TRACKING ALASKA’S PORPHYRY CU SYSTEMS WITH DETRITAL MAGNETITE GEOCHEMISTRY AND (U-TH)/HE DATING

Alaska is the frontier of untapped US domestic mineral resources, but understanding links between the geologic and metallogenic frameworks is hampered by poor rock exposure and remote field sites. Magnetite (Mt) occurs in a variety of (non)mineralized rock types, carries geochemical signatures unique to formation conditions, and is amenable to (U-Th)/He (He) dating. Here, we develop and test a novel workflow for applying coupled detrital Mt (DMt) geochemistry and He dating as a tool for tracking DMt provenance in Alaska’s Yukon-Tanana Upland. We present DMt minor and trace element geochemistry measured by electron probe microanalyzer (EPMA) from stream sediments (N=9 samples, n=684 grains) that drain the Taurus porphyry Cu-Mo(-Au) deposit and environs, complementary textural and inclusion count data, and EPMA analyses from local hydrothermal porphyry-related (HTP) and metamorphic sources (N=20, n=164). Principal component analysis and Gaussian mixture modeling of DMt EPMA data resolves at least nine geochemical populations across our dataset, with any given population comprising 0% to 57% of an individual sample. We interpret the geologic significance of these populations with Mg-in-Mt temperatures (T_Mg), published geochemical discriminant plots, and inclusion and boundary phase counts. High-temperature (T_Mg ≥650℃) populations have mean geochemical compositions compatible with derivation from felsic and intermediate igneous sources. In contrast, two low-temperature populations (T_Mg<500℃) yield mean geochemical compositions compatible with Taurus HTP Mt and comprise 18-33% of grains with sulfide inclusions and(or) intergrown chlorite and titanite (compared to 2-11% in other populations). Other low-temperature populations are more geochemically similar to local metamorphic sources. Across our dataset, HTP Mt comprises ~≥30% of DMt adjacent to mineralized rocks but is identifiable up to ~40 km downstream (~2-6% of grains) of presently known sources. Mt grains representative of each population have been isolated for He dating. Analyses are ongoing and we anticipate these data will inform the formation and(or) exhumation of DMt populations. Our results suggest DMt is a promising tool for tracking the spatiotemporal distribution of porphyry systems in Alaska and possibly other regions.