A METAL RHYTHM: MAGMATIC EVOLUTION OF THE SANTA RITA PORPHYRY Cu DEPOSIT, NM, USA FROM HIGH-PRECISION ZIRCON GEOCHRONOLOGY AND GEOCHEMISTRY

Models for porphyry copper formation rely on factors such as crustal architecture, changes in magma genesis and the degree and lifespan of magmatic fractionation to explain the formation of economic porphyry systems relative to barren systems. Accurately constraining the magmatic lifespan and sources of mineralizing intrusions is crucial for assessing models for deposit formation, because the duration of magmatic processes may control the total metal endowment of individual deposits by enriching ore through overprinting of discrete hydrothermal-magmatic systems. The Paleocene Santa Rita porphyry copper deposit, located in New Mexico, USA, has been studied for decades, and serves as an archetype for porphyry copper deposit models. However, existing chronology for the deposit lacks the precision necessary to deconvolve its complex magmatic history.

New high-precision zircon Chemical Abrasion-Isotope Dilution-Thermal Ionization Mass Spectrometry (CA-ID-TIMS) U-Pb geochronology and Hf isotope geochemistry from rocks associated with porphyry mineralization at Santa Rita indicate intrusive assembly, porphyry mineralization and skarn formation was protracted and episodic over >2 Myr of deposit assembly. However, there is no evidence in the crystal record indicative of a sustained upper crustal parental magma chamber; single crystal U-Pb zircon age distributions from temporally disparate mineralizing intrusions do not yield consistent or considerable overlap. Instead, these zircon data suggest individual batches of mineralizing magma formed discretely between 59.905 ± 0.028 and 57.744 ± 0.035 Ma and repeatedly sampled a juvenile source, as indicated by overlapping Hf isotopic compositions. Pre- and post-mineralization magmas emplaced 100’s of kyr before and after mineralization host considerably more xenocrystic zircon inheritance from Proterozoic basement, and therefore reflect different magma genesis or evolution relative to the mineralizing intrusions. Altogether, this dataset and ongoing work suggest Laramide porphyry copper formation in Arizona and New Mexico resulted from a shift in magma genesis lasting millions of years, and that protracted magmatic assembly drives overall deposit endowment.