LEAD AWAY FROM THE PORPHYRY: UNDERSTANDING THE SOURCE OF HYDROTHERMAL MINERALS IN THE QUESTA PORPHYRY MO DEPOSIT

The formation of porphyry Mo deposits is associated with hydrothermal fluid exsolution from crystallizing magmas. However, the role of meteoric water during molybdenite deposition remains uncertain. We present new Pb-isotope measurements of molybdenite and carbonate minerals from the Oligocene Questa porphyry-Mo deposit in New Mexico, and compare them to published isotopic data from wall rocks, productive intrusive rocks and barren late intrusions. The data suggest that the role of meteoric water varies during the T-t history of the system and becomes increasingly important at low-T.

Molybdenite at Questa precipitated during multiple events at temperatures between ~550 and 375°C. The Pb isotopic composition of the molybdenite ranges from ²⁰⁶Pb/²⁰⁴Pb = 17.93- 18.08 and ²⁰⁸Pb/²⁰⁴Pb = 37.30-37.47. Carbonate veins, that precipitated at low-temperatures (220-75°C), have Pb isotopic compositions ranging from ²⁰⁶Pb/²⁰⁴Pb = 17.95-18.11 and ²⁰⁸Pb/²⁰⁴Pb = 37.40-37.63. The Pb-isotope compositions of both molybdenite and carbonate minerals are more radiogenic than the mineralizing intrusions and are shifted toward the isotopic composition of the host-rock (Precambrian amphibolite and Oligocene andesite and rhyolite). Thus, the lower-T carbonates show a greater influence of wallrock interaction that we suggest is via introduction of meteoric water. This is also supported by published stable isotope data from Questa. The temperature-compositional trend suggests that the deposit incorporated meteoric fluids throughout its formation, with an increased contribution at lower temperatures. Further, these data suggest that molybdenite may be resistant to repetitious input of hydrothermal fluids and can be a useful tool for tracking source inputs in molybdenite-bearing ore systems.