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Fluid Chemistry, Structural Setting, and Emplacement History of the Rosario Cu-Mo Porphyry and Cu-Ag-Au Epithermal Veins, Collahuasi District, Northern Chile

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Masterman, GJ and Cooke, DR and Berry, RF and Walshe, JL and Lee, AW and Clark, AH (2005) Fluid Chemistry, Structural Setting, and Emplacement History of the Rosario Cu-Mo Porphyry and Cu-Ag-Au Epithermal Veins, Collahuasi District, Northern Chile. Economic Geology, 100 (5). pp. 835-862. ISSN 0361-0128

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Abstract

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was the earliest formed alteration product but has been replaced by biotite-albite.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 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 % NaCl).
Molybdenite was precipitated in quartz veins that formed between the potassic and intermediate argillic alteration
events. These fluids were 350 degrees to 400 degrees 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 degrees to 400 degrees C from fluids with a salinity of 10 wt percent NaCl. The pyrophyllite-dickite assemblage
formed between 250 degrees and 320 degrees 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 degrees to 300 degrees 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.

Item Type: Article
Keywords: fluid inclusions structure telescoping high sulfidation
Journal or Publication Title: Economic Geology
Page Range: pp. 835-862
ISSN: 0361-0128
Identification Number - DOI: 10.2113/100.5.835
Date Deposited: 26 Sep 2007
Last Modified: 18 Nov 2014 03:22
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