NEW GEOCHRONOLOGIC AND GEOCHEMICAL CONSTRAINTS ON THE FORMATION OF CU ORE-BEARING PORPHYRY DIKE UNITS, YERINGTON, NV

The Yerington Pit copper mine in northern Nevada has a long, well-documented history. However, a precise age of the oldest and highest grade ore-bearing unit—the QMP-1 porphyry dike—is lacking. Several surrounding units were dated by bulk U-Pb zircon analyses (Dilles and Wright 1988), but those data are difficult to interpret and indicate substantial Pb loss. It is also unclear why Cu content decreases in surrounding cogenetic porphyry dikes. We present in-situ SHRIMP U-Pb ages of zircons that were chemically abraded and thermally annealed in order to minimize analyses of grains that have undergone Pb-loss and analytical artifacts that can occur from measurements of radiation-damaged or metamict portions of grains. We also measured trace element concentrations in zircon and oxygen isotope ratios by SIMS from the QMP-1 dike, related surrounding dikes, and the host pluton to construct an enhanced geochemical picture of the Yerington Pit porphyry system, with the goal of identifying trends leading to more units that are chemically similar to QMP-1 and show promise for Cu mineralization. In particular, we examine the role of oxidation state in the host magma(s) using the magnitude of Ce and Eu anomalies in zircon. Zircon Ce/Ce* ratios are elevated in oxidizing environments and Eu/Eu* is depleted in more reduced magmas. Understanding why the QMP-1 dike is Cu-enriched compared to the surrounding cogenetic dikes is of interest not only for the Yerington system, but for Cu porphyry systems in general. By gaining a better understanding of geochemical conditions in melts that source economically viable deposits, it may be possible to discern ore-bearing versus barren rocks earlier in the exploration process.