Stable isotope and geochronological study of the Mawchi Sn-W deposit, Myanmar: implications for timing of mineralization and ore genesis

Myint, A. Z., Yonezu, K., Boyce, A. J. , Selby, D., Scherstén, A., Tindell, T., Watanabe, K. and Swe, Y. M. (2018) Stable isotope and geochronological study of the Mawchi Sn-W deposit, Myanmar: implications for timing of mineralization and ore genesis. Ore Geology Reviews, 95, pp. 663-679. (doi: 10.1016/j.oregeorev.2018.03.014)

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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 °C) and salinities (4.5–15.7 wt% NaCl equiv), while the sulfide stage is characterized by lower temperatures (Th = 175–260 °C) and moderate to low salinity (2.5–13 wt% NaCl equiv). The mean δ34S of all sulfides is 2.9 ± 2.9‰, which is suggestive that the overall system is dominated by magmatic sulfur. The similarity of δ34S values in galena and Pb-Bi sulfides (−1.3 to 2.7‰) suggest that sulfur, and inference the Pb and Bi were transported by a common fluid, probably of magmatic origin. The calculated δ18OH2O of the hydrothermal fluid associated with cassiterite and tourmaline is 7.3–8.4‰. Scheelite and quartz deposition is characterized by lighter δ18OH2O values (2.1–4.9‰) indicating that the ore fluid might be mixed with another source of water, perhaps, meteoric. Hydrogen isotopic compositions (δD = −51 to −121‰) again indicate that origin of the ore fluid is magmatic, but δD (∼−120‰) is low enough to support the δ18OH2O data that suggests, in part, a meteoric water component to the hydrothermal fluids at Mawchi.

Item Type:Articles
Additional Information:Sincere thanks are due to the Japan International Cooperation Agency (JICA) for the PhD scholarship for AZM and Global COE Program of Kyushu University for support for the field work. The Society of Economic Geologists Foundation is thanked for their student research grant to AZM. DS acknowledges the Total Endowment Fund.
Glasgow Author(s) Enlighten ID:Boyce, Professor Adrian
Authors: Myint, A. Z., Yonezu, K., Boyce, A. J., Selby, D., Scherstén, A., Tindell, T., Watanabe, K., and Swe, Y. M.
College/School:College of Science and Engineering > Scottish Universities Environmental Research Centre
Journal Name:Ore Geology Reviews
ISSN (Online):1872-7360
Published Online:24 March 2018
Copyright Holders:Copyright © 2018 Elsevier B.V.
First Published:First published in Ore Geology Reviews 95:663-679
Publisher Policy:Reproduced in accordance with the copyright policy of the publisher

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