Geologic observations in the San Marcos area, Coahuila, Mexico: the case for sediment-hosted stratiform copper–silver mineralization in the Sabinas basin during the Laramide orogeny

Abstract

The sediment-hosted stratiform copper–silver mineralization in the San Marcos area of Coahuila, northeastern Mexico occurs predominantly at an Early Cretaceous redox boundary between footwall siliciclastic red beds of the San Marcos Formation and hanging-wall carbonate strata of the Cupido Formation in the Sabinas basin. The hypogene mineralization is mainly present as chalcocite-group minerals, with additional bornite and chalcopyrite, and everywhere occurs in both disseminated and vein/veinlet forms. Supergene copper-bearing oxides (malachite, chalcanthite, azurite, chrysocolla) are, however, the dominant surface expression of the mineralization. Additional sediment-hosted stratiform copper–silver mineralization also occurs, albeit erratically, in lower strata of the Sabinas basin as well as in veins in basement granitoids, thus spanning ~3000 m of basin stratigraphy. Where best developed, the stratiform mineralization displays intense structural control proximal to the regional San Marcos fault system. This major bounding fault, regional in nature and with numerous periods of activity, controlled the evolution of the Sabinas basin. Structural controls on mineralization include stacked, shallow-angle, bedding-parallel, northeast-vergent thrust faults and associated drag folds, in addition to numerous, steeply-dipping, northeast-trending copper-bearing veins and veinlets. The mineralized veins and veinlets, and the bedding-parallel thrusts display mutually crosscutting relationships. These elements are all consistent and in harmony with a regional northeast-trending direction of horizontal shortening accompanying reverse motion of the San Marcos fault system. Inversion along the San Marcos fault system, and the entire Sabinas basin in the Paleogene from ~60 to 40 Ma, resulted from wholesale contractional deformation during the Laramide (Mexican) orogeny. Hence, emplacement of the sediment-hosted stratiform copper–silver mineralization at San Marcos, and elsewhere in the larger Coahuila region, is interpreted as a natural corollary of large-scale, metal-bearing fluid expulsion, migration, and precipitation resulting from orogenic shortening, lithostatic loading, and squeezing of the Sabinas basin during Mexican orogen construction. Although sedimentation of the host strata in the Sabinas basin took place in a pericratonic setting associated with the opening of the Gulf of Mexico, sediment-hosted stratiform copper-silver mineralization occurred during orogenic uplift and conversion of the original basin into an orogen-foreland pair, with similarities to some of the world´s largest sediment-hosted stratiform copper provinces.