Geology, Mineralization, Alteration, and Structural Evolution of the El Teniente Porphyry Cu-Mo Deposit

Abstract

El Teniente is a typical porphyry Cu-Mo deposit-in terms of its alteration and sulfide assemblage zonation, association with felsic intrusions, and predominance of quartz vein-hosted copper mineralization. It is anomalous in size, with >94 million metric tons (Mt) of contained fine copper making it the worlds largest known porphyry Cu deposit. There is an intimate spatial and temporal association between all stages of mineralization and latest Miocene to early Pliocene felsic intrusions at Teniente. Most of the copper was emplaced during the late magmatic stage (5.9-4.9 Ma), contemporaneously with intrusion of the dacite porphyry dike and dacite pipes into a mafic to intermediate sill-stock complex. Mineralization of the late magmatic stage is mainly hosted by a quartz-anhydrite-dominated stockwork associated with K-feldspar alteration in the dacites and Na-K-feldspar, biotite, and propylitic alteration of the mafic intrusive package. Minor copper-mineralized hydrothermal biotite-cemented breccias formed at this time. The late magmatic stage was followed by two stages of mineralized phyllic alteration, referred to as the principal hydrothermal (4.9-4.8 Ma) and late hydrothermal (4.8-4.4 Ma) stages, during which thicker, Cu-rich veins were emplaced. A 1,200-m-wide breccia pipe, the Braden Breccia, formed during the late hydrothermal stage and appears to have destroyed a large amount of ore from the center of the deposit. The late magmatic and principal hydrothermal vein stages have predominantly concentric and radial vein orientations centered on the Braden Pipe. Most of the concentric veins are shallowly dipping, whereas the radial veins are subvertical. We present a model in which vein distributions were controlled by the local stress regime generated by the intrusion of a large, deep magma chamber that is interpreted to be the source of the dacites, the Braden Pipe, and ultimately, the copper and molybdenum mineralization. The late hydrothermal veins are steeply inward dipping and concentric to the Braden Pipe. In contrast to the late magmatic and principal hydrothermal vein stages, radial veins and shallow-dipping concentric veins are rare, consistent with formation during a stage of subsidence due to relaxation of intrusion-induced stresses. Resurgence of the magma chamber reactivated the steep concentric structures in a reverse sense, and a build up of magmatic and/or fluid pressure resulted in explosive brecciation and fluidization, producing the Braden Pipe. A predominantly late set of northeast-trending faults, associated with movements on the district-scale Teniente fault zone, is the only evidence for far-field stresses exceeding local stresses in the deposit.