Porphyry copper and tourmaline breccias at Los Bronces-Rio Blanco, Chile

Abstract

The Los Bronces-Rio Blanco deposit is located on the west side of the Andes in central Chile about 69 km from Santiago. Los Bronces is a breccia complex superimposed on the west side of an earlier major porphyry copper system. The Rio Blanco mine currently exploits the north-central part of this porphyry deposit and has started operating a large copper-bearing tourmaline breccia, Sur-Sur, about 2 km south of the present mine.The Los Bronces-Rio Blanco deposit was formed on the east side of the San Francisco batholith. This intrusion is strongly peraluminous and has a calc-alkaline composition with an alkali-calcic affinity. The batholith took a minimum of 11.5 m.y. to form from the early Miocene (20.1 m.y.) to the late Miocene (8.6 m.y.). The porphyry copper mineralization, alteration, and copper tourmaline breccias were formed over a period of at least 2.5 m.y. between 7.4and 4.9 m.y. ago.A postmineral volcanic neck or diatreme at La Copa erupted within, and removed a large segment of, the northern part of the porphyry copper system, thus marking the last evidence of magmatic activity in the area. K-Ar age determinations of biotites indicate the diatreme erupted in the early Pliocene between 4.9 and 3.9 m.y. ago.The prebreccia porphyry system exhibits propylitic, sericitic, silicic, and potassic alteration. A unique alteration feature of this system is the replacement of mafic minerals by specularitc and/or tourmaline within the propylitic zone. The porphyry system contains disseminated and stockwork copper-iron-molybdenum sulfide mineralization within an area of about 12 km 2 .Los Bronces is composed of at least seven different copper-bearing tourmaline breccias that form one large contiguous kidney-shaped body about 2 km long and 0.7 km wide, at the present erosion surface. The breccia body crops out at elevations between 4,150 and 3,450 m. The various breccias are characterized by their locations, matrices, clasts, shapes, types, and degrees of mineralization and alteration. The breccias are usually monolithic but in some cases are bilithic or heterolithic with most clasts consisting of quartz monzonite or andesitc with locally minor amounts of quartz latite porphyry, monzodiorite, and vein quartz. The breccia matrices consist of variable amounts of quartz, tourmaline, specularitc, anhydrite, pyrite, chalcopyrite, bornitc, molybdenite, sericite, chlorite, and rock flour.The seven different breccias types are identified from oldest to youngest as Ghost, Central, Western, Infiernillo, Anhydrite, Fine Gray, and Donoso. The breccia complex has sharp contacts with the surrounding intrusive rocks and andesites. Internally, the breccia contacts are locally well defined, but elsewhere they coalesce, interfinger, or display gradational contacts. Breccias at Los Bronces are interpreted as being emplaced explosively, followed by collapse after pressure release of hydrothermal fluids.The primary mineral distribution is best known in the Donoso breccia which has been the center of mining activity since its discovery in 1864. In spite of the coarse and irregular nature of the sulfides in the matrix, chalcopyrite, pyrite, and specularitc at the 3,670-m, open-pit operating level show a tendency to be distributed in irregular shells in which one of the three minerals predominates in any one shell. The transitions between shells are rapid. Several semiellipsoidal shells of alternating high and low copper grades are also apparent from the copper distribution of underground level 3640 and from various cross sections. The shells are approximately vertical, subparallel to the Donoso breccia contacts, which dip inward.Secondary enrichment enhanced the primary grade in the southern two-thirds of the Los Bronces breccia complex and in much of the surrounding porphyry copper system. The degree and depth of enrichment are a function of breccia and fracture permeability and extend to a depth of more than 500 m in certain favorable sectors. The shape and depth of the enrichment blanket and overlying leached capping suggest-that the enrichment process is related to the present ground-water regime and is still active.