The Mulatos group of concessions covers the Mulatos deposit and satellite gold systems known as Cerro Pelon, La Yaqui, El Carricito, El Halcon, Las Carboneras, El Jaspe, Puebla, Los Bajios, and La Dura. Minas de Oro Nacional, the Mexican subsidiary of Alamos, controls mineral rights for all concessions comprising the Mulatos Group of Concessions.
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Summary:
The Mulatos mineral deposits are large epithermal, high-sulfidation, disseminated gold deposits. Gold mineralization is closely associated with silicic alteration within extensive areas of argillic and advanced argillic alteration. The Mulatos deposit proper is composed of the contiguous Estrella, El Salto, Mina Vieja, and Puerto del Aire Mineral Resource areas. The Escondida deposit is the faulted extension of the Mina Vieja and El Salto sub-deposits and is believed to be continuous to the northeast with 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 Mulatos Group of Concessions, and generally within 20 km from the Mulatos deposit, geologically similar high sulfidation gold deposits, occurrences, or prospects are known.
Gold deposits of the Mulatos district are considered to be high sulphidation-state epithermal systems. Epithermal precious metal systems may be classified as high, intermediate, and low sulphidation styles. They are characterized by the sulphidation state of the hypogene sulphide mineral assemblage, and show general relations in volcano-tectonic setting, precious and base metal content, igneous rock association, proximal hypogene alteration, and sulphide abundance. Ore in all occurrences is of the type formed under epizonal conditions, that is, generally within 2 km of the paleo-surface.
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 protolith (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% sulphide as pyrite with various amounts of enargite and tetrahedrite. The principal gold-bearing host rock is interpreted as favoured for mineralization due to relatively high primary porosity and its intense fracturing.
Gold mineralization within the Mulatos deposit 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% of the contained gold within the deposit. There are 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 (in those specific deposits where it has been removed).
Mineralization of the La Yaqui Grande High-Sulfidation Epithermal Deposit
La Yaqui Grande (LYG) is a new discovery 9 km southwest of the Mulatos Mine. LYG is a high- sulfidation epithermal Au-Ag deposit situated on the northern end of the Mexican Au-Ag belt along the southern arm of the Laramide porphyry belt in the Sierra Madre Occidental volcanic province.
Gold mineralization is hosted by residual vuggy quartz and quartz-dickite-alunite advanced alteration in the upper Mulatos Dacite. This silicic alteration formed by intense hypogene acidic leaching of porphyritic rocks, below a cap of barren massive silica. 40Ar/39Ar age determinations of coarse crystalline hydrothermal alunite from LYG yielded a plateau age of 57.34±0.09 Ma, and a fine-grained alunite matrixsupported silica breccia returned total gas age of 55.20±0.06 Ma. Hydrothermal alunite is common at LYG, in contrast to pyrophyllite-dominant alteration at the Mulatos Mine.
Puerto del Aire
The Puerto del Aire sulphide hosted gold deposit currently consists of four zones: PDA, PDA Extension, GAP-Victor and Estrella. The deposit is located between the current Mulatos deposit and the past producing Victor Deposit. Of the four zones, PDA, is the largest, hosting 78% of the current mineral resource.
The zones are generally stratiform, dipping gently to the north-east. PDA has a strike length of approximately 1100 m and has a pinch-and-swell geometry due primary and post mineralization faulting. The zone thickness ranges from 20 m to over 120 m. GAP – Victor has a strike length of approximately 850 m and thickness ranges from 15 to 50 m.
Lithologically, the mineralization at PDA is hosted in either a porphyritic dacite unit or a phreatic breccia below the Tplt rhyolite. Alteration consists of vuggy silica in the center and advanced argillic alteration at the borders of the system. Hydrothermal breccias are centered around faults, which are interpreted as the conduits for gold.
Gold mineralization is associated with multiple alteration styles, including:
• Vuggy silica and advanced argillic alteration with dickite filling vugs and fractures,
• Advanced argillic alteration in hydrothermal breccia with a silica-dickite cement, and strong dickite in fractures, and
• Advanced argillic alteration with dickite crystals and dickite in fractures.
The best mineralization is also associated with barite infilling vugs and fractures.