The Cretaceous sediment-hosted copper deposits of San Marcos (Coahuila, Northeastern Mexico): An approach to ore-forming processes

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• 作者：Donaj&#xed ; Garc&#xed ; a-Alonso^a ; ^b ; Carles Canet^b ; ^{ccanet@geofisica.unam.mx"" rel=""nofollow} ; Eduardo Gonzá ; lez-Partida^c ; Ruth Esther Villanueva-Estrada^b ; Rosa Mar&#xed ; a Prol-Ledesma^b ; Pura Alfonso^d ; Juan Antonio Caballero-Mart&#xed ; nez^e ; Rufino Lozano-Santa Cruz^f
• 关键词：Red-beds ; Stratabound deposits ; Silver deposits ; Supergene alteration ; Rifting ; Transgressive
• 刊名：Journal of South American Earth Sciences
• 出版年：2011
• 出版时间：April 2011
• 年：2011
• 卷：31
• 期：4
• 页码：432-443
• 全文大小：2698 K

文摘

In the San Marcos ranges of Cuatrociénegas, NE Mexico, several sediment-hosted copper deposits occur within the boundary between the Coahuila Block, a basement high mostly granitic in composition and Late Paleozoic to Triassic in age, and the Mesozoic Sabinas rift basin. This boundary is outlined by the regional-scale synsedimentary San Marcos Fault. At the basin scale, the copper mineralization occurs at the top of a 1000 m thick red-bed succession (San Marcos Formation, Berrisian), a few meters below a conformable, transitional contact with micritic limestones (Cupido Formation, Hauterivian to Aptian). It consists of successive decimeter-thick roughly stratiform copper-rich horizons placed just above the red-beds, in a transitional unit of carbonaceous grey-beds grading to micritic limestones. The host rocks are fine- to medium-grained arkoses, with poorly sorted and subangular to subrounded grains. The detrital grains are cemented by quartz and minor calcite; besides, late iron oxide grain-coating cement occurs at the footwall unmineralized red-beds. The source area of the sediments, indicated by their modal composition, is an uplifted basement. The contents of SiO₂ (40.70–87.50 wt. % ), Al₂O₃ (5.91–22.00 wt. % ), K₂O (3.68–12.50 wt. % ), Na₂O (0.03–2.03 wt. % ) and CaO (0.09–3.78 wt. % ) are within the ranges expected for arkoses. Major oxide ratios indicate that the sedimentary-tectonic setting was a passive margin.

The outcropping copper mineralization essentially consists in a supergene assemblage of chrysocolla, malachite and azurite. All that remains of the primary mineralization are micron-sized chalcocite grains shielded by quartz cement. In addition, pyrite subhedral grains occur scattered throughout the copper-mineralized horizons. In these weathered orebodies copper contents range between 4.24 and 7.72 wt. % , silver between 5 and 92 ppm, and cobalt from 8 to 91 ppm. Microthermometric measurements of fluid inclusions in quartz and calcite crystals from footwall barren veinlets gave temperatures of homogenization between 98 °C and 165 °C, and ice-melting temperatures between −42.5 °C and −26.1 °C.

The primary copper mineralization formed during the early diagenesis, contemporary with the active life of the Sabinas Basin. The mineralizing fluids were dense, near neutral, moderately oxidized brines that originally formed from seawater that, driven by gravity, infiltrated to the deepest parts of the basin and dissolved evaporites. As a result, they became hydrothermal fluids of moderate temperature capable of leaching high amounts of copper. The source of this metal could be mafic detrital grains and iron oxides of the underlying Jurassic and Lower Cretaceous red-beds. Copper precipitation took place when the brines passed through the redox boundary marked by the transition from red- to grey-beds. The upward movement of the brines was promoted by a high heat flow that allowed their convective circulation and their ascent along the synsedimentary San Marcos Fault.