Abstract The Mulatos district is a volcanic-hosted, advanced argillic, gold enargite system of late Oligocene age, located in the northern Sierra Madre Occidental volcanic province of Sonora, Mexico. Hypogene mineralization is associated with rhyodacite domes and major faults. Gold is associated with pyrite ± enargite in distinct pods of vuggy silica-pyrophyllite-diaspore-dickite in altered dacite-rhyodacite volcanic rock. Past production of more than 300,000 oz Au and reserves of more than 2.3 Moz make the district one of the largest gold systems in northern Mexico and one of the larger advanced argillic gold systems in the world. To the west of Mulatos, five other similarly altered systems are present, and these systems provide additional insight into the genesis and possible variations in mineralization, level of exposure, and physio-chemical conditions of formation. Unlike many acid-sulfate systems, hypogene alunite is uncommon at Mulatos and instead the main alteration mineral is pyrophyllite. The district was tilted ˜15° to 25° NE after mineralization, exposing >1 km of a mineralized and variably altered section. Advanced argillic alteration (>3 km 2 ) can be traced laterally outward through intermediate argillic (>5 km 2 ) into chlorite-montmorillonite ± epidote. Prominent silicified ridges and red (oxidation of pyrite) hills with kaolinite and scattered barite veinlets characterize the surface expression above ore zones. The age of mineralization is bracketed between 31.6 Ma mineralized tuffs and 25 Ma crosscutting and overlying unaltered basaltic andesites. Ore minerals include free gold, Au-rich pyrite, enargite, sphalerite, and less commonly, tennantite, Au telluride, covellite, and chalcopyrite. Elevated concentrations of Ag, As, Au, Ba, Cu, Hg, Mo, Sb, and Te are common in a 2-km 2 alteration zone surrounding the mineralized centers. Mass balance calculations based on whole-rock studies of progressively altered samples show decreasing Ca, K, and Na and increasing Si and Al associated with intensifying acid leaching. The apparent increase in Si and Al is likely a consequence of cation leaching related to the low-pH hydrothermal fluids rather than element addition. Early Au with pyrite, followed by auriferous pyrite + enargite ± Ag sulfosalts, and late Au-containing barite make up the three principal ore stages. Stratigraphic reconstructions show that the tops of the shallowest orebodies are structurally controlled, thin, high-grade pyrite-barite, Au telluride, and Au pyrite + quartz veins formed at a depth of <200 m, whereas the top of the main Mulatos orebody (Cerro Estrella) formed at ˜600 m and continues downward for >400 m. Deep mineralization is dominantly lithologically controlled with large, low-grade (1–2 g/t Au) substratiform horizons coincident with stratigraphic contacts and specific lithologic facies. Higher grade quartz-pyrophyllite-pyrite-Au subvertical elongate zones feed the lateral mineralization. Cerro Estrella ore horizons are not distinctive veins, but 10- to >80-m-wide vuggy silica-pyrite pods surrounded by dickite-pyrophyllite-pyrite zones. Beneath the Au ore, rare chalcopyrite veinlets with traces of quartz-illite selvages cut dacite flows and possible dikes. Phase equilibria indicate that hydrothermal fluids were extremely acidic (pH <2) with temperatures of ˜260° to 300°C. Stable isotopes suggest fluid mixing between magmatic and meteoric components, with increasing meteoric input during waning stages, including the period of high-grade Au barite mineralization. Sulfur isotopes of near zero for pyrite (δ 34 S = –4 ‰) and 18 per mil for coexisting barite give equilibrium temperatures of 260° ± 10°C and are consistent with a magmatic sulfur source. Supergene oxidation in the upper 100 to 200 m of mineralized zones has redistributed copper into small chalcocite layers and liberated Au from pyrite-forming native Au + earthy brown hematite in an oxidized cap.

Abstract The Mulatos district is a volcanic-hosted, advanced argillic, gold enargite system of late Oligocene age, located in the northern Sierra Madre Occidental volcanic province of Sonora, Mexico. Hypogene mineralization is associated with rhyodacite domes and major faults. Gold is associated with pyrite ± enargite in distinct pods of vuggy silica-pyrophyllite-diaspore-dickite in altered dacite-rhyodacite volcanic rock. Past production of more than 300,000 oz Au and reserves of more than 2.3 Moz make the district one of the largest gold systems in northern Mexico and one of the larger advanced argillic gold systems in the world. To the west of Mulatos, five other similarly altered systems are present, and these systems provide additional insight into the genesis and possible variations in mineralization, level of exposure, and physio-chemical conditions of formation. Unlike many acid-sulfate systems, hypogene alunite is uncommon at Mulatos and instead the main alteration mineral is pyrophyllite. The district was tilted ∼15° to 25° NE after mineralization, exposing >1 km of a mineralized and variably altered section. Advanced argillic alteration (>3 km 2 ) can be traced laterally outward through intermediate argillic (>5 km 2 ) into chlorite-montmorillonite ± epidote. Prominent silicified ridges and red (oxidation of pyrite) hills with kaolinite and scattered barite veinlets characterize the surface expression above ore zones. The age of mineralization is bracketed between 31.6 Ma mineralized tuffs and 25 Ma crosscut-ting and overlying unaltered basaltic andesites. Ore minerals include free gold, Au-rich pyrite, enargite, sphalerite, and less commonly, tennantite, Au telluride, covellite, and chalcopyrite. Elevated concentrations of Ag, As, Au, Ba, Cu, Hg, Mo, Sb, and Te are common in a 2-km 2 alteration zone surrounding the mineralized centers. Mass balance calculations based on whole-rock studies of progressively altered samples show decreasing Ca, K, and Na and increasing Si and Al associated with intensifying acid leaching. The apparent increase in Si and Al is likely a consequence of cation leaching related to the low-pH hy-drothermal fluids rather than element addition. Early Au with pyrite, followed by auriferous pyrite + enar-gite ± Ag sulfosalts, and late Au-containing barite make up the three principal ore stages. Stratigraphic reconstructions show that the tops of the shallowest orebodies are structurally controlled, thin, high-grade pyrite-barite, Au telluride, and Au pyrite + quartz veins formed at a depth of <200 m, whereas the top of the main Mulatos orebody (Cerro Estrella) formed at ∼600 m and continues downward for >400 m. Deep mineralization is dominantly lithologically controlled with large, low-grade (1–2 g/t Au) substratiform horizons coincident with stratigraphic contacts and specific lithologic facies. Higher grade quartz-pyrophyllite-pyrite-Au subvertical elongate zones feed the lateral mineralization. Cerro Estrella ore horizons are not distinctive veins, but 10- to >80-m-wide vuggy silica-pyrite pods surrounded by dickite-py-rophyllite-pyrite zones. Beneath the Au ore, rare chalcopyrite veinlets with traces of quartz-illite selvages cut dacite flows and possible dikes. Phase equilibria indicate that hydrothermal fluids were extremely acidic (pH <2) with temperatures of ∼260° to 300°C. Stable isotopes suggest fluid mixing between magmatic and meteoric components, with increasing meteoric input during waning stages, including the period of high-grade Au barite mineralization. Sulfur isotopes of near zero for pyrite (δ 34 S = −4 ‰) and 18 per mil for coexisting barite give equilibrium temperatures of 260° ± 10°C and are consistent with a magmatic sulfur source. Supergene oxidation in the upper 100 to 200 m of mineralized zones has redistributed copper into small chalcocite layers and liberated Au from pyrite-forming native Au + earthy brown hematite in an oxidized cap.

Abstract The Buckskin Range lies approximately 4 km west of the Yerington porphyry copper district and hosts the Artesia Lake and Fulstone Spring volcanic sequences that structurally overlie the Yerington batholith. Hy-drothermal alteration minerals characteristic of advanced argillic, sericitic, and marginal porphyry copper-type alteration assemblages have been detected via infrared spectrometry, X-ray diffraction, petrography, micro-probe analysis, and hand-lens based-field mapping in the central Buckskin Range. It is postulated that high-level alteration in the Artesia Lake Volcanics may be contemporaneous with the main event of sericitic alteration and pyrite deposition in the deeper porphyry copper environment. The presence of sericitic alteration underlying or overprinting hypogene advanced argillic assemblages may imply that fluids responsible for porphyry copper mineralization have ascended to epithermal depths. The spatial relationships of hydrothermal alteration in the Buckskin Range suggest an evolution of low-pH, sulfide-bearing fluids to nearly neutral, oxide-rich hydrothermal fluids. Sulfide-rich, feldspar-destructive advanced argillic and sericitic alteration is crosscut and overlain by feldspar-stable, oxide-rich sericite-hematite-chlorite alteration. Sericite-hematite-chlorite alteration is abruptly overlain by potassium-added, feldspar-stable calcite-chlorite-hematite alteration, produced by late sodic-calcic or potassium-enriched fluids possibly derived from sedimentary or evaporitic brines.

Abstract Phyllic and argillic alteration at Bingham consists of smectite, sericite, and kaolinite. X-ray diffraction and microprobe studies show that sericite in these alteration assemblages is illite interstratified with 10 percent or less smectite. Illite occurs as a replacement of plagioclase phenocrysts, in veins, and as pervasive disseminations in the rock along with smectite and minor kaolinite. The illite/smectite interstratification is a mixture of 1M and 2M polytypes and generally contains more smectite in the finer size fractions. Structural formulae for the illite show less K and Al and more Si than muscovite. The illites also contain octahedral Mg and an octahedral layer charge. The Kubler crystallinity index increases systematically as particle size decreases. Smectite dominates in pervasive alteration of whole rock, plagioclase phenocrysts, and one vein. Illite is the major constituent of vein clays. The smectite replacing plagioclase and whole rock is montmorillonite and smectite in the vein is beidellite. The distribution of smectite, illite, and kaolinite at present mining levels appears to be different than reported in earlier studies in that smectite may be a major component of phyllic and argillic alteration while kaolinite is a minor component. Sample traverses on pit benches through the quartz monzonite porphyry show no systematic variations in mineral abundances. Measurements of clay alteration of phenocrysts in quartz monzonite porphyry in drill hole 50 from an elevation of 5,730 to 3,260 feet show an increase in smectite content downward to the core of the quartz monzonite porphyry, just below present mining levels, and then a decrease to minimum values near the lower contact with monzonite. Copper grades decrease systematically throughout this interval. This pattern of smectite distribution is not observed in horizontal traverses on bench levels across the quartz monzonite porphyry. 40 Ar/ 39 Ar age spectrum analysis on a hydrothermal biotite from altered quartz monzonite porphyry yields an isochron age of 37.07 ± 0.21 Ma. Sericite and K-feldspar phenocrysts from this sample also yield apparent ages of ~37 Ma, but have complex age spectra. The apparent age concordance for these three minerals suggests a hydrothermal event in the quartz monzonite porphyry that produced biotite and sericite at indistinguishable times and may have partially reset the K-feldspar. In comparison, an isochron age for a 10 to 20 mm split of vein sericite in monzonite from the present pit bottom is 37.81 ± 0.20 Ma, whereas a maximum age of a vein sericite in monzonite from the 6440 level is 39.71 ± 0.34 Ma. Coupled with the results of Moore and Lanphere (1971) and the Warnaars et al. (1978) data compilation, these new results allow for a minimum duration of ~1 Ma for the hydrothermal activity at Bingham.