Iron oxide-copper-gold deposits of the Mantoverde area, northern Chile: Ore genesis and exploration guidelines.

摘要

The Mantoverde area, located in the III Region of northern Chile from latitudes 22° to 27°S, hosts numerous Lower Cretaceous iron oxide-Cu-(Au) prospects and deposits assigned to the "iron oxide-copper-gold" (IOCG) clan. They range from Cu-poor bodies of magnetite-apatite-actinolite (e.g. , Carmen) to chalcopyrite-rich, hematite-cemented breccias and hematite veins (e.g., Mantoverde, Cerro Negro and Palmira). The most important Cu-Au deposits are associated with the main or, more commonly, subsidiary structures of the plate boundary-parallel Atacama Fault System, and are hosted by Middle to Upper Jurassic andesites and Lower Cretaceous quartz dioritic plutons. In the Mantoverde area, the rocks exhibit a sequence of alteration stages, involving early spilitization and hydrolysis, both followed by low-grade metamorphism to the lowermost greenschist facies. These background assemblages were overprinted by four stages associated with IOCG-type alteration and mineralization (i.e., Stages I to IV).; The delta34S and delta18O values of hydrothermal minerals in the Mantoverde deposit and its satellites indicate that the incursion of externally-derived fluids (i.e., seawater or evaporite-derived brines) was a prerequisite for Cu (-Au) mineralization, a previously unrecognized feature in the Mantoverde district. Supporting this genetic model, the trace-element compositions of hypogene pyrite and chalcopyrite, determined by laser ablation-ICP-MS, indicate that the chemistry of the ore-fluids changed during the evolution of the hydrothermal system.; Exploration for IOCG-type deposits in calc-alkaline volcano-plutonic terrains is facilitated by the integration of molar element ratio analysis and whole-rock oxygen isotope geochemistry. This method differentiates the least altered volcanic and plutonic rocks, from moderately-altered volcanic affected by regional spilitization and low-grade metamorphism. A third group comprises strongly K-feldspathized, Fe-metasomatized and chloritized rocks localized in mineralized centres. No recognizable alteration haloes are associated with the low-temperature Cu-(Au) ore stage, but the hydrolytically-altered rocks preceding it have distinctive lithogeochemical and isotopic signatures recording equilibrium with non-magmatic fluids. These parameters can be used to vector onto economic IOCG type mineralization in Andean, calcalkaline terrains.; Comparison of the compositions of outcrop samples and lag talus composite samples indicates that, in this now hyperarid terrain, the latter constitute a reliable and effective sampling medium in regional exploration programs for "Mantoverde" type IOCG deposits.