Geology, Structural Setting, and Mineralization of the Dolores District, Chihuahua, Mexico
William J. Overbay, Tench C. Page, Dennis J. Krasowski, Mark H. Bailey, Thomas C. Matthews, 2010. "Geology, Structural Setting, and Mineralization of the Dolores District, Chihuahua, Mexico", Northern Sierra Madre Occidental Gold-Silver Mines, Mexico
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The Dolores project is located in the Sierra Madre Occidental mountain range of northern Mexico, near the historic mining district of Dolores in the state of Chihuahua. Exploratory drilling by Minefinders Corporation, Ltd., began in 1996 and has resulted in the discovery of one of Mexico's largest undeveloped gold-silver deposits.
The regional geologic history of the area is dominated by three phases of volcanism: (1) A period of intermediate composition volcanism that resulted in deposition of voluminous andesitic flows and volcaniclastics interlayered with lesser amounts of felsic ash (the Lower volcanic series). This occurred from ˜46 to 35 Ma, (2) The first phase was closely followed by eruption of dominantly felsic ash flow tuffs and flows of rhyolitic to latitic composition (the Upper volcanic series), which occurred between ˜35 and 27 Ma, (3) Finally, intermittent eruption of basaltic andesite in thin flows occurred from <27 to 3 Ma.
Deposition of the Baucarit Formation, a conglomeratic basin-fill sedimentary unit with thin interlayers of basalt, also occurred in down-dropped basins during the Pliocene to Pleistocene, or approximately 5 to 1 m.y. ago.
Definitive age dates for the mineralization at Dolores have yet to be established. Ages of vein-style Ag-Au mineralization throughout the Sierra Madre Occidental are reported to be between about 49 to 27 Ma (Clark et al., 1979). Geologic observations at Dolores suggest that mineralization occurred following the episode of voluminous andesitic volcanism and generally at the same time as deposition of the overlying latitic pyroclastic tuffs of the Lower volcanic series, because mineralization and alteration are generally confined to the andesites and the lowermost portion of the volcaniclastic rocks. It follows that the Dolores mineralization occurred prior to deposition of the Upper volcanic series and dates from about 38 to 35 Ma of age.
Within the district, regional north-northwest-trending structures controlled emplacement of a series of porphyritic andesitic to latitic dikes and sills in conjunction with emplacement of several larger hypabyssal north-northwest-elongate, domal intrusive bodies that formed during deposition of the Lower volcanic series. Epithermal, low-sulfidation fluids, believed to be associated with the waning stages of the intrusive episode, deposited quartz-adularia and precious metals. Wider zones of mineralization formed within areas of higher permeability and where boiling and episodic hydrothermal brecciation were focused in areas of greater structural complexity. Consequently, the mineralization occurs both within high-level stockworks, breccias, and disseminations formed near the contact of the felsic volcaniclastic rocks with the underlying andesites and within more tightly confined north-northwest-trending feeders that continue to depth.
Gold predominates in the higher levels of the system and can be found across widths of 100 m or more at an average grade of from 1 to up to 2 g/t. Mineralized feeders occur below these zones and can be from 2 to more than 20 m in width, with gold content of up to 10 to more than 200 g/t and silver content of 1 to more than 5 kg/t. In the studied resource area, Ag/Au ratios appear to be zoned about a central domal intrusive and vary from about 100:1 near the intrusive to less than 10:1 to the north and south. Trace element geochemistry includes variable Hg, As, and Sb in the higher elevations with increasing amounts of Pb, Zn, and minor Cu at depth.
A combined program of reverse circulation and core drilling totaling 61,441 m in 291 holes has revealed mineralization that occurs within an area that is approximately 2,800 m long by more than 600 m across. Additional drill intercepts and surface geochemical assays outside of this area indicate the potential to increase the resource base throughout an overall area that is approximately 4,000 m by 1,200 m.
A resource study within the most densely drilled 1,900 by 300 m area was completed in 2000. Economic analysis, based on work by a major international engineering firm, indicates that a bulk-minable resource of approximately 67 Mt, at a gold equivalent (Aueq) grade of 1.85 g/t, can be developed by open-pit mining methods. The total calculated resource within the study area is 100.1 Mt containing 2.45 Moz of gold and 129.7 Moz of silver, or 4.62 Moz of Aueq at a 60:1 ratio.
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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