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. Mineral rights for all concessions comprising the Mulatos Group of Concessions are controlled by Minas de Oro Nacional, the Mexican subsidiary of Alamos.
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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.
Residual vuggy quartz with gold is common at La Yaqui Grande deposit and zones outward to quartz-dickite ±alunite, quartz-kaolinite, and illite-smectite±chlorite. Coarse alunite is present on fractures in ore zones, and patchy quartz-alunite is found locally at depth (patchy alunite is considered an indication of the epithermal-porphyry transition, cf., Gustafson et al., 2004), pyrophyllite is absent. Advanced argillic quartz-dickite-alunite alteration provides significant gold resource at La Yaqui Grande.
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.
Volcanic rocks at LYG are subdivided into compositionally, texturally, and temporally distinct rock units that crop out across the Mulatos mining district. The Lower Mulatos Andesite represents the oldest rocks (~75-62 Ma) and consists of fine-grained andesitic lavas and pyroclastic rocks interbedded with epiclastic rocks. Upper Mulatos Dacite (~61-58 Ma) overlies Mulatos Andesite and consists of porphyritic andesite and dacitic flow-domes with related pyroclastic rocks that host the gold ore. Massive silica lies atop mineralized Mulatos Dacite and consists of stratified polymictic and silicified breccia containing massive and vuggy silica fragments. Mulatos Dacite and massive silica are intruded by Yaqui dacite porphyry (60.82±0.76 Ma), followed by emplacement of the Yaqui granodiorite (58.0±0.6 Ma). The rhyodacitic Estrella Tuff (55.01±0.13 Ma) overlies upper Mulatos Dacite and represents the final explosive Laramide eruption at Mulatos. Yaqui dacite porphyries display hydrothermal quartz alunite-dickite alteration, whereas Estrella Tuff and Yaqui granodiorite are unaltered to weakly altered with illite-chlorite-smectite. Post-Laramide late Eocene to Oligocene rhyolitic ash-flow tuffs of the Nopal Rhyolite (~36-32 Ma) overlie the Estrella Tuff with unconformity and a hiatus of ~20 million years. These rocks represent the onset of the middle Cenozoic caldera-forming ignimbrite flare up and are unaltered at LYG.
Extensional tectonics (28-16 Ma) post-date Mulatos ore and the caldera eruptions, juxtapose the stratigraphy and rotated bedding (20° to 35° ENE) exposing advanced argillic alteration below barren massive silica. Basins were filled with post-mineral Oligo-Miocene deposits of basaltic-andesite lavas (28-25 Ma) interbedded with rhyolitic tuff and well-bedded conglomerates. Basaltic-andesite dikes crosscut the advanced argillic alteration.