Modification of the crust: Mineralization and alteration in long-lived magmatic centers

Dense drill core and subsurface mapping at the Questa molybdenum (Mo) mine combined with regional structural tilt and high-precision U-Pb zircon geochronology, permit detailed 4-D documentation of plutonic rocks in the Latir magmatic center, New Mexico. The Questa porphyry Mo deposit was emplaced from 24.9-24.5 Ma by five intrusive units and is underlain by several post-mineralization sheets. Field relations indicate rapid cooling of magmas and associated hydrothermal networks, with discrete periods of wall rock modification and ore deposition. Therefore it is unlikely a sustained magmatic conduit focusing degassing of a long-lived, fractionating magma chamber formed the Questa deposit. Productive magmatism at the Questa porphyry Mo deposit has distinctly different isotopic compositions than barren magmatism before and after mineralization. Altogether, the rocks define a temporal trend of varying Sr, Nd and Pb isotopic compositions following peak volcanism at the Latir magmatic center. Elsewhere in the Latir, Au-mineralization occurred at 23.8 Ma, shortly followed by Ag-mineralization, and a second episode of Mo-mineralization at 20.52 Ma. All have distinct isotopic compositions that are also distinct from bookending barren magmatism. Overall, the Questa batholith was assembled through downward stacking of discrete pulses of magma over 8 Ma, amalgamating to an estimated 500 km3 batholith as suggested by Bouguer gravity data. There are no thermochronologic data from the Questa batholith to suggest the formation of plutons and mineralizing intrusions resulted from fractionation of a long-lived, high-melt fraction magma chamber. Together, these results indicate it is unlikely that fractionation of a long-lived magma chamber is a dominant mechanism in the generation of porphyry ore deposits. Instead, it is more likely that changes in magma generation, (e.g., magma source, degree of partial melting, volatile influx), control the heterogeneities within long-lived magmatic centers, particularly in porphyry magma systems.Doctor of Philosoph