Banded Iron Formation to Blue Dust: Mineralogical and geochemical constraints from the Precambrian Jilling-Langalata Deposits,Eastern Indian Craton

Numerous economic deposits of high-grade iron ores occur in the Singhbhum-Orissa Craton, in Eastern India. The deposits are mainly located in the Jilling-Langalata, Noamundi and Joda areas which are part of the eastern limb of the regional horseshoe shaped synclinorium, where millimetre to centimetre scale Archaean Banded Iron Formation units have been converted to steel grey, iron rich fine grained powder, known as Blue Dust. Field observations and subsequent laboratory investigations indicate that in this region, the Blue Dust deposits occur as pockets or lenses of varying dimensions and are randomly oriented. However, in most cases the Blue Dust deposits are found above the Fe-rich primary host rock known as Banded Haematite Jasper. Mineralogical observations indicate that the Blue Dust is mainly composed of haematite, martite and goethite while quartz and kaolinite are the gangue minerals. Silica removal is the primary control of iron enrichment. Geochemical and field observations indicate that the Blue Dust in these deposits is regarded to be of supergene-modified hydrothermal origin. In the first stage, the early hydrothermal process affects the primary unaltered Banded Iron Formation by simultaneously oxidising magnetite to martite and replacing quartz with hydrous iron oxides. In the second stage, the supergene processes upgrade the hydrous iron oxides to fine grain microplaty haematite. The supergene process causes the leaching of remnant silica from hydrothermally upgraded iron ore under a suitable Eh and pH condition and leads to the formation of Blue Dust.