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 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).

Abstract Epithermal deposits form in tectonically active arc settings and magmatic belts at shallow crustal levels as the products of focused hydrothermal fluid flow above, or lateral to, magmatic thermal and fluid sources. At a belt scale, their morphology, geometry, style of mineralization, and controls by major structural features are sensitive to variations in subduction dynamics and convergence angle in arc and postsubduction settings. These conditions dictate the local kinematics of associated faults, influence the style of associated volcanic activity, and may evolve temporally during the lifetime of hydrothermal systems. Extensional arc settings are frequently associated with arc-parallel low- to intermediate-sulfidation fault-fill and extensional vein systems, whereas a diversity of deposit types including intermediate-sulfidation, high-sulfidation, and porphyry deposits occur in contractional and transtensional arc settings. Extensional rift and postsubduction settings are frequently associated with rift-parallel low-sulfidation vein deposits and intermediate- and high-sulfidation systems, respectively. At a district scale, epithermal vein systems are typically associated with hydrothermal centers along regional fault networks, often coexistent with late fault-controlled felsic or intermediate-composition volcanic flow domes and dikes. Some districts form elliptical areas of parallel or branching extensional and fault-hosted veins that are not obviously associated with regional faults, although veins may parallel regional fault orientations. In regional strike-slip fault settings, dilational jogs and stepovers and fault terminations often control locations of epithermal vein districts, but individual deposits or ore zones are usually localized by normal and normal-oblique fault sets and extensional veins that are kinematically linked to the regional faults. Faults with greatest lateral extent and displacement magnitude within a district often contain the largest relative precious metal endowments, but displacement even on the most continuous ore-hosting faults in large epithermal vein districts seldom exceeds more than several hundred meters and is minimal in some districts that are dominated by extensional veins. Veins in epithermal districts typically form late in the displacement history of the host faults, when the faults have achieved maximum connectivity and structural permeability. While varying by district, common unidirectional vein-filling sequences in low- and intermediate-sulfidation veins comprise sulfide-bearing colloform-crustiform vein-fill, cockade, and layered breccia-fill stages, often with decreasing sulfide-sulfosalt ± selenide abundance, and finally late carbonate-fill; voluminous early pre-ore barren quartz ± sulfide fill is present in some districts. These textural phases record cycles associated with transient episodes of fluid flow triggered by fault rupture. The textural and structural features preserved in epithermal systems allow for a field-based evaluation of the kinematic evolution of the veins and controlling fault systems. This can be achieved by utilizing observations of (1) fault kinematic indicators, such as oblique cataclastic foliations and Riedel shear fractures, where they are preserved in silicified fault rock on vein margins, (2) lateral and vertical variations in structural style of veins based on their extensional, fault-dominated, or transitional character, (3) extensional vein sets with preferred orientations that form in the damage zones peripheral to, between, or at tips of fault-hosted veins, and (4) the influence of fault orientation and host-rock rheology and permeability on vein geometry and character. Collectively, these factors allow the prediction of structural settings with high fracture permeability and dilatancy, aiding in exploration targeting. Favorable structural settings for the development of ore shoots occur at geometric irregularities, orientation changes, and vein bifurcations formed early in the propagation history of the hosting fault networks. These sites include dilational, and locally contractional, steps and bends in strike-slip settings. In extensional settings, relay zones formed through the linkage of lateral fault tips, fault intersections, and dilational jogs associated with rheologically induced fault refraction across lithologic contacts are common ore shoot controls. Upward steepening, dilation, and horsetailing of extensional and oblique-extensional fault-hosted vein systems in near-surface environments are common and reflect decreasing lithostatic load and lower differential stress near surface. In these latter settings, the inflection line and intersections with branching parts of the vein system intersect in the σ 2 paleostress orientation, forming gently plunging linear zones of high structural permeability that coincide with areas of cyclical dilation at optimal boiling levels to enhance gangue and ore precipitation. The rheological character of pre- and syn-ore alteration also influences the structural character, morphology, and position of mineralized zones. Adularia-quartz-illite–dominant