Tourmaline in Meta-Evaporites: Perspectives from Namibian Tourmalinites