9418301 Einaudi The link between acid-sulfate alteration and gold deposition in the epithermal environment, and between epithermal- and porphyry-type alteration-mineralization will be studied in the Potrerillos district, Chile. Documentation of porphyry- epithermal transitions is difficult because most districts lack exposure of the deep plutonic source region. The district was chosen because of excellent exposures of these many ore-types and because in many localities they are superimposed, allowing determination of relative age. Absolute timing of igneous and hydrothermal events will be documented by high precision Ar-Ar age dating in the context of mapped cross-cutting relations. Geochemical factors affecting fluid evolution and the partitioning of Cu and Au between different environments, will be assessed by detailed field mapping and core logging, and by petrographic, analytical, and fluid inclusion studies in conjunction with data on phase equilibria. Deposits known as "Cordilleran base-metal lodes", are superimposed on, and genetically linked to, porphyry copper deposits. Less well known are the analogous lode deposits, also porphyry-related, that occur in carbonate wall rocks. This deposit type, and its link to proximal skarn and to distal quartz-pyrite-gold replacement deposits will be studies in the Superior district, Arizona by similar methods. Results of this research will contribute to the understanding of sources and physiochemical evolution of crustal fluids linked to subvolcanic environments.
