IRON-OXIDE(-CU-AU) (“IOCG”) VEIN SYSTEMS NEAR COPIAPO, CHILE

The Chile coastal batholith near Copiapó generated many hydrothermal systems (Barton et al., this volume); among the products are multiple styles of IOCG deposits. The Ojancos Viejo district and nearby areas contain intrusion and volcanic-hosted IOCG vein systems (>30 Mt @ 1.5% Cu). These IOCG veins paragenetically resemble larger systems in the region; an improved understanding of their time-space evolution should help address questions related to IOCG genesis. IOCG deposits, which are characterized by voluminous hydrothermal magnetite or hematite ± Cu, Au, REE and other elements, exhibit many styles including breccias, stockworks, veins and massive bodies. The relatively simple geometries and small size of veins offer advantages for study of IOCG zoning and genesis; they also provide a useful comparison with epithermal Ag-Au veins.

New mapping, core logging and petrography indicate that Copiapó vein systems form swarms of individual (0.5-10 m wide) veins and sheeted veins that are up to 5 km long, and can be exposed over a vertical interval of >1 km. The veins and district-scale alteration that envelops them span relatively deep exposures in intrusive rocks to shallower exposures in supracrustal rocks. The veins show repeated opening and deposition; occur in faults of small displacement, are typically breccia-rich (hydrothermal and tectonic) and are best developed on extensional jogs. Magnetite or hematite ± quartz are abundant at deeper levels and calcite at shallower levels with accessory Cu-Fe sulfides and various Ca-Mg-K-Na silicates. Scap-Act±Tit alteration changes upward to Olig-Act±Tit to shallow Ab-Chl with locally intense hydrolytic alteration, including advanced argillic assemblages, with K alteration localized on structures.

Intramineral dikes indicate igneous heat was the principal source of energy driving circulation of oxidized, S-poor, metal-rich non-magmatic brines (evidenced by S, Sr, O isotopes and petrology). Vapor-rich and brine-rich fluid inclusions indicate boiling in the veins. IOCG veins have parallels in style and geometry to low sulfidation Ag-Au epithermal systems perhaps reflecting a common uppermost crustal setting, but differ in their geochemistry reflecting the large differences in fluid salinities. Supported by NSF EAR08-38157 and Freeport-McMoran Exploration.