The Candelaria-Punta del Cobre Iron Oxide Cu-Au(-Zn-Ag)Deposits, Chile

Abstract

Several iron oxide-rich Cu-Au(-Zn-Ag) deposits define an approximately 5-km-wide and at least 20-km-long belt along the eastern margin of the coastal batholith near Copiapó, Chile. This belt includes the large Candelaria mine and a group of middle- and small-sized mines in the Punta del Cobre district, which is located about 3 km northeast of the Candelaria deposit. Estimated geologic resources of the belt are on the order of 700 to 800 million metric tons (Mt) at 1.0 percent Cu. The ore occurs in veins, breccia, and stringer bodies, and in replacement bodies that are roughly concordant to bedding. The orebodies are hosted mainly by volcanic and volcaniclastic rocks of the Punta del Cobre Formation and, in places, also occur in volcaniclastic intercalations in the lower part of the overlying Early Cretaceous Chañarcillo Group. Most of the larger orebodies in the belt are located where northwest-trending brittle faults intersect the contact between massive volcanic and volcaniclastic rocks. These northwest faults and a major northeast-trending ductile shear zone control portions of the ore of the Candelaria deposit. Chalcopyrite is the only hypogene Cu mineral. The Cu-Au ore is characterized by abundant magnetite and/or hematite and by locally elevated concentrations of Ag, Zn, Mo, and light rare earth elements. The ore is hosted mainly in zones with biotite-potassium feldspar ± calcic amphibole ± epidote alteration at Candelaria. In the Punta del Cobre district, ore in the deeper parts of the deposits is similarly associated, whereas at shallow levels it occurs in zones of biotite-potassium feldspar, or albite-chlorite ± calcite alteration. Mineralization at Candelaria-Punta del Cobre took place under relatively oxidized conditions manifested by the formation of specular hematite. In parts of the district, the pseudomorphic replacement of early specular hematite by magnetite during the main iron oxide formation marks a shift toward more reduced conditions or higher temperatures. The bulk of the magnetite probably formed at temperatures of about 500° to 600°C. The main sulfide stage followed with formation of pyrite and chalcopyrite at temperatures of >470° to 328°C. Subsequent martitization of the magnetite points to a temperature decrease. Cooling of the hydrothermal system is also indicated by the homogenization temperatures of <236°C of saline fluid inclusions in late-stage calcite. Oxygen isotope combined with microthermometric data suggest that magmatic fluids or nonmagmatic fluids equilibrated with magmatic silicates were dominant during the main copper mineralization. Relatively light oxygen isotope signatures of fluids in equilibrium with late calcite suggest mixing with a nonmagmatic fluid (e.g., basinal brines or meteoric waters) during the late stages of hydrothermal activity. Sulfur isotope ratios of chalcopyrite, pyrite, pyrrhotite, and sphalerite from the Bronce, Candelaria, Las Pintadas, Santos, and Socavón Rampa deposits range from δ34SCDT values of –0.7 to +3.1 per mil. This narrow range of sulfu...

The boron isotope geochemistry of tourmaline-rich alteration in the IOCG systems of northern Chile: implications for a magmatic-hydrothermal origin

“…The alteration assemblages are biotite-magnetite-actinolite±K feldspar± epidote±quartz in the deep zones and biotite-K feldspar± quartz in the shallow ones; the sulfide assemblage consists of chalcopyrite and pyrite with magnetite commonly replacing earlier hematite. The overlying limestone is irregularly replaced by a nearly barren calcic skarn (Marschik and Fontboté 2001a). Marschik and Fontboté (2001a) reported minor tourmaline in the earliest hydrothermal assemblage, but we found in our study that it is fairly common within the deep parts of the deposit.…”

“…The overlying limestone is irregularly replaced by a nearly barren calcic skarn (Marschik and Fontboté 2001a). Marschik and Fontboté (2001a) reported minor tourmaline in the earliest hydrothermal assemblage, but we found in our study that it is fairly common within the deep parts of the deposit.…”

“…This is the case for the Cloncurry district in Australia and the Skellefte district in Sweden (Mark et al 2006;Williams et al 2003Williams et al , 2005. Thus, current models for IOCG systems fluctuate between wholly magmatic (Pollard 2006;Tornos et al 2010), those that invoke circulation of basinal brines with the igneous rocks acting exclusively as heat engines Johnson 1996, 2000) or intermediate models in which a magmatichydrothermal event is followed by the invasion of the system by basinal brines (Marschik and Fontboté 2001a).…”

“…Tourmaline-rich rocks are widespread in magmatichydrothermal systems of the Andes, including porphyry copper and epithermal deposits (Sillitoe and Sawkins 1971), but only recently has tourmaline been described as an abundant hydrothermal mineral in IOCG-like systems (Tornos et al 2010); previous works (Benavides et al 2007;Marschik and Fontboté 2001a) have quoted it as an accessory phase in some zones of the IOCG deposits of the Copiapó area. This study reports the first tourmaline compositional and δ 11 B data for several IOCG deposits of the Coastal Cordillera of the Andes.…”

“…Fluid composition and stable isotope data are somewhat ambiguous. At Candelaria, fluid inclusions indicate the circulation of CO 2 -bearing fluids having salinities between 25 and 47 wt.% (Marschik and Fontboté 2001a) and suggest the presence of a first pulse of magmatic brines that was gradually replaced by Ca-rich fluids of basinal derivation. However, both metals and sulfur are interpreted to be of magmatic origin.…”