GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 171-8
Presentation Time: 9:00 AM-6:30 PM


CHANNER, Michael A.1, AULT, Alexis K.1, REINERS, Peter W.2 and STEARNS, Michael A.3, (1)Department of Geology, Utah State University, 4505 Old Main Hill, Logan, UT 84322, (2)Department of Geosciences, University of Arizona, Tucson, AZ 85721, (3)Geology and Geophysics, University of Utah, 135 S 1460 E, Salt Lake City, UT 84112-0111,

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.