Multistage intrusion, brecciation, and veining at El Teniente,Chile: evolution of a nested porphyry system

The El Teniente copper-molybdenum deposit is hosted by the late Miocene Teniente Mafic Complex, a largely subvolcanic package of primarily basaltic-andesite porphyry sills and stocks that were emplaced within the mid-late Miocene Farellones Formation. In the late Miocene-Pliocene, a series of intermediate felsic plutons were intruded into the Teniente Mafic Complex. These are spatially associated with magmatichydrothermal breccias and multiple vein types that form individual mineralized complexes. Here we present detailed observations on mineralogy, textures, and intrusion, breccias, and vein crosscutting relationships to constrain the nature and relative timing of magmatic and hydrothermal events. Our revised classification defines 13 vein types, divided into three main stages: (1) premineralization biotite and/or K-feldspar ± quartz-anhydrite-albite-magnetite-actinolite-epidote veins that formed prior to emplacement of mineralized intrusions and breccias; (2) main mineralization stage veins that grade from gangue-dominated quartz-anhydrite veins ± potassic alteration halos into sulfide-dominated veins with phyllic alteration halos; and (3) late mineralization veins containing sulfosalts. Five breccia types have been observed in the deposit, typically forming individual, vertically zoned complexes, spatially associated with individual intrusions and overlapping in time: (1) igneous-cemented breccias, (2) K-feldspar?cemented breccias, (3) biotite-cemented breccias, (4) anhydrite-cemented breccias, and (5) tourmaline-cemented breccias. Breccia cements display a similar paragenetic evolution to main mineralization stage veins, indicating a close genetic link between them. A distinction can be made between early premineralization vein types (types 1-2) that represent veins formed, possibly deposit-wide, prior to emplacement of intrusion-breccia complexes, late pre- and main mineralization vein types (types 3-8), which are interpreted to reflect the repeated cycle of fluid release associated with each mineralized intrusive complex, and late mineralization vein types (types 9-10) that represent a single event linked to the emplacement of the Braden Breccia Pipe. The geologic evidence indicates a close spatial and temporal relationship between emplacement of shallow level, felsic-intermediate pipelike intrusions and the development of igneous and mineralized magmatichydrothermal breccias and vein halos. The magmatic-hydrothermal transitions observed in these complexes indicate that the deposit formed from a series of localized pulses of magmatic-hydrothermal activity which followed rather similar evolution paths. Single, deposit-wide models of fluid evolution and mineralization are therefore inappropriate. We conclude that El Teniente represents a nested but otherwise rather typical porphyry Cu-Mo system, unusual only in that the Teniente Mafic Complex provided a particularly efficient physical trap in terms of pervasive fracturing during intrusion of magmatic-hydrothermal breccia complexes and as an effective chemical trap for deposition of sulfides. The overlapping of mineralized envelopes from successive fertile intrusions and the absence of barren, intermineral porphyries resulted in its unusual size and high hypogene grades. © 2010 Society of Economic Geologists, Inc.