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Northern Sierra Madre Occidental Gold-Silver Mines, Mexico

Abstract

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

  1. Page 1
    Abstract
    E-mail, staude.john-mark.jg@bhp.com.auPresent address: BHP Minerals, Avenida Americo Vespucio Sur 100, Piso 8, Las Condes, Santiago, Chile.

    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 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 >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 <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.

  2. Page 19
    Abstract

    The Dolores project is located in the Sierra Madre Occidental Range in the State of Chihuahua, northern Mexico, at latitude 29± 00' N, longitude 108±32' W. It is in the municipality of Madera, about 94 km by road (45 km by air) southwest of the town of Madera, and 250 km west of the city of Chihuahua.

  3. Page 29
    Abstract

    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.

  4. Page 45
    Abstract

    Pinos Altos is Toronto-based Agnico-Eagle Mines Limitedâ∈™s first mine in Mexico, and has the potential to become one of the companyâ∈™s largest gold producers. The 100%- owned Pinos Altos operation is located in the Sierra Madre mountain range of northern Mexico, 220 km west of the state capital, Chihuahua, and approximately 2,000 m above sea-level. A permanent camp has been built in the village of Cahuisori, 6 km from the mine site, including employee accommodations and offices. Pinos Altos had more than 728 permanent employees at end of 2009.

    Mining will be from a series of open pits starting with Santo Nino, as well as an underground mine and a number of nearby, stand-alone satellite open pit-heap leach operations, starting with Creston Mascota in 2011. Average annual production from Pinos Altos is anticipated to be about 170,000 oz of gold and 2.5 Moz of silver through the year 2028.

  5. Page 59
    Abstract

    The Ocampo mine is located at approximately 28±12.5' latitude and 108±25' longitude in western Chihuahua State, Mexico. The property lies SW of the capital city of Chihuahua.

    Driving directions: From Chihuahua, drive 285 km west along federal Hwy 16 to Cahuisori, then 25 km south on a Gammon Lake-maintained gravel road.

    The small town of Ocampo, population 600, lies in a deep valley above Gammon Goldâ∈™s Ocampo underground mine. The town and mine are connected to the National Electrical Grid (CFE). Other industries in the area are forestry, ranching, and farming. Gammonâ∈™s workforce comes from all across Mexico. Mexican men and women make up 99% of the workforce.

  6. Page 83
    Abstract

    The silver and gold deposits of the world-class Palmarejo district are typical intermediate sulfidation-style epithermal precious metal occurrences hosted in Cretaceous- to Tertiary-age volcanic and intrusive rocks, of the Lower Volcanic Complex (LVC), and now host one of the newest and largest silver and gold producers in Mexico. The district, along with a surface and underground mine and new 6,000 tpd ore processing facility, is owned and operated by Coeur dâ∈™Alene Mines Corporation through its wholly owned subsidiary, Coeur Mexicana S.A. de C.V. Palmarejo is located in the southwest portion of the State of Chihuahua in the Sierra Madre Occidental (SMO) epithermal belt (Fig. 1).

  7. Page 95
    Abstract

    The E1 Sauzal gold-silver deposit is situated in the western Sierra Madre Occidental of Mexico, near the border between the states of Chihuahua and Sinaloa (Fig. 1). The deposit was discovered in 1995 by Francisco Gold during grass-roots exploration centered on the historic silver district of Batopilas, about 15 km to the NE (Fig. 1), (Charest et al., 2004). Colonial-era and other significant workings were completely absent at El Sauzal. Geologists travelling by mule and on foot in the deep, roadless canyon of the Rio Urique made the discovery by direct rock-chip sampling of resistant, highly altered outcrops. Between 1996 and 1999, a camp was established and a total of 188 diamond-drill holes were completed. Drilling was focused on the potential for shallow, oxidized ores and was largely guided by gold in rock-chip and channel samples.

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