38 Magma, Heat, Fluids, and Critical Resources Across Tectonic Settings, Lithospheric Realms, Volcanic Systems, and Geologic Timescales (Posters)

Tuesday, 16 April 2024
8:00 AM - 12:00 PM
Grand Ballroom

AN ANALYTICAL ANALYSIS OF ALKALI FELDSPARS FROM LOW SULFIDATION EPITHERMAL SYSTEMS ALONG THE MIOCENE YELLOWSTONE LIP: UNDERSTANDING THE ROLES OF ARGON DIFFUSION AND CRYSTALLINE DEFECTS IN 40AR/39AR AGE VARIATIONS

Paper number: 38-8
Program: Theme Sessions
Time: 8:00 AM – 12:00 PM

Authors

Abstract

The initiation of the Yellowstone hotspot (ca. 16.5 Ma) and the accompanying magmatism during the mid-Miocene is responsible for the heat and metals required to develop numerous epithermal Au-Ag deposits in the OR-ID-NV area. Hydrothermally precipitated alkali feldspars co-precipitated with ores (‘adularia’) are commonly dated with 40Ar/39Ar geochronology to constrain the temporal evolution of magmatic regions and to aid in the development of genetic models for exploration and understanding ore genesis. In the present study, samples of adularia were collected from the Silver City District, Idaho (the Trade Dollar Mine) and the Jumbo Mine of Humboldt County, northeastern Nevada. The adularia crystals of each mine are typically fresh, euhedral, and optically clear to milky, and appear ideal for age dating. Previous XRD work on adularia from these epithermal systems reveals a sanidine structure suggestive of rapid crystallization and cooling. 40Ar/39Ar ages of adularia from Jumbo seem consistent with the inferred regional history and have yielded ages of ca. 16.5 Ma. However, 40Ar/39Ar ages for adularia from the Silver City district have a wide range (16.1-15.3 Ma) and are generally much younger than the earliest rhyolites and basalts of the district (also ca. 16.5 Ma). This raises the question of why the 40Ar/39Ar ages of these areas are misaligned and do we understand what 40Ar/39Ar ages of adularia represent? In-depth TEM and XRD analysis of adularia (in progress) from the epithermal systems may reveal lattice defects which may facilitate 40Ar loss below a nominal 200°C closure temperature. We intend to also induce artificial 40Ar loss in adularia and other sanidine samples with controlled heating experiments. This study is focused on quantifying the diffusion of 40Ar in adularia from multiple sites and developing a characterization of feldspars that will further the scientific understanding of the mechanisms for 40Ar diffusion. The improved understanding of 40Ar diffusion generated by this study will improve the accuracy of 40Ar/39Ar geochronology and improve the genetic modeling of epithermal deposits.

Geological Society of America Abstracts with Programs. Vol. 56, No. 2

doi: 10.1130/abs/2024SE-398545

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