[SRD35-06] A new orogenic gold belt in mainland SE Asia: The Vangtat gold deposit in southern Laos
キーワード：Vangtat deposit, Orogenic gold, Hydrothermal alteration, Graphite, K-Ar age dating
The Vangtat orogenic gold deposit is located within a low-grade metamorphic (greenschist facies) terrane along the western margin of Kontum massif, which is characterized by subduction and collision orogenesis of the Poko suture zone (PKSZ) during the Indosinian orogeny (270 to 230 Ma). Gold mineralization follows the N-S striking Vangtat shear zone, which is subparallel to the regional PKSZ, and extends for several kilometers along strike and a few hundred meters across. The Vangtat shear zone cuts across an early- to mid-Palaeozoic metamorphosed volcano-sedimentary sequence consisting of greenschist, pelitic schist, slate, quartzite, and metagabbro. The deposit is hosted in pelitic schist, which is made up of chlorite, muscovite, quartz, albite with minor epidote, carbonates and K-feldspar, and has been deformed with lineation and foliation formed by chlorite and muscovite.
Hydrothermal alteration and gold mineralization occur within the shear zone with a progression from proximal to distal assemblages; the intensity of alteration decreases with distance from the gold mineralized bodies. Highest grade gold mineralization (avg. >3 g/t Au) occurs in discrete quartz-sulfide veins that cut the metamorphic foliation of the immediate host rock. The quartz-sulfide veins occur in a zone 3+ kilometers long and up to 300 meters wide. The veins are surrounded by a graphite-carbonate alteration envelope, in which graphite and carbonates penetrate the host pelitic schist, especially where the foliation planes are enhanced by microshears. Precipitation of graphite and carbonates is commonly associated with sulfide dissemination and lower grade (<1 g/t) gold mineralization. Adjacent to the graphite-carbonate zone the host pelitic schist is weakly hydrothermally altered by an assemblage similar to the major silicates of the host pelitic schist and minor amounts of hydrothermal pyrite, titanite, rutile, ilmenite, hematite, magnetite, and barite.
Pyrite is the major sulfide associated with gold mineralization and is accompanied by minor chalcopyrite and arsenopyrite. Other sulfides, such as bismuthinite, galena, sphalerite, and pyrrhotite, occur as trace amounts in inclusions in pyrite. Gold in the Vangtat deposit occurs as anhedral grains in sulfides, most of the gold inclusions appear to have been incorporated during the initial growth of the sulfides. Gold also occurs in fractures, as a coating on the exterior surface of sulfides, and in the matrix of fractured sulfides. Rarely visible gold grains occur in the quartz-sulfide veins and along the alteration halo. Gold to silver ratio ranges from 5:1 to 25:1, with an average of 10:1.
The fluid inclusion study of quartz-sulfide veins shows that the fluids responsible for the mineralization are characterized by an aqueous-carbonic fluid with relatively consistent H2O-NaCl-CO2 and minor CH4-N2, low to moderate salinity (0.7 to 10.0 wt% NaCl equivalent) that homogenized to liquid at moderate temperatures (190 to 320°C). The S isotope compositions of pyrite from quartz-sulfide veins, alteration halo and host rock range from 1.3 to 10.7‰, suggesting a magmatic sulfur origin.
K-Ar age dating on pure white mica mineral separates from the quartz-sulfide veins in the Vangtat shear zone resulted in two age ranges: 235 to 205 Ma and 348 to 280 Ma. The timing of gold mineralization is interpreted to be constrained by the younger episode (235 to 205 Ma) which corresponds to the final stage of the collision of the Indosinian orogeny. The older episode (348 to 280 Ma) may represent the age of well-crystallized muscovite, suggesting the mixing of muscovite from the host rocks.
Hydrothermal alteration and gold mineralization occur within the shear zone with a progression from proximal to distal assemblages; the intensity of alteration decreases with distance from the gold mineralized bodies. Highest grade gold mineralization (avg. >3 g/t Au) occurs in discrete quartz-sulfide veins that cut the metamorphic foliation of the immediate host rock. The quartz-sulfide veins occur in a zone 3+ kilometers long and up to 300 meters wide. The veins are surrounded by a graphite-carbonate alteration envelope, in which graphite and carbonates penetrate the host pelitic schist, especially where the foliation planes are enhanced by microshears. Precipitation of graphite and carbonates is commonly associated with sulfide dissemination and lower grade (<1 g/t) gold mineralization. Adjacent to the graphite-carbonate zone the host pelitic schist is weakly hydrothermally altered by an assemblage similar to the major silicates of the host pelitic schist and minor amounts of hydrothermal pyrite, titanite, rutile, ilmenite, hematite, magnetite, and barite.
Pyrite is the major sulfide associated with gold mineralization and is accompanied by minor chalcopyrite and arsenopyrite. Other sulfides, such as bismuthinite, galena, sphalerite, and pyrrhotite, occur as trace amounts in inclusions in pyrite. Gold in the Vangtat deposit occurs as anhedral grains in sulfides, most of the gold inclusions appear to have been incorporated during the initial growth of the sulfides. Gold also occurs in fractures, as a coating on the exterior surface of sulfides, and in the matrix of fractured sulfides. Rarely visible gold grains occur in the quartz-sulfide veins and along the alteration halo. Gold to silver ratio ranges from 5:1 to 25:1, with an average of 10:1.
The fluid inclusion study of quartz-sulfide veins shows that the fluids responsible for the mineralization are characterized by an aqueous-carbonic fluid with relatively consistent H2O-NaCl-CO2 and minor CH4-N2, low to moderate salinity (0.7 to 10.0 wt% NaCl equivalent) that homogenized to liquid at moderate temperatures (190 to 320°C). The S isotope compositions of pyrite from quartz-sulfide veins, alteration halo and host rock range from 1.3 to 10.7‰, suggesting a magmatic sulfur origin.
K-Ar age dating on pure white mica mineral separates from the quartz-sulfide veins in the Vangtat shear zone resulted in two age ranges: 235 to 205 Ma and 348 to 280 Ma. The timing of gold mineralization is interpreted to be constrained by the younger episode (235 to 205 Ma) which corresponds to the final stage of the collision of the Indosinian orogeny. The older episode (348 to 280 Ma) may represent the age of well-crystallized muscovite, suggesting the mixing of muscovite from the host rocks.