Geology, geochronology and structural evolution of La Escondida copper district, Northern Chile

La Escondida district is one of the largest copper concentrations within the late Eocene-early Oligocene porphyry copper metallogenic belt of northern Chile. The district occurs in the north-trending Domeyko Cordillera, an orogenic chain associated with the Domeyko Fault System since the uppermost Late Cretaceous. The results of a 1:25,000-scale geological mapping of La Escondida district have revealed an intricate juxtaposition of Late Paleozoic to Eocene sedimentary, volcanic and intrusive rock packages. These occur in fault-bounded structural panels and their locations can be predicted underneath the post-Eocene unconsolidated alluvial blanket. Previously defined and new geological units define two major tectonostratigraphic cycles in La Escondida region: a Late Carboniferous-Permian to Triassic-age Pre-Andean (Gondwanian) cycle; and Late Triassic-Present Andean cycle. The former includes La Tabla Formation, which resulted from widespread terrestrial volcanism and cogenetic granitoids. Minor, weakly mineralised porphyry copper-type intrusions and Michigan-style copper occurrences are part of this cycle. The Pre-Andean cycle was characterised by caldera complexes, resurgent intrusions (domes and plutons) and fluvio-lacustrine sedimentation. Major and trace element compositions of volcanic and intrusive rocks are consistent with a destructive continental margin setting. Calc-alkaline, dominantly siliceous explosive volcanism and I-type intrusions were emplaced in a mature magmatic arc governed by an extensional regime. The Andean cycle in La Escondida district started with deposition of a mixed calcareous-siliciclastic sequence on the eastern margin of a back-arc basin, between the Late Triassic and Neocomian. An almost 2,000 m thick pile of sedimentary rocks was deposited and subsequently folded and thrusted by the 90-80 Ma Peruvian compressional tectonic phase. Three successive magmatic events occurred from 81 Ma to 34 Ma. These events were characterised by volcanic and intrusive episodes that have geochemical signatures typical of arc and transitional/back-arc settings. During the Late Cretaceous-lowermost early Paleocene, three north- to northeast elongated plutonic complexes were emplaced (81-79 Ma Sombrero, 77-72 Ma Cerro Bayo and 66-64 Ma Torcaza complexes) that range in composition from monzogabbro, monzodiorite to monzogranite and dacite porphyries. Andesitic volcaniclastic rocks of Las Torres igneous complex were deposited between ca. 70 and 66 Ma, within a small tectonovolcanic basin. Effusive volcanism was preceded and probably accompanied by the intrusion of a north-northeast-oriented dyke swarm of monzogabbro, diorite and andesite, between 82-70 Ma. Structural data suggest that these north- to northeast elongated intrusions were emplaced and an extensional to transtensional stress regime. The dykes were deformed by the K-T deformational tectonic phase at ca. 64-62 Ma, although those bodies emplaced early were affected by the Peruvian tectonic movements. The Paleozoic and Mesozoic rocks are unconformably overlain by the late Paleocene-early Eocene Augusta Victoria Formation. This unit consists of extensive ignimbrite flows, coarse-grained volcano-sedimentary rocks and subordinate basaltic andesite and andesite, and was deposited between 57 and 53 Ma. Minor intrusive activity at 60-61 Ma produced andesite dikes and other subvolcanic bodies as a precursor to widespread volcanism. Subsequent E-W contractional deformation produced large-scale open folds with axes that slightly plunge to the north. Folding and associated reverse faulting were followed by dextral and sinistral strike-slip faulting that only produced minor offsets. The middle-late Eocene magmatism started with the intrusion of the 43-41 Ma Cerro Rincones plutonic complex, the largest in La Escondida district. It contains nine phases that range from diorite, through monzodiorite and granodionte, to quartz-monzorute and micrograrute. Although early discrete phases were preferentially emplaced along fold axes in the Augusta Victoria Formation, larger phases display ring-shapes in plane-view. Emplacement of the porphyry Cu-style intrusive complexes at La Escondida and Escondida Norte-Zaldivar deposits occurred between 38 and 34 Ma, and the Domeyko Fault System was a major control on the mineralising intrusions. The kinematic history of its eastern branch, the Escondida master fault, consisted of an early compressional stage (<51-50 Ma), followed by dextral strike-slip transpressional activity (ca. 40-37 Ma) during the emplacement of Escondida Norte-Zaldivar porphyries and later sinistral transpressional movement (> 37 Ma) that controlled the intrusion of La Escondida porphyry (ca. 38-34 Ma). Deformation continued up to ca. 20 Ma, with a minumum average exhumation rate of ca 0.05 km/m.y. Significant extensional and left-lateral transtensional movements occurred on La Escondida master fault at ca. 18-10 Ma (Quechua deformation phase) that produced the triangular pull-apart basin of the Salar de Hamburgo. Dissection of the Hamburgo basin occurred during the early Miocene by NNE-striking faults. The 39-38 Ma Capella stocks, 38-36 San Carlos strata volcaniclastic rocks and microdiorite bodies were respectively emplaced prior to and during mineralisation at La Escondida and Escondida Norte-Zaldivar. A large family of post-mineralisation porphyritic diorite, andesite-diorite porphyry and andesite pyroxene dykes and minor stocks were also intruded between 38 and 35 Ma. Major and trace elements compositions of the Eocene intrusions suggest increasing pressure and depth of magma source from the pre-to post-mineralisation magmatic products. There was a change from a dry magmatic source regime (pre-mineralisation Cerro Rincones and Capella intrusions), through wet magma (syn-to post-mineralisation porphyrite diorite, andesite-diorite porphyry and San Carlos strata), and back to dry magma (post- minerailsation pyroxene andesite porphyry dyke), based on the available geochemical data. Appreciable Late Paleozoic crustal inheritance has been detected during analyses of zircons from the Andean magmatic rocks, revealing a crustal role in the petrogenesis of these predominantly asthenospheric mantle wedge-derived rocks. Two sub-belts of Eocene hydrothermal alteration systems in La Escondida district have been identified during this study: (1) an eastern sub-belt that includes La Escondida and Escondida Norte-Zaldivar deposits and several penpheral mineralised intrusions; and (2) a poorly constrained western sub-belt that consists of advanced argillic altered and silicified rocks, the most significant of which is the Chimborazo high sulfidation epithermal AuCu-(Fe) deposit. Future exploration should be focused along the La Escondida master faults in the eastern sub-belt beneath the extended Late Cenozoic alluvial cover as these faults appears to have localised the most productive mineralised systems in the district.