GEOLOGY AND MINERALIZATION
The Mulatos District occurs within the Sierra Madre Occidental volcanic province. Deposits in the district are hosted within a mid-Tertiary dacitic to rhyodacitic volcanic dome complex and are considered high-sulphidation, epithermal deposits. Gold mineralization is closely associated with silicic and advanced argillic alteration occurring near the upper contact of a rhyodacite porphyry and in overlying dacite flows and volcaniclastic rocks. All lithologic units of the dome complex are intensely altered. Alteration assemblages are typical of high-sulphidation deposits and show zonation patterns from distal propylitic alteration through illite to kaolinite to dickite/pyrophyllite to pervasive, vuggy silica alteration. Gold is predominantly hosted within this silicic alteration. The altered and mineralized units are locally overlain by a thick section of unaltered volcanic rocks that are believed to be post-mineral in nature. Tilting and post-mineral normal faulting associated with late Tertiary extensional (basin and range) tectonics have affected both the mineralized flow dome complex and overlying volcanic rocks. North-northwest and northeast trending faults cut all rocks in the Mulatos area. The Mulatos mine is exposed in the footwall uplift of the Mulatos extension fault.
The Mulatos deposit proper is composed of the contiguous Estrella, El Salto, Mina Vieja, and Puerto del Aire resource areas. The Escondida deposit is the faulted extension of the Mina Vieja and El Salto deposits and mineralization is believed to be continuous to the northeast to the Gap, El Victor and San Carlos mineralized areas. Although zones are often bounded by post-mineral faults, together they form a trend of 2.7 km of gold mineralization starting at the north end of the Estrella pit to the San Carlos deposit.
Within the larger Salamandra Concessions, and generally within 20 km from the Mulatos deposit, geologically similar high-sulfidation gold deposits, occurrences, or prospects are known. The principal ones, some of which are in the process of being evaluated and/or drill-tested, are: Cerro Pelon, La Yaqui, El Realito, El Carricito, El Halcon, Las Carboneras, El Jaspe, Puebla, Los Bajios, and La Dura.
Gold deposits of the Mulatos district are high-sulphidation, epithermal, disseminated gold deposits. Precious metal mineralization at Mulatos is associated with intense silicic alteration (mostly vuggy silica), advanced argillic alteration, and the presence of hydrothermal breccias. The original protoliths (dacite porphyry flow/tuff, coarse grained volcaniclastic rocks, breccias), as indicated by surface mapping and core drilling, may have contained in the order of 2-3 percent sulphide as pyrite with various amounts of enargite and tetrahedrite.
Gold mineralization occurs primarily within areas of pervasive silicic alteration of the volcanic host rocks, and to a lesser extent, within advanced argillic alteration assemblages proximal to silicic alteration. The gold-bearing advanced argillic zones are dominated by pyrophyllite or dickite alteration. Silicic rocks host approximately 80 percent of the contained gold within the deposit.
Staude (2001) describes three main mineralization assemblages. From oldest to youngest they are: 1) quartz + pyrite + pyrophyllite + gold; 2) quartz + pyrite + kaolinite + gold + enargite; 3) kaolinite + barite + gold. Free gold is commonly found in hematite-filled fractures. Gold also occurs in pyrite, as gold/silver telluride minerals, and possibly as a solid solution in some copper sulphide minerals. Supergene oxidation and perhaps remobilization and secondary enrichment of gold have been ongoing since the post-mineral volcanic cover was removed.
Gold mineralisation exists in oxide, mixed oxide/sulphide, and sulphide ore types, with pyrite as the primary sulphide mineral. The deposits are amenable to cyanidation in all ore types, but gold extraction decreases with decreasing levels of oxidation.