Abstract

The Copper Cliff porphyry copper prospect, located 48 km east-southeast from Missoula, Montana, presents two distinct phyllic alteration events: an early green-colored expression, locally texturally destructive and associated with specular hematite and hypogene copper mineralization, and a later white variety generally structurally controlled and associated with pyrite. Variations in the mineral chemistry of white micas were evaluated by short-wave infrared spectroscopy, electron microprobe analysis, and laser ablation-inductively coupled plasmamass spectrometry (LA-ICP-MS). The analytical results allow the identification of a systematic compositional variation of the white micas, possibly controlled by redox changes in the system. The early green phyllic expression consists of Fe-bearing white micas characterized by longer Al-OH absorption wavelength (2,206–2,210 nm) and formed by oxidized magmatic fluids, whereas the late white phyllic phase contains white micas enriched in V and Sc with shorter Al-OH absorption wavelengths (2,197–2,206 nm) formed under less oxidizing conditions. Based on LA-ICP-MS trace element analyses of white micas, Cu concentrations decrease exponentially with distance from the porphyry center, in contrast to Zn, which tends to increase. The application of the Cu/Zn ratio of white micas in a manner analogous to the chlorite proximitor equation of Wilkinson et al. (2015) allows the estimation to the center of the hydrothermal system within a radial distance of approximately 710 m in samples of the early green phyllic alteration phase and within approximately 1,300 m in samples of the late white phyllic alteration style. Therefore, the Cu/Zn ratio in white micas at Copper Cliff deposit may provide a useful tool for targeting the hydrothermal center.

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