Modeling Indium Enrichment in the Mount Pleasant Ore System

The Mount Pleasant deposits are granite-associated Sn-W-Mo-In deposits in New Brunswick, Canada. The granites formed during three intrusive stages, of which the first two were ore-forming and generated the Fire Tower Zone (FTZ) and the North Zone. The goal of this study is to characterize the host topaz/A-type granite and elucidate the source of indium. Host granite mineralogy in the FTZ has been analyzed by EPMA and LA-ICP-MS. The primary mineralogy of the granite comprises two feldspars (potassium feldspar and albite), quartz, topaz, mica, and minor fluorite. Textural relationships of topaz to other phases indicate that it is magmatic and has been largely altered to sericite + fluorite. The mica is lithium- and rubidium-rich (2.4 wt% Li₂O; 1 wt% Rb₂O) and falls along the siderophyllite-polylithionite join. These characteristics are consistent with a highly evolved magmatic system, which ultimately deposited indium-bearing sulfides. The indium is likely magmatic; however, the size and indium concentration of the source region for the magmas is unconstrained. These variables can be evaluated by combining published whole rock chemical data, resource estimates, estimates of the mass of emplaced granite, and models for topaz/A-type granite formation. The conceptual model presented here begins with the partial melting of a mid-lower crustal ferrodiorite (Christiansen et al., 2007) with 0.05 ppm indium (Rudnick and Gao, 2003), producing a granitic magma with 0.05 to 0.2 ppm indium. This magma fractionally crystallizes forming a granite, as well as a residual topaz/A-type magma with 0.2 to 1 ppm indium. Based on these estimates and estimates of indium tonnage, the residual ore-forming magma for the Mount Pleasant deposits was between 2 and 38 km³. The topaz/A-type magma fractionally crystallizes, becoming volatile saturated at 80% melt remaining, and complete crystallization removes 97% of the indium from the melt into the volatile phase. Mica is the only phase sequestering indium (bulk partition coefficient of 0.004) and has a predicted indium concentration of 0.09 to 0.7 ppm, which is similar to the observed concentration of 0.35 ± 0.02 ppm. The size of the source magma is neither particularly large nor rich in indium, indicating that average magma compositions can form indium deposits under favorable conditions.