Abstract
The Grasshopper prospect, located 23 km west-southwest from Dillon, Montana, presents exposed zones of phyllic alteration assemblages comprising the early and late phyllic styles. The mineral chemistry of white micas from both phyllic alteration zones was evaluated by short-wave infrared spectroscopy, electron microprobe analysis, and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). The early phyllic expression consists of white to green micas characterized by longer Al-OH absorption wavelengths (2,204–2,210 nm), whereas the late phyllic phase contains white micas with shorter Al-OH absorption wavelengths (2,197–2,204 nm). Correlation with electron microprobe data found that the Tschermak substitution in the white micas is mainly controlled by Mg concentrations. Based on LA-ICP-MS data, higher Mn and Sr concentrations characterize white micas from the early phyllic alteration, whereas higher concentrations of B, Ba, Cr, Cs, Cu, Li, Rb, Sc, Sn, Ti, Tl, V, and W are present in white micas from the late phyllic style. Systematic zoning patterns of trace element concentrations in white micas from the early and late phyllic alteration styles were confirmed at Grasshopper. In general, increasing trends toward the center of the system were observed in V, Cu, Sc, Sn, W, and Zn, whereas increasing trends outward from the hydrothermal center were reported in Li and Cs. Comparison of the trace element concentrations of white micas from the early phyllic style from the barren system of Grasshopper and the mineralized system of Copper Cliff indicates significant differences in Zn, Cr, B, Tl, Sn, and Cs. Therefore, we propose a preliminary discrimination (Zn + Cr + B vs. Tl + Sn + Cs) plot that can be used to differentiate white micas from the early phyllic alteration among mineralized and weakly to unmineralized systems.
