Alteration and Mineralization of the Deep Extension of the Giant Chuquicamata Porphyry Copper Deposit, Northern Chile
Abstract
Porphyry copper deposits are the major source of copper and molybdenum in the world. The deposits consist of large volumes of rock altered by hydrothermal fluids released by shallow, intermediate composition magma chambers, with metallic mineralization present in veins, breccias, and disseminations; ore grades are typically 0.3-1.0 wt.% Cu. The subject of this study, the Chuquicamata porphyry copper deposit in northern Chile, is one of the most productive such deposits in the world, with over 45 million tons of Cu produced since large scale, open pit mining started early in the XXth century. Current production at Chuquicamata is from an underground, block caving operation that will extend the life of the mine for at least 40 years.CLike most world-class porphyry deposits, the Chuquicamata Cu-Mo deposit in northern Chile had a complex evolution. The deposit evolved through the superimposition of magmatic and hydrothermal stages structurally controlled by a large, regional-scale fault system (Domeyko Fault) and a subordinate, district-scale fault (West Fault). The objective of this study is to better understand the evolution of the deposit, with focus on its underground reserve, by identifying the: i) nature, ii) timing and iii) metal contribution of the stages that formed the deposit. This objective was achieved through: a) use of the comprehensive, time-tested Chuquicamata mine Block Model, b) review of the abundant, existing geochronological data, and c) collection and mineralogical geochemical analysis of a set of representative samples.Excluding a younger, supergene stage that is minimally present in the underground reserve, the evolution of the Chuquicamata deposit can be summarized as follows: A poorly understood stage of pre-Chuquicamata mineralization is hypothesized based on a cluster of secondary biotite and K-feldspar dates between ~34 and 32.8 Ma that have been identified marginal to the core of the deposit. Stage 1 forms a proximal zone of secondary biotite, K-feldspar, albite (potassic alteration), and disseminated chalcopyrite-pyrite, and is dated at 32.1-30.4 Ma . Based on the mine block model, Stage 1 contributes 17 percent of the Cu endowment in the underground reserve at an average grade of 0.46% Cu. Marginal to zone of potassic alteration, an envelope of chlorite ± albite, calcite, and ankerite (propylitic) is estimated to add negligible amounts of copper.- A Stage 2 of fine-grained quartz and green-gray white mica group minerals with chalcopyrite and bornite in veinlets and disseminations overprints Stage 1. This is a high-grade stage (avg. 0.89% Cu) that is estimated to contribute 8 percent of the copper in the deposit; however, this is a minimum number because it is possible that this stage is responsible but unrecognized for part of the copper present in subsequent stages. This stage is enriched in Te.- Stage 3 is characterized by a hydrothermal pulse of quartz-molybdenite veining within the core of the deposit and has been dated at 32.9-31.7 Ma. This stage contributes about 5% of the copper at average grade of 0.51% Cu. This stage is enriched with Mo, Ag, As, Bi, Re, Sb, and Se.- Stage 4: This is the main mineralization stage at Chuquicamata. It is dominated by formation of white mica-quartz (phyllic alteration) and sulfide mineralization of pyrite> chalcopyrite and bornite, plus high sulfidation-state minerals, including digenite, enargite and covellite. A large number of white mica radiometric ages (40Ar39Ar) corresponding to this stage range between 31.8 and 30.5 Ma. Stage 4 is divided into three sub-stages, each with increasing alteration strength and degree of copper mineralization: Stage 4A (weak alteration in haloes), and Stage 4B and 4C (both strong alteration) are estimated to have an average grade of 0.61, 0.72, and 1.26% Cu, and contribute 19, 20, and 30 percent of the copper in the underground reserve. Stage 4C is associated with the highest copper grades and also high concentrations of other elements, including As, Mo, S, Se, Sn, W, In, Fe, K, P, and Co.
Subject Area
Geology|Environmental science|Geochemistry
Recommended Citation
Araya Torres, Claudio Andres, "Alteration and Mineralization of the Deep Extension of the Giant Chuquicamata Porphyry Copper Deposit, Northern Chile" (2023). ETD Collection for University of Texas, El Paso. AAI30818658.
https://scholarworks.utep.edu/dissertations/AAI30818658