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 6 0-84% heavy and resistant minerals is recorded in association with qua rtzite and metagreywacke of the Proterozoic Miri Group from the Boleng area in Fast 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, ve ry heavy (sp. gr. 3.96-4.66), massive (at places, banded) and exhibit clastic texture with rounded to subrounded and well sorted fine grains of heavy minerals pointing to a long distance of transportation. Mine ralogically, it consists of 46-61% opaques (mostly Ti-hematite), 9-13% rutile, 3-9% cyrtolite, 0.74.5% monazite and 0.2-0.9% thorite, all se t in 10-30%, sericitic matrix. Chemically, it is markedly different fr om the widespread unconsolidated coastal and inland placers, and analy ses 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 l ight 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, c onsolidated placers (fossil placers) are monazite-rich, whereas uncons olidated placers are, zircon-rich and viewed in this light, the cyrtol ite (zircon)-rich fossil placer from the Boleng area is a rare occurre nce.