Geology of the Josemaría Porphyry Copper-Gold Deposit, Argentina: Formation, Exhumation, and Burial in Two Million Years

Abstract

The Josemaría porphyry copper-gold deposit is located in the Frontal Cordillera of San Juan Province, Argentina, near the present-day northern limit of the Chilean-Pampean flat-slab segment of the central Andes, and midway between the Maricunga and El Indio metallogenic belts. The deposit is centered on small, multiphase dacite porphyry intrusions that were emplaced at the contact between rhyolitic volcanic and tonalitic plutonic rocks of Late Permian to Triassic age. The earlier, more intensely quartz ± magnetite-veined porphyry phases and contiguous wall rocks display a telescoped sequence of alteration-mineralization zones, from shallow advanced argillic (mainly quartz-pyrophyllite) and underlying sericitic to deeper chlorite-sericite and minor remnant potassic. All the alteration types are mineralized, but the highest copper and gold grades are present as a low-arsenic, high-sulfidation assemblage in the quartz-pyrophyllite and sericitic zones. The outermost parts of the copper-gold zone are overlapped by a pronounced molybdenum-bearing annulus. New U-Pb zircon ages show that the deposit was formed at ~25 to 24.5 Ma, partially unroofed during continued NNE-striking, high-angle reverse faulting, and then unconformably overlain by red-bed conglomerate and sandstone capped by andesitic and dacitic tuff and lava. The andesite reported an age of ~22.35 Ma. A second, discrete pulse of currently undated, advanced argillic alteration, accompanied by minor high-sulfidation enargite mineralization, locally affected the southern periphery of the deposit, including its postmineral cover. Following erosional removal of the volcano-sedimentary strata from the northern and central parts of the deposit, the NNE-trending fault zone underwent minor normal displacement and localized economically significant supergene chalcocite enrichment. However, probably because of the rapidity of deposit unroofing, supergene processes were barely able to keep pace with erosion, resulting in a thin supergene profile over much of the exposed deposit. The southern part of the deposit remains beneath the postmineral cover and, hence, escaped the enrichment. Josemaría is unusual among the many central Andean porphyry copper deposits formed during rapid uplift because it preserves evidence for not only alteration-mineralization telescoping but also exceptionally rapid postmineral exhumation and subsequent burial beneath thick volcano-sedimentary cover. Unroofing of porphyry copper deposits in 1 to 2 m.y. is more typical of the high erosion rates that characterize pluvial tropical climates than the semiarid conditions that prevailed during and since the formation of Josemaría.