NATURE AND ORIGIN OF THE SIERRA MOJADA NON-SULFIDE ZN DEPOSITS, COAHUILA, MEXICO

The Sierra Mojada district consists of multiple types of mineral concentrations ranging from polymetallic sulfide deposits, “non-sulfide Zn” deposits (separate smithsonite and hemimorphite zones), and a Pb carbonate manto hosted by Upper Jurassic to Lower Cretaceous carbonates. The district is typically grouped with other polymetallic carbonate replacement deposits of southwestern North America, but the intrusive rocks that commonly are associated with these types of deposits are not known at Sierra Mojada. The Sierra Mojada district located near the boundary of the Coahuila Platform and the Sabinas Basin that formed during Late Jurassic and Cretaceous tectonic extension. The regional E-trending San Marcos fault is thought to have acted as the major conduit for basinal fluids, responsible for local dolomitization, sulfide mineralization, and petroleum in the region.

The polymetallic sulfides are the stratigraphically lowest and structurally highest ore zone and occur north of the San Marcos fault. Non-sulfide Zn concentrations occur in younger strata south of the fault at lower elevations. The Iron Oxide Manto consists of stratabound zones dominantly of hemimorphite pore-filling in Fe-oxide-rich dolostones giving the ore a distinct red to orange color. The Smithsonite Manto has distinct karst features, including internal sediments interbanded with smithsonite. Dendrites of Mn-Fe oxides are encased in banded and colloform smithsonite. Hemimorphite occurs in the Smithsonite Manto suggesting that they may have coprecipitated, although locally hemimorphite cross-cuts smithsonite bands. Associated minerals include barite and calcite that seem to have formed later than the major Zn mineral formation.

Preliminary isotope analysis reveals that some Sierra Mojada smithsonites have δ¹⁸O values that are lower than most supergene deposits. The low oxygen isotope values require either ¹⁸O-depleted waters if oxidation occurred at temperatures of less than 20°C or elevated temperatures during oxidation if waters had isotope compositions similar to present-day ground waters. Studies are in progress to constrain the relationships among the ore zones, including their fluid character and mineralization timing in the context of regional history.