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

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.