, with some Stage 1 pyrite grains containing inclusions of argentiferous
galena. Stage 1 fluids are inferred to have been moderately acidic to near-neutral, reduced
(H2S-predominant) moderate to high temperature waters that transported Ag as AgC12-(aq) and Au as AuHS(aq)· Silver deposition was promoted by dilution of saline magmatic waters
during mixing with meteoric waters. Gold deposition may have occurred by boiling, based
on observations of bladed calcite and abundant vein breccias.
Stage 2 fluid inclusions hosted by quartz, calcite and sphalerite have homogenisation
temperature from 207° to 367°C and salinities from 0.8 to 7.0 wt percent NaCl equiv.
The presence of bladed calcite provides evidence for boiling in the upper levels of the
veins, as does the systematic temperature decrease noted in Stage 2 fluid inclusions with
respect to elevation. Stronger correlations of Ag and Au in Stage 2 pyrite grains compared
to Stage 1 suggest the occurrence of electrum inclusions and may imply more alkaline conditions,
with some Ag transported as Ag(HSh and co-precipitating with gold transported
as Au(HS)2- during boiling. However, the Stage 2 fluid inclusion salinity array provides
strong evidence for fluid mixing, as do the ranges of <5 180H2o values calculated for Stage 2
quartz (3 .15 to 11.03 %o) and calcite ( -0.3 to 9.0 %o ), suggesting that mixing of magmatichydrothermal
and meteoric waters was the main cause of Ag precipitation during Stage 2,
and implying that most of the Ag in Stage 2 was transported as AgC12-. In the lower levels
of the mineralized veins, Stage 2 pyrite grains show a positive correlation of Ag with Cu
(consistent with chloride complexes), whereas in the upper levels, Ag in pyrite is wellcorrelated
with Sb and Se.
Salinity data from barren Stage 3 veins provide further evidence for fluid mixing in
the Palmarejo district, with temperatures from 230° to 388°C and salinities from 2.7 to 22.8
wt percent NaCl equiv. The <5 180H2o values from Stage 3 calcite range from 0.58 to 12.8 %o,
again consistent with mixing of magmatic-hydrothermal and meteoric waters.
Precious metal mineralisation in the epithermal veins of the Palmarejo district formed
in structural traps due to a combination of fluid mixing and boiling. Mineralised ore shoots
pinch out towards the surface, with clay blooms in some cases the only surficial indications
of large epithermal veins at depth. Recognition of favourable structural orientations, local
perturbations of the district-scale structural framework and subtle alteration features are
keys to the discovery of similar epithermal veins elsewhere in NW Mexico.

Item Type: Thesis (PhD)
Keywords: Ore deposits, epithermal, geochemistry, silver, isotopes, fluid inclusions
Copyright Holders: The Author
Copyright Information:

Copyright 2014 the author

Date Deposited: 25 Sep 2015 03:52
Last Modified: 11 Mar 2016 05:52
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