Geologic evolution of the Escondida area, northern Chile: A model for spatial and temporal localization of porphyry Cu mineralization

A program of geologic mapping and lithogeochemical and geochronological sam pling has been carried out over a 745-km(2) area of the Atacama Desert surr ounding the porphyry Cu deposits at Escondida, Zaldivar, and Chimborazo (Co rdillera de Domeyko, northern Chile). Tile pur-pose of this study was to ex amine tile regional tectonic and magmatic setting of this preeminent porphy ry Cu district for evidence of features or processes that might explain the giant scale of mineralization at Escondida and provide predictive tools fo r exploration in other areas. Tile geologic history of this area as recorded by exposed rocks begins with voluminous, intermediate to felsic Permo-Carboniferous volcanism (La Table Formation), and these rocks appear to constitute the crystalline basement throughout much of the porphyry belt of nor-them Chile. Geochemically: they are I-type in character, but the parental magmas were relatively dry, and thus did not generate effective magmatic-hydrothermal systems (few signific ant ore deposits are known to be associated with them). Andean cycle are magmatism began in the Triassic, centered on the La Negra magmatic are (now located near tile Chilean coast). Farther inland, near Es condida, back-are processes led to the eruption of intermediate to felsic l avas and tuffs and the deposition of marine sediments in rift basins. Closu re of these basins in the Late Cretaceous resulted in deformation of tile v olcano-sedimentary sequences and was followed by emplacement of small alkal i gabbro stocks and dikes. The axis of arc magmatism moved eastward in the Paleocene (Central Valley a re) and produced widespread calc-alkaline intermediate to felsic volcanism through to the Eocene. East- to northeast-directed convergence maintained a dextral transpressive regime during this period, and early movements in th e West Fissure zone, a corridor of orogen-parallel faults that runs the len gth of the Cordillera de Domeyko (over 1,000 km), reflect this couple. At t ile end of the Eocene, however, stresses in tile are appear to have relaxed , and by the late Oligocene, strike-slip movement along tile West Fissure z one had reversed to sinistral. This period of stress relaxation at the end of the Eocene period coincided with the voluminous emplacement of dioritic magmas at shallow crustal levels and also with porphyry development, Six samples of hornblende from these diorites field Ar-40/Ar-39 dates betwe en 38.28 +/- 0.32 and 36.94 +/- 0.46 Ma (2 sigma). Porphyry emplacement at Escondida, Zaldivar, and Chimborazo was coeval with this dioritic magmatism at similar to 38 Ma. Where plutonism was intense, the dioritic magma is in terpreted to have evolved by processes of assimilation and fractional cryst allization to more felsic compositions characteristic of the ore-forming po rphyry intrusions. Whole-rock trace element data indicate that hornblende f ractionation was an important control on chemical evolution of the diorites and attests to high-magmatic water contents (less than or equal to4 wt % H 2O). Volatile saturation would have occurred during further differentiation of these magmas, evidence for which is provided by tile porphyry ore depos its, Porphyry emplacement was localized within a broad zone of intersection betw een die West Fissure zone and a regionally extensive northwest-trending str uctural corridor (the Archibarca lineament). It is proposed that the geomet ry of this junction was conducive to tile formation of transtensional pull- apart structures during relaxation or reversal of dextral shear on the West Fissure zone. Such dilational structures would have focused the ascent and pooling of magma in tile upper crust and maximized de potential for format ion of magmatic-hydrothermal ore deposits. The formation of giant porphyry systems such as Escondida is, therefore, co nsidered to be the result of a fortuitous coincidence of processes, includi ng generation of suitable volumes and compositions of magma, appropriate li thospheric stress conditions, and structural focusing of emplacement; in ad dition, the development of thick supergene enrichment blankets has been cri tical to the economic value of these deposits, None of these contributory p rocesses are in themselves unusual or rare, but because they are largely in dependent of one another, their constructive cooperation in ore formation i s not necessarily repeatable at different places and at different times, th us explaining the relative rarity of giant porphyry deposits.