R. Singh et al., RADIOACTIVE FOSSIL PLACER (CYRTOLITE-HEMATITE IRONSTONE) FROM THE BOLENG AREA, EAST SIANG DISTRICT, ARUNACHAL-PRADESH, INDIA, Journal of the Geological Society of India, 43(4), 1994, pp. 407-413
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.