ZIRCON AS A RECORDER OF MAGMATIC REDOX IN EOCENE SILICIC INTRUSIONS OF THE GREAT BASIN

Tracking the oxidation state of magmas during crustal passage and assimilation is not only important for our understanding of magma differentiation, but also for ore formation. Changes in magmatic oxidation state control the associated fluid chemistry that dictates the transport and enrichment of metals near the surface. The Eocene arc of the Great Basin exposes many high-level intrusions that are related to mineralization including Carlin-type gold deposits. Moreover, the arc runs perpendicular to a former passive margin with predominantly anoxic sediments deposited in the West and shelf and platform deposition in the East. Thus, magmas traversing the crust are potentially distinctly modified in Nevada (West) compared to Utah (East), and therefore these different pathways may be responsible for the distinct metal endowment with gold-rich deposits in Nevada contrasting dominantly copper deposits found in Utah.

We tested the role of crustal interaction on magma oxidation state using Ce speciation in zircon determined by x-ray near-edge absorption spectroscopy (XANES) on Eocene intrusions of similar age (35-40 Ma). Being an alteration-resistant mineral, zircon retains a record of the magmatic evolution of the magma in addition to information about magma differentiation and crystallization temperatures. Our results across the Great Basin demonstrate that magmas in the Eastern part of the basin became increasingly oxidized with differentiation, supporting the scavenging of Cu in upper level magmatic-hydrothermal systems. While magmas in the West start out at similar intermediate oxygen fugacity, they get reduced during differentiation, facilitating the concentration of Au during ore formation. We argue that the signal in the Ce-speciation is derived from the gradients in the upper crust with which mantle-derived magmas interacted prior to emplacement.