SIERRA MOJADA AG-PB-ZN DISTRICT, COAHUILA: EVOLUTION OF AN ENDMEMBER POLYMETALLIC CARBONATE REPLACEMENT MINERALIZATION SYSTEM?

After more than a century of production, the Ag-Pb-Zn orebodies of the Sierra Mojada District remain rather enigmatic as to their genetic affiliations. The orebodies are hosted by Cretaceous carbonates and are located near the boundary of the Coahuila Platform and the Sabinas Basin. Several regional and topical compilations have placed Sierra Mojada with other polymetallic carbonate replacement deposits (CRD) that are abundant in northern Mexico. However, the Sierra Mojada district lacks some features typical of classic CRD, notably including evidence of magmatism contemporaneous with mineralization. Some regional studies have suggested that Sierra Mojada and some other deposits in Coahuila have stronger affinities with basinal brine systems of the type that formed the amagmatic carbonate-hosted Mississippi Valley (MVT) Pb-Zn systems in Paleozoic shelf carbonate strata of the U.S. midcontinent and other regions. Yet the abundance of Ag, as well as a suite of other elements, at Sierra Mojada is not typical of the mineralogically simple classic MVT systems. However, other Zn-Pb deposits within sedimentary basins are Ag-rich, and even oilfield brines and Zn-Pb mineralization in the modern Gulf of Mexico Basin are Ag-bearing.

A significant issue concerning genetic studies of the Sierra Mojada mineralization is that the ore-bearing strata are near surface and have been deeply oxidized, a key process in the development of the two major orebodies mined to date: the original Pb carbonate manto and the non-sulfide Zn mantos. Recent studies in concert with a major minerals exploraton program have contributed new information about the character of the hypogene Sierra Mojada mineralization. While definitive evidence is elusive, the Sierra Mojada orebodies appear to have a genetic component involving basinal brines, and the district could represent an amagmatic endmember of the polymetallic carbonate-replacement type ore systems.