Radioactive fossil placer (cyrtolite-hematite ironstone) from the Bolenga area, east Siang district, Arunachal Pradesh, India

A zirconium-rare earth (RE)-thorium-uranium fossil placer with about 60-84% heavy and resistant minerals is recorded in association with quartzite and metagreywacke of the proterozoic miri group from the Boleng area in East Siang district of Arunachal Pradesh. The heavy minerals occur as thin layers with variable thickness of a fraction of a cm to a maximum of 12 cm. The mineralized rocks are hard, highly compact, very heavy (sp. gr. 3.96-4.66), massive (at places, banded) and exhibit elastic texture with rounded to subrounded and well sorted fine grains of heavy minerals pointing to a long distance of transportation. Mineralogically, it consists of 46-61% opaques (mostly Ti-hematite), 9-13% rutile, 3-9% cyrtolite, 0.7-4.5% monazite and 0.2-0.9% thorite, all set in 10-30% sericitic matrix. Chemically, it is markedly by different from the widespread unconsolidated coastal and inland placers, and analyses about 43-62% iron oxides, 1.7-5.4% ZrO2, 0.5-3.1% (RE,Y)2O3, 0.1-0.5% ThO2 and 0.01-0.9% U3O8. Chondrite-normalized REE-patterns show light REE enrichment and heavy REE depletion, with (Ce/Yb)cn ratio of 9-10 and markedly negative Eu-anomaly (Eu/Eu : 0.18-0.22). Generally, consolidated placers (fossil placers) are monazite-rich, whereas unconsolidated placers are, zircon-rich and viewed in this light, the crytolite (zircon)-rich fossil placer from the Boleng area is a rare occurrence. (author). 16 refs., 5 figs., 1 tab