ARGON DIFFUSION IN POTASSIUM FELDSPARS FROM LOW SULFIDATION EPITHERMAL SYSTEMS ALONG THE MIOCENE YELLOWSTONE LIP: UNDERSTANDING VARIATIONS IN 40AR/39AR AGES

Large Igneous Provinces (LIPs) are widely recognized as sources of heat and metals in the evolution of hydrothermal ore deposits throughout Earth’s history. Mid-Miocene magmatism in western North America resulted in development of the Columbia River LIP, the inception of the Yellowstone hotspot, and numerous epithermal Au-Ag deposits in the OR-ID-NV region. ⁴⁰Ar/³⁹Ar ages of hydrothermal K-feldspars in Au-Ag bearing veins (commonly referred to as ‘adularia’) are useful in understanding the relationship of mineralization to the broader regional magmatic evolution and to assist targeting of efforts in exploration and mine development. The Silver City District of southwestern Idaho has many historic mines and deposits of low sulfidation epithermal quartz-adularia veins that have yielded approximately $3B (present value) of Au-Ag ore. XRD analyses show that at least some of the Sliver City adularia are end-member high sanidine, with crystals ranging up to 3 mm in diameter that typically appear very fresh, euhedral and optically clear. This adularia occurs in veins that locally contain boiling textures (e.g., bladed calcite pseudomorphs) that presumably formed at low temperatures, below that which would favor loss of ⁴⁰Ar from coarse sanidine in a short-lived thermal event. Although they seem ‘ideal’ to date, and they are highly radiogenic and yield very precise ages, previous ⁴⁰Ar/³⁹Ar analyses for adularia from the Silver City mines (Aseto, 2012, AU M.S. thesis) yielded ages ranging from 16.076±0.08 to 15.367±0.061 Ma - notably younger than the regional volcanic rocks - and ages for individual crystals from single hand-samples were found to vary by up to ~ 0.7 ka. These intercrystalline variations in sanidine age could be created by variations in time of growth, loss of ⁴⁰Ar by diffusion during regional cooling, non-diffusive fluid-assisted loss of ⁴⁰Ar from earlier formed crystals, and combinations of these processes. This study is focused on understanding the mineralogy and 40Ar* transport mechanisms within adularia from the Silver City and associated, regional mining districts to better understand the timing and context of Au-Ag mineralization in this classic setting and to enhance the usefulness of ⁴⁰Ar/³⁹Ar age determinations of adularia as important tools in the precious metal exploration of LIPs in general.