Hydrothermal alteration haloes and enlarged soil anomalies over concealed talc bodies, southwestern Montana

Abstract

Anomalous concentrations of talc have been found in residual soil associated with buried and blind talc bodies in southwestern Montana. In order to better understand the intensity and distribution of the residual talc, rock samples obtained by means of drilling have been studied. In general, except for massive talc, the rock is calcitic or dolomitic marble. In the weathering environment, the carbonate is leached away leaving the insoluble material as residual soil. This being the case, the minerals of primary interest are in the acid insoluble portion of the drilling samples. X-ray diffraction analysis shows talc to comprise 50% or more of the acid insoluble residue obtained from the marble. Talc and chlorite occur in some parts of the zone in sufficient quantities to be visually identified in the hand samples. In addition to talc and chlorite, minor amounts of chlorite/smectite and corrensite-type mixed-layer minerals occur randomly. The outer zone shows talc averaging less than 20% of the insoluble. Generally an amount of smectite±vermiculite equal to that of talc is present. Talc and other layer-silicate minerals cannot be identified in hand sample. The alteration zonation appears to be compatible with origin by hydrothermal fluids having decreasing temperature away from the talc zone. In deposits studied, the size of the alteration zones appears to be directly related to the size of the talc body. Because the insoluble minerals end up in the residual soil as a result of weathering and because the analytical procedures for Montana soils involve nonfractionated bulk samples, a large zone of talc-rich residual soil will be found to overlie and surround most buried or blind bodies suggesting a much larger body than actually present. However, such soil zones also offer sizeable targets for exploration.