INTEGRATED GEOCHEMISTRY AND (U-TH)/HE DATING OF DETRITAL MAGNETITE AS A PROVENANCE TOOL IN ALASKA’S GEOLOGIC AND METALLOGENIC TERRANES

Understanding relations between interior Alaska’s geologic and metallogenic terranes is challenged by poor rock exposure and remoteness, motivating development of detrital mineral tools for refining the footprint of mineral resources, and linking these to a regional tectonic framework. Here, we test a novel workflow for integrated geochemistry and (U-Th)/He (He) dating as a tool for inferring detrital magnetite (DMt) provenance in mineralized regions, with the ~70 Ma Taurus porphyry Cu-Mo(-Au) deposit as a natural laboratory. Minor- and trace-element compositions of DMt isolated from modern stream sands were measured by electron probe microanalyzer (EPMA; N=9 samples, n=684 grains) and complemented by inclusion counts and EPMA analyses of local hydrothermal porphyry (HTP) and metamorphic sources (N=20, n=164). Principal component analysis and Gaussian mixture modeling of DMt EPMA data resolves at least ten geochemical populations within our samples. These populations are compatible with derivation from HTP, metamorphic, and intermediate to felsic igneous sources. Across our dataset, HTP Mt comprises ~≥30% of DMt adjacent to mineralized rocks but diminishes (~2-6% of grains) several km downstream. He dates from Mt grains (n=48) representative of HTP, igneous, and metamorphic populations are highly dispersed, but show coherent modes within each population. Igneous Mt is <1 Ma and ~55 Ma, corresponding to known phases of volcanism in the region. HTP Mt dates are predominantly ~70 Ma, consistent with the known age of Cu mineralization, but several grains yield ~50-45 Ma dates. Metamorphic Mt dates range from ~120-45 Ma. Regionally distributed biotite and white mica ⁴⁰Ar/³⁹Ar and apatite fission-track dates are ~130-110 Ma and ~55-45 Ma, respectively, and we thus interpret these He date modes in DMt to reflect variable exhumation across each stream catchment. Prior work attributes ~55-45 Ma exhumation to regional faulting, and we suggest this same phase of deformation exhumed portions of the altered and mineralized halo at Taurus. DMt is thus a promising target phase for (1) tracking the spatiotemporal distribution of porphyry systems, and (2) linking the formation and exhumation of these systems to a regional tectonic history, in Alaska and other regions.