The 45 km 2 map area is situated at the south end of the Hot Creek Range in central Nevada, ~16 km east of the buried leading edge of the Mississippian Roberts Mountains thrust. Three eastward-trending left-slip faults divide the area into four structural blocks. The southernmost block is occupied solely by upper Oligocene volcanic rocks. The narrow northernmost block, now occupied surficially by valley fill and volcanic rocks, represents the south end of the main part of the Hot Creek Range, from which the study area is offset. The middle two blocks display different aspects of the eastward-traveled outer crater rim created by the ca. 382 Ma (early Late Devonian, middle Frasnian) Alamo impact. The Alamo impact was produced by a 5-km-diameter bolide, most likely a comet, which excavated a transient submarine crater 44–65 km in diameter. Comparison of thin (8–12 m) Alamo Breccia deposits in the northern of the middle two blocks with a more easterly, thick (35–42 m) Alamo deposit in the main Hot Creek Range, 4 km north of the map area, suggests that these blocks traveled many kilometers eastward. The northern of the middle two blocks contains a large olistolith capped by the thin breccia, whereas the southern block contains a larger olistolith lacking an Alamo Breccia cap. Three Devonian pulses of the Antler orogeny are better documented in the chapter on the Bisoni-McKay area. Here, the first Antler pulse in latest Middle Devonian time is obscured within an ~9 m.y. hiatus enlarged by excavation of the Alamo impact crater. The second Antler pulse is recorded by the ~4 m.y. hiatus produced by the regional unconformity between the lower and upper members of the Woodruff Formation. The third Antler pulse is documented by an ~8 m.y. regional hiatus between the Mississippian Webb Formation and Upper Devonian Woodruff Formation. In previous papers, we had interpreted this pulse to initiate the Antler orogeny.

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.