CRITICAL ELEMENT POTENTIAL OF TERTIARY INTRUSIVE COMPLEXES IN THE TRANS-PECOS REGION OF TEXAS: A COMPARISON OF THE SILICA-UNDERSATURATED ALKALINE CORNUDAS COMPLEX

The Trans-Pecos region of Texas hosts more than 100 Paleocene to Miocene age intrusive and extrusive igneous bodies related to the transition from subduction to rifting along the southwest margin of North America. Many of the localities, including Round Top and other laccoliths of the Sierra Blanca Complex, Marble Canyon and Cave Peak, and the Davis Mountain rhyolites are hypabyssal to extrusive, sub-volcanic to volcanic, and mostly felsic in composition. Despite the numerous magmatic rocks in the Trans-Pecos area and contiguous regions, only a few have been analyzed for a complete geochemistry suite, including rare earth (REE) and other critical elements, or recognized to have REE mineral occurrences.

The Sierra Blanca Complex comprises 5 rhyolite laccoliths, whereas Cave Peak (rhyolite and alkali-feldspar granite) and Marble Canyon (monzodiorite, monzonite and quartz syenite) are zoned and formed from multiple phases. The Davis Mountains comprise a series of extrusive flood rhyolite, trachyte and tuffs. The Cornudas igneous complex consists of 9 intrusions on the Texas-New Mexico border (6 in New Mexico, 3 in Texas) and 6 more intrusions ~10 km south-southeast in the Sierra Tinaja Pinta and Cornudas Station areas. The intrusions in Texas range from phonolite, analcime syenite, and analcime-nepheline syenite to pyroxene trachyte and olivine-analcime trachyte. The northern Trans-Pecos magmatism occurred over a relatively short timeframe. Cave Peak and Marble Canyon have recorded emplacement ages between 36.1 ± 0.09 Ma and 34.8 ± 0.4 Ma, the Sierra Blanca Complex was emplaced at 36.2 Ma ± 0.6 Ma, the Davis Mountain rhyolites erupted at 37–36 Ma, and the Cornudas Complex ages range from 36.0 ± 1.1 Ma to 31.6 ± 0.9 Ma.

Geochemically, each of these complexes represent peraluminous (A/CNK >1.0), alkalic, extensional, within-plate tectono-magmatic affinity. Round Top in the Sierra Blanca Complex has long been identified as a potential REE deposit with Total REE+Y ranging from 249-518 ppm (the 43-101 report indicates 512–672 ppm total rare earth oxides and 84–199 ppm Y). Cave Peak REE+Y ranges from 250–953 ppm, and the Davis Mountain rhyolites ranges from 263–930 ppm. The Cornudas complex has the highest average Total REE+Y ranging from 413–807 ppm (average = 609; n=23).