(U-TH)/HE AND 4HE/3HE CONSTRAINTS ON SECONDARY FE-OXIDE MINERALIZATION IN SOUTHWESTERN NEW MEXICO

Temporal constraints on paleofluid flow and mineralization are important for understanding a variety of upper-crustal processes including ore genesis and magma-related hydrothermal circulation. Hematite commonly co-precipitates with economically valuable mineral phases in ore deposits along veins, fracture systems, and faults and is amenable to (U-Th)/He dating. Prior work indicates hematite, commonly a poly-crystalline aggregate, exhibits poly-domain diffusion behavior and a range of He closure temperatures from ~50-250 °C depending on aliquot grain size distribution. We apply this method to a suite of hematite-coated fracture surfaces near Lordsburg, New Mexico, located in the Pyramid Mining District and the transition between the Rio Grande rift and Basin and Range provinces, to constrain the timing and evaluate the significance of this hematite mineralization. Integrated field and scanning electron microscopy (SEM) observations provide macroscopic and microscopic context for hematite formation as well as preliminary constraints on the temperature sensitivity of dated hematite aliquots. Millimeter-thick botryoidal hematite coats fractures that crosscut a ~60-54 Ma porphyritic rhyolite. SEM imaging reveals this botryoidal hematite is composed of stacked ~≥200 nm-thick sublayers of densely-packed, radiating, blade- to rod-like crystals, likely yielding a low hematite He closure temperature. The upper surfaces of many of these layers exhibit brilliant iridescent patches with color variations on the millimeter to meter scale. Preliminary hematite (U-Th)/He dates from two hematite-coated surfaces are reproducible at ~1.4 Ma and ~1.3 Ma. These dates are appreciably younger than published low temperature thermochronology dates of ~20-10 Ma, which record regional cooling. This suggests that the hematite He results may record the timing of hematite mineralization, which is concomitant with regional volcanism. Hematite He dates may also reflect He loss, possibly from hot fluids or from the dominance of low He retentivity domains. Additional bulk hematite (U-Th)/He dates from a larger sample suite, He diffusion kinetic information from ⁴He/³He experiments, and apatite and zircon (U-Th)/He thermochronology data from the host rock will shed light on these possible scenarios.