Gahnite from metamorphosed massive sulphide deposits and rare-element pegmatites: Development of discriminators based on bedrock and overburden samples

Gahnite, ZnAl₂O₄, is a potential indicator mineral for metamorphosed massive sulphide deposits and rare-element pegmatites due to its hardness and chemical stability in surficial environments. Gahnite grains were recovered from bedrock samples from the Langlade massive sulphide occurrence in Quebec, the Geco massive sulphide deposit and the Separation Lake rare element pegmatite area, both in northwestern Ontario. Gahnite was also recovered from overburden samples from Langlade and Separation Lake. Compositionally, gahnite recovered from metamorphosed massive sulphide occurrences is relatively rich in MgO (>2.0 wt.%) and depleted in ZnO (<32.7 wt.%) relative to that recovered from peraluminous granite (MgO 0.3–0.7 wt.%, avg. ZnO 32.8 wt.%) and rare-element pegmatite (MgO < 0.3 wt.%, avg. ZnO 37.3 wt.%). Gahnite recovered from overburden showed a wider dispersion and overlap in mineral chemistry relative to that recovered from bedrock. It was determined that inclusion assemblages within gahnite appear to reflect host-rock assemblages and may be the most useful criteria in distinguishing between environments. Inclusions such as sulphides, magnetite and corundum occur more frequently in metamorphosed massive sulphide deposits, while rare-element bearing phosphates, muscovite, nigerite and feldspar are generally more abundant in rare-element pegmatites and related peraluminous granites. These results suggest that the inclusion assemblages in addition to the composition of gahnite may be useful for using gahnite as an exploration tool.