Epithermal Style Iron Oxide(-Cu-Au) (=IOCG) Vein Systems and Related Alteration

Abstract

The Copiapó region in northern Chile contains numerous intrusion- and volcanichosted IOCG vein systems. These veins share many features with larger IOCG systems in the region (e.g., Candelaria, Punta del Cobre), including abundant hydrothermal magnetite or hematite ± Cu, Au, REE, and other elements, and exhibit similar styles of mineralization including voluminous breccias, stockwork, and massive veins. The relatively simple geometries and small size of veins offer advantages for study of zoning and genesis in an IOCG system; and, they also provide an interesting counterpoint to classic epithermal Ag-Au veins. The vein systems exhibit systematic patterns in the alteration and mineralization zoning in both time and space. Deeper exposures are characterized by high-temperature styles of sodic and sodic(-calcic) alteration with Fe and Cu depleted vein fill assemblages. This passes upwards through a proximal zone of magnetite-dominated vein fill with sparse to absent copper, and into a magnetite-dominated, copper-bearing portion of the vein. Copper is best developed at intermediate to shallow levels in association with the hematite-dominated portions of the system. More distal, carbonate dominated facies with minor hematite and chalcopyrite are also present. Shallow levels of the vein system may be characterized by a low-sulfur style of advanced argillic alteration, that may be stratabound, in discordant breccia bodies, or structurally controlled on faults. The assemblages differ from other ore forming environments by their lack of sulfide and/or sulfate minerals, and the abundance of hypogene iron oxide phases (hematite and/or magnetite). Vein systems are dominated by brecciation events that record repeated, cyclic pulses of mineralizing fluids. Stable and radiogenic isotopic analyses, combined with fluid inclusion and mineral phase equilibria indicate the fluids were hypersaline brines (generally >40 wt% NaCl(eq)) over a temperature range of 200º-450ºC. The shallow formation, structural styles, repeated mineralization events, and size of the IOCG vein systems have many parallels to the classic precious-metal rich Ag-Au epithermal systems. Nonetheless, the two types of veins differ in their geochemistry, reflecting the large differences in fluid salinities, commonly <10 wt% NaCl(eq) in epithermal settings as compared to 15 to > 50 wt% NaCl(eq) in IOCG systems.