Geology of the El Abra porphyry copper deposit, Chile

Abstract

The El Abra porphyry copper deposit, located 42 km north of Chuquicamata, was mined on a small scale in the past, and several studies of the deposit were carried out prior to 1972. From 1972 to 1975, the Corporacion Nacional del Cobre de Chile completed an evaluation program known as the El Abra Project.In common with several other major porphyry copper deposits in the region, El Abra is related to a regional structure known as the West Fissure and is emplaced in Tertiary intrusive rocks of intermediate composition, which in turn form part of an intrusive and volcanic complex, the oldest members of which are Jurassic in age. The orebody is controlled by a west-northwest structural trend which continues to the east where it is related to other minor hydrothermal occurrences.The host rocks for the mineralization range from diorite to syenite. Late in the magmatic cycle, these rocks were altered, first by a period of K-feldspar-quartz addition and later by a period of strong biotization. The intrusive complex, called the El Abra diorite, is intruded by several dacitic necks which produced a remobilization of potassic alteration, developing biotite breccias on their tops. Age relations are obscured by early alteration stages, but it can be concluded that El Abra primary mineralization was almost synchronic with the Chuquicamata orebody. Primary mineralization related to potassic alterations exhibit high copper/iron ratios, with increasing iron in time. The dacitic porphyries have their own zoning pattern.Hydrothermal alteration and mineralization is found inside the deposit as bands controlled by structures and as discontinuous occurrences in fringe zones. There, quartz veins with sericitic alteration in host rocks show a pyrite-rich assemblage, but only the roots of the phyllic zone are now exposed. Argillic and supergene alterations coexist with phyllic alteration in the same structures, being hydrothermal events obscured by this superimposition. The lack of pyrite and the basicity of host rocks had an unfavorable effect on supergene leaching and enrichment. However, an important oxidation blanket which has an area of one square kilometer covers primary sulfides. The dominant oxide mineral is chrysocolla.Alteration and mineralization at El Abra has been interpreted as a deep exposure of a porphyry copper due to erosion of upper zones. A multiple intrusion-alteration-mineralization model is proposed, which is essentially the classical model repeated under slightly different conditions during time. The deep exposure of the orebody in a porphyry copper column in this deposit approaches a magmatic environment with the geometric patterns deformed by premineral structural control and by the complexity of the intrusion-alteration history.