Geology of the El Teniente orebody with emphasis on wall-rock alteration

This paper discusses new geological information on El Teniente which has become available since the publication of Howell and Molloy (1960) and reviews our present knowledge of wall-rock alteration and sulfide distribution.The regional geology is characterized by a thick section of premineral volcanic rocks with intercalations of continental sediments which are intruded by felsic to intermediate intrusive bodies. The age of these rocks varies between Upper Cretaceous and Quaternary, and they have been separated into two stratigraphic units. The older unit is the intensely folded Coya-Machali formation of Upper Cretaceous age. Overlying this and located above an angular unconformity is the Farellones formation of lower Tertiary age which constitutes the main host rock of the orebody. Quaternary laharic breccias and andesitic flows occur locally above the Farellones unit.In the mine area, these two formations are intruded by a quartz-diorite-dacite intrusive complex which is related to the alteration-mineralization of the area. This igneous complex forms part of an 80-km-long intrusive belt distributed along a N5 degrees W trend. The mineralized area is cut by the Braden formation, a brecciated circular inverted cone surrounded by a 20- to 40-meter-thick brecciated belt.Three hypogene alteration phases and one supergene phase have been recognized in the orebody. The hypogene phases are potassic, quartz-sericitic, and propylitic. The supergene phase is mainly argillic.Potassic alteration is defined by the presence of secondary biotite, K-feldspar, anhydrite, and lesser amounts of quartz, sericite, carbonates, rutile, and apatite. The quartzsericite alteration is denned by the assemblage quartz, sericite, and pyrite with minor anhydrite, tourmaline, calcite, and traces of pyrophyllite. The propylitic phase is defined by the association of chlorite, epidote, calcite, magnetite, pyrite, and minor amounts of sericite, quartz, and anhydrite. The supergene argillic alteration is recognized by the presence of kaolinite with lesser montmorillonite and alunite.Hypogene mineralization forms a zonal pattern with bornite in the central part surrounded by a zone of chalcopyrite with lesser pyrite passing abruptly to a marginal zone with only pyrite. Within the outer rim of the Braden formation is a zone of late-stage mineralization composed of tennantite-tetrahedrite, pyrite, gypsum, and ankerite. The latest feature in the history of formation of the deposit is a stage of secondary enrichment. This secondary process, besides producing an important increase in copper grade and the development of an argillic alteration phase, caused anhydrite leaching the lower limit of which defines the base of the supergene enrichment zone and the top of the primary zone.On the basis of the new information, a genetic model for El Teniente is presented which includes an explanation of intrusion and late-magmatic hydrothermal processes.

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