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 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.

Epithermal ore deposits have traditionally been the most economically important in México, with renowned world-class deposits like those in the Pachuca–Real del Monte, Guanajuato, Fresnillo, Taxco, Tayoltita, and Zacatecas districts. Whereas in certain areas (like the Great Basin in Nevada) intermediate and low sulfidation deposits have been found to be mutually exclusive in time and space; in the case of epi thermal deposits in México, the intermediate and low sulfidation types do not appear to be mutually exclusive and, to the contrary, they coexist in the same regions, formed during the same time spans, and even occur together within a single deposit. These deposits are all Tertiary in age, ranging from middle Eocene to early Miocene, with the possible sole exception of a Paleocene deposit. Their space and time distribution follows the evolution of the continental arc volcanism of the Sierra Madre Occidental and Sierra Madre del Sur. The vast majority of epithermal deposits in México belong to the intermediate (IS) or low (LS) sulfidation types; only a few high sulfidation (HS) deposits have been described in the NW part of the country (e.g., El Sauzal, Mulatos, Santo Niño, La Caridad Antigua, all of them in Sonora and Chihuahua). Because most epithermal deposits in México exhibit composite characteristics of both IS and LS mineralization styles (as well as scarce characteristics of HS), they cannot be simply characterized as IS (polymetallic deposits associated with the most saline brines) or LS deposits (mainly Ag and Au deposits associated with lower salinity brines). Thus, in this paper we propose to use an empirical classification for IS + LS deposits (that is, alkaline/neutral epithermal deposits) into three types of mineralization; namely, A, B, and C. Type A (or IS type) comprises those deposits that generally formed at greater depths from highly saline but unsaturated brines and contain exclusively from top to bottom IS styles of mineralization with a consistent poly-metallic character. Type B (or LS-IS type) comprises those deposits that exhibit dominant LS characteristics but have polymetallic IS roots (Zn-Pb-Cu); this is the most widespread type of epithermal mineralization in México. Types A and B generally exhibit mineralogic and/or fluid inclusion evidence for boiling. Type C (or LS type) comprises those deposits that exhibit only LS styles of mineralization, formed generally by shallow boiling of low salinity fluids, and have relatively high precious metal and low base metal contents. In this paper, we also review other known or attributable aspects of Mexican epithermal deposits, including ore and gangue mineralogy and their evolution in time and space, structure, geothermometry, stable iso topic composition of mineralizing fluids and other components of the deposits, chemistry and sources for mineralizing fluids, and the plausible mechanisms for the mobilization of deep fluid reservoirs and for mineral deposition in the epithermal environment.

Abstract The Guadalupe de Los Reyes project is situated 100 km north of the city of Mazatlan on the border between the states of Sinaloa and Durango, Mexico. The project covers approximately 88 km 2 and hosts four major epithermal gold- and silver-bearing quartz veins, breccias, and stockwork zones with a combined strike length in excess of 14 km. Historically, three of these structures supported underground mining operations. The Guadalupe structure was mined as early as 1772, with the most active period being between 1872 and 1938. The Guadalupe de Los Reyes mine reported production of 15,500 kg (525,000 oz) of gold and 1.28 Mkg (41 Moz) of silver. Gold-silver mineralization within the district is associated with northwest- to west-northwest-striking, moderately southwest-dipping, dip-slip faults that cut basalt, andesite, and dacite of the Upper Cretaceous to early Tertiary Lower Volcanic sequence. These structures initially provided channelways for injection of dacite dikes and subvolcanic dacite porphyry stocks, possibly representing the roots of a dacite flow-dome complex. Continued fault movement prepared these structures for subsequent emplacement of low-sulfide, quartz-adularia, gold- and silver-bearing epithermal veins. Approximately 6.5 km of mineralized structure had preliminary drill testing. The geometry of the vein breccia zones in relationship to topography would potentially allow for exploitation using both open-pit and underground mining techniques. Tonnage and grade calculations based on geologic block models have outlined a resource of 24,400 kg (758,000 oz) of gold and 495,000 kg (15.9 Moz) of silver. The Guadalupe de Los Reyes district gold-silver epithermal system is closely associated with the last volcanic event in the Lower Volcanic sequence and has similarities to several other volcanic-hosted gold-silver deposits in the Sierra Madre Occidental such as the nearby Tayoltita deposit.

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.