Geology, mineralogy, geochemistry, and genesis of the Vangtat gold belt, Southeastern Laos

• Material Type: 博士論文
• Title: Geology, mineralogy, geochemistry, and genesis of the Vangtat gold belt, Southeastern Laos
• Author/Editor: BOUNLIYONG, PATTHANA
• Author Heading: BOUNLIYONG, PATTHANA
• Publication, Distribution, etc.: 秋田大学
• Publication Date: 2021-03-22
• Publication Date (W3CDTF): 2021-03-22
• Alternative Title: ラオス南東部バンタット金鉱化帯の地質、鉱物、地球化学および成因
• Degree grantor/type: 秋田大学
• Date Granted: 2021-03-22
• Date Granted (W3CDTF): 2021-03-22
• Dissertation Number: 甲第1369号
• Degree Type: 博士（資源学）
• Conferring No. (Dissertation): 甲第1369号
• Text Language Code: eng
• Target Audience: 一般
• Note (General): The Indochina Terrane is a major tectonic unit of mainland Southeast Asia, is bound to the north by the South China Terrane and to the west by the Sibumasu Terrane, with the Truong Son fold belt, and the Loei and Sukhothai fold belts marking broad collision zones with the two continental terranes. Broadly coincident with these tectonic collisions, several internal domains were amalgamated to the Indochina Terrane during the Indosinian orogeny. Magmatism, metamorphism, deformation, and hydrothermal alteration along the domain boundaries surrounding and within the Indochina Terrane resulted in the formation of a variety of precious- and base-metal deposits, notably porphyry-related skarn, epithermal, sediment-hosted, and orogenic gold deposits. The collision orogenesis between the eastern extension of Indochina Terrane with the Kontum Massif resulted in the formation of the Poko suture zone and associated structures, including the Vangtat shear zone, which the subject of this research as the host of a recently discovered orogenic gold belt. A newly discovered orogenic gold belt consists of the Thongkai-Ok, Vangtat, and Gieng Dat deposits, occurs within the Vangtat shear zone and is called as the Vangtat orogenic gold belt. This research aims to (1) clarify the nature and genesis of primary gold mineralization in the Vangtat gold belt, and (2) constrain the age of gold mineralization and to better understand the relationship between Southeast Asia tectonics and mineralization. Base on the study of host geology, geochronology, ore and gangue mineralogy, hydrothermal alteration, fluid inclusion, and sulfur isotope. The Vangtat gold belt is hosted in the early- to mid-Palaeozoic regional metamorphic belt, located in the western part of the Poko suture zone, western margin of the Kontum Massif. The Vangtat gold deposit is one of several actively mined deposits and represented the main economic orebody of the Vangtat gold belt. Two main rock units are found in the deposit scale: the lower unit is made up of metagabbro and the upper unit is composed of metasedimentary lithologies, including pelitic schist, slate, metasandstone and quartzite. The lithological units around the Vangtat deposit, as well as other hard-rock (primary) gold deposits in the district, are covered by a thick (~ up to 50 m) blanket of weathering and supergene oxidation, which led to local gold remobilization and concentration of secondary gold in surficial detrital sediments. This alluvial gold is recovered by artisanal miners through the district and marks the first recorded gold mining in the region. Pelitic schist is the main mineralized host rock of the Vangtat deposit, which is made up of foliated muscovite-chlorite with finely interlayer quartz and plagioclase. Pelitic also contained a minor amount (<1 vol%) of carbonaceous or graphitic materials, which frequently interleaved in the foliated chlorite-muscovite. Within progressive metamorphism, the layers of graphite were segregated and associated with foliation planes of pelitic schist, and became deformed, including folding and development of microshears, indicates that graphite transformation during the regional metamorphism. Raman spectrometry and its parameters were estimated the temperature of graphite transformation and provided a temperature range of 350-410 ºC. This temperature range is consistent with the major components of the pelitic schist, suggesting that graphite within the pelitic schist was metamorphosed or transformed under the greenschist facies metamorphic grade. The development of shear structures within the Vangtat shear zone intersected pelitic schist marked the favorable site for gold deposition in the Vangtat gold belt, in which the development of shear structure would create an open space and permeability for hydrothermal fluid transports ore mineral constituents upon entering an ore trap. Vein marked the center of the hydrothermal activity and represented the fluid pathway, hydrothermal alteration and fluid-rock interaction likely occurred perpendicular to the veins (fluid conduits). Innermost alteration is marked by the graphite-carbonate alteration envelope and laterally extended to the adjacent wall rocks. Moreover. the hydrothermal evolution transected two different types of wall rock lithologies within the contrasting components (e.g. metagabbro in the lower portion and pelitic schist in the upper portion) displayed a variety of alteration assemblages. High-grade gold mineralization (> 3g/t Au on average; reaching values >100 g/t Au in selected hand specimens and drill core intercepts) occurs only in the quartz-sulfide-graphite-carbonate-white mica-chlorite veins. Lower-grade gold mineralization (avg. < 3g/t Au) occurs in a graphite-carbonate-quartz-sulfide hydrothermal alteration zone that forms an envelope around the veins and grades into a sub-economic to barren altered pelitic schist. Gold is closely associated with sulfide, identically gold concentration decreases together with sulfide content from the quartz-sulfide vein at the center towards the marginally altered host rocks. Pyrite is the main host for gold mineralization in the Vangtat deposit. Most of the auriferous pyrite is homogeneous in texture (i.e. rare of zoning texture is observed), coarse-grained up to ~1 cm, consistent in composition, and less of other elements in solid solution. Other sulfides, such as chalcopyrite, arsenopyrite, sphalerite, galena, bismuthinite, and pyrrhotite occur as minor and trace amounts incorporated with pyrite, most likely occur as inclusion in pyrite. These typical characteristics of auriferous pyrite possibly suggest that pyrite has formed during the hydrothermal processes, which allows the crystallization of pyrite slowly growth and most of the trace elements are partitioned into the separated phases rather than incorporated in solid solution in pyrite. The co-existing and textural relationship of pyrite and arsenopyrite in the mineralized vein suggest an equilibrium condition among them, which applies to estimate the trapping temperature of ore formation in the Vangtat deposit. The range of As content in arsenopyrite plotted in the pyrite-arsenopyrite phase of the Fe-As-S system diagram estimated the temperature range of 335 to 385 ºC. Gold mainly occurs as particle grains inclusions in sulfide, predominantly pyrite, appearing to have been incorporated during the crystallization of sulfides. Free-gold is rarely observed in the mineralization zone. Gold to silver ratio ranges from 5:1 to 25:1, with an average of 10:1. Fluid inclusions trapped in quartz crystals from mineralized veins represented the nature of hydrothermal fluid responsible for gold mineralization in the Vangtat deposit, Vangtat gold belt, indicating that ore-forming fluid has the homogenization temperature range of 190 to 325 ºC (240 to 250 ºC is observed for the majority of fluid inclusions) and salinity range of 0.7 to 10.0 wt% NaCl equivalent (4 to 6 wt% NaCl equivalent is observed for the majority of fluid inclusions). The gas compositions are CO2, CH4, N2, and H2S. The composition of the ore-forming fluid is considered to have been generated via metamorphic dehydration and devolatilization. The presence of these gas compositions in the ore-forming fluid may indicate the reaction of fluid-wall rocks played an important role in gold precipitation. Depth of ore formation is translated using the pressure range of the ore formation, which was estimated by the correlation temperature between (1) homogenization temperature (240 to 250 °C) deduced for microthermometric data, and (2) trapping temperature (336-384 °C) obtained from arsenopyrite geothermometry. Providing the pressure range of 330 to 400 Mpa and translated to the depth range of 10 to 12 km (based on a typical lithostatic pressure gradient of 100 MPa=3 km). Gold is carried as the sulfide complex along with the ore fluid. The precipitation of gold is related to both (1) The destabilization of sulfide complex by sulfidation reaction, which involves the interaction of Fe-bearing mineral (e.g. chlorite) in the host rock and S-bearing fluid, and (2) Changing the physical-chemical of the ore-forming fluid, which relates to the reductive reaction with the carbonaceous material or graphite dominant in the host rock and alteration envelope that facilitated gold extraction. The sulfur isotope composition of auriferous pyrite at the Vangtat deposit ranges from +1.3 and +10.7 ‰, a narrow range is confined from +4 to +6 ‰ for the most population data and corresponding to the high-grade gold assay. These sulfur isotope compositions are not related to the sedimentary sulfur through the geological time, together with a narrow range of sulfur isotope composition may indicate the single source of sulfur. Hence, the input of sulfur sourced from the metasedimentary host rocks can not explain in the sulfur isotope data. However, these sulfur isotope compositions are compatible with the sulfur isotope composition range of the most igneous rocks, and it could potentially be the source of sulfur and ore components. The presence of hydrothermal white mica-bearing auriferous vein from the Thongkai-Ok deposit was subjected for K-Ar dating in order to constrain the age of gold mineralization in the Vangtat gold belt. The results provided a wide range of ages from 348 to 206 Ma. Within the interpretation of excess argon derived from contaminated quartz as an impurity of samples defined the youngest age (206 Ma) of a quartz-free fraction was the closest age of gold mineralization in the Vangtat gold belt. A 206 Ma age of gold mineralization in the Vangtat gold belt, southeast Laos is consistent with regional geological and geochronological data and indicates a post-collisional, post-peak metamorphism hydrothermal gold event that is potentially younger than much of the orogenic gold mineralization in nearby Vietnam. In conclusion, the nature and genesis of gold mineralization in the newly discovered Vangtat gold belt have many similarities to the orogenic gold deposits globally.
• DOI: 10.20569/00005717; https://doi.org/10.20569/00005717
• Persistent ID (NDL): info:ndljp/pid/11679171; https://dl.ndl.go.jp/pid/11679171
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• Date Accepted (W3CDTF): 2021-06-07T02:06:26+09:00
• Date Created (W3CDTF): 2021-06-03
• Format (IMT): PDF
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• Periodical Title (URI): http://dx.doi.org/10.20569/00005717; http://hdl.handle.net/10295/00005717
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