SMALL, EPHEMERAL AND SHALLOW RELATIONSHIPS: AMALGAMATING MO-MINERALIZING MAGMAS AT THE QUESTA PORPHYRY MO DEPOSIT

Upper crustal magmatism and mineralization at the Questa Climax-type porphyry Mo deposit in New Mexico are documented. The formation of low angle normal faults in the Questa-Latir range predates mineralization, presenting an opportunity to study structurally-intact porphyry magmatism. Subsurface mapping and core analyses reveal that Mo-mineralization occurred through episodic emplacement of at least six intrusive units. Mineralizing intrusions are underlain by two post-mineralization sheets and cut by late-stage barren dikes. The magmatic system includes an upper-crustal laccolith that was structurally focused along wall rock anisotropies. Mineralization is spatially associated with productive intrusions, and the most significant Mo enrichments are structurally associated with the smallest intrusions. Previous zircon geochronology indicates a mineralized intrusion was emplaced at 24.9 Ma, and a granite porphyry sheet was emplaced at 24.5 Ma. New high precision zircon geochronology of a small mineralizing intrusion (24.734 ± 0.051 Ma) containing clasts of earlier producive sheets and a dike that cuts ore-grade (24.556 ± 0.031 Ma) confirm that mineralization waned beginning around 24.5 Ma. Thus, alteration and mineralization occurred incrementally over ~ 400 ka.

The small volume intrusions associated with high-grade mineralization cooled rapidly, as evidenced by multiple internal contacts within sheets and rebrecciation textures. Due to the cross cutting relationships and the rapid cooling of small volume magmas, we suggest that mineralization occurred without significant involvement from fluids generated deeper in the crust. Emplacement of a structurally deeper granite porphyry sheet post-dates productive magmatism, was coeval with barren dikes and therefore did not serve as a magmatic conduit for mineralizing fluids. This suggestion that Climax-type Mo is the result of small volume, upper crustal magmas is in agreement with conclusions made on similar cross sectional work at the Henderson deposit.