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

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.