13:45 〜 15:15
[HRE11-P06] Mineralogy and geochemistry of the Pilok Sn-W deposit, Kanchanaburi, Thailand
キーワード：Pilok, Thailand, Sn-W, geochemistry, mineralogy, S-isotope
The Pilok Sn-W deposit, located within the Southeast Asian Tin Belt, is one of the most important Sn-W deposits in Thailand. It consists mainly of northwest-southeast trending quartz veins, which follow the Three–Pagoda fault zone. The quartz veins are hosted in a Late Cretaceous S-type granite that intruded into Paleozoic metasediments. The granite is fine- to coarse-grained, displaying both porphyritic and equigranular textures. However, updated data on the mineralogy and geochemistry of the deposit are limited. Therefore, this study aimed to investigate the mineralogical, textural, and geochemical characteristics of the Pilok deposit using optical petrography, scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), inductively coupled plasma mass spectroscopy (ICP-MS), and sulfur isotope analyses.
The main mineralization occurs as greisen and veins. The greisen-type mineralization is characterized by abundant tourmaline and muscovite, minor anhedral wolframite and K-feldspar, and trace amounts of rutile, zircon, monazite, and xenotime. On the other hand, the vein-type mineralization consists of early quartz + tourmaline ± muscovite veins and late quartz-sulfide-cassiterite veins. Microscopic observation shows no occurrence of ore minerals in the early quartz + tourmaline ± muscovite veins. In contrast, the quartz-sulfide-cassiterite veins have abundant ore minerals, such as arsenopyrite, pyrite, pyrrhotite, cassiterite, and several Bi-sulfosalts. Cassiterite occurs as either euhedral to subhedral grains or as inclusions within arsenopyrite.
The whole-rock geochemical analyses of the granite provide evidence for the genesis of the Sn-W mineralization as follows. The granite displays a positive correlation of both the high-field strength elements (HFSE) and the large-ion lithophile elements (LILE) with Sn and W. This suggests that Sn and W were concentrated in residual magmas of granite through fractional crystallization. The results indicate a negative correlation between Sr and Rb/Sr, which supports the occurrence of fractional crystallization in the source magma. Also, a positive correlation between Sn and Rb/Sr confirms an enrichment of Sn through fractional crystallization. The sulfur isotope data from pyrite and arsenopyrite in the quartz-sulfide-cassiterite veins indicate a narrow δ34SCDT range from -1.9 to -1.2 ‰ for pyrite and -1.2 to -0.2 ‰ for arsenopyrite, suggesting a relatively homogeneous source of sulfur, likely derived from magma.
The main mineralization occurs as greisen and veins. The greisen-type mineralization is characterized by abundant tourmaline and muscovite, minor anhedral wolframite and K-feldspar, and trace amounts of rutile, zircon, monazite, and xenotime. On the other hand, the vein-type mineralization consists of early quartz + tourmaline ± muscovite veins and late quartz-sulfide-cassiterite veins. Microscopic observation shows no occurrence of ore minerals in the early quartz + tourmaline ± muscovite veins. In contrast, the quartz-sulfide-cassiterite veins have abundant ore minerals, such as arsenopyrite, pyrite, pyrrhotite, cassiterite, and several Bi-sulfosalts. Cassiterite occurs as either euhedral to subhedral grains or as inclusions within arsenopyrite.
The whole-rock geochemical analyses of the granite provide evidence for the genesis of the Sn-W mineralization as follows. The granite displays a positive correlation of both the high-field strength elements (HFSE) and the large-ion lithophile elements (LILE) with Sn and W. This suggests that Sn and W were concentrated in residual magmas of granite through fractional crystallization. The results indicate a negative correlation between Sr and Rb/Sr, which supports the occurrence of fractional crystallization in the source magma. Also, a positive correlation between Sn and Rb/Sr confirms an enrichment of Sn through fractional crystallization. The sulfur isotope data from pyrite and arsenopyrite in the quartz-sulfide-cassiterite veins indicate a narrow δ34SCDT range from -1.9 to -1.2 ‰ for pyrite and -1.2 to -0.2 ‰ for arsenopyrite, suggesting a relatively homogeneous source of sulfur, likely derived from magma.