Contact metasomatism and ore deposition: Concepcion Del Oro, Mexico

Concepcion del Oro, the second largest copper producing district of Mexico, contains sulfide and iron oxide minerals disseminated in skarn matrix surrounding a granodiorite stock. The age (K-Ar) of the stock is 40 ± 1.2 m.y. and the end of mineralization occurred at least 38 ± 1.2 m.y. ago, indicating that mineralization was approximately contemporaneous with emplacement. Far from the stock wollastonite is prominent, although temperatures were too low to produce it in a closed environment. Streaming, ascending fluids presumably removed C02 as rapidly as formed, thereby facilitating the reaction between limestone and enclosed siliceous layers. With increasing metasomatism andradite garnet and other calc-silicates formed, mainly within limestone but also replacing adjacent granodiorite. Magnetite, the earliest metallic mineral, occurs in limestone as well as skarn, but pyrite, specularite, chalcopyrite, and the sulfo-salts (listed in approximate order of deposition) are restricted to skarn. Variations of oxygen fugacity and temperature during mineralization are indicated by at least four alternating generations of magnetite and hematite. The range in f02 is 10-18 to 10-25 atm. Coexisting pyrite during late-stage mineralization fixes the fS2 at 10-6 ± 2 atm. Quartz was deposited after most metallic minerals. Fluid inclusion filling temperatures indicate a formation temperature of 350° ±35° C, the minimum temperature of mineralization. Pyrrhotite indicates a (questionable) formation temperature of 500° C and a heat-flow estimate demonstrates that the maximum temperature at the igneous contact was approximately 500° C, suggesting that metallic mineralization was initiated at the highest temperatures within the contact aureole. Nearby mesothermal to epithermal deposits at Providencia are related to those at Concepcion del Oro. Both deposits are adjacent to essentially identical stocks, occur within the same sedimentary horizons and both occur at or close to the igneous contacts. A genetic correspondence between these deposits, as well as between hydrothermal and contact meta-somatic ores in other areas, suggests that the two types of deposits are fundamentally related. Moreover, measured temperatures of Concepcion del Oro ores fall within the range accepted for hypothermal deposits. It is suggested that these contact metasomatic deposits are properly classified as a particular type within the larger genetic group of high temperature hydrothermal deposits.