16:00 〜 16:15
[HRE12-08] Geochemistry and U-Pb geochronology of granites from the Pilok tin-tungsten deposit, Kanchanaburi, Thailand
キーワード：Pilok Sn-W deposit, granite, geochemistry, zircon U-Pb age
The Pilok Sn-W deposit, located within the Southeast Asian Tin Belt, is one of the most important Sn-W deposits in Thailand. The main Sn-W mineralization in the Pilok deposit occurs as veins, greisen, and veinlets. Quartz veins are hosted in granite that intruded into the Paleozoic metasediments. This study aims to investigate the mineralogy, geochemistry, and ages of granite based on petrography, X-ray fluorescence spectrometry (XRF), solution inductively coupled plasma mass spectrometry (ICP-MS), scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS), cathodoluminescence (CL), and U-Pb dating by laser ablation (LA) ICP-MS.
The granites in the Pilok deposit are divided into biotite-muscovite granite and tourmaline-biotite-muscovite granite based on their mineralogy. Phenocrysts of both granites are predominantly fine- to coarse-grained, displaying porphyritic and equigranular textures. The biotite-muscovite granite is composed mainly of quartz, plagioclase, K-feldspar, and muscovite, with accessory zircon, biotite, apatite, and opaque minerals. The tourmaline-biotite-muscovite granite has a mineral composition similar to that of biotite-muscovite granite, except for the occurrence of tourmaline. Bulk chemical compositions of all the granites show SiO2 (72.3-75.4 wt.%), K2O (4.5-5.6 wt.%), Na2O (2.3-3.7 wt.%), Fe2O3 (1.1-2.1 wt.%), MgO (0.1–0.2 wt. %), and CaO (0.4–0.6 wt.%). Whole-rock chemistry indicates that the granites are peraluminous (ASI = 1.2 - 1.5) and calc-alkaline in composition, classifying them as an S-type granite. Additionally, the granites exhibit low CaO/Na2O ratios (0.12–0.22), suggesting that they derived from the partial melting of pelitic rocks, which is further supported by the high Rb/Sr (5.3–26) and Rb/Ba (2.36–10.6) ratios. Zircon U-Pb dating yielded 211.7 ± 0.72 Ma and 211.1 ± 1.5 Ma for the biotite-muscovite granite and tourmaline-biotite-muscovite granite, respectively. The ages indicate Late Triassic magmatism, which implies a post-collisional setting for the granites in the Pilok area and likely for the associated Sn-W mineralization. This suggests that these peraluminous S-type granites were formed during the Sibumasu-Indochina collision driven by the closure of the Paleo-Tethys Ocean. Additionally, an older age of 422 Ma was obtained from inherited zircon, likely reflecting remnants of pre-existing crustal material incorporated into the magma during its evolution.
The granites in the Pilok deposit are divided into biotite-muscovite granite and tourmaline-biotite-muscovite granite based on their mineralogy. Phenocrysts of both granites are predominantly fine- to coarse-grained, displaying porphyritic and equigranular textures. The biotite-muscovite granite is composed mainly of quartz, plagioclase, K-feldspar, and muscovite, with accessory zircon, biotite, apatite, and opaque minerals. The tourmaline-biotite-muscovite granite has a mineral composition similar to that of biotite-muscovite granite, except for the occurrence of tourmaline. Bulk chemical compositions of all the granites show SiO2 (72.3-75.4 wt.%), K2O (4.5-5.6 wt.%), Na2O (2.3-3.7 wt.%), Fe2O3 (1.1-2.1 wt.%), MgO (0.1–0.2 wt. %), and CaO (0.4–0.6 wt.%). Whole-rock chemistry indicates that the granites are peraluminous (ASI = 1.2 - 1.5) and calc-alkaline in composition, classifying them as an S-type granite. Additionally, the granites exhibit low CaO/Na2O ratios (0.12–0.22), suggesting that they derived from the partial melting of pelitic rocks, which is further supported by the high Rb/Sr (5.3–26) and Rb/Ba (2.36–10.6) ratios. Zircon U-Pb dating yielded 211.7 ± 0.72 Ma and 211.1 ± 1.5 Ma for the biotite-muscovite granite and tourmaline-biotite-muscovite granite, respectively. The ages indicate Late Triassic magmatism, which implies a post-collisional setting for the granites in the Pilok area and likely for the associated Sn-W mineralization. This suggests that these peraluminous S-type granites were formed during the Sibumasu-Indochina collision driven by the closure of the Paleo-Tethys Ocean. Additionally, an older age of 422 Ma was obtained from inherited zircon, likely reflecting remnants of pre-existing crustal material incorporated into the magma during its evolution.