Scheelite and coexisting F-rich zoned garnet, vesuvianite, fluorite, and apatite in calc-silicate rocks from the Mogok metamorphic belt, Myanmar: Implications for metasomatism in marble and the role of halogens in W mobilization and mineralization

Abstract

Scheelite, which is an important ore of tungsten and colored gemstone, is well developed in the calc-silicate rocks from the Mogok metamorphic belt (MMB), Myanmar. In this study, the textural, mineralogical, and compositional characteristics of scheelite and its associated minerals were systematically investigated to constrain the petrogenesis of scheelite-bearing calc-silicate rocks and the tungsten transfer and mineralization mechanism in a hydrothermal–metasomatic system. The petrological evidence, bulk and mineral geochemical signatures, and mass-transfer calculations indicate that the calc-silicate rocks formed by local metasomatism of marble via the introduction of an externally derived Si–Al–Fe–W–F-bearing, H2O-rich fluid phase. The distinct compositional zonations [F, Fe, Ca, and heavy rare earth elements (HREEs)] of garnet in the calc-silicate rocks record a two-stage metasomatic process and significant compositional variation in the associated fluid. The late-stage metasomatic fluid that led to the formation of the F-rich garnet rims, scheelite, and most of the calc-silicate minerals has noticeably higher fluorine activity (aF−), oxygen fugacity (fo2), and HREE content than the early-stage metasomatic fluid responsible for the garnet cores.The MMB scheelite exhibits typical “skarn-type” compositional characteristics with a high LaN/YbN ratio (100–180), a negative Eu anomaly (δEu=0.3–0.5), and a high Mo content (1100–1330ppm). These geochemical signatures are primarily controlled by the protolith, metasomatic fluid, redox conditions, and coexisting mineral phases. The enrichment of rare earth elements (REEs) and high field strength elements (HFSEs) in the MMB scheelite was dominated by two substitutionreactions: Ca2++W6+=REE3++HFSE5+ and 3Ca2+=2REE3++□Ca (where □Ca is a Ca-site vacancy). Considerable amounts of F and OH in the metasomatic fluid substituted for O in the garnet via the substitute reaction 4(F, OH)−=4O2−+Si4+, leading to a significant enrichment of F (up to 1.2wt.%) and OH (up to 0.32 for nOH) and a negative correlation between F and Si in the garnet.Detailed petrographic observations show that the occurrence of scheelite in the MMB calc-silicate rocks is always associated with the growth of F-rich minerals such as garnet rims (0.8–1.2wt.% F), vesuvianite (2.4–2.6wt.% F), fluorite (48–49wt.% F), apatite (3.9–4.1wt.% F), and titanite (2.6–3.4wt.% F). These textural characteristics, combined with the positive correlation of whole-rock F and W (as well as Sn, Mo) contents in the calc-silicate rocks, indicate that the elevated F contents increased the solubility of W in the infiltrating fluid, thereby allowing the W transfer in the hydrothermal–metasomatic system. The mineralization of scheelite was triggered by the crystallization of F-rich minerals during the formation of the calc-silicate rocks, which caused F depletion and consequent saturation of W in the metasomatic fluid. Our results suggest that, in the MMB metasomatic system, F rather than Cl is the key fluxing compound that facilitates the transfer of W and the mineralization of scheelite.