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.
Porphyry Cu (±Mo ±Au) and epithermal Au-Ag deposits are major sources of mined metals (>$80 B global annual value) and are commonly formed by magmatic-hydrothermal fluids derived from hydrous magmas in Phanerozoic convergent margin settings (subduction-related). These ore deposits contain up to 120 million tonnes of contained copper and 1 billion tonnes sulfur as copper and iron sulfide minerals deposited in in volumes of rock exceeding a cubic kilometer. The ore metals and sulfur derived from magmas are transported and precipitated by hydrothermal aqueous chloride solutions. The igneous rock assemblages associated with porphyry mineral deposits are common in modern convergent margin settings, but while many have produced acidic magmatic fluids, very few, past or present, have produced sufficient metal, chlorine, and sulfur enrichments necessary to engender an ore deposit. The reasons for this remain uncertain. We report SHRIMP-RG ion microprobe analyses of hafnium, titanium and rare earth element (REE) abundances in zircon, a nearly ubiquitous and robust trace mineral in crustal magmas ranging from erupted (volcanic) andesite to rhyolite, and intrusive diorite to granite. Comparison of the compositions of zircons in ore-forming and barren granitic plutons indicate that ore-forming granites crystallized at relatively low temperature and have relatively small negative europium anomalies (mostly EuN/EuN* ≥0.4). We interpret these small zircon europium anomalies to indicate oxidizing magmatic conditions and hypothesize that in many cases this reflects oxidation due to SO2 degassing from magmas with a relatively low Fe/S ratio. Oxidation of europium and iron (Eu2+ --> Eu3+ and Fe2+ --> Fe3+) in the melt is produced by reduction of magmatic sulfate (S6+O42-) to SO2 (S4+) upon degassing. As a consequence of S-degassing and magmatic oxidation, Eu3+ is incorporated into zircon, and Eu2+ is not available to be incorporated into plagioclase and therefore zircon crystallizes with a minimal negative europium anomaly. This interpretation reinforces the important role of oxidized sulfur-rich fluids derived from magmas in porphyry and epithermal mineral deposit formation. Zircon compositions thus may be used to identify ancient magmas that released significant amounts of SO2-rich gases and are potentially a valuable tool for mineral exploration. For example, regional surveys of zircon composition from granites or volcanic rocks or detrital grains in soils and stream deposits derived from granitic sources may define prospective terranes for mineral deposits even where deposits are hidden beneath sedimentary cover. The project provided financial support and mentoring for one female post-doctoral scholar and one female graduate student. In addition, both these and several other students (1 finished MS, 1 in progress BS, 2 in progress MS, and one PhD student) received training in zircon processing and U/Pb age and trace element analyses. The PI teaches courses and mentors students on projects involving mineral deposits. Therefore, the research project has helped train several geologists who are or will be employed in the minerals economy, academia, or government. It has helped OSU develop protocols for relatively low-cost trace element and U/Pb age analyses of zircon using Laser Ablation-ICP-MS techniques.
