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

• Published in: Journal of South American earth sciences 2011-04, Vol.31 (4), p.432-443
• Main Authors: García-Alonso, Donají, Canet, Carles, González-Partida, Eduardo, Villanueva-Estrada, Ruth Esther, Prol-Ledesma, Rosa María, Alfonso, Pura, Caballero-Martínez, Juan Antonio, Lozano-Santa Cruz, Rufino
• Format: Article
• Language: English
• Subjects: Red-beds; Rifting; Silver deposits; Stratabound deposits; Supergene alteration; Transgressive
• ISSN: 0895-9811; 1873-0647
• DOI: 10.1016/j.jsames.2011.02.012

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 SiO 2 (40.70–87.50 wt.%), Al 2O 3 (5.91–22.00 wt.%), K 2O (3.68–12.50 wt.%), Na 2O (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 bec