PETROLOGICAL AND GEOCHRONOLOGICAL INVESTIGATIONS ON THE CU- AND AU-DEPOSIT MINA COBRIZA (SAN LUIS POTOSI, MEXICO)
The Cu- and Au-deposit Mina de Cobriza is the northern part of a mining complex in Santa María de La Paz, in the northern part of the Mexican state San Luis Potosí. Samples of two different parts of the Cobriza intrusion and the related skarn deposit were taken below ground and analysed concerning petrological and chemical composition as well as crystallization and cooling age to reconstruct the geodynamical intrusion and skarn alteration of the area.
The hydrothermal skarn deposit were developed after the formation of the intrusive rocks in the southern Sierra del Fraile into the mid-Cretaceous marine sediments. The intrusive rocks as well as the surrounding rocks are affected by metasomatic processes. These transformed rocks can be divided into two different types, the exoskarn consisting prograde andradites and hedenbergites and the endoscarn with a quartz, alterated plagioclase, K-feldspar and low Mg hedenbergite content. In addition both skarns contents of pyrite, chalcopyrite, and bornite. Using petrological and geochemical analyses the skarns can be determined as Cu-Zn-Au-Fe skarns. These skarn types are typical for subduction zones related to oceanic crusts.
The U-Pb dating of the intrusives indicates an emplacement event at the postlaramide magmatic event at 35.5 Ma (LA SF ICP-MS, zircon single grain analyses). These can be confirmed by the geochemical composition of the granitoids indicating that the magmas were formed by complex processes derived from partial melting of the mantle above the subducted Farallon plate in a post-orogenic, extensional setting. In addition the intrusions show a small variation in composition and belong to a high K rich calc-alkaline and peralumious to metaluminous magmatic sequence. The regional and local geological framework allowed rapid magma emplacements with assimilation continental crust material throughout fractional crystallization. These processes of the derivation of the intrusive rocks are completed on to a rapid cooling within 1.5 Ma determined by a K-Ar biotite dating age at 34.0 Ma.