THERMAL HISTORY OF LA COLORADA AG-DEPOSIT, MEXICO

La Colorada is a silver deposit located on the eastern part of the Sierra Madre in Mexico. The ore is predominantly hosted in carbonate and quartz veins, and breccias within a predominantly normal fault system. Veins are hosted in Cretaceous limestone, Tertiary volcanic units, and in some locations fault material. Mineralization is consistent with that of an epithermal system, and appears to be synchronous with faulting in parts of the deposit. Cataclasized sulfides (some of which are ore minerals) within the brecciated material are cut in some locations by later carbonate and/or quartz veins containing euhedral sulfides (some of which are also ore minerals). The ore mineralogy consists of ‘fahlores’ [(Ag,Cu)₁₀(Fe,Zn)₂(As,Sb)₄S₁₃ ], polybasite (with a minor pearcite component), acanthite, native silver, and pyrargyrite. The initial temperature of deposition is likely greater than 300-350^oC, as constrained by the maximum observed silver content of ‘fahlore’ in the pyrargyrite—‘fahlore’—miargyrite and ‘fahlore’—polybasite —pearcite –fcc- or bcc-(Ag,Cu)₂S solid solution assemblages. Only some of the ‘fahlore’ compositions preserve the higher temperatures, and appear to be encapsulated in tectonized clasts. Initial temperatures preserved in these ‘fahlores’ are higher than what has previously been recognized in the district. Silver contents in the polybasite-pearcite solid solution indicate retrograde re-equilibration to ambient conditions. The lower silver compositions in some ‘fahlores’ indicate that parts of the deposit re-equilibrated at sub 200^oC for an extended period of time, and thus the composition of some minerals currently being mined are not original. Specifically, some late stage minerals with high silver content are likely the result of re-equilibration of mineral assemblages at moderately elevated temperatures.