Publication details
Hydrothermal replacement of garnet by tourmaline - an example from LCT pegmatites in Sahatany Valley, Madagascar
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| Year of publication | 2014 |
| Type | Appeared in Conference without Proceedings |
| MU Faculty or unit | |
| Citation | |
| Description | In the pegmatites Tsarafara Nord and Tamponilapa, both tending to elbaite subtype, garnet and albite located close to Li-mineralized zone were partially replaced by a mixture of blue tourmaline and quartz. Spessartine-dominant garnets from the pegmatites Tsarafara Nord (Sps48-60Alm32-42Grs8-10) and Tamponilapa (Sps43-62Alm2244Grs3-21Prp0-3) are associated with primary tourmaline (schorl to Mg-rich schorl). Secondary tourmaline replacing garnet are of schorl to Fe-rich fluor-elbaite composition. Albite is close to the end member composition. The minerals from the Tsarafara Nord are slightly more fractionated with higher (Li, Mn, F) and less contaminated (elevated Ca, Mg, Ti) from host rock relative to the Tamponilapa. Textural relations showing replacement of garnet and albite by tourmaline and quartz suggest the simplified reaction: 24 NaAlSi3O8 + 3 (Mn1.6Fe1Ca0.4) Al2Si3O12 + 12 H3BO3 + 2 LiF => 4 (Na0.5Ca0.3v0.2) (Al1.5Fe0.75Li0.5Mn0.25) Al6 (Si6O18) (BO3)3 (OH3.5F0.5) + 46 SiO2 + 11 Na2SiO3 + 3.8 MnO2 + 0.4 O + 9 H2O + 4 H+ It is calculated based on stable = immobile Al, Fe and Ca and idealized compositions of primary garnet, albite and secondary tourmaline, which is generally enriched in Al, F, Li, and Mn and depleted in Ti, Mg and Fe compared to primary tourmalines. They are only sporadically associated with garnet and very likely do not participate in the replacement reaction. Because we assume a direct replacement of primary garnet and albite, the elevated contents of Al, Mn and Ca in secondary tourmalines reflects the compositions of primary garnet (Mn, Al, Fe, Ca) and of residual fluids (B, F, Li, H2O). Very low to negligible concentrations of Mg in secondary tourmaline suggest closed system to host rock during this alteration. The excess of Na and Mn may be dissolved in fluids and transported away as e.g., soluble sodium silicates. Apparently, no Mn-bearing minerals occur close to garnet as well. |