The Upper Conglomerate and its importance to the Sierra Mojada Ag-Zn deposit system, Coahuila, Mexico

The Sierra Mojada Ag-Pb-Zn-(Cu) district is located in Coahuila, Mexico approximately 200 kilometers south of the United States border. The district was originally discovered in 1879 and has produced approximately 10 million short tons of arsenian silver, zinc, lead, and copper ore. The Sierra Mojada district is situated on the northern edge of the Coahuila block at the southern boundary of the Sabinas basin along the trace of the San Marcos fault system. The mineral deposits at Sierra Mojada are primarily hosted in Lower Cretaceous limestone and dolomitized limestone strata that were faulted and folded during the Laramide orogeny. This study focused on the Upper Conglomerate, a previously poorly known unit in the district. Geological mapping and logging of recent drill core indicates that the Upper Conglomerate can be distinguished from rocks of the somewhat lithologically similar San Marcos Formation that forms the base of the Cretaceous sedimentary section. The Upper Conglomerate was deposited unconformably on Cretaceous carbonate rocks. It contains a distinctive suite of Permo-Triassic igneous clasts, some up to boulder size, that do not have a known source in the Sierra Mojada district. The sizes of clasts indicate that the Upper Conglomerate was deposited near the source. Regional geology suggests the source for the material in the Upper Conglomerate may have been a horst buried under alluvial cover in the northern portion of the district. Alteration and weak sulfide mineralization of the Upper Conglomerate above mineralized zones in the underlying Cretaceous carbonate rocks suggests that mineralization in the Sierra Mojada district occurred after deposition of the Upper Conglomerate. If the Upper Conglomerate was deposited prior to mineralization it suggests an early Tertiary age for the unit. Though the intrusive source for the Sierra Mojada deposit remains unknown, mapping of alteration intensity within the Upper Conglomerate suggests a causative intrusion may be present at depth on the western side of the deposit area. The location of the intrusion may have been controlled by a series of northeast trending faults that form a major structural domain boundary within the district. Sierra Mojada is distinguished from other high-temperature, carbonate-hosted Ag-Pb-Zn-(Cu) in northern Mexico by the extreme development of supergene oxidation. This study suggests that the supergene oxidation was related to significant Quaternary to recent normal faulting and uplift.