TRACING MOLYBDENITE: A NEW LOOK AT THE LEAD ISOTOPE COMPOSITIONS OF PORPHYRITIC INTRUSIONS AND ASSOCIATED HYDROTHERMAL MINERALS IN THE QUESTA MO DEPOSIT

Porphyry ore deposits are magmatic-hydrothermal systems that supply the majority of the world’s copper and molybdenum. Whole rock and feldspar radiogenic isotope data are often used to better understand the origin of magmas and ore metals in these systems. The Questa porphyry Mo deposit is an ideal setting to evaluate the isotopic evolution of a dynamic magmatic-hydrothermal system due to the availability of high-precision zircon geochronologic data for much of the magmatic system, including samples from drill cores. Exploratory Pb isotopic studies of molybdenite from Questa suggested an offset from whole rock values of mineralizing intrusions. In this study, we analyzed the Pb isotopes of molybdenite extracted from multiple Questa rock samples to test hypotheses about the origin of the Pb isotopic compositional differences.

The Questa Mo deposit consists of a structurally complex series of mineralizing porphyries that intruded Precambrian metamorphic basement and Oligocene volcanic rocks. Hydrothermal alteration associated with the Questa deposit extends for hundreds of meters (vertically and laterally) beyond the ore bodies. Lead concentrations of the molybdenite range from 95-830 ppm and the molybdenite generally has higher ²⁰⁸Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁶Pb/²⁰⁴Pb compared to the mineralizing intrusions. Molybdenite was extracted from both andesitic wallrock and the mineralizing rhyolite porphyries, but there is no observed correlation between the vein host rock and Pb isotopic composition. One explanation for the offset is that alteration of feldspars by late hydrothermal fluids may have disturbed the Pb and Sr isotopic composition of the whole rocks. Thus, Pb isotopes in molybdenite, which is often encased in quartz veins in porphyry deposits, may be a more reliable indicator of the source composition of the mineralized intrusions. Analysis of Pb isotopic compositions of wallrocks proximal to the deposit and low temperature hydrothermal minerals in the ore system is underway. These data will allow us to investigate whether the offset in Pb isotopic compositions between molybdenite and the mineralizing intrusions is due to late-stage overprinting by fluids that circulated through the wallrock or evidence of distinct sources for the molybdenite-bearing fluids and the silicate magmas.