Geochemistry of detrital zinc-rich chromite in conglomerates from eastern India

Abstract

Radioactive conglomerate overlies Archaean basement Bonai granite, located on the western part of the eastern Indian shield. The conglomerate contains well-rounded, fractured Mg-poor (< 0.18 wt.% MgO), aluminous (up to 21.14 wt.% Al 2 O 3 ) zincian chromite (> 35.00 wt.% Cr 2 O 3 ) containing up to 15.5 wt.% ZnO. This is the first reported occurrence of detrital zincian chromite having such unusually high ZnO from India, and to our knowledge, the third reported occurrence in radioactive quartz pebble conglomerate after Witwatersrand, South Africa and Tarkwa, Ghana. Zincian chromite grains are either clean or contains exsolution blebs/lamella of rutile. The conglomerates bear testimony of post-depositional hydrothermal fluid influx and fluid-induced mineral alterations. The strong negative correlation between Zn and Fe 2+ and between Al and Cr and Al and Fe 3+ , and strong positive correlation between Zn and Al suggest secondary incorporation of Zn and Al by substituting Fe 2+ and Cr (and Fe 3+ ) respectively leading to partial transformation of Fe(Cr, Al) 2 O 4 towards ZnAl 2 O 4 composition.The chromite grains were most possibly derived from komatiite. The timing of Znenrichment, either at the provenance prior to sedimentation or at the depositional site postdating sedimentation, remains unresolved.