Mineral chemistry of Lithium micas and geochemistry of Lithium-bearing pegmatites and Khao Po Granites in Western Granitic Belt, Phang-Nga Province, Thailand

Abstract

Lithium-bearing pegmatites in Takua Thung District, Phang-Nga Province, are one of the lithium-deposit sites in southern Thailand. These pegmatites intruded into sedimentary country rocks near the Khao Po granites, a part of the Western Granitic Belt, and parallel to the Khlong Marui Fault Zone to the northeast trend. Representative samples of lithium-bearing pegmatites and Khao Po granites are studied based on petrography, whole-rock geochemistry, and mineral chemistry of lithium micas in lithium-bearing pegmatites. Lithium-bearing pegmatites are made up of quartz, K-feldspar, and albite with accessory minerals: lepidolite, zinnwaldite, topaz, tourmaline, beryl, apatite, fluorite, microlite, and cassiterite in different proportions. Based on their distinct appearance in studied samples, accessory mineral assemblages can be categorized into six groups, including Groups I, II, III, IV, V, and VI. Group I contains topaz, ± beryl, ± microlite, and ± apatite. Group II consists of tourmaline, ± topaz, ± microlite, and ± apatite. Group III is made up of microlite and ± beryl. Group IV comprises fluorite, ± microlite, and ± apatite. Group V includes beryl, ± microlite, and ± apatite. Group VI consists of topaz, tourmaline, and beryl. In addition, lepidolite, zinnwaldite, and cassiterite are accessory minerals which can be found in all groups. The mineral chemistry of lithium micas is illustrated by the following chemical formulas: lepidolite, K2.0(Li2.4-3.0Al2.3-3.4)(Si6.4-6.7Al1.3-1.6)O20(OH,F)4; and zinnwaldite, K1.7-2.4(Li1.7-2.3Al3.0-4.1)(Si5.8-6.4Al1.6-2.2)O20(OH,F)4. The geochemical analysis indicates that these pegmatites can be classified as the lepidolite-subtype pegmatites or the Li-Cs-Ta (LCT) pegmatites, with peraluminous S-type characteristics derived from partial melting of metasediments. Their emplacement is approximately 2 – 4 kbar and 450 – 650°C. By the difference in SiO2 of whole rocks, they can be grouped into two groups, comprising low SiO2 lithium-bearing pegmatites (LSLP) and high SiO2 lithium-bearing pegmatites (HSLP). Khao Po granites are mainly syenogranite, slightly quartz syenite and monzogranite, divided into two groups by the difference in textures and mineral constituents, including porphyritic granites and non-lepidolite pegmatites. Porphyritic granites contain quartz, plagioclase, orthoclase, microcline, biotite, tourmaline, muscovite, ± garnet, ± zircon, ± rutile, ± apatite, ± opaque minerals, and ± topaz in different proportions. Non-lepidolite pegmatites are made up of quartz, K-feldspars, plagioclase, muscovite, garnet, ± biotite, ± zircon, and ± opaque minerals, and ± allanite in different proportions. Due to the distinct TiO2, Khao Po granites can be divided into two groups, containing low TiO2 Khao Po granites (LTKG) and high TiO2 Khao Po granites (HTGK). Their bulk-rock composition suggests that Khao Po granites are S-type peraluminous granites, derived from partial melting of metapelites and metagreywackes. Although Khao Po granites and lithium-bearing pegmatites have some geochemical characteristics in common, they are not congenitally related to each other, as suggested by the difference in major and trace elements of the bulk rock and their REE patterns. The formation of lithium-bearing pegmatites may possibly be related to other granitic plutons in the Western Granitic Belt or linked to the formation of the Khlong Marui Fault Zone.