REGIONAL GEOLOGIC SETTING OF EPITHERMAL GOLD DEPOSITS, CHILE

Epithermal gold deposits in Chile are Cretaceous to Cenozoic in age. The ores and the volcanic rocks with which they are associated were generated at a noncollisional, ocean-continent convergent plate margin. They are related to long, slightly sinuous north-northeast-trending magmatic belts roughly parallel to the Pacific coast. Most of the deposits are associated with eroded volcanic centers and subvolcanic porphyries. North-south variations in the nature, extent, and timing of magmatism control the succession of mineralization events. In the Late Cretaceous, vein districts (e.g., El Bronce) and hydrothermal alteration zones were formed in central Chile. Paleocene-Eocene volcanic rocks constitute a large, diffuse volcanic field in the Antofagasta region (21-degrees-26-degrees-S), which contains epithermal precious metal and porphyry copper deposits associated with coeval extrusive rocks (e.g., Faride, San Cristobal, Guanaco, and Cachinal de la Sierra). In the late Eocene-early Oligocene, volcanism diminished and finally ceased in northern Chile, probably as a consequence of highly oblique plate convergence. Oligocene magmatism was restricted to emplacement of porphyries carrying copper deposits along transtensional zones within a strike-slip fault system. One of these deposits (Potrerillos), hosted by sedimentary rocks, developed epithermal gold mineralization in its cupola (El Hueso). The late Oligocene to mid-Miocene magmatic province of north-central Chile (26-degrees-31-degrees-S) coincides approximately with the modern, nonvolcanic, flat slab segment of the Chilean Andes and hosts the largest number of known epithermal developments. Deposits such as El Indio-Tambo, Nevada, La Pepa, Lobo, Marte, and La Coipa are hosted by Oligocene-Miocene volcanics. The deposits are associated with large zones of argillic and advanced argillic alteration, in some places related to shallow-level stocks and flow domes. A younger event of alteration-mineralization is recognized in the eroded cores of middle Miocene composite volcanoes, which cap some of the older altered areas (e.g., Refugio, La Laguna). The main alteration event began at 23 Ma, continued after a major deformation event initiated at 18 to 16 Ma, and ended at 11 to 8 Ma. The precious metal deposits and mineralized zones of the Oligo-Miocene province were exposed by erosion after the cessation or eastward shift of volcanism in the late Miocene, which was probably due to shallowing of the subduction zone between 26-degrees-30 and 33-degrees-S. Late Miocene to Holocene magmatism in Chile is present in two segments beneath which the dip of the Benioff zone remains steep: north of 27-degrees-S (Central Volcanic zone, CVZ) and south of 33-degrees-S (Southern Volcanic zone, SVZ). The Central is characterized by widespread Neogene hydrothermal alteration zones and native sulfur deposits associated with composite volcanoes, calderas, subvolcanic intrusions, and active geothermal fields which are superimposed on the Oligocene-Miocene volcanic belt. These features are believed to represent a very shallowly eroded epithermal environment. In this setting the late Miocene Choquelimpie deposit is located in the exposed core of a volcano.