Geological context,mineralization, and timing of the Juramento sediment-hosted stratiform copper–silver deposit,Salta district,northwestern Argentina |
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| Authors: | C Gustavo Durieux Alex C Brown |
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| Institution: | (1) Department of Civil, Geological and Mining Engineering, école Polytechnique de Montréal, P.O. Box 6079, Station Centre-Ville, Montreal, Quebec, H3C 3A7, Canada;(2) Present address: Anglo American Exploration (Canada) Ltd., Suite 800-700 West Pender St., Vancouver, British Columbia, V6C 1G8, Canada |
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| Abstract: | The Juramento deposit in northwestern Argentina exhibits several readily visible general characteristics of sediment-hosted
stratiform copper (SSC) mineralization. It consists of fine-grained disseminated base-metal sulfides within marine to lacustrine
graybeds (the basal whitish Late Cretaceous Lecho Sandstone and shallow-water carbonates of the overlying Maastrichtian Yacoraite
Formation) that overlie a thick sequence of redbeds (the Pirgua Subgroup). The property has been examined and drilled in three
successive exploration programs as a possible analog of world-class mineralization in the copperbelts of central Africa and
the Kupferschiefer. The present report provides specific field and laboratory results that confirm the classification as SSC-type
mineralization. The host graybeds are the basal sandstone and overlying oolitic and stromatolitic units of the Yacoraite Formation,
which are shown from textural studies to be carbonaceous and to have initially contained very fine-grained, disseminated,
syndiagenetic pyrite. These sediments would have been sufficiently porous and permeable in early diagenetic time to allow
an infiltration of metalliferous fluids from the underlying redbeds, resulting in the observed progressive replacement of
in situ pyrite by common base-metal sulfides (sphalerite, galena, argentiferous tetrahedrite, and copper-rich sulfides: first
chalcopyrite, then bornite, and finally chalcocite). Sulfur isotope analyses indicate that a portion of the sulfur of ore-stage
sulfides is isotopically heavier than that of pyrite, possibly due to a contribution from associated gypsum. Ore-stage sulfides
are zoned vertically and obliquely through the mineralized zones, from cupriferous sulfides at low stratigraphic levels to
lead- and zinc-rich mineralization above, with unreplaced pyrite remaining within upper Yacoraite strata. The zoned sulfides
and their replacement textures, the peneconformable configuration of the mineralized zones, and the position of ore-stage
mineralization adjacent to a stratigraphically defined redox transition from redbeds upward into graybeds indicate an overprint
of copper (and accompanying ore-stage metals) on originally pyritic graybeds. The influx of ore-stage metals, presumably in
an oxidized low-temperature brine, terminated with a silicification event that effectively sealed the host carbonates. These
observations and the overall genetic interpretation are consistent with the general deposit-scale genetic model for early
diagenetic SSC mineralization. The regional geologic context is also consistent with its classification as a SSC deposit:
It is hosted by post-oxyatmoversion sediments and was formed in association with evaporites at a low paleolatitude in a major
intracontinental rift system. |
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| Keywords: | Sediment-hosted stratiform copper Sulfur isotopes Juramento Salta Argentina |
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