Marble-hosted noble spinel deposits from the Luc Yen District (Vietnam): mineral equilibria involving corundum and spinel

Thermodynamic analysis is carried out for the model systems: (1) Na–Ca–Mg–A1–Si–CO2–H2O and (2) Ca–Mg–A1–Si–CO2–H2O. In each systems the mineral equilibria involving dolomite, calcite, spinel, corundum, pargasite, hydroxylclinohumite, and diaspore are considered. A diagram of the multisystem in coordinates μH2O-μCO2is constructed. Analysis showed a significant influence of chemical potentials of water and carbon dioxide in the mineral-forming medium on stability of corundum and spinel in marbles. The conditions of the spinel formation during metamorphism of protolith consisting of carbonate sediments with evaporites lenses and terrigenous material (smectites, bauxites, etc.), are considered in detail. The conditions of the replacement of spinel by corundum during retrograde metamorphism are determined depending on the mole fraction of CO2 in the fluid and temperature at pressure of 5 kbar.