Marble-Hosted Noble Spinel Deposits from the Luc Yen District (Vietnam): Mineral Equilibria Involving Corundum and Spinel

Thermodynamic analysis of (1) the Na–Ca–Mg–A1–Si–CO2–H2O and (2) Ca–Mg–A1–Si–CO2–H2O model systems has been carried out to establish the conditions for the spinel and corundum formation in dolomitic marbles. The mineral equilibria involving dolomite, calcite, spinel, corundum, pargasite, hydroxyl clinohumite, and diaspore have been investigated. A diagram of the multisystem in the $${{\mu }_{{{{{\text{H}}}_{{\text{2}}}}{\text{O}}}}}$$ – $${{\mu }_{{{\text{C}}{{{\text{O}}}_{{\text{2}}}}}}}$$ coordinates has been built, the analysis of which has showed a significant influence of the water and carbon dioxide chemical potentials in the mineral-forming medium on the stability of corundum and spinel in marbles. The conditions for the spinel formation at the metamorphism of protolith consisting of magnesium-enriched carbonate sediments with evaporite lenses and impurities of a terrigenous material (smectites, bauxites, etc.) have been discussed in detail. The conditions for the replacement of spinel by corundum at the retrograde metamorphism have been established depending on the CO2 mole fraction in the fluid and temperature at a pressure of 5 kbar.