Magmatic-Hydrothermal Evolution of the Pampa Escondida Porphyry Copper Deposit Northern, Chile

Abstract

The Pampa Escondida porphyry copper deposit (Pampa) of northern Chile records the dynamic relationship between magmatism and hydrothermal activity that occurred between 37.6±0.5 Ma and 28.4±2.9 Ma Ma in the central portion of the Escondida district, the largest currently operating copper mine in the world. Five distinct magmatic-hydrothermal cycles can be distinguished during the genesis of the 7,378 million metric ton (at 0.47 % copper) ore body. The focus of this study is to describe the magmatic-hydrothermal evolution of Pampa, underscore key evolutionary moments that conflict with simplified genetic models for porphyry copper deposits (PCDs), and propone the development of descriptive models that incorporate multiple absolute and relative timing data sets at individual PCDs. The goals of this study are: 1) to map (through 1:250 scale logging of diamond drill core) and describe all Eocene intrusive phases related to the development of the Pampa Escondida deposit; 2) to record stable mineral assemblages present in observed veins and then classify veins based on their stable assemblages; 3) to constrain the multi-stage formation of the mineralogically complex and early-formed transitional potassic-advanced argillic-sericite alteration associated with green sericite (GREEN SER) veinlets; 4) to propose genetic mechanisms for hydrothermal corundum that occurs in intense hypogene potassic alteration; 5) to improve understanding of the precise nature of gold mineralization and define the diversity of hydrothermal alteration with which it is associated ; 6) to interpret spatial concentration of and cross cutting relationships among vein types to infer their association with the major intrusive phases; 7) to obtain lowtemperature thermochronologic ages to complement U-Pb in zircon ages of previous studies; and 8) to synthesize these data into a genetic framework for the evolution of Pampa. Results from field data indicate that at least sixteen distinct classes of hydrothermal veins can be macroscopically distinguished. Petrographic, mineral chemistry, and fluid inclusion 16 microthermometric data show that multiple classes of hydrothermal veins formed from dynamic, multi-stage processes that spanned at least several magmatic-hydrothermal cycles. GREEN SER veinlets are a prime example of a vein class that formed through such a multi-stage process. The alteration halos of GREEN SER veinlets display several distinct stable silicate assemblages that record equilibrium with both the liquid and vapor phases of high-salinity aqueous fluids of magmatic origin. Synthesizing these data show that hydrothermal alteration related to the five magmatichydrothermal cycles at Pampa is expressed as four potassic-hydrolytic couplets and as a single, terminal volatile-rich pulse of alteration, which produced a series of internally zoned phreatic breccias. The bulk mass of Cu-Fe sulfides was deposited in the potassic and hydrolytic phases of the second and third magmatic-hydrothermal cycles; most of the gold is associated with bornite in four of the five vein classes that are associated with the potassic phases of the second and third cycles; and most of the molybdenite was deposited during the transition from potassic to hydrolytic phases and during the hydrolytic phase of the fourth cycle. These temporal patterns of metal introduction resemble those observed at many continental and continental arc porphyry copper and porphyry copper-gold deposits.