Peralkalinity in peraluminous granitic pegmatites. I. Evidence from whewellite and hydrogen carbonate in fluid inclusions

Fluid inclusions in pegmatite minerals were studied using Raman spectroscopy to determine the carbon species. Carbon dioxide is very abundant in the aqueous liquid and vapor phases. Occasionally, CH4 was found in the vapor. In the aqueous liquid, HCO3- was detected in fluid inclusions in tantalite-(Mn) from the Morrua Mine and in late-stage quartz from the Muiane pegmatite and the Naipa Mine, all in the Alto Ligonha District, Mozambique. Moreover, we observed a carbonate (calcite group) in fluid inclusions in garnet from the Naipa Mine and in beryl from the Morrua Mine, both in the Alto Ligonha District, Mozambique, and a calcite-group carbonate and whewellite [CaC2O4 center dot H2O] in fluid inclusions in topaz from Khoroshiv, Ukraine. The occurrence of oxalate is interpreted to be due to a reaction of some form of carbon (possibly CO or bitumen) with a peralkaline fluid. Our results support the hypothesis that, although counterintuitive, hydrogen carbonate-rich peralkaline fluids may locally be involved in the evolution of peraluminous granitic pegmatites, in which peralkaline minerals are normally absent or very rare.

Automated summary

In this study, fluid inclusions in pegmatite minerals were analyzed using Raman spectroscopy. It was found that the aqueous liquid and vapor phases contained a high amount of carbon dioxide, and occasionally methane was detected in the vapor. HCO3- ions were identified in the fluid inclusions in tantalite-(Mn) from the Morrua Mine and in late-stage quartz from the Muiane pegmatite and the Naipa Mine, suggesting the involvement of hydrogen carbonate-rich fluids in the evolution of peraluminous granitic pegmatites.