A GRANITE PEGMATITE: AN UNLIKELY PLACE TO FIND EVIDENCE OF A PRECAMBRIAN BLACK SHALE

The 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. It has also been investigated for its 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-aged 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 there, and examining the composition and source of the associated granite batholiths, multiple lines of evidence were found that suggests a Precambrian black shale in the area of the BHP.

The Badu Hill Pegmatite is exposed largely as an open pit mine. The walls were mapped in several sections and then analyzed for percentages of different minerals including biotite, feldspar, quartz, fluorite, and pyrite. Mineral samples were taken from a number of sections and analyzed with a hand-held XRF analyzer in bench-top mode. There is abundant sulfide mineralization at the BHP including pyrite and chalcopyrite, and documented high concentrations of uranium, some of which is linked to zircons and other accessory minerals.

Assimilation – Fractional Crystallization (AFC) modeling of uranium and other element concentrations showed that without the addition of a higher uranium component, like a black shale, it would have been implausible to get the high concentrations of uranium and sulfide minerals seen at 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 the BHP is the Honey Formation of the Packsaddle Schist. The Honey Formation is characterized by interfingered marble and graphite bands and may have an organic-rich shale as a protolith. The associated granite, Town Mountain Granite of the Llano Uplift, is typically classified as an A-Type granite, but the BHP is classified as metaluminous. There has been no indication of a Precambrian black shale in Texas until now, but at the Badu Hill Pegmatite in Llano County, there is ample evidence to support the hypotheses that one did exist.