Geology of Porphyry Cu-Au and Epithermal Cu-Au-Ag Mineralization at Filo del Sol, Argentina-Chile: Extreme Telescoping During Andean Uplift

Abstract

Filo del Sol is a composite porphyry-epithermal deposit, straddling the frontier between Argentina and Chile at latitude 28°29′ S, that has attracted a great deal of recent attention because of several drill intersections in excess of 1 km long with unusually high Cu, Au, and Ag grades. The deposit is part of the 8.5-km-long, N- to NE-trending Filo del Sol alignment of porphyry and high-sulfidation epithermal centers, which, in turn, is located in the newly defined Vicuña metallogenic belt that unites the well-known late Oligocene to middle Miocene Maricunga and El Indio belts. The deposit is hosted by Permian felsic volcanic rocks intruded by Triassic monzogranite and, nearer the surface, by shallowly dipping Late Cretaceous volcano-sedimentary rocks. These lithologic units were intruded by several generations of mafic dikes and high-level sills and, in the middle Miocene, by a parallel swarm of composite diorite porphyry dikes. An intermineral magmatic-hydrothermal breccia body is closely associated with the porphyry dikes and subsequently cut at shallow levels by a smaller, finer-grained breccia of phreatic and, possibly, phreatomagmatic origin. Early Cu-Au mineralization, mainly as chalcopyrite, is associated with K-feldspar- and biotite-bearing potassic alteration and accompanying A-type quartz-veinlet stockworks. The potassic zone was massively overprinted and extensively reconstituted by a zone of vuggy residual quartz and silicification in the core of the deposit, flanked by quartz-alunite as part of a more extensive lithocap. These alteration types host high-sulfidation Cu-Au-Ag mineralization as pyrite with early enargite and later bornite, chalcocite, covellite, and numerous Ag-bearing sulfosalts, with the highest grades typically confined to vuggy residual quartz. Largely barren, steam-heated alteration is preserved above and overprinted on the lithocap. The results of radiometric dating (six U-Pb and 22 Re-Os ages) show that porphyry intrusion and potassic alteration began at ~15 Ma and lasted for nearly 1 m.y., although most of the molybdenite samples reported ages from 14.7 to 14.4 ± 0.06 Ma. This short interval coincided with a pulse of regional compressive tectonism accommodated by high-angle, thick-skinned, reverse faulting of basement-cored blocks, and concomitant uplift and exhumation. This uplift gave rise to ~1 km of erosion during formation of the Filo del Sol deposit, thereby accounting for the extreme telescoping of high-sulfidation over porphyry mineralization. Spatial association of these middle Miocene intrusion centers with dated middle Permian and Eocene mineralization suggests deep-seated structural control of the Filo del Sol alignment. Ongoing supergene processes under low-pH conditions produced a near-surface resource dominated by chalcanthite, with minor underlying chalcocite enrichment, as well as partially oxidizing a shallow zone of high-grade Ag mineralization.