alteration, common to higher-temperature upflow zones central to intermediate- and low-sulfidation epithermal vein deposits, behaves as a brittle, competent medium enabling maintenance of fracture permeability. Lateral to and above these upflow zones, lower-temperature argillic alteration assemblages are less permeable and aid formation of fault gouge that further focuses fluid flow in higher-temperature upflow zones. Fault character varies spatially, from entirely breccia and gouge distally through progressively more hydrothermally lithified fault rocks and increasing vein abundance and diminishing fault-rock abundance proximal to ore shoots. In poorly lithified volcaniclastic rocks or phreatic breccia with high primary permeability, fault displacement may dissipate into broader fracture networks, resulting in more dispersed fluid flow that promotes the formation of disseminated deposits with low degrees of structural control. In disseminated styles of epithermal deposits, mineralization is often associated with synvolcanic growth faults or exploits dikes and phreatic breccia bodies, feeding tabular zones of advanced argillic and silicic alteration that form stratabound replacement mineralized zones. In lithocap environments common to high-sulfidation districts, early, laterally continuous, near-surface barren zones of advanced argillic alteration and silicification form near the paleowater table above magmatic-hydrothermal systems. In many high-sulfidation deposits, these serve as aquitards beneath which later hydrothermal fluids may localize mineralization zones within permeable stratigraphic horizons, although deeper mineralization may also be present within or emanating from faults unrelated to lithocap influence. Silicified lithocaps may contain zones with high secondary structural permeability that localize ore through the formation of zones of vuggy residual quartz and/or elevated fracture densities in the rheologically competent silicified base of the lithocap, often along or emanating laterally outward from ore-controlling faults. Syn-ore faults in such settings may form tabular, intensely silicified zones that extend downward below the lithocap.

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 Fluid inclusion and metal-ratio data have been compiled for 52 low-sulfidation precious metal and base metal-rich low-sulfidation epithermal deposits in Mexico. Precious metal deposits typically have inclusion salinities that range from 0 to 7.5 wt percent NaCl equiv, whereas base metal-rich deposits have salinities that are as high as 23 wt percent NaCl equiv. Salinities are typically high in fluids included in sphalerite, suggesting a genetic relationship between base metal mineralization and saline fluids. Silver/gold and Ag + Au/Pb + Zn + Cu ratios correlate with fluid inclusion salinity, a relationship that underscores the importance of chloride complexing in base metal transport and polymetallic mineralization. Fluid inclusion gas chemistry of 21 low-sulfidation epithermal deposits plotted on N 2 -Ar-He and N 2 -Ar-CH 4 diagrams indicate that meteoric, mantle or evolved crustal, and magmatic fluids were present in the ore-forming hydrothermal systems, although in different proportions in individual deposits. The N 2 /Ar ratios of sulfide mineral fluid inclusions are all higher than that of air-saturated water, indicating a mag-matic source, whereas a significant proportion of inclusions in barren gangue minerals have N 2 /Ar ratios near that of air-saturated water. Plots of N 2 /Ar vs. H 2 S/Ar show a correlation between N 2 and H 2 S concentrations. The data suggest that low-sulfidation epithermal deposits in Mexico comprise both meteoric waters and magmatic waters, with a significant contribution of H 2 S of magmatic origin. New oxygen and hydrogen isotope data are presented for seven deposits. Fluids responsible for precious metal and base metal deposition contain consistently heavy oxygen isotope signatures and shifts as high as +10 to +20 per mil from the meteoric water line, regardless of host rock type. Boiling and/or water-rock interaction processes alone cannot explain adequately the consistently heavy oxygen isotope signatures of Mexican low-sulfidation deposits. Rather, these results are best accounted for by a significant contribution of magmatic waters to the deep fluid, subsequently modified by water-rock interaction, boiling, and mixing with meteoric water. A classification of low-sulfidation deposits of Mexico is presented based on depth of formation and whether or not boiling is thought to have occurred in the system. Three end-member types are recognized: shallow with boiling, deep with boiling, and deep without boiling. In shallow-formed deposits boiling fluids rise to depths of <500 m below the paleowater table, and ore occupies a vertical range of a few hundred meters. In deep-formed deposits, boiling occurs at temperatures that may exceed 300°C, and ore is generally deposited between 400 and 1,000 m from the paleowater table as fluids rise within the area of liquid-vapor immiscibility. Vein deposits related to fluids that rise within the liquid-phase field and do not reach the field of liquid-vapor immiscibility deposit ore at depths of >1,000 m below the paleo-water table.