Fluid inclusions in ruby from Asian marble deposits: genetic implications

Primary fluid inclusion (FI) assemblages in ruby from marble at Mogok and fifteen other deposits from central and Southeast Asia were studied by microthermometry, Raman spectroscopy and crush-leach analysis. Microthermometry combined with Raman spectroscopy investigations of primary FI in Mogok rubies indicated the simultaneous trapping of carbonic FI in the system CO2–H2S (± COS ± S8) and multi-solid carbonic FI in the system Na–K–Ca–CO3–SO4–NO3–Cl–F ±(CO2–H2S). The multiple solids are mainly mixtures of Na-Ca-Al carbonates such as shortite and dawsonite, sulphates, phosphates, nitrates, fluorides, and chlorides such as halite. They represent the trapped recrystallised residues of molten salts formed during the metamorphism of evaporite lenses (chlorides and sulphates) intercalated in the carbonate rocks. Raman spectroscopy of solids in FI from all Asian rubies hosted in marbles identified both (i) accidentally trapped minerals mainly calcite, dolomite and rutile, and (ii) daughter phases, i.e., native sulphur, diaspore and boehmite, and Na-K-Ca chlorides, anhydrite, dawsonite, shortite and fluorides. Crush-leach analysis of ruby crystals identified chloride as the dominant anion, but sulphate and nitrate are present at only slightly lower concentrations, whereas they are the dominant species in some Pakistani deposits. The Na–K–Ca–Li–CO3–SO4–NO3–Cl–F molten salt assemblages and the presence of nitrate and phosphate indicate the deposition of the original sediments in a coastal sabkha with ephemeral marine and continental water flooding. Input of V- and Cr-bearing argillite to the salty mudflats insured the formation of rubies in marble during Cenozoic metamorphism.