GEOCHEMICAL MODELING OF TRACE ELEMENTS AT THE BADU HILL PEGMATITE, CENTRAL TEXAS

The ~1.1 Ga Badu Hill Pegmatite (BHP) is located on a narrow isthmus between the Lone Grove and Kingsland granite plutons in Llano County, Texas. It has known high concentrations of Rare Earth Elements (REEs), as evidenced by the presence of gadolinite and bastnasite, and elevated uranium (U) content. The pegmatite is likely a cupola where late-stage fluids derived from extensive crystallization of a granitic magma chamber collected and crystallized. Oxygen isotope analyses show that the minerals in the BHP are ~3 per mil heavier than the corresponding granite, similar to what is observed in other pegmatites.

Preliminary results indicate that the total U in the pegmatite exceeds what could be produced from extensive crystallization of a host pluton using reasonable distribution coefficients. Thus the amount of U and likely some other “incompatible” elements must be due to one of the following: 1) assimilation of country rock which could enrich the magmatic system, 2) multiple mineralizing events which might add elements from fluids related to additional pluton crystallization, 3) enrichment of some elements in an immiscible liquid in the granitic magma chamber.

There is some evidence for all three scenarios. 1) The Honey Formation within the Packsaddle Schist is found locally and has a high concentration of graphite. This graphite may be related to organic-rich shale deposit which would have high concentrations of some elements (e.g. U, REE, Mo?, V?) that are also elevated in the BHP. Other workers present oxygen isotope ratios for the Packsaddle Schist and radiogenic isotopic studies that seem to preclude significant assimilation of country rock. 2) It is unclear if the BHP is directly related to the crystallization of the Lone Grove or Kingsland pluton but there is evidence of crosscutting and offset quartz veins indicating a second, later stage mineralization that may be a result of fluids from the other nearby pluton crystallization. 3) Sulfide pods are pervasive in the BHP and if these are the result of an immiscible liquid within the magma chamber, they may have enriched the BHP with respect to some elements.