This three-day field trip was possible by air travel from one location to another, visiting the following mines: La India, Pinos Altos, Palmarejo, and La Cienega. Detailed road logs and descriptions provide an excellent overview of the mines and surrounding areas.

This three-day field trip was possible by air travel from one location to another, visiting the following mines: La India, Pinos Altos, Palmarejo, and La Cienega. Detailed road logs and descriptions provide an excellent overview of the mines and surrounding areas.

Abstract The San Dimas district is one of the major silver and gold producers in the world. The high-grade silver-gold deposits are of the low sulfidation, adularia-sericite epithermal type, formed during the final stages of igneous and hydrothermal activity of small Eocene quartz monzonitic and andesitic intrusions. The veins are hosted by tuffs, flows, and agglomerates of Eocene age, belonging to the Lower Volcanic Group, and are unconformably overlain by the younger Miocene Upper Volcanic Sequence. K-Ar age studies were done with the purpose of investigating the relationship between the magmatic and hydrothermal events within the district. Three episodes of intrusion took place in the district. K-Ar ages of the Piaxtla intrusions suggest that the batholithic complex was emplaced about 45.1 Ma. The second event was the Intrusive andesite, giving ages ranging from 39.9 to 37.9 Ma. The last intrusion event was the Arana quartz monzonite. K-Ar ages of these rocks range from 38.1 to 36.6 Ma. Ages obtained for the vein stage of economic mineralization ranged from 39.1 to 31.9 Ma throughout the district. Alteration and mineralization occurred within a 0.3 to 3.4 Ma time span after the emplacement of the Intrusive andesite and the Arana quartz monzonite. These last two intrusive events are genetically related to ore mineralization.

Abstract The Santa Rita mine is an epithermal precious metal deposit of the adularia-sericite type, and a new discovery within the San Dimas district. Ore is hosted by east-northeast to northeast and north-northwest-striking normal faults that cut andesitic and rhyolitic tuffs, agglomerates, and lava flows of Paleocene age. The mining project started in 1984 with surface geology, recognizing local structures during district-scale mapping, followed by a diamond drill program, and later with 2.5 km of adit to cut the veins. Total production, as of January 1999, was about 10.2 Moz of silver and 64,800 oz of gold from about 617,000 metric tons (t) of ore. The ore is sent to the Tayoltita mill, and represents 30 percent of the total production at Tayoltita. The total historical production of the San Dimas district is around 20.3 × 10 6 kg of silver and 284 × 10 3 kg of gold from about 19 Mt of ore. The stages of vein formation consist of an early quartz stage, an ore stage, and a late quartz stage. The ore stage is subdivided into quartz-chlorite-adularia, quartz-rhodonite, and quartz-calcite substages. Most of the ore consists of sulfide-bearing quartz veins with a variety of textures typical of this type of deposit. Silver occurs in acanthite, argentite, stromeyerite, jalpaite, pearceite-polybasite, and native silver. Gold occurs in electrum accompanied by minor amounts of base metal sulfides. The radiometric age of the deposit is around 38 Ma. Geochemical studies were done in two of the major veins of the deposit. One hundred five samples were analyzed for Ag, Au, Pb, Zn, Cu, As, Sb, Mn, Mo, and Hg. The data were analyzed using the total raw data and data subpopulations representing samples from above, within, and below the ore zone. Three major associations were determined from the geochemical data: (1) an Ag-Au association that defines the ore zone, (2) a Zn-Pb-Cu association, and (3) an As-Sb association. Microthermometric measurements of fluid inclusions were carried out to evaluate the role of temperature in controlling mineralization. Fluid inclusion studies suggest that the quartz-chlorite-adularia substage was deposited primarily around 223°C from fluids with salinities of 2.7 wt percent NaCl equiv, the quartz-rhodonite substage mineralization was formed primarily around 239°C from fluids with salinities of 3.2 wt percent NaCl equiv, and the quartz-calcite substage mineralization was deposited at 227°C from fluids with salinities of 0.7 wt percent NaCl equiv. The late quartz stage was deposited around 228°C from fluids with salinities of 3.3 wt percent NaCl equiv. Temperatures and salinities of the substages remained constant throughout ore deposition. Evidence of boiling, characterized by coexisting gas and liquid-rich fluid inclusions, is spatially sporadic. Ore deposition is inferred to have occurred mainly in response to boiling and fluid mixing mechanisms.

Abstract The La Ciénega gold-silver veins are low-sulfidation, epithermal-type deposits related to a rhyolitic flow-dome complex. Ore deposits in the La Ciénega district consist of a series of subparallel, northwest– southeast-trending veins which are hosted in andesitic and rhyolitic rocks that belong to the Tertiary-age Upper and Lower Volcanic sequences. The veins are vertical to steeply dipping structures which consist of quartz, generally containing gold-silver and less than 10 percent combined lead, zinc, and copper sul-fide, and sulfosalt minerals. Supergene enrichment is present in the upper parts of the known ore shoots. Gold-silver production from the La Ciénega orebodies is equivalent to 106,000 oz Au/yr, and La Ciénega has become the largest gold-producing district in the Sierra Madre Occidental province. The new discovery of adjacent mineralized veins extends the mine life for 10 years.

Abstract The Tayoltita Mining Unit in Durango, a mining-metallurgical industry that develops silver and gold deposits owned by Compañía Minera MSL, S. A. de C. V., is located 125 km northwest of the harbor town and prominent tourist center of Mazatlán, Sinaloa, and 150 km west-northwest of Durango, capital of Durango State (Fig. 1). A commercial airline based at Tayoltita offers year-round daily flights, stopping at Mazatlán and Durango on alternate days. The bulk of the raw material for the mining-metallurgical operation, as well as heavy machinery and equipment, are landtransported on heavy trucks along the annually reconditioned Rio Piaxtla bed between San Ignacio, Sinaloa, and Tayoltita; traffic usually ceases in June at the start of the rainy season. The climate at Tayoltita is subtropical; temperatures range from a minimum of 10°C on winter nights to a maximum midday 40°C in summer. Regional annual precipitation is 69 cm, with afternoon torrential showers during the summer and lighter rains in late November and early February. The topography of the mine area is extremely rugged. Elevations fluctuate from 450 m at Tayoltita to 1,985 m at the highest mineworks in Sierra Soledad. Tablelands surrounding the Rio Piaxtla Canyon on the north, south, and east are 2,400 m high on the average, with a maximum elevation of 3,150 m at Cerro Huehuente, 19 km east of Tayoltita.

Abstract The Velardeña Mining Unit, owned by Minerales Metálicos del Norte, S.A., a member of Grupo Industrial Minera México, S.A. de C.V., is located in northeastern Durango State within the Cuencamé municipality, 150 km northeast of the city of Durango and 85 km south-southwest of Torreón. Geographic coordinates of Velardeña Village, in the vicinity of the mineworks, are 25°04′N and 103°44′W. Altitude above sea level ranges from 1,400 m in the valley separating Sierra de Santa Maria and Sierra de San Lorenzo to about 2,000 m at the San Lorenzo peaks. Climate and vegetation are typical of northern Mexico’s semidesert zone. Federal Highway 49 runs approximately 6 km northwest of Velardeña, crossing the Durango-Torreón railroad track 2.5 km farther on at Pedriceña, near which is a small light-aircraft runway. Velardeña has telephone and telegraph facilities, schools, and other services for a population of 5,000, most of whom depend on the mines for a living (Fig. 1).