Characterizing Zinc-Rich Chromite Cores and Uvarovite-Grossular Garnet Mantles from the Pikoo Property, Central Eastern Saskatchewan, Canada

Abstract

Abstract Zinc-rich chromite, (Fe,Zn)Cr2O4, is an important repository for chromium (Cr) that has been observed sporadically worldwide. As another source for Cr, green uvarovite garnet, ideally Ca3Cr2(SiO4)3, is the rarest variety among anhydrous garnets. Here, we present a detailed petrographic, mineralogical, and geochemical characterization of 71 uvarovite garnets with zinc-bearing chromite cores recovered from the Pikoo Property (central eastern Saskatchewan), which also hosts recently discovered kimberlites proven to be diamondiferous. In this work, euhedral to anhedral unzoned chromite occurs as cores or irregular inclusions enclosed by uvarovite–grossular mantles. They contain moderate to high Cr (41.63–66.70 wt.% Cr2O3; Cr/(Cr+Al) = 0.64–0.99), Fe2+ (16.71–28.67 wt.% FeO), and Zn (1.64–15.52 wt.% ZnO) contents, accompanied by an appreciable amount of Mn (0.63–2.32 wt.% MnO). The chromite core with the highest Zn content gave the structural formula (Zn0.409Fe2+0.555Mg0.018Mn0.019)Σ1.00(Cr1.174Al0.674Fe3+0.152)Σ2.00O4, which corresponds to Zn-rich chromite with a minor proportion of other endmembers (e.g., hercynite, FeAl2O4). Zinc enrichment in chromite is most likely the result of metasomatic replacement of primary chromite involving the action of Zn-rich fluids. The Zn-rich chromite cores are commonly porous and demonstrate streaking in the 2D diffraction pattern that is attributable to non-uniform strain. The strain measurements suggest the involvement of fluid-present plastic deformation during metasomatic alteration. The garnets are compositionally zoned aggregates and commonly contain inclusions. Chemical formulae indicate that they are mainly members of the uvarovite–grossular series (up to 93% mol.% Uv) enriched in Ca (22.99–35.57 wt.% CaO) and Cr (up to 28.10 wt.% Cr2O3), but consistently depleted in Mg (mean = 0.10 wt.% MgO) and Ti (mean = 0.26 wt.% TiO2). Most garnet aggregates consist of uvarovite and grossular that exhibit a core-rim zoning pattern characterized by an increase in grossular proportion at the expense of the uvarovite component. Morphological characteristics, textural relations, and compositional trends suggest that uvarovite garnet formed through interaction of Zn-rich chromite with grossular. This mineral assemblage is not genetically related to the Pikoo kimberlite and represents an independent evolutionary event. The use of Zn-bearing chromite as a new kimberlite indicator mineral therefore should be carefully evaluated prior to exploration practices.