THE CENTRAL TEXAS LEAD DISTRICT: GEOLOGIC, GEOCHEMICAL, AND GENETIC COMPARISONS TO THE SUPERGIANT MVT DEPOSITS OF SOUTHEAST MISSOURI

Most Mississippi Valley Type deposits are greatly enriched with Zn relative to Pb, with the notable exception of the Pb-dominant deposits in Cambrian strata around the Ozark Uplift in southeast Missouri. Lead-dominant occurrences around the Llano Uplift in central Texas have attracted intermittent exploration over the past century because of the similarities of their general geologic setting to the supergiant southeast Missouri deposits. Although the Central Texas district has not proved to have commercial significance, the distribution of Pb prospects suggests a regional mineralization system with a comparable footprint to southeast Missouri. Central Texas deposits are hosted by dolomitized Upper Cambrian carbonates of the Moore Hollow Group, commonly adjacent to local granite basement highs near pinchouts of the basal Hickory Sandstone. The eroded roots of the late Paleozoic Ouachita Orogenic Belt are buried beneath Cretaceous and younger strata to the east and south of the Llano Uplift. Faults of late Paleozoic age are common in the Llano area, and Ouachita tectonic events have been tied to the migration of basinal brines into older Paleozoic units flanking the Llano Uplift, with consequent diagenetic alteration, including MVT mineralization.

The Pb isotopic data for galenas from the Central Texas district are relatively unradiogenic and are reasonably well correlated on a plot of ²⁰⁶Pb/²⁰⁴Pb vs. ²⁰⁷Pb/²⁰⁴Pb. Although the data set is not large, individual prospects tend to have consistent values within the total district range. Central Texas Pb data are unusual for MVT deposits from the southern midcontinent, in that they plot below the Stacey and Kramers silicate earth evolution curve. These data can be modeled as a two-stage system, with a first-stage corresponding to the age of the Grenville basement and the second stage corresponding to the age at which Pb was removed from the source. These data support a genetic model of a Pb-dominant mineralization system related to a granitic basement source being mobilized by basinal fluids driven by the late Paleozoic Ouachita Orogeny.