Contrasting Cenozoic Magmatism and Related Hydrothermal Systems, Western USA

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Part 1:Water- and sulfur-rich andesite and dacite magmas in the Andes, Cascades, Aleutians and other Circum-Pacific convergent-margin tectonic settings are genetically related to magmatic-hydrothermal fluids that produce numerous economic mineral deposits. Porphyry copper-molybdenum-gold mineral deposits are the principal sources of mined copper and molybdenum, and together with epithermal mineral deposits contribute much the world's gold and silver. Annual raw metal production value is more than $25 billion (USA) and and globally $100 billion (global). Many important deposits occur in the western USA, and there remains potential to find more deposits with improved geologic understanding. We plan to study volcanic and granitic rocks that are closely associated with these deposits in the western USA, with the goal of a better understanding of the magmatic processes that produce the hydrothermal fluids (hot, water-rich, and metal-bearing solutions) that are responsible for these mineral deposits. The study will investigate the petrology and geochemistry of Eocene volcanic rocks related to the Carlin gold deposits (100 million ounces of gold contained), Nevada, and Miocene volcanic rocks and granites related to porphyry copper deposits of the western Cascades of the Pacific Northwest. We plan to collect samples of these volcanic and igneous rocks and analyze them for major and trace elements composition, the composition of mineral constituents, the geochemical composition of the trace mineral zircon, and isotopic composition (of strontium and neodymium) of the rock. These data will allow characterization of the composition of the magmas and lead to a better understanding of their origins and what magmatic processes are essential to for large economic mineral deposits. Of particular interest is the understanding of the water content, oxidation state, and the content of ore components such as sulfur, copper, and gold, and how these contents may be governed by contributions from basaltic magmas derived from the upper mantle or from the upper crust. A potential economic outcome is increased understanding of the magmatic processes that form economic hydrothermal mineral deposits. Such knowledge will benefit the US minerals industry by broadening exploration tools. The PI does geology coursework and training of students, many of whom seek employment in exploration, exploitation, and environmental remediation of mineral deposits. The PI annually co-teaches an 8-day field mapping of ore deposits course that has enrolled in the past 10 years 250 professionals from 25 nations and 100 students. Since 2005, the PI has advised 17 graduate students (8 MS, 2 PhD completed) and 7 undergraduate theses, 2 international scholars, and two postdocs (10 of 28 female). The PI has 30 years of success obtaining industry sponsorship, which provides pathways for students to obtain practical experience in the minerals industry and opening the way for future collaborative efforts.Part 2: The planned research builds on and expands our recent NSF-sponsored study that reported SHRIMP-RG ion microprobe analyses of hafnium, titanium and rare earth element abundances of zircon, a nearly ubiquitous and robust trace mineral in crustal magmas (Dilles et al., 2015). 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 (Ballard et al., 2001). This small Eu anomaly may result from water-rich silicic magmas that suppress Eu-rich plagioclase crystallization and also lead to large magmatic Sr/Y ratios characteristic of ore-forming intrusions. Second, the small zircon europium anomaly may indicate oxidizing magmatic conditions, and we hypothesize that this partly reflects oxidation due to SO2 degassing or ore fluids from magmas with a relatively low Fe/S ratio. Oxidation of europium and iron in the melt is produced by reduction of magmatic sulfate (S6+) to SO2 (S4+) upon degassing. This interpretation reinforces the important role of oxidized sulfur-rich fluids 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 regional surveys of zircon composition are potentially a valuable tool for mineral exploration. The new research will extend understanding of zircon geochemistry and magmatic compositions of the important Carlin gold district of northern Nevada, and investigate the Cascade volcanoes of Oregon and Washington which contain relatively few porphyry copper deposits relative to Arizona-New Mexico-Utah-Montana and the Andes of Peru-Chile.