Geochemical Variations in Biotite Outward from the Toquepala and Cananea Porphyry Copper Deposits

摘要

Biotite geochemical surveys surrounding Cordilleran porphyry copper deposits of Toquepala, Peru and Cananea, Mexico show systematic compositional changes in the elements Mg, Fe, Ti, Mn, and F in zones exterior to silicate alteration (propylitic). In these zones, Mg, Ti, and F are increased in concentrations while Fe and Mn are decreased. Chlorine shows no systematic changes toward ore. Spatially, the distribution of anomalous composition biotites occurs as far as 6 km outward from the ore centers within pre-mineral, biotite-bearing rocks. The biotite in these zones is phlogopitic, low in Fe, and texturally and chemically distinct from the low-Ti, hydrothermal biotite generated within and near the interior of the deposits. Fluorine especially shows anomalous intra-crystal and intra-sample variations in concentrations from samples within the anomalous zones. Because the anomalous composition biotite occurs peripheral to the porphyry deposits, emplacement of the ore system itself is presumed to be the cause of chemical variations. The variations are demonstrated from thin-section to map scale and together with anomalous biotites in rocks of varied type and pre-mineral age, argue for a unique fluid origin, one that chemically modifies the original magmatic biotite crystals and possibly forms new ones. The areas of anomalous biotite cover multiple square kilometers, appear to be of metasomatic origin, and, therefore, suggest a remarkable degree and extent of rock permeability and intimate fluid-mineral interaction. The same chemical variations in biotite presented here, to varying degrees, have also been documented surrounding other porphyry copper deposits in the American Cordillera. In largely unaltered rocks outward from visible hydrothermal alteration, the anomalous composition biotite suggests a novel exploration tool for detecting the proximity of buried or covered porphyry copper deposits.