El Tesoro Exotic Copper Deposit, Antofagasta Region, Northern Chile
Authors: Mora Rodrigo , Artal Jorge , Brockway Humberto , Martínez Edmundo , Muhr Ricardo 
Published: February 2020

The El Tesoro exotic copper deposit is located approximately 70 km south of Chuquicamata, northern Chile. Prior to the involvement of Anaconda Chile S.A. (Anaconda) in 1990, the area was known historically as a group of small copper occurrences, where small-scale mining had taken place intermittently since 1886. Between 1980 and 1990, exploration programs undertaken by several companies, including Anaconda, targeted the source of the exotic copper mineralization. In 1990, a renewed exploration effort by Anaconda, originally designed to outline a small oxide copper resource around the old workings at El Tesoro, was quickly expanded after intersecting significant ore-grade mineralization in the first two drill holes. Follow-up exploration and delineation, carried out in several stages between 1990 and 1995, included 54,500 m of reverse-circulation drilling at El Tesoro. Discovery of 206 million metric tons (Mt) of 0.86 percent Cu was the direct result of geologic mapping and modeling, strongly supported by company management. Mining of the El Tesoro deposit by Compaúía Minera El Tesoro (61% Antofagasta Minerals S.A., 39% Compañía Contractual Minera Leonor) commenced in November 2001, with current production amounting to ~85,000 t of copper cathode per year. With the present mining schedule of 25,000 t/d, mine life is estimated to be 19 yrs.

Regional geology is characterized by a late Paleozoic intrusive and volcanic basement and several sedimentary and volcanic sequences of Mesozoic age, which are locally intruded by Cretaceous granitoids. An early Tertiary volcanic sequence unconformably overlies the Mesozoic units. A series of flow-dome complexes of middle to late Eocene age is the youngest bedrock component. Local cover comprises a thick sequence of moderately consolidated gravel of middle to late Tertiary age, which is blanketed by a 10-Ma ignimbrite. A locally important north-northeast-trending fault zone, part of the regionally extensive Domeyko fault system, transects the district and offsets all units, including a post-10-Ma pediment surface. These faults display both strike-slip and reverse motion and were accompanied by minor folding of the gravel sequence.

The El Tesoro deposit is completely contained in the Tertiary gravel sequence. It is composed of two main, northeast-trending, northwest-dipping mantos with a maximum thickness of 150 m and an areal extension of 2 × 2.5 km. The host gravel is dominated by crudely stratified, pebbly to blocky horizons interbedded with coarse-grained sand and silt, some of which are calcareous. Clast composition is varied and allows definition of two main units, namely Gravas I and Gravas IL The basal Gravas I sequence includes several subunits of which the Gravas de Pórfido is the main host to the exotic copper mineralization. The Gravas II is essentially postmineralization in timing. Mineralization is dominated by paratacamite and atacamite, accompanied by subordinate chrysocolla and copper wad. These mineral(oid)s occur as cement to the gravel, the clasts of which were not mineralized, although very locally they contain encapsulated chalcocite and copper oxide remnants. Mineralization is clearly controlled by original host porosity and permeability, with silty horizons generally being barren. A copper-depleted, hematitic ferricrete horizon that is locally interbedded with gravels in stratigraphically higher parts of the El Tesoro deposit correlates laterally with nearby exposures of Gravas Rojas. Broad ore mineral zoning comprises a central zone of paratacamite and atacamite concentrated in upper horizons and a zone of chrysocolla that dominates the lower parts of the deposit. Copper wad is common and is irregularly distributed, although it generally tends to occur along the fringes of individual mineralized lenses.

El Tesoro is a classic exotic copper deposit formed by lateral migratión of copper-charged solutions and subsequent deposition of copper species in lithologically favorable alluvial gravel horizons. Its dominant mineralogy and mineralogic zoning, as well as the nature of the host gravel, suggest that copper precipitation took place in a distal environment, located perhaps several kilometers from a porphyry-related chalcocite enrichment source area.