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,*Centre for Ore Deposit Research, University of Tasmania, Private Bag 79, Hobart, Tasmania 7001, Australia
CSIRO Division of Exploration and Mining, P.O. Box 1130, Bentley, Western Australia 6102, Australia
Department of Geological Sciences and Geological Engineering, Queens University, Kingston, Ontario, Canada K7L 3N6
Corresponding author: e-mail, glen.masterman{at}bolnisigold.com.mx
The Rosario Cu-Mo-Ag deposit is located in the Collahuasi district of northern Chile. It comprises high-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 Mt at 0.93 percent copper occurs in the district at the nearby Ujina and Quebrada Blanca porphyry deposits. The Rosario reserve contains over 95 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 southwest-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 Paragenetic relationships indicate that magnetite was the earliest formed alteration product but has been replaced by biotite-albite.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 early hydrothermal fluid was a hypersaline brine (40–45 wt % NaCl) that coexisted with vapor between 400° and >600°C. Weakly mineralized illite-chlorite (intermediate argillic) alteration of the early K and Na silicate assemblages was caused by moderate temperature (250°–350°C), moderate-salinity brines (10–15 wt % NaCl). Molybdenite was precipitated in quartz veins that formed between the potassic and intermediate argillic alteration events. These fluids were 350° to 400°C with salinities between 10 and 15 wt percent NaCl.
Porphyry-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° to 400°C from fluids with a salinity of 10 wt percent NaCl. The pyrophyllite-dickite assemblage formed between 250° and 320°C from dilute (5 wt % NaCl) fluids. An upward-flared zone of muscovite-quartz-pyrite altered rocks surrounds the fault-controlled domain of advanced argillic alteration. Thick veins (0.5–2 m wide) of fault-hosted massive pyrite, chalcopyrite, and bornite precipitated brines with a salinity of 30 wt percent NaCl at temperatures of 250° to 300°C.
Pressure-depth estimates indicate that at least 1 km of rock was eroded at Rosario between formation of the K-Na silicate and advanced argillic assemblages. This erosion was rapid, occurring over a period of 1.8 m.y. The Rosario Porphyry intruded immediately after 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 porphyry 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.
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