14:30 〜 14:45
[HRE12-03] Characteristics of mineralization at the Doi Ngom W-Sb-F deposit, northern Thailand
キーワード：Doi Ngom deposit, breccia, tungsten, antimony, Sukhothai, Thailand
The Doi Ngom W-Sb-F deposit in northern Thailand is one of the hydrothermal mineral deposits in the Sukhothai Fold Belt which is a volcanic arc resulted from the subduction of the Paleo-Tethys oceanic plate beneath the Indochina Terrane, followed by the collision of the Sibumasu Terrane and the Indochina Terrane after the closure of the Paleo-Tethys. Tungsten as wolframite (i.e., ferberite-hübnerite solid-solution) is hosted by quartz-wolframite cemented polymictic breccia as a cement, whereas antimony and fluorine as stibnite and fluorite, respectively, are hosted by mineralized carbonaceous mudstone. The lithologic units lie in the NE-SW trend along faults with the same trend. Quartz-wolframite cemented polymictic breccia is between Triassic granite in the east, and Triassic marine-derived mudstone in the west with the mineralized carbonaceous mudstone between them. Triassic limestone is exposed in the north of the deposit. This research aims to reveal characteristics of mineralization at the Doi Ngom W-Sb-F deposit, including classification of ore-hosting breccia, based on petrography, mineralogy, and sulfur isotope signatures.
Breccia and mudstone were collected from both outcrops and stockpiles of the mining site. There are three types of breccia in the study area, 1) matrix-supported monomictic breccia, 2) matrix-supported polymictic lithic breccia, and 3) quartz-wolframite cemented polymictic breccia. The matrix-supported monomictic breccia is composed of poorly sorted angular-rounded clasts of mudstone without cement, indicating tectonic breccia formed by seismic rupture-driven fragmentation and attrition during faulting. The matrix-supported polymictic breccia is composed of poorly sorted angular clasts of mudstone, tuff, and granite and 5–10 vol% cement of microcrystalline quartz, indicating high-energy, explosive hydraulic ± seismic rupture-driven fragmentation. The quartz-wolframite cemented polymictic breccia is composed of poorly sorted angular clasts of mudstone and vuggy quartz (i.e., intensively silicified volcanic rock) and cement of microcrystalline quartz (10–20 µm in size) and fine-grained wolframite (250–500 µm in size), likely indicating hydrothermal breccia. The mudstone includes 1) carbonaceous mudstone with dissemination of stibnite and fluorite, and 2) unmineralized mudstone. Wolframite has a small variation of Fe/(Fe + Mn) atomic ratio, 0.80 to 0.93 indicating ferberite and a single stage deposition. Pyrite from the mineralized carbonaceous mudstone can be classified into two types, 1) detrital pyrite with a corroded texture, and 2) epigenetic pyrite with euhedral shape. Co/Ni ratio of both pyrite types is less than 1. δ34S values of stibnite in the mineralized carbonaceous mudstone have a narrow variation from −10.7 ‰ to −8.8 ‰. A comparison of the S-isotope data of stibnite with common geological reservoirs and the absence of metamorphic rock in the study area suggest sedimentary origin of sulfur for stibnite. Combined petrographic, mineral chemistry, and isotopic evidence suggests that wolframite precipitated from hydrothermal fluids as a breccia cement, while antimony and fluorine were likely leached from sedimentary rocks and precipitated as stibnite and fluorite associated with pyrite in the mineralized carbonaceous mudstone through the carbonization process.
Breccia and mudstone were collected from both outcrops and stockpiles of the mining site. There are three types of breccia in the study area, 1) matrix-supported monomictic breccia, 2) matrix-supported polymictic lithic breccia, and 3) quartz-wolframite cemented polymictic breccia. The matrix-supported monomictic breccia is composed of poorly sorted angular-rounded clasts of mudstone without cement, indicating tectonic breccia formed by seismic rupture-driven fragmentation and attrition during faulting. The matrix-supported polymictic breccia is composed of poorly sorted angular clasts of mudstone, tuff, and granite and 5–10 vol% cement of microcrystalline quartz, indicating high-energy, explosive hydraulic ± seismic rupture-driven fragmentation. The quartz-wolframite cemented polymictic breccia is composed of poorly sorted angular clasts of mudstone and vuggy quartz (i.e., intensively silicified volcanic rock) and cement of microcrystalline quartz (10–20 µm in size) and fine-grained wolframite (250–500 µm in size), likely indicating hydrothermal breccia. The mudstone includes 1) carbonaceous mudstone with dissemination of stibnite and fluorite, and 2) unmineralized mudstone. Wolframite has a small variation of Fe/(Fe + Mn) atomic ratio, 0.80 to 0.93 indicating ferberite and a single stage deposition. Pyrite from the mineralized carbonaceous mudstone can be classified into two types, 1) detrital pyrite with a corroded texture, and 2) epigenetic pyrite with euhedral shape. Co/Ni ratio of both pyrite types is less than 1. δ34S values of stibnite in the mineralized carbonaceous mudstone have a narrow variation from −10.7 ‰ to −8.8 ‰. A comparison of the S-isotope data of stibnite with common geological reservoirs and the absence of metamorphic rock in the study area suggest sedimentary origin of sulfur for stibnite. Combined petrographic, mineral chemistry, and isotopic evidence suggests that wolframite precipitated from hydrothermal fluids as a breccia cement, while antimony and fluorine were likely leached from sedimentary rocks and precipitated as stibnite and fluorite associated with pyrite in the mineralized carbonaceous mudstone through the carbonization process.