Mineral chemistry of gahnite from the Lalor metamorphosed VHMS deposit, Snow Lake, Manitoba

Gahnite (ZnAl2O4) is a common accessory mineral at the Lalor auriferous Zn-Cu metamorphosed VHMS deposit (Snow Lake, Manitoba). To evaluate factors influencing its crystal chemistry, gahnite representing a range of textures, host mineral assemblages, and whole-rock compositions were analyzed for major, minor, and trace elements. The analyzed grains span the range of Ghn63-75Her15-22Spl10-18 and are un-zoned with respect to Zn, Fe, and Mg. A moderate positive correlation exists between Mg in gahnite and whole-rock MgO (R2 = 0.66). The minor- and trace-element chemistry of the Lalor gahnite is dominated by Mn (400–2600 ppm), Si (<25–250 ppm), and V (<25–2300 ppm). Based on the limited variability in gahnite major-element composition, as well as similar partitioning coefficients of Zn and Fe between sphalerite-gahnite pairs (indicating comparable metamorphic conditions of crystallization for the analyzed gahnite), metamorphic grade is interpreted to have had the strongest influence on gahnite major-element chemistry. Most sphalerite occurs with pyrite and pyrrhotite, an assemblage that would have buffered fS2 and fixed the Zn:Fe ratio in sphalerite, which also could have contributed to the narrow compositional range observed in gahnite. Magnesium was not an essential component of the sphalerite-consuming, gahnite-producing reactions, so its concentration in gahnite was more readily affected by whole-rock Mg. A small proportion of gahnite grains may have formed from the destabilization of silicates (staurolite and biotite), rather than sphalerite. These possible gahnite-forming reactions (sphalerite- versus biotite- or staurolite-consuming) appear to have had the strongest control on gahnite minor- and trace-element chemistry, as gahnite formed from sphalerite desulfidation reactions shows a range in Mn (450–2600 ppm) and restricted V/Mn values (<0.5), while gahnite interpreted to have formed from the dehydration of biotite and staurolite shows restricted Mn (<430 ppm) and a range of V/Mn values (0.75–5.5). Further work is recommended to investigate the possibility of using gahnite trace-element signatures (such as with Mn and V) to discriminate between gahnite that crystallized in sphalerite-rich and sphalerite-barren environments, as this concept has potential for application to exploration using detrital gahnite.