40AR/39AR DATING OF COARSE ADULARIA FROM EPITHERMAL AU-AG DEPOSITS ASSOCIATED WITH THE EARLY MIOCENE YELLOWSTONE HOTSPOT

Adularia is a common accessory mineral accompanying Au-Ag mineralization in epithermal, low sulfidation deposits, and many studies have estimated the timing of precious mineral deposition by dating adularia with the ⁴⁰Ar/³⁹Ar method. Coarse, hydrothermal adularia is nominally a monoclinic, K-rich feldspar of intermediate order (essentially an orthoclase), but it can have structural states varying from maximum microcline to high sanidine. Coarse adularia of some epithermal Au-Ag deposits is associated with crystal habits and textures typical of near-surface geothermal systems (e.g., J. Saunders, 1990). ⁴⁰Ar/³⁹Ar ages of such adularia would be expected to be the age of adularia growth, assuming the nominal ⁴⁰Ar diffusion characteristics for adularia are comparable to those of typical feldspars. Coarse adularia from the northern Great Basin of Nevada (D. Unger, M.S. thesis, 2008) and the Silver City district of southwestern Idaho (C. Aseto, M.S. thesis, 2012) were dated by the ⁴⁰Ar/³⁹Ar method with laser fusion and incremental heating of single crystals in the ANIMAL facility. The adularia ages determined are consistent with the hypothesis that the epithermal mineralization is contemporaneous with the early initiation and subsequent migration of the Yellowstone hotspot volcanism (e.g., Brueseke et al., 2007). In detail, the adularia crystals were generally clear to milky white, 1-4 mm in their longest dimension, and deposited in colloform layers and masses bounded by quartz or crystallized as cavity-filling, euhedral rhombic crystals. The age measurements are generally consistent with crystals that contain little inherited ⁴⁰Ar and that constitute closed systems, with resulting precision (standard deviation) that is commonly within 0.2% of the measured age. However, the variability of single crystal adularia ages of a given deposit is commonly much greater. The observed variations in age are considered to result from differences in the physical grain size and structural state of single crystals and correlated variations in diffusion characteristics, together with the protracted heat flow of attendant regional volcanism and extension from 16.5-15.0 Ma. This hypothesis can be tested in this and similar settings by future studies that collect structural data and ages for single feldspar crystals.