Zinc-rich chromite from Paleoproterozoic conglomerates at Tarkwa gold mine, Ghana

Zn-rich chromite is an extremely rare detrital constituent in gold-bearing meta-conglomerates of the Paleoproterozoic Tarkwaian Group of Ghana. Electron-microprobe analyses of the chromite give a mean Zn content of 13.30 wt.% ZnO, with the general formula (Fe0.64Zn0.36)(Sigma 1.00)(Cr1.45Al0.33Fe0.22)O-4. The highest measured Zn content, 19.42 wt.% ZnO, yields the structural formula (Zn0.51Fe0.49)(Sigma 1.00)(Cr1.12Al0.68Fe0.20)(Sigma 2.00)O-4, which corresponds to an (Fe,Al)-rich zincochromite. Reflectance values for this composition, measured between 400 and 700 nm in air, decrease steadily from 17.0 to 14.6% with increasing Zn content. The chemical data demonstrate a large field of solid solution between chromite and zincochromite, and an extended miscibility gap in the system (Zn,Fe)Cr2O4 - ZnFe2O4 - ZnAl2O4, at least above 50 at.% (Zn,Fe)Cr2O4. The origin of the Zn-rich chromite at Tarkwa is uncertain. In the paleosource area of the Tarkwaian conglomerates, a few examples of ultramafic rocks are known, but these carry Zn-poor chromite.