Cu, Mn, and Ag mineralization in the Quebrada Marquesa Quadrangle, Chile: the Talcuna and Arqueros districts |
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Authors: | R Oyarzun L Ortega J Sierra R Lunar J Oyarzun |
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Institution: | (1) Departamento de Cristalografía y Mineralogía, Facultad de C.C. Geológicas, Universidad Complutense, 28040 Madrid, Spain e-mail: oyarzun@eucmax.sim.ucm.es, ES;(2) Departamento de Cristalografía y Mineralogía, Facultad de C.C. Geológicas, Universidad Complutense, 28040 Madrid, Spain, ES;(3) Departamento de Minas, Facultad de Ingeniería, Universidad de La Serena, Casilla 554, La Serena, Chile, CL |
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Abstract: | The Quebrada Marquesa Quadrangle in Chile exhibits a series of mineralizations comprising manto-type manganese and copper
deposits of Lower Cretaceous age, and copper and silver veins of Tertiary age. The deposits are hosted by volcanic and volcaniclastic
units of the Arqueros (Hauterivian-Barremian) and Quebrada Marquesa (Barremian-Albian) Formations. Three episodes of manganese
mineralization (Mn1-3) are recognized within the study area. Hydrothermal activity leading to episodes 1 and 3 was of minor importance, while the
second one (Mn2) gave rise to major manto-type deposits of both manganese and copper in the Talcuna mining district. Extensional faulting
during Tertiary time resulted in block faulting and the unroofing of the oldest andesitic volcanics and marine sediments (Arqueros
Formation). This episode was accompanied by magmatic and hydrothermal activity leading to vein formation in the Arqueros (Ag)
and Talcuna (Cu) districts. The latter veins cross-cut the previous manto-type copper deposits. Ore mineralogy is similar
in both styles of mineralization (manto- and vein-type) and consists mainly of chalcopyrite and bornite, with variable amounts
of galena, tetrahedrite (vein-related), chalcocite, sphalerite, pyrite, hematite, digenite and covellite. Alteration processes
at Talcuna can be divided into two categories, those related to the Lower Cretaceous manto-type episode (LK alteration: chlorite-epidote-calcite-albite, prehnite, zeolite), and those associated with the locally mineralized normal faults of
Tertiary age (Tt alteration: chlorite-calcite, sericite). The Arqueros silver veins display an ore mineralogy consisting of arquerite, argentite, native
silver, polybasite, cerargyrite and pyrargyrite-proustite; associated alteration includes strong chloritization of the country
rock. The manto-type deposits formed from fluids of salinity between 11 and 19 wt.% NaCl equivalent and temperatures between
120 and 205 °C. Mineralizing fluids during the vein-type stage circulated at lower temperatures, between 70 and 170 °C, with
salinity values in a wide range from 3 to 27 wt.% NaCl equivalent. This distribution of salinities is interpreted as the result
of the complex interplay of two different processes: boiling and fluid mixing; the former is considered to control the major
mineralogical, textural and fluid inclusion features of the vein-type deposits. We suggest that the Lower Cretaceous mineralization
(manto-type stage) developed in response to widespread hydrothermal activity (geothermal field-type) involving basinal brines.
Received: 18 July 1997 / Accepted: 28 January 1998 |
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