Evidence of Precambrian Organic-Rich Shales in a Granitic Pegmatite, Llano County, Texas
Badu Hill Pegmatite (BHP), found in Llano County, Texas, is an NYF (Niobium, Yttrium, and Fluorine) pegmatite that has been mined in the past for its high-grade feldspar crystals and investigated for its rare earth element (REE) potential. Understanding the petrogenesis of pegmatites and associated ore bodies is critical to finding and developing resources. This research examines multiple lines of evidence for the assimilation of Precambrian-age black shale to help produce the suite of minerals found at the BHP. By examining the surrounding country rock types, determining chemistries of the suite of minerals found at BHP, and examining the composition and source of the associated granite batholiths, multiple lines of evidence were found suggesting previously unknown Precambrian black shales in this area.
There is abundant sulfide mineralization at the BHP, including pyrite and chalcopyrite, and documented high concentrations of uranium, some of which is associated with zircons and other accessory minerals. Assimilation–Fractional Crystallization (AFC) modeling of uranium and other elements show that without the addition of a higher uranium component, like organic-rick black shale, it would have been implausible to get the high concentrations of uranium seen in the BHP. An investigation into black shales revealed their propensity to retain sulfur and uranium that has precipitated from the surrounding marine water. The country rock surrounding BHP is the Honey Formation of the Packsaddle Schist. The Honey Formation is characterized by interfingered marble and graphite bands that may have organic-rich shales as a protolith. The associated granite, Town Mountain Granite of the Llano Uplift, is typically classified as an A-Type granite, but BHP is classified as metaluminous. There has been no indication of Precambrian black shales in Texas until now, but at the Badu Hill Pegmatite in Llano County there is ample evidence to support the hypothesis that one did exist.
AAPG Datapages/Search and Discovery Article #90219 © 2015 GCAGS, Houston, Texas, September 20-22, 2015