Fluid chemistry, structural setting, and emplacement history of the Rosario Cu-Mo porphyry and Cu-Ag-Au epithermal veins, Collahuasi district, northern Chile

The Rosario Cu-Mo-Ag deposit is located in the Collahuasi district of northern Chile. It comprises hi-h-grade Cu-Ag-(Au) epithermal veins, superimposed on the core of a porphyry Cu-Mo orebody. Rosario has mining reserves of 1,094 million metric tons (Mt) at 1.03 percent copper. An additional 1,022 Nit at 0.93 percent copper occurs in the district at the nearby, Ujina and Quebrada Blanca porphyry deposits. The Rosario reserve contains over 9.5 percent hypogene ore, whereas supergene-sulfide ores dominate at Ujina and Quebrada Blanca. Mineralized veins are hosted within Lower Permian volcanic and sedimentary, rocks, Lower Triassic granodiorite and late Eocene porphyritic quartz-monzonite. The Rosario fault system, a series of moderate south-west-dipping faults, has localized high-grade Cu-Ag-(Au) veins. At Cerro La Grande, similar high-grade Cu-Ag-(Au) veins are hosted in north-northeast-trending sinistral wrench faults. Normal movement in the Rosario fault system is interpreted to have been synchronous with sinistral strike-slip deformation at La Grande, Hydrothermal alteration at Rosario is characterized by a K-feldspar core, focused in the Rosario Porphyry that grades out to a secondary biotite-albite-magnetite assemblage. Paragenetic relationships indicate that mag netite was the earliest formed alteration product but has been replaced by biotite-albite. Vein crosscutting relationships indicate that K-feldspar formed during and after biotite-albite alteration. Chalcopyrite and bornite were deposited in quartz veins associated with both K-feldspar and biotite-albite assemblages. The Cady hydrothermal fluid was a hypersaline brine (40-45 wt % NaCl) that coexisted with vapor between 400 degrees and > 600 degrees C. Weakly mineralized illite-chlorite (intermediate argillic) alteration of the early K and Na silicate assemblages was caused by moderate temperature (250 degrees-350 degrees C), moderate-salinity brines (10-15 wt % NaG). MolYbdenite was precipitated in quartz veins that formed betweeu the potassic and intermediate argillic alteration events. These fluids were 350 degrees to 400 degrees C with salinities between.1.0 and 15 wt percent NaCl. Porphry=style ore and alteration minerals were overprinted by structurally controlled quartz-alunite-pyrite, pyrophyllite-dickite, and muscovite-quartz (phyllic) alteration assemblages. The quartz-alunite-Pyrite alteration formed at 300 degrees to 400 degrees C from fluids with a salinity of 10 wt percent NaCl. The pyrophyllitc-dickite assemblage formed between 250 degrees and 320 degrees C from dilute (5 wt % NaCl) fluids. An upward-flared zone of muscovite-quartz-p pyrite altered rocks surrounds the fault-controlled domain of advanced argillic alteration. Thick veins (0.5-2 in wide) of fault-hosted massive pyrite, chalcopyrite, mid bornite precipitated brines with a salinity of 30 wt percent NaCl at temperatures of 250 degrees to 300 degrees C. Pressure-depth estimates indicate that at least 1 km of rock was eroded at Rosario between fori nation of the K-Na silicate and advanced argillic assemblages. This erosion was rapid, occurring over a period of 1.8 The Rosario Porphyry intruded immediately alter the Incaic tectonic phase, implying that it was emplaced as the Domeyko Cordillera underwent gravitational collapse, expressed as normal faults in the upper crust. Gravitational sliding potentially accelerated exhumation and helped to promote telescoping of the high-sulfidation environment onto the Rosario Porphyry The hydrothermal system responsible for porphry Cu mineralization at Rosario was partially exhumed prior to the formation Of high-sulfidation ore and alteration assemblages. This implies that emplacement of a second blind intrusion occurred somewhere beneath the Rosario and Cerro La Grande high-sulfidation vein systems and is supported by the fault geometry and Zoning of precious metals and sulfosalts at the district scale.