Geologic, geochemical, and fluid inclusion studies of the tin granites from the Bujang Melaka Pluton, Kinta Valley, Malaysia

Abstract

The Bujang Melaka pluton, which has an area of 20 X 10 km, is located on the eastern side of the Kinta Valley, Malaysia. It is a typical example of the granite magmatism of this mining area. The Kinta Valley granites belong to the Permo-Triassic Main Range batholith or western granite belt of peninsular Malaysia. The Bujang Melaka pluton is composed mainly of S-type biotite granites, which cover a narrow compositional range, and was emplaced into a sequence of Devonian to Lower Permian sediments.The main intrusive phase of the pluton is a megacrystic (microcline) medium-grained biotite granite which contains abundant metasedimentary xenoliths. It is peraluminous (average A/KNC = 1.03) and has characteristics common of granites derived from a sedimentary parent rock. Its high initial 87 Sr/ 86 Sr ratio of 0.7193 and the low magnetic susceptibility of 0.2 X 10 (super -3) SI units are indicative of a significant crustal component.The megacrystic medium-grained biotite granite is a little-evolved phase, as can be shown by its relatively high concentrations of compatible elements such as Fe, P, Sc, Ti, V, and Zr.The megacrystic medium-grained biotite granite differentiated into nonmegacrystic biotite granite and (tourmaline-) muscovite granite, thereby increasing the concentration of Sn and other incompatible elements. A low-salinity, acid, Sn-bearing, aqueous fluid phase, partly mixed with CO 2 , separated from the crystallizing magma. The residual fluids interacted with the solidified part of the granite, cooled, and deposited cassiterite in the 440 degrees to 240 degrees C temperature range at pressures of approximately 1 kbar. The crystallization of cassiterite was followed by arsenopyrite, pyrite, chalcopyrite, sphalerite, galena, Bi sulfosalt, and native Bi. The style of mineralization is a variable, greisen type in the muscovite granite in the center of the pluton and a vein type in the megacrystic medium-grained biotite granite close to the contact with the sedimentary rocks.The tin granites from the Bujang Melaka pluton are similar to many other tin granites in the concentrating of Sn by fractional crystallization and by fluid-rock interaction. However, the outstanding feature of the pluton is the high level from which this concentration process started, as demonstrated by the high content of tin in the megacrystic medium-grained biotite granite (27 ppm Sn). Most tin granites in the world would have half this value or less at the same differentiation stage, taking their TiO 2 concentration as the differentiation index. The granite magma must have originated in a segment of the crust which was already enriched in tin.