Radioactive Fossil Placer (Cyrtolite-Hematite Ironstone) from the Boleng 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 mineralised rocks are hard, highly compact, very heavy (sp. gr. 3.96-4.66), massive (at places, banded) and exhbit clastic 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 different from the widespread unconsolidated coastal and inland placers, and analyses about 43-62% iron oxides, 1.7-5.4% Zr0₂, 0.5-3.1 % (RE,Y)₂O₃, 0.1-0.5% ThO₂, and 0.01-0.9% U₃O₈. Chondrite-normalizcd REE-pattems 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 cyrtolite (zircon)-rich fossil placer from the Boleng area is a rare occurrence.