gems, corundum, Burma.12, geology.11, Mogok, Mogok Metamorphic Belt,

The primary occurrence of ruby in the Mogok area of northern Myanmar is exclusively found in marble, together with spinel–forsterite-bearing marble and phlogopite–graphite marble. These marble units are enclosed within banded biotite–garnet–sillimanite–oligoclase gneisses. Samples analyzed for C–O stable isotopes display two trends in δ¹³C–δ¹⁸O variation attributable to fluid–rock interaction. Ruby-bearing marble and phlogopite–graphite marble show coupled C–O depletion, whereas spinel–forsterite-bearing marble exhibits δ¹⁸O depletion with nearly constant δ¹³C values. Ruby formation may have resulted from CO₂-rich fluid–rock interaction, while the spinel–forsterite-bearing assemblage appears related to CO₂-poor fluids; both fluid types likely originated externally. Graphite Raman spectral thermometry indicates temperatures below 607 °C for phlogopite–graphite marble (and probably ruby-bearing marble) and below 710 °C for spinel–forsterite-bearing marble. Contrasting C/O diffusion between graphite/ruby/spinel/forsterite and calcite, local isotopic variations produced by non-pervasive fluid infiltration, and open-system isotopic disturbance during cooling may have influenced C/O isotopic fractionations among minerals. The high formation temperatures inferred for ruby and spinel/forsterite suggest parental fluids related to nearby igneous intrusions and/or metamorphic processes. Whether the two fluid types were genetically linked remains unclear from the present data.