Understanding 40Ar/39Ar Age Variations in Potassium Feldspars from Low-Sulfidation Epithermal Systems in the Miocene Yellowstone Hotspot

Abstract

The inception of the Yellowstone hotspot resulted in the Columbia River Large Igneous Province (LIP), Mid-Miocene magmatism in western North America, and numerous epithermal Au-Ag deposits in the Oregon-Idaho-Nevada 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 assisting in targeting exploration and mine development efforts. Although they seem ‘ideal’ to date, and they are highly radiogenic and yield precise ages, 40Ar/39Ar analyses for adularia from the Trade Dollar Mine (Silver City District (SCD), Idaho) and the Jumbo Mine (Nevada) yield ages ranging from 16.27 ±0.11 to 15.79±0.02 Ma, and 16.86±0.13 to 16.15±0.07 Ma respectively. These ages are 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 Ma. These age dates could indicate a remarkably long-lived system, with episodic precipitation of adularia. However, it is more likely that these intercrystalline variations in adularia ages are created by varying diffusive properties between adularia crystals, in combination with episodic precipitation of adularia. X-ray diffraction (XRD) data indicate adularia in the SCD, and the Jumbo Mine are high sanidine (face-centered monoclinic, C2m), suggesting rapid crystallization. Rapid crystallization would prevent the ordering of Al into its preferred T1o site and generate defects in the crystal lattice. Fluid inclusion homogenization temperatures from the SCD range from 180-285°C (Halsor et al., 1988; Aseto, 2012), at or above the calculated closure temperatures (200-266°C) for adularia from the Trade Dollar Mine. This means that upon crystallization, the adularia crystals were partially open systems that lost some radiogenic 40Ar. The range of 40Ar/39Ar closure temperatures are interpreted to be created by effective diffusion dimensions that vary from crystal to crystal. These varying effective diffusion dimensions are likely due to the rapid precipitation of adularia from solution, generating lattice vacancies, and linear and planar defects. This study concludes that 40Ar/39Ar ages reflect the age at which the individual crystal cooled past its respective closure temperature, and not the Au-Ag mineralization age.