Mesozoic orogenic gold deposits in Mexico

Abstract

Although orogenic gold deposits in Mexico were mostly formed during the Cenozoic, they started forming during the Late Cretaceous, or perhaps even earlier. Mylonitic zones in the Oaxacan metamorphic complex span either Permo-Triassic or Triassic-Jurassic ages, but none of these have been formally associated with the existing orogenic gold occurrences. Given the current scarcity of age data for them, any interpretation concerning the age patterns or distribution of orogenic gold deposits in Mexico other than those in the Sonoran belt is hitherto deemed to be precarious. A few known Mesozoic examples are part of the Quitovac–La Choya deposits in the Caborca Orogenic Gold Belt (COGB) in Sonora, and Los Uvares in the Los Cabos Block (southernmost part of the Baja California Peninsula). The COGB constitutes the most economically important and geologically consistent mineralized area in Mexico with respect to orogenic gold deposits. Also, the COGB arguably correlates with the Mother Lode trend in California and Nevada, whereas the cluster of deposits in the Los Cabos Block may have correspondent similarly mineralized areas in mainland southwestern Mexico (Jalisco Block). Other areas with orogenic gold deposits in Mexico, which have been traditionally overlooked or scantly mentioned, are the northernmost part of the Baja California Peninsula, and the Arteaga and Oaxacan metamorphic complexes, although no geochronological data are available from these. Besides the overly lack of such data, one of the typical problems concerning the characterization and metallogenesis of orogenic deposits in Mexico has been their inaccurate or incorrect ascription to other genetic paradigms or types of deposits, such as epithermal or porphyry Cu–Au deposits, therefore misguiding exploration efforts.The formation of orogenic gold deposits in Mexico has been commonly associated with the Laramide orogeny, in association with cataclastic or mylonitic deformation and shear zones. Their depth of formation, under lithostatic pressure, has ben estimated at a range between ~3 and ~10km. Alteration assemblages in them are generally dominated by quartz±“sericite”±pyrite, and may be carbonate-rich. The hypogene mineralogy of orogenic gold deposits in Mexico is normally dominated by sulfides like pyrite, galena, sphalerite and whereas gold normally occurs as “electrum”, also known as unspecified Ag-rich gold, Au-rich silver, or as free gold. The latter may either be a hypogene or supergene mineral, in which case it is likely to be due mostly to the leaching-off of Ag from “electrum”. Te-rich associations are rare and have complex parageneses that consist typically of silver and/or gold tellurides. The mineralizing fluid precipitation into quartz veins occurred at temperatures ~350°C and are CO2-rich deeply seated crustal brines from metamorphic sources, with salinities typically ranging between ~2 and ~10wt.% NaCl equiv. but attaining up to ~33wt.% NaCl equiv.