Characterization and timing of the Cave Peak porphyry molybdenum system, Culberson County, Texas
Creators: name: Ugurhan, Mert
Publisher: name: The University of Texas at Austin
Year: 2022
Subjects: subject: Porphyry mo
subject: Geochemistry
subject: FOS: Earth and related environmental sciences
subject: U-Pb geochronology
subject: Molybdenum isotopes

description: The Cave Peak molybdenum deposit is a breccia-hosted porphyry system located on the eastern flank of the Sierra Diablo mountains in west Texas. The deposit belongs to the class of high-grade, rift-related, fluorine-rich Mo deposits commonly referred to as Climax-type porphyry Mo systems that are products of highly evolved rhyolite magmatism. The Cave Peak intrusive system consists of an outer rhyolite breccia mass with a complex intrusive core, overprinted by multiple hydrothermal events. Molybdenite is the primary Mo-bearing mineral and occurs as stockwork veinlets commonly with quartz, biotite and fluorite and as fine disseminations. At least three spatially and temporally different Mo zones were identified by exploration in the 1960s (Sharp, 1979). Elevated concentrations of F, Nb, Sn, and W are also present. The Marble Canyon Stock is an unmineralized, compositionally zoned, monzodiorite and monzonite to quartz syenite pluton located 1.5 km southwest of Cave Peak; previous studies have suggested a genetic link between the two plutons. Petrographic and geochemical studies were conducted on core samples to understand the 3D nature of Cave Peak system. The deposit shows a within-plate geochemical affinity, average (n=10) Nb, Rb, and Sr concentrations of 730, 690, and 61 ppm, respectively, and a highly differentiated magma series. Rare earth element (REE) concentrations are elevated, with average REE+Y concentrations of 758 ppm (n=10) and 437 ppm (n=9) for Cave Peak and Marble Canyon, respectively. Curvilinear trends are observed between Marble Canyon and Cave Peak rocks when major oxides plotted against silica on variation diagrams. Trace element plots of Rb, U, Th, and Mo also show these linear trends implying a link between Cave Peak and Marble Canyon. Quartz syenite and monzonite from the Marble Canyon Stock yielded zircon U-Pb ages of 36.2 ± 0.15 Ma and 36.1 ± 0.09 Ma, respectively. The youngest major intrusion at Cave Peak, an alkali feldspar granite porphyry, has a zircon U-Pb age of 34.8 ± 0.4 Ma, further suggesting these intrusions are related. The δ⁹⁸Mo of seven Cave Peak molybdenite samples range between -0.39 ± 0.06 ‰ and +0.77 ± 0.11 ‰ which indicates a total Mo isotope variation of 1.16 ‰. The δ⁹⁸Mo of early upper Mo zone molybdenite samples (n=4) range between -0.26 ± 0.08 ‰ and 0.77 ± 0.11 ‰. One transitional intermediate Mo zone sample yielded δ⁹⁸Mo value of +0.19 ± 0.07 ‰ and two late lower Mo zone molybdenite samples yielded δ⁹⁸Mo values of -0.09 ± 0.08 ‰ and -0.39 ± 0.06 ‰. This dataset may record the progressive evolution of fluids within the system and suggest that fluids exsolved from early magmas were enriched in heavier Mo isotopes. Minerals preferentially incorporated heavier Mo isotopes during fractional crystallization leaving the melt enriched in lighter Mo isotopes. During the later stages of hydrothermal events, molybdenite incorporated lighter Mo isotopes. Tectonic setting of the ore-bearing porphyries also appears to influence Mo isotopic compositions. Molybdenite from the Red Hills deposit, a Laramide subduction-related porphyry Cu-Mo system in southern Trans-Pecos Texas, has δ⁹⁸Mo values of -0.25‰ to -0.51‰ that are more typical of porphyry mineralized systems. The heavier δ⁹⁸Mo values of the rift-related Cave Peak mineralization suggest an atypical evolution of this fluorine-rich magmatic system.