Melt inclusion and mineral geochemical analyses supporting the evaluation of petrogenesis, degassing, and metallogenic potential of mid-Cenozoic rhyolite magmas in northern Nevada, USA (ver. 2.0, March 2025)

This data release presents geochemical analyses of silicate melt inclusions, host quartz phenocrysts, and biotite phenocrysts in samples collected by the U.S. Geological Survey (USGS) from mid-Cenozoic rhyolitic rocks from northern Nevada.
Igneous intrusions and volcanic rocks in this study encompass nineteen rhyolitic samples from five magmatic centers across northeastern Nevada. Rhyolites were emplaced throughout northeastern Nevada over about an 8 million year timespan (41–33 mega-annum) in a variety of eruptive styles and volumes, ranging from meter-wide dikes to caldera-forming ignimbrites, and were roughly contemporaneous with regional ore deposition.
Samples were prepared for petrographic and scanning electron microscope (SEM) analysis, as well as analysis by electron microprobe (EMP), Fourier transform infrared (FTIR) spectroscopy, laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), and sensitive high-resolution ion microprobe-reverse geometry (SHRIMP-RG). See Process Steps in the metadata for detailed descriptions of analytic procedures.
SUMMARY OF RESULTS
Silicate melt inclusions are rhyolite to high-silica rhyolite. Major element abundances are broadly consistent with crystallization of major and minor phases observed in hand sample, including quartz, sanidine, plagioclase, and biotite. Melts are medium-K to very high-K, ferroan, and range from calcic to alkalic compositions. A few melts are metaluminous to slightly peraluminous, but most are strongly peraluminous. Rhyolite and high-Si rhyolite melts display wide variations in trace element concentrations. Chondrite-normalized multi-element diagrams generally show moderate to deep troughs in Ba, Sr, Ce, P, and Ti, and some notable enrichments in Rb, Ba, Th, U, Nb, Ta, La, and Y relative to average upper crust. Rare earth element patterns typically have gentle to steep negative slopes, with light rare earth elements (LREEs) enriched relative to heavy rare earth elements (HREEs). Samples with higher overall REE concentrations tend to have flatter patterns with more pronounced negative Eu anomalies. The rhyolite melts contain a wide range of dissolved volatile concentrations. Measured dissolved H2O contents range from about 0.3 to 4.7 weight percent (percent) (mean about 2.4 percent) and dissolved CO2 concentrations range from about 20 to 950 parts per million (ppm) (mean about 190 ppm). Fluorine and Cl concentrations are moderate to high, from about 0.13 ppm to 1.1 percent (mean about 0.3 percent) and about 130 ppm to 0.2 percent (mean about 650 ppm), respectively. Sulfur contents are low, about 2 to 140 ppm (mean about 25 ppm). Lithium concentrations are low to moderate, from about 1 to 770 ppm (mean about 155 ppm). Concentrations of Ti in host quartz range from 6 to 95 ppm Ti with a mean of 36 ppm. Sparse biotite phenocrysts display a broad range of MgO (about 2-14 percent), FeO* (about 14-36 percent), and contain variable amounts of F (about 0.1-3.8 percent) and Cl (about 0.05-0.4 percent).
Ore metal concentrations in rhyolite melts range from &lt;3-266 ppb Au (median = 16 ppb), &lt;16-44 ppb Te (median = 29 ppb), &lt;0.2-2.2 ppm Sb (median = 0.29 ppm), 0.2-6.3 ppm Tl (median = 1.2 ppm), 0.5-7.4 ppm W (median = 2.5 ppm), and 0.5-22 ppm As (median = 1.7 ppm), &lt;0.02-1.5 ppm Ag (median = 0.13 ppm), 0.1-9 ppm Mo (median = 2 ppm), 0.7-25 ppm Sn (median = 4.5 ppm), 1-82 ppm Pb (median = 31 ppm), 0.2-2.3 ppm Co (median = 0.5 ppm), &lt;0.04-39 ppm Cu (median = 2 ppm), 4-18 ppm Ni (median = 7 ppm), and 10-153 ppm Zn (median = 42 ppm). Metal concentrations in melt inclusions range widely regardless of their age and timing relative to nearby Carlin-type Au mineralization (i.e., pre-, syn-, or post-ore).
These major, trace, and volatile element data from quartz-hosted rhyolitic melt inclusions, host quartz, and biotite indicate that mid-Cenozoic rhyolitic magmas are remarkably diverse. This compositional diversity suggests varied evolutionary pathways as magmas ascended through the crust, mobilizing magmas from different reservoirs with distinct characteristics. The evident indifference of Carlin-type Au systems to a specific silicic magma “flavor” seems to imply that if magmas are involved in their genesis, they perhaps do not need to be compositionally specialized.

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Organization: Department of the Interior