THERMOCHRONOLOGIC CONSTRAINTS ON SECONDARY FE-OXIDE MINERALIZATION IN SOUTHWESTERN NEW MEXICO

The transition between the Rio Grande rift and Basin and Range provinces in southwestern New Mexico is the site of protracted volcanism from ~60 Ma to 500 ka and associated epithermal mineralization. New (U-Th)/He (He) dates from fracture-hosted hematite in the Lordsburg Mining District of SW New Mexico are hypothesized to record mineralization related to hydrothermal circulation. Integrated field, hematite microtextural, and zircon geo- and thermochronologic data aid in hematite He data interpretation. Hematite fills NE–trending fractures that crosscut a brecciated rhyolite. Some fractures preserve open voids and hematite is not deformed post-precipitation. Scanning electron microscopy reveals the mm- to cm-thick botryoidal hematite comprises stacked ≥200 nm-thick sublayers of densely-packed, radiating, blade- to rod-like crystals ≤100 nm in diameter, corresponding to inferred bulk He closure temperatures of ≤40 ºC. Upper surfaces of many of these layers exhibit iridescent patches with color variations on mm- to m-scales. Forty-nine hematite aliquots from 10 samples yielded reproducible intrasample He dates from 3.4 ± 0.30 to 0.8 ± 0.02 Ma (mean ± 2s), with no age difference between iridescent and noniridescent samples from the same fracture fill. New LA-ICP-MS U-Pb zircon dates (~90–54 Ma) and previously published dates (~56–52 Ma) overlap with new zircon He dates of 51 ± 0.14 Ma and 51 ± 4.6 Ma from the from two rhyolite host rock samples. New U-Pb zircon results are complicated, but the youngest dates from both samples of 54 ± 1.3 Ma are inferred to represent the eruption age. This, along with lack of zircon He date-eU and date-grain size trends, implies host rock zircons have not experienced appreciable post-crystallization He loss, limiting surface erosion since ~50 Ma. Although partial hematite He loss from the fine-grained, low-retentivity domains could be triggered by regional thermal effects including nearby ~500 ka volcanism, the hematite textural observations, hematite He and zircon He data patterns, and regional thermochronology data imply hematite He dates record fracture-hosted epithermal mineralization.