Re‐evaluating genetic models for porphyry Mo mineralization at Questa, New Mexico: Implications for ore deposition following silicic ignimbrite eruption

Abstract

AbstractThe Questa porphyry Mo deposit in New Mexico provides a unique opportunity to study the relationship between pluton assembly and mineralization in a long‐lived volcanic field. Magmatism along the caldera margin initiated at ~ 25.20 Ma and continued for ~ 770 ka. During this time, the emplacement of mineralizing intrusions progressed westward and culminated in the assembly of the Questa Mo deposit between 24.76 Ma and 24.50 Ma. Molybdenite Re/Os geochronology shows that mineralization occurred in multiple pulses without thermal resetting of the chronometer. Because most of the molybdenite samples used in this study are from previous fluid inclusion studies, we treat Re/Os molybdenite as a new thermochronometer. Molybdenite Re/Os ages are integrated with zircon U/Pb ages to evaluate the cooling histories within the Mo deposit. This study suggests that individual cycles of magma emplacement and mineralization cooled rapidly. In contrast to prior genetic models for the Questa Mo deposit, these data show that the mineralizing intrusions were generated via rapid melt generation, separation, and intrusion into the shallow crust without involvement in a long‐lived magma chamber. It is proposed that the anomalously high magma flux event associated with ignimbrite eruption transferred materials (Mo, volatiles) from the upper mantle necessary for immediately subsequent mineralization. Partial melting and scavenging within a deep‐crustal hybridized zone generated Mo‐rich magma that ascended to form the Questa deposit. Moreover, this hypothesis predicts an important connection between caldera‐forming systems and porphyry‐style mineralization that could be incorporated into future exploration models.