CHARACTERIZING ZIRCON PORPHYRY COPPER FERTILITY CHARACTERISTICS IN THE EVOLVING LARAMIDE ARC, EAST-CENTRAL ARIZONA

Porphyry copper deposit formation depends on particular magmatic features related to oxidation state, temperature, metal, water, chlorine, and sulphur contents. Zircon trace element signatures and grain morphology are robust indicators of many of these attributes and can be interpreted to inform on a magma’s fertility, or on it’s potential to form a porphyry copper deposit. However, the scale of variability within an evolving arc, along an arc segment or across a migrating arc has not been established. Identifying fertile magmatic signatures and interpreting favourable magmatic processes requires that they can be resolved from background arc signatures. Furthermore, to use zircon trace element signatures as a porphyry exploration tool, sampling needs to be able to quickly evaluate large regions, such as drainage basins, ideally with a single sample. This requires evaluating trace element in zircon signatures from within a population of contemporary fluvially-transported detrital grains.

We evaluated a segment of the Laramide arc in east-central Arizona that hosts numerous porphyry copper deposits. The region records a period of porphyry copper associated magmatism from ca. 69 to 61 Ma. The porphyry copper deposits are associated with a range of lithologies within granodioritic to granitic plutons, and have been dismembered and tilted during Mid-Cenozoic normal faulting which has resulted in a range of exposure depths from <1 to >10 km. The magmas are interpreted to have variable sources and evolution, and there is a high degree to inherited zircons within the melts. In situ rock samples were obtained from various plutonic bodies, and sand samples were collected from several drainages. Together these samples characterize the background arc signature and identify the nature of the anomalous ‘target’ fertile signature.