ACCESSORY MINERALOGY OF REE GRANITE FROM THE LLANO AREA OF TEXAS

Precambrian granite plutons in the the Llano area of Texas are host to small REE granitoid pegmatites. The best known is the Baringer Hill pegmatite that was mined for yittria early in the last century. REE mineralization in the pegmatites is described as a late-stage feature. Early-formed parts of the pegmatite were devoid of the REE mineralization. To test the hypothesis of late stage crystallization of REE minerals we studied the accessory mineralogy of a Llano granite outcrop devoid of obvious pegmatite.

A small outcrop of red, fine to coarse-grained biotite granite intruded into high grade metamorphic rocks of the Valley Spring Domain about 7 km southeast of Llano, Texas was selected for study. Large hand samples of granite were collected and polished thin sections prepared for study. A crude, grain-sized defined layering is evident in some samples. Zones with the largest grains (about 10 mm) occasionally contain miarolitic cavities. The granite is composed mostly of red alkali feldspar. The feldspar is colored red by numerous small (microns)hematite inclusions. Quartz is common and graphic granite textures are developed near the coarser grained zones. Minor amounts of plagioclase and biotite also occur in the granite.

Electron microprobe studies revealed trace amounts of zircon, monazite, Y-xenotime, and NbTa oxides in the granite. The niobates were most common in or near the biotite grains. The other phases were found as inclusions in the alkali feldspar and quartz or along grain boundaries. Radiogenic zircon with elevated contents of uranium, and thorium (∑UThO = 33-36 wt. %) is overgrown by less radiogenic zircon (∑UThO = 0.4-2.0 wt.%). Radiogenic zircon is metamict with variable Zr and Si contents. Some low uranium zircon grains have inclusions of radiogenic zircon, Y-xenotime, and uraninite. Monazite and Y- xenotime contain elevated contents of REE (∑REO = 34-56 wt. %). Niobates are complex with variable Nb/Ta ratios (Nb/Ta = 0.8-128) and high REE contents (∑UThO = 11.1-29.1 wt.%).

Enrichment in REE is not confined to fractionated portions of Llano pegmatites. The occurrence of radiogenic phases and REE-enriched accessory minerals in the granite confirm REE enrichment is a feature of the Llano granitoid rocks and not just the pegmatites.