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GEOREF RECORD

Evolution of magmatic vapor to gold-rich epithermal liquid; the porphyry to epithermal transition at Nevados de Famatina, northwest Argentina

C. Pudack, W. E. Halter, C. A. Heinrich and T. Pettke
Evolution of magmatic vapor to gold-rich epithermal liquid; the porphyry to epithermal transition at Nevados de Famatina, northwest Argentina
Economic Geology and the Bulletin of the Society of Economic Geologists (July 2009) 104 (4): 449-477

Abstract

Fluid inclusions in the subeconomic porphyry Cu-Mo-Au system at Nevados de Famatina and closely associated high-sulfidation epithermal Cu-Au-(Ag-As-Sb-Te) veins at La Mejicana, northwest Argentina, were studied to reconstruct the evolution of hydrothermal fluids from their deep magmatic source to the shallow epithermal environment. Field geology, vein petrography, fluid inclusion microthermometry, and single-inclusion microanalysis by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) were combined to determine the evolution of pressure, temperature, and ore metal concentrations in the fluids. Cathodoluminescence imaging complements transmitted light petrography to constrain the successive stages of quartz formation and the entrapment sequence of the fluid inclusion populations. Aqueous liquid inclusions of approximately 5 wt percent NaCl equiv salinity trapped in quartz-sericite-pyrite (QSP) veins between 360 degrees and 325 degrees C contain unusually high concentrations of Cu (400-5,000 ppm), As (approximately 200 ppm), Sb and Te (both up to 100 ppm), and Au (several ppm) along with other ore-forming elements. These veins are paragenetically transitional between subeconomic porphyry copper mineralization exposed in the valley floor, and high-sulfidation epithermal veins with high Au grades, which are preserved along a high ridge adjacent to the porphyry stock. Low-density vapor and hypersaline liquid (i.e., brine) inclusions were trapped in early-formed quartz of the porphyry stockwork veins, between 450 degrees and >600 degrees C. The brine contains lower Cu and Au concentrations than coexisting vapor inclusions and texturally even earlier inclusions of intermediate density that are trapped in phenocrysts and some stockwork veins; the latter may be equivalent to a parental fluid. The low- and intermediate-density fluids compositionally overlap in salinity with the aqueous liquids recorded by the later transitional quartz-sericite-pyrite veins. Fluid inclusions and geologic time relations indicate that the transitional quartz-sericite-pyrite veins were the channelways for metal-rich aqueous liquids that generated the high-sulfidation epithermal Cu-Au deposit. These mineralizing liquids are interpreted to have evolved by continuous density increase ("contraction") from a low-salinity, S-rich magmatic fluid of low to intermediate density. This vaporlike fluid was produced by exsolution from a magma that existed at greater depth during the late stages of cooling of the magmatic-hydrothermal complex, and probably underwent minor brine separation prior to reaching the present level of exposure. Epithermal mineral precipitation occurred upon dilution of the low-salinity magmatic fluid with meteoric water, which entered the hydrothermal system as it was cooled and successively eroded during continued magmatic fluid ascent.


ISSN: 0361-0128
EISSN: 1554-0774
Coden: ECGLAL
Serial Title: Economic Geology and the Bulletin of the Society of Economic Geologists
Serial Volume: 104
Serial Issue: 4
Title: Evolution of magmatic vapor to gold-rich epithermal liquid; the porphyry to epithermal transition at Nevados de Famatina, northwest Argentina
Affiliation: ETH Zurich, Department of Earth Sciences, Isotope Geochemistry and Mineral Resources, Zurich, Switzerland
Pages: 449-477
Published: 200907
Text Language: English
Publisher: Economic Geology Publishing Company, Lancaster, PA, United States
References: 75
Accession Number: 2009-070277
Categories: Economic geology, geology of ore deposits
Document Type: Serial
Bibliographic Level: Analytic
Annotation: Includes appendix
Illustration Description: illus. incl. sects., 2 tables, geol. sketch maps
S30°45'00" - S27°00'00", W68°30'00" - W65°00'00"
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, Copyright, Society of Economic Geologists. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 200938
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