Gold metallogeny of Chile; an introduction

Abstract

The Chilean Andes attained the status of a major gold province as a result of exploration successes in the late 1970s and early 1980s. The 18 principal deposits, each with > or = 10 metric tons of gold, contain > 1,000 metric tons of gold. The principal deposits are mainly of epithermal and porphyry type, with one major pluton-related vein and one distal contact metasomatic deposit. At least half the gold deposits are observed or reasonably inferred to be parts of porphyry systems. High sulfidation (acid sulfate) epithermal deposits are located above and low sulfidation (adularia-sericite) epithermal and contact metasomatic deposits around porphyry-type mineralization, which itself may be either rich or poor in gold.High sulfidation deposits dominate the epithermal category and include a broad spectrum of mineralization styles. These range from vein systems through hydrothermal breccias to complex combinations of stockworks, disseminated zones, breccias, and/or veins. Low sulfidation deposits are generally smaller than those of high sulfidation type and consist mainly of veins. Porphyry-type deposits, including newly recognized gold-only examples at Marte and Lobo, accompany quartz stockworks developed in dioritic to quartz dioritic porphyry stocks and their andesitic wall rocks. Gold + or - copper is introduced with K silicate alteration, which is commonly overprinted by intermediate argillic assemblages.Chilean gold deposits are parts of volcanoplutonic arcs that were generated from the Mesozoic to the Miocene, although most are younger than the mid-Cretaceous transition from an extensional to a compressional stress regime at the central Andean convergent margin. The high sulfidation and porphyry-type gold deposits are mainly Miocene in age. Most Miocene and some older gold deposits are associated with stratovolcanoes and/or dome complexes, but major Cenozoic ash-flow calderas appear to be poorly mineralized.Paucity of gold deposits in the coastal zone of Chile, concentration of high sulfidation and porphyry-type deposits in the Andean Cordillera near or above 4,000 m above sea level, and presence of low sulfidation deposits farther west than those of high sulfidation type may all be explained in terms of a progressive west to east decrease in depth of erosion as well as of mineralization age. Miocene gold deposits postdate much or all of the main episode of supergene copper and silver enrichment in northern and central Chile which took place during the Oligocene and early Miocene and, although in part extensively oxidized, underwent no gold enrichment and only minor, if any, silver enrichment. In contrast, gossans developed in the upper parts of gold veins in the coastal zone of northern Chile are products of this major supergene event and appear to have undergone additional surficial gold enrichment by chloride brine movement during the last 15 m.y. of hyperaridity.The Andean Cordillera of Chile is the world9s premier high sulfidation epithermal province. During the generation of the province in the Miocene, low sulfidation epithermal silver deposits generally deficient in gold were forming in southern Peru and southern Bolivia in association with more felsic igneous suites, which are richer in potash (alkali calcic), and in the tin-silver belt of southern Bolivia, are more reduced (ilmenite series).