CO2–H2S–COS–S8–AlO(OH)-bearing fluid inclusions in ruby from marble-hosted deposits in Luc Yen area, North Vietnam

Fluid inclusions have been studied in rubies from marble-hosted deposits in the Luc Yen mining district, North Vietnam. Raman and Infrared spectroscopy combined with microthermometry investigations on primary and secondary fluid inclusions provided evidence of CO2–H2S–COS–S8–AlO(OH)-bearing fluids with native sulphur and diaspore daughter minerals, without visible water. Diaspore appeared as a nonvisible film of 2–3 μm coating the wall of the fluid inclusion cavity and native sulphur was generally nucleated during Raman irradiation. The presence of diaspore and COS in the fluid inclusions indicates that water was present in the paleofluid. The mole fractions of H2O and CO2 are around 10−2 and the expected concentration of water in the fluid inclusions is in the 1–10 mol.% range. Crush-leach identified sulphates and chlorides that are assigned to the presence of anhydrite and Na–Ca–Cl salts found by SEM in the ruby crystals. The CO2-rich inclusions which do not coexist with an aqueous phase by immiscibility process demonstrate that ruby grew from this CO2-rich and water-poor fluid at equilibrium with Na–Ca–Cl salts. Thermal reduction of evaporitic sulphates based on an initial assemblage of anhydrite, calcite and graphite, originating from the metamorphism of organic matter is proposed to explain the original fluid chemistry of these marble-hosted rubies. The marbles acted as a closed system and the carbonic composition of the parent fluids in ruby indicates that aluminium can be transported in CO2-rich fluids at high P–T.