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

The Sierra Mojada district in west central Coahuila comprises several types of mineral concentrations ranging from polymetallic carbonate-replacement sulfide deposits, “non-sulfide Zn” (NSZ) deposits, and a Ag-rich Pb carbonate deposit hosted by Lower Cretaceous carbonates. The NSZ mineralization consists of smithsonite, hemimorphite, and Zn clays (sauconite) associated locally with calcite and Mn-Fe-oxides The NSZ deposits consist of the Smithsonite Manto and the Iron Oxide Manto, also referred to as the Red Zinc Zone. The Smithsonite Manto shows karst features, including internal sediments interbanded with smithsonite. The Iron Oxide Manto consists of strata-bound zones dominantly of hemimorphite that fill pores in Fe-oxide-rich strata.

Smithsonite in the Smithsonite Manto forms botryoidal aggregates of scalenohedral or rhombohedral microcrystals and banded colloform or massive smithsonite in open spaces. Smithsonite in the Iron Oxide Manto occurs as rhombic microcrystals grown in pore spaces or finely intergrown with Fe-oxides. Both Fe-poor and Fe-rich smithsonite are found in the Iron Oxide Manto. Under optical-CL, smithsonite displays complex growth zoning that can be related to variable trace element content. LA-ICP-MS analyses of banded smithsonite reveals lower Mn and Fe contents in blue luminescent smithsonite compared to pink to bright red luminescent smithsonite. Based on oxygen isotope data for smithsonite (δ¹⁸O_vSMOW = 21.9 ± 0.5 ‰) in comparison with the modern groundwater composition at nearby Cuatro Ciénegas, the Sierra Mojada smithsonite is interpreted to have formed from groundwater at slightly elevated temperatures. Similar Pb isotopic compositions of smithsonite and cerussite to Sierra Mojada galenas (average ²⁰⁶Pb/²⁰⁴Pb = 18.679) suggest the source of metals in the NSZ deposits was from the polymetallic sulfide deposits.