Date of Award:

5-2017

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Geosciences

Department name when degree awarded

Geology

Committee Chair(s)

Alexis K. Ault

Committee

Alexis K. Ault

Committee

James Evans

Committee

Dennis Newell

Abstract

Southwestern New Mexico experienced protracted volcanism from ~60 Ma to 500 ka and associated epithermal mineralization. We apply hematite (U-Th)/He (hematite He) thermochronology to fracture-hosted hematite in the Lordsburg Mining District to resolve the timing of mineralization related to hydrothermal fluid circulation. We interpret hematite He dates with integrated field and structural observations, scanning electron microscopy to characterize hematite texture and grain size distribution, and zircon U-Pb and zircon (U-Th)/He (zircon He) chronology to constrain the timing of host rock formation and the ambient low-temperature thermal history, respectively. Undeformed hematite fills fractures cut a brecciated rhyolite and preserve open voids. Scanning electron microscopy reveals botryoidal hematite comprises stacked sublayers of ultra-fine grained crystals, corresponding to low He retentivity hematite. Forty-nine hematite aliquots from 12 samples, across seven fracture coatings yield reproducible mean hematite He dates from 3.4 to 0.8 Ma. U-Pb zircon geochronology dates range from ~90 to ~54 Ma, excluding ~1.3–1.0 Ga zircon components. These data overlap previously published dates (~56–52 Ma) from the host rhyolite overlap with new zircon He dates of ~51 Ma from two rhyolite host rock samples. The mean of the youngest U-Pb zircon dates is 54 ± 1.3 Ma and is inferred to represent the rhyolite eruption age. We suggest the rhyolite has remained in the near surface since emplacement and that hematite He dates record ~3.5–1 Ma epithermal mineralization. Younger dates likely reflect partial diffusive He loss from the fine-grained and thus low He retentivity hematite. Our data suggests the hematite fracture-fills are unaffiliated with Paleocene-Eocene economic mineralization.

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