Abstract

Geological, petrographical and geochemical investigations in several tin mines located in the Kathu Valley on Phuket Island resulted in the recognition of two main types of tin-bearing pegmatites. The two types are tin-tantalum pegmatites (or albite-muscovite pegmatites) and tin-rare earth element (REE) pegmatities (or orthoclase-lepidolite pegmatites). The mineralization of Sn-REE pegmatites is more complex than of Sn-Ta pegmatites.

Petrological stidies show that in Sn-Ta pegmatites, quartz (15-45%), and albite (15-30%), K-feldspar (10-50%) and muscovite (5-30%) are the main minerals, and the common heavy minerals are garnet (spessartine-almandine, concentration in 20kg samples up to 8%), cassiterite (up to 0.35%; 0.05% on an average), columbite-tantalite (up to 0.1%), tourmaline (up to 0.2%) and zircon (up to 0.1%). The zircon crystals are short-prismatic, earthy and brownish, and are typical for Sn-Ta pegmatites.

In the Sn-REE pegmatites under investigation, lepidolite (up to 20-80% in 20kg samples) is dominant. Quartz (15-30%) and orthoclase (5-30%) are the other main minerals. The associated heavy minerals are garnet, tourmaline, cassiterite, monazite/xenotime, tantalite, fergusonite, samarskite and struverite. Fluorite is abundant and topaz occurs occasionally in Sn-REE pegmatites.

All the tin-bearing pegmatites in the Kathu Valley occur in sediments or meta-sediments of the Phuket Group, late Carboniferous to Permian in age. It appears that the two main types were formed at differing distances from the source of pegmatitic fluids and are generally controlled by fractures. The Sn-Ta pegmatites occur in areas rather close to the granite body while the more complex Sn-REE pegmatites are located at greater distance (2-4 km) from the source granite.

 


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