Resumo

CAMPRUBI, Antoni  e  ALBINSON, Tawn. Epithermal deposits in Mexico- update of current knowledge, and an empirical reclassification. Bol. Soc. Geol. Mex [online]. 2006, vol.58, n.1, pp.27-81. ISSN 1405-3322.  https://doi.org/10.18268/bsgm2006v58n1a2.

Epithermal ore deposits have traditionally been the most economically important in Mexico, with renowned world-class deposits as those in the Pachuca - Real del Monte, Guanajuato, Fresnillo, Taxco, Tayoltita, and Zacatecas districts. It is also one of the most interesting topics for both scientific research and exploration on ore deposits, especially in the light of the genetic links with other deposit types such as metalliferous porphyries and skarns. Additionally, recent rearrangements of denominations and definitions for the types and subtypes of epithermal deposits (Einaudi et al., 2003; Sillitoe and Hedenquist, 2003), with consequent inclusivities and exclusivities between them, are going to be a matter of discussion for a while, as any newly set model needs to be completed with further evidence. Sillitoe and Hedenquist (2003) indicated a close association between porphyry and high and intermediate sulfidation deposits whereas, in certain areas (as 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 epithermal deposits in Mexico, 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, and 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 Mexico belong to the intermediate (IS) or low sulfidation (LS) types, and only a few high sulfidation (HS) deposits have been described in the NW part of the country (i.e. El Sauzal, Mulatos, Santo Niño, La Caridad Antigua, all of them in Sonora and Chihuahua). As most epithermal deposits in Mexico exhibit composite characteristics of both IS and LS mineralization styles (as well as scarce characteristics of HS), they can not 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 four 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, with or without evidence for boiling, and contain exclusively from top to bottom IS styles of mineralization with a consistent polymetallic character. Type B (or LS-IS type) comprises those deposits that exhibit dominant LS characteristics but have polymetallic IS roots (Zn-Pb-Cu), and is the most widespread type of epithermal mineralization in Mexico. Types A and B generally exhibit 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 brines, and have the relatively highest precious metal and lowest base metal contents.

In this paper, although not necessarily as part of the above classification, 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 isotopic 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 at the epithermal environment. In this review the intent is to show what is known today of epithermal deposits in Mexico, and to point out the gaps that remain in their knowledge. Detailed multidisciplinary studies are available only for a handful of literally hundreds of epithermal deposits in the country, whose study may potentially provide valuable information about the origin and evolution of these deposits as well as about the metallogenic provinces that contain them.

Palavras-chave : Epithermal deposits; Mexico; continental arc volcanism; Tertiary; intermediate sulfidation; low sulfidation; high sulfidation; polymetallic; base metals; precious metals; Silver; Gold.