Bulletin of the Geological Society of Malaysia, Volume 19, April 1986, pp. 261 – 280
Department of Geology, Faculty of Science, Chulalongkom University
Abstract: Approximately three-fourth of the area, 210 sq km, in the central part of Phuket Island is covered with granitic rocks of Kathu Plutons which range in ages from Cretaceous to Tertiary. Five types of granite are recognized in the field, namely, coarse-grained porphyritic biotite granite (G-1), fine-to medium-grained biotite granite (G-2), medium-to coarse-grained mica granite (G-3), fine-to medium-grained mica granite (G-4), and fine-grained mica-tourmaline granite (G-5).
The G-1 granites are the major phase and are composed mainly of plagioclase, biotite, microline, quartz, sphene, and allanite. The non-porphyritic G-2 granites are mineralogically similar to the porphyritic G-1 granites. The G-3 granites are distinguished from the G-1 and G-2 granites by the lower color index and the presence of muscovite, tourmaline and garnet. The G-4 and G-5 granites are petrographically similar to the G-3 granites except for the presence of commonly unzoned plagioclase and the widespread occurrence of muscovite, tourmaline, and fluorite. Intense effects of metasomatic and pneumatolytic alterations are also additional characteristics of the G-4 and G-5 granites. The contact relationships of the G-1 & G-2 with the G-3, G-4 and G-5 are sharp in many parts of the area.
Petrochemically, the granites are peraluminous. The G-1 and G-2 granites are regarded as tin-barren granites whereas the G-4 and probably G-3 and G-5 are tin-bearing granites. The tin-bearing granites, compared with the tin-barren ones, are more intensely altered and are more differentiated as evidenced by higher contents of SiO2, K2O, Rb, Nb, and Sn and lower TiO2, FeO (total), AI2O3, MgO, MnO, CaO, P2O5, Sr and Ba contents. The tin deposits are principally found in greisenized G-4 granites and pegmatites.