Geology, Geochemistry, and Formation of Au-(Cu) Mineralization and
Advanced Argillic Alteration in the Mulatos District, Sonora, Mexico
Authors: Staude John-Mark 
Published: February 2024

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 &gt;1 km of a mineralized and variably altered section. Advanced argillic alteration (&gt;3 km2) can be traced laterally outward through intermediate argillic (&gt;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 &lt;200 m, whereas the top of the main Mulatos orebody (Cerro Estrella) formed at ˜600 m and continues downward for &gt;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 &gt;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 &lt;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 (δ34S = –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.