ALBERT

Description: We report Raman spectroscopic analyses and elastic modeling of mineral inclusions from three skarns in the Ludwig area, Yerington District, Nevada. Raman spectroscopy shows that at ambient conditions, apatite inclusions in garnet from the shallowest skarn, Casting Copper, retain tensile pressures of –75 ± 30 MPa (2σ; n = 33). Apatite inclusions in garnet from the intermediate-depth Douglas Hill deposit retain pressures of –162 ± 24 MPa (2σ; n = 15). Apatite inclusions in garnet from the deepest skarn, Hill 5923, retain pressures of –177 ± 21 MPa (2σ; n = 20). Quartz inclusions in garnet retain pressures of –111 ± 31 MPa (2σ; n = 7) and –152 ± 86 MPa (2σ; n = 10) for the Douglas Hill and Hill 5923 deposits, respectively. Quartz inclusions in garnet from Hill 5923 retain pressures of –152 ± 86 MPa (2σ; n = 10). The higher tensile stress within inclusions from the deeper skarns is related primarily to the higher formation temperatures of these deeper skarns. Elastic modeling of apatite-in-garnet and quartz-in-garnet from the three deposits is in good agreement with stratigraphic paleodepth estimates and corroborates higher formation temperatures for the deeper skarns. Fluid inclusions from these skarns show a bimodal temperature distribution, with inclusions interpreted as primary showing homogenization by critical phenomena at approximately 400 °C, whereas lower-temperature liquid-rich inclusions are interpreted as secondary. Isochore modeling of the critical-density inclusions is in good agreement with the results from quartz and apatite inclusions in garnet. Overall, these results indicate that mineral inclusion thermobarometry is an effective method of estimating formation pressures of otherwise inscrutable metasomatic rocks.

Description: Texturally destructive phlogopite-rich alteration occurs as a narrow, curvilinear zone with a width of 〈0.5 to 3 m exposed discontinuously over ∼1 km along strike in eastern exposures of the Yerington batholith, Yerington district, Nevada. The phlogopite is preferentially oriented and defines foliation, suggesting that the alteration zone has accommodated structural deformation. The mineral association consists of near-endmember phlogopite with local clusters of euhedral dravitic to oxy-dravitic tourmaline crystals. Tourmaline-hosted fluid inclusions are high-density aqueous inclusions that generally homogenize between 230 and 330 °C and contain ∼7–20 wt.% NaCl equivalent. The alteration is hosted within the Luhr Hill porphyritic granite and, although timing of the alteration is unclear, it likely postdates and is unrelated to well-known porphyry systems in the district. The phlogopite-rich assemblages represent an unusual style of Mg-K-rich alteration of a granitoid without exposed adjacent ultramafic or dolomitic carbonate units. Phlogopite-rich alteration is potentially related to the circulation of moderately saline to hypersaline external fluids through the Luhr Hill porphyritic granite.