Characteristics of granitic rocks of Bangka Island, Indonesia, and their associated mineralization

SITHA, Kong, Dr. Lucas Donny Setijadji

2009 | Tesis | S2 Teknik Geologi

Bangka island is located at southeast Sumatra island, Indonesia. This island consists of several granitic plutons which belong to main range, and eastern belt of Southeast Asian granite belt trending from Burma, Thailand, and Malaysia and associate with tin mineralization. The main sources of tin production of this island are from placer deposits mostly from the northern part. Recently granites of this tropical region are subjected to the study of REE mineralization as the evidences from REE deposits in Sn-bearing granites and their weathered crusts, Sn placer of southern Thailand. Hence 95 samples from bulk rocks and soils are collected throughout the island for analyses such as magnetic susceptibility, XRF, ICP-MS, and fluid inclusions. The purposes are to study the characteristics of Bangka granitic rocks, the tectonic setting during the emplacement of the granites, and the associated mineralization, particularly, Sn and REE. Granitic rocks of Bangka island are characterized by metaluminous and peraluminous monzo-granites. These granites are classified as high K- and shoshonite type. The emplacement of the studied granites was in the syn-collisional as well as within-plate environment. These granitic magmas derived from two major sources, i.e. garnet bearing melt (lower continental crust) and plagioclase bearing melt (upper continental crust <40 km). The latter underwent higher degree of magmatic fractionation. Besides these source was characterized by extensive fluid. Significant tin anomaly is found in quartz vein in weathered granite body (Jangkang and Tinggi island), stream sediment (Sungai Liat), and beach sand (Toboali). Tin mineralization was formed in the low temperature (210-220 oC) and salinity (5-6 wt. % NaCl equivalent). Crustal involvement, syn-collision environment, and high degree of plagioclase fractionation play important role in tin enrichment. Besides Pb anomaly is also observed as the by-products of tin such as in Jangkang and Tinggi island. REE (including Y) in Bangka granites are mostly higher than 200 ppm and locally up to 649 ppm (Toboali). REE enrichment is found mostly in horizon C, stream sediment (Sungai Liat), beach sand (Toboali), and quart veinlet in metasedimentary rocks at Tinggi island. REE is revealed as depleted in the granitic magmas with increasing degree of magmatic fractionation.

Kata Kunci : Granitic rocks,Bangka island,Sn and REE mineralization