Northern Sierra Madre Occidental Gold-Silver Mines, Mexico
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 km2) can be traced laterally outward through intermediate argillic (>5 km2) 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-km2 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
