THE EL INDIO-PASCUA AU BELT, CHILE/ARGENTINA: REGIONAL AND LOCAL CONTROL OF EPITHERMAL MINERALIZATION BY PEDIMENT INCISION

Major, largely high-sulfidation, epithermal Au (-Ag, Cu) mineralization in the Central Andes between Lats. 29°15' and 30°S was entirely associated with small volumes of dacitic magma emplaced at 6-9.5 Ma immediately prior to cessation of normal arc magmatism above a flattening slab, and following several episodes of barren hydrothermal activity. Economic deposits formed uniquely beneath the Middle Miocene (12.5-14 Ma) Azufreras-Torta Pediplain, but during its degradation by 6-10 Ma Los Ríos pediment-valleys. All major clusters of ore deposits are located on the margins of regional fault corridors which controlled the Late Miocene planar erosion, and within 1 km of the backscarps of the young pediments. Moreover, progressive incision of the pediments was accompanied by a headward migration of the hydrothermal focus. Thus, the 9.4 Ma Lama prospect is situated 1-2 km E of, and "downstream" from, the 8.6 Ma main-Pascua orebody, and the subeconomic 9.6 Ma Vacas Heladas prospect was emplaced 6 km downstream from the 8-8.7 Ma Tambo deposit cluster.

The deposits are inferred to have formed in direct response to changes in the hydrodynamic regime induced by pediment incision into previously existing planar landforms. Precipitation of Au, Ag, and Cu from the mineralizing fluids was optimized adjacent to the backscarps of incising valley-pediments, through boiling due to rapid lowering of the water-table, and/or mixing with meteoric water due to increased downward-lateral flow of groundwater. This concept may be applicable to other epithermal districts in semi-arid cordilleran environments where hypogene mineralization is not associated with large-volume volcanism.