Intrusive history of the Oligocene Questa porphyry molybdenum deposit, New Mexico

Subsurface mapping and core analyses of upper crustal intrusions and mineralization at the Questa porphyry molybdenum deposit, New Mexico, reveal that Mo-mineralization occurred through episodic emplacement of at least six intrusive units. The structure of intrusions associated with the Questa deposit is documented in a series of detailed cross sections and visualized with a 3D animation. Mineralizing intrusions are underlain by two post-mineralization intrusions and cut by late-stage barren dikes. The plutonic complex was structurally focused along a system of preexisting flat-lying faults and their associated fractures. Mineralization is spatially associated with specific intrusive units in the subsurface, and the highest Mo ore grades within established ore blocks are structurally associated with the smallest intrusions. Existing U/Pb thermal ionization mass spectrometry (TIMS) zircon geochronology in conjunction with new relative chronology presented herein indicate that mineralization began before 24.91 Ma. We present three new chemical abrasion U/Pb TIMS zircon ages-one from an amphibole-bearing intrusion associated with high-grade mineralization (dark-matrix porphyry, 24.74 ± 0.37 Ma), a rhyolite dike that cuts ore-grade rocks (24.50 ± 0.02 Ma), and an equigranular granite discovered during deep drilling (23.67 ± 0.02 Ma). The dark-matrix porphyry contains clasts of an earlier amphibole-free intrusion that is spatially associated with low-grade mineralization. Thus, mineralizing intrusions were, in part, intruded into slightly older porphyries, confirming that episodic mineralization continued after 24.91 Ma. The age of the barren dike (24.50 ± 0.02 Ma) is indistinguishable from that of a previously dated granite porphyry that is associated with low-grade mineralization (