Linkages between volcanotectonic setting, ore-fluid compositions and epithermal precious metal deposits

Epithermal Au and Ag deposits of both vein and bulk-tonnage styles may be broadly grouped into high-, intermediate-, and low-sulfidation types based on the sulfidation states of their hypogene sulfide assem-blages. The high- and low-sulfidation types may be subdivided using additional parameters, particularly related igneous rock types and metal content. Most high-sulfidation deposits are generated in calc-alkaline andesitic-dacitic arcs characterized by near-neutral stress states or mild extension, although a few major deposits also occur in compressive arcs charac-terized by the suppression of volcanic activity. Rhyolitic rocks generally lack appreciable high-sulfidation deposits. Highly acidic fluids produced the advanced argillic lithocaps that presage high-sulfidation miner-alization, which itself is due to higher pH, moderate- to low-salinity fluids. Similar lithocaps in the Bolivian Sn-Ag belt, some mineralized with Ag and Sn, accompany reduced, ilmenite series magmatism. Intermediate-sulfidation epithermal deposits occur in a broadly similar spectrum of andesitic-dacitic arcs but commonly do not show such a close connection with porphyry Cu deposits as do many of the high-sulfidation deposits. However, igneous rocks as silicic as rhyolite are related to a few intermediate-sulfidation deposits. These deposits form from fluids spanning broadly the same salinity range as those responsible fo