Myanmar is endowed with abundant Sn-W mineralization, pre-eminent amongst which is the world-class Mawchi deposit. In the Mawchi area, N-S trending vertical or steeply dipping quartz veins are hosted by both Eocene granite and Carboniferous to Early Permian metasediments. Three stages of ore formation are recognized; (i) tourmaline-cassiterite stage (ii) main ore stage and (iii) sulfide stage. Tourmaline, cassiterite and pyrite-I are early-formed minerals and are representative of the first stage. Their deposition continued together with wolframite, scheelite, molybdenite, arsenopyrite, pyrite-II, fluorite and danalite, which form the second stage. This was followed by the successive deposition of sulfides such as chalcopyrite, sphalerite, galena and Pb-Bi sulfides. A 40Ar/39Ar magmatic biotite plateau age of 41.50 +/- 0.16 Ma (MSWD = 2.5) is determined for the Mawchi biotite granite which we interpret as the time the granite cooled through the biotite blocking temperature, and is consistent with a previously reported LA-ICP-MS U-Pb zircon concordia age of 42.72 +/- 0.94 Ma (MSWD = 2). A molybdenite Re-Os model age of 42.4 +/- 1.2 Ma indicates that Sn-W mineralization was synchronous with late Eocene granitic magmatism. Our 40Ar/39Ar hydrothermal muscovite plateau ages from the tourmaline granite (40.14 +/- 0.14 Ma; MSWD = 1.48) and quartz vein selvages (40.80 +/- 0.12 Ma; MSWD = 0.47) define the timing of hydrothermal alteration and simultaneous veining that accompanied the late stage of ore forming at Mawchi. Fluid inclusion microthermometry from cassiterite, scheelite, quartz, and fluorite reveals that ore fluids in the cassiterite-tourmaline stage and main ore stage are characterized by moderate homogenization temperatures (Th = 260-345 degrees C) and salinities (4.5-15.7 wt% NaCl equiv), while the sulfide stage is characterized by lower temperatures (Th = 175-260 degrees C) and moderate to low salinity (2.5-13 wt% NaCl equiv). The mean delta S-34 of all sulfides is 2.9 +/- 2.9 parts per thousand, which is suggestive that the overall system is dominated by magmatic sulfur. The similarity of delta S-34 values in galena and Pb-Bi sulfides (-1.3 to 2.7 parts per thousand) suggest that sulfur, and inference the Pb and Bi were transported by a common fluid, probably of magmatic origin. The calculated delta O-18(H2O) of the hydrothermal fluid associated with cassiterite and tourmaline is 7.3-8.4 parts per thousand. Scheelite and quartz deposition is characterized by lighter delta O-18(H2O) values (2.1-4.9 parts per thousand) indicating that the ore fluid might be mixed with another source of water, perhaps, meteoric. Hydrogen isotopic compositions (delta D = - 51 to - 121 parts per thousand) again indicate that origin of the ore fluid is magmatic, but delta D (similar to-120 parts per thousand) is low enough to support the delta O-18(H2O) data that suggests, in part, a meteoric water component to the hydrothermal fluids at Mawchi. Myanmar is endowed with abundant Sn-W mineralization, pre-eminent amongst which is the world-class Mawchi deposit. In the Mawchi area, N-S trending vertical or steeply dipping quartz veins are hosted by both Eocene granite and Carboniferous to Early Permian metasediments. Three stages of ore formation are recognized; (i) tourmaline-cassiterite stage (ii) main ore stage and (iii) sulfide stage. Tourmaline, cassiterite and pyrite-I are early-formed minerals and are representative of the first stage. Their deposition continued together with wolframite, scheelite, molybdenite, arsenopyrite, pyrite-II, fluorite and danalite, which form the second stage. This was followed by the successive deposition of sulfides such as chalcopyrite, sphalerite, galena and Pb-Bi sulfides. A 40Ar/39Ar magmatic biotite plateau age of 41.50 +/- 0.16 Ma (MSWD = 2.5) is determined for the Mawchi biotite granite which we interpret as the time the granite cooled through the biotite blocking temperature, and is consistent with a previously reported LA-ICP-MS U-Pb zircon concordia age of 42.72 +/- 0.94 Ma (MSWD = 2). A molybdenite Re-Os model age of 42.4 +/- 1.2 Ma indicates that Sn-W mineralization was synchronous with late Eocene granitic magmatism. Our 40Ar/39Ar hydrothermal muscovite plateau ages from the tourmaline granite (40.14 +/- 0.14 Ma; MSWD = 1.48) and quartz vein selvages (40.80 +/- 0.12 Ma; MSWD = 0.47) define the timing of hydrothermal alteration and simultaneous veining that accompanied the late stage of ore forming at Mawchi. Fluid inclusion microthermometry from cassiterite, scheelite, quartz, and fluorite reveals that ore fluids in the cassiterite-tourmaline stage and main ore stage are characterized by moderate homogenization temperatures (Th = 260-345 degrees C) and salinities (4.5-15.7 wt% NaCl equiv), while the sulfide stage is characterized by lower temperatures (Th = 175-260 degrees C) and moderate to low salinity (2.5-13 wt% NaCl equiv). The mean delta S-34 of all sulfides is 2.9 +/- 2.9 parts per thousand, which is suggestive that the overall system is dominated by magmatic sulfur. The similarity of delta S-34 values in galena and Pb-Bi sulfides (-1.3 to 2.7 parts per thousand) suggest that sulfur, and inference the Pb and Bi were transported by a common fluid, probably of magmatic origin. The calculated delta O-18(H2O) of the hydrothermal fluid associated with cassiterite and tourmaline is 7.3-8.4 parts per thousand. Scheelite and quartz deposition is characterized by lighter delta O-18(H2O) values (2.1-4.9 parts per thousand) indicating that the ore fluid might be mixed with another source of water, perhaps, meteoric. Hydrogen isotopic compositions (delta D = - 51 to - 121 parts per thousand) again indicate that origin of the ore fluid is magmatic, but delta D (similar to-120 parts per thousand) is low enough to support the delta O-18(H2O) data that suggests, in part, a meteoric water component to the hydrothermal fluids at Mawchi.
