Trace Element Characteristics of Zircon: A Means of Assessing Mineralization Potential of Granitoid Intrusions and Porphyries

Oregon State University, Corvallis OR

Investigators

Abstract

Intellectual Merit. Globally, hydrous and sulfur-rich arc magmas in convergent margin settings are genetically related to magmatic-hydrothermal fluids that produce porphyry Cu(-Mo-Au), epithermal Au(-Ag), and a variety of other types of mineral deposits types. Although in theory an average arc-type granodiorite can generate such a deposit, most intrusions in arc crust do not produce mineral deposits and are economically barren. intermediate to silicic magmas are both strongly oxidized and, although they are emplaced and crystallized in the upper continental crust, they ultimately derive from deeper seated mantle-sourced basaltic to andesitic magmas. A systematic study of the trace element compositions and ages of zircons from representative mineralized and barren intrusions is proposed to evaluate the utility of this mineral as a tracer for mineralization. Samples from a range of well characterized porphyry and related systems in the Western USA and South America will be targeted for study. Zircon is well-known as a robust mineral that retains its magmatic composition through subsequent cooling, reheating, or hydrothermal events. A suite of trace elements including Hf, Y, Sc, V, Ti, and the rare earth elements can be accurately analyzed in situ in zircon via the ion microprobe (SHRIMP-RG) and laser ablation-ICP-MS methods. Additionally, U-Th-Pb isotopic age information can be collected simultaneous or sequentially. The Hf/Zr ratio increases during crystallization of magma, and temperature can be monitored using the Ti-in-zircon geothermometer. Thus, zircon offers the opportunity to constrain time-temperature-composition paths for intrusive systems. The rare earth element systematics in zircons will also be evaluated as a tracer of redox conditions in the intrusive magmas. For example, initial work on porphyry intrusions at El Salvador, Chile, indicates that the Eu anomaly measured in zircons from mineralized intrusions is small compared to barren intrusions, and likely results from a combination of higher oxidation state and magmatic differentiation in the middle or deep crust where plagioclase is absent. Broader Impacts. This proposal will contribute to understanding of the processes that lead to formation of porphyry-type mineral deposits, which are the principal global sources of copper and molybdenum, and significant byproduct gold, silver, and other metals with annual global production in excess of $100 billion. The project will also support one post-doctoral researcher and support teaching efforts by both PI?s in economic geology and geochemistry at OSU that are essential training for geoscientists required to find, exploit and remediate mineral deposits. The work will also involve collaboration with industry, providing new pathways for students to obtain practical experience in the minerals industry and opening the way for future collaborative efforts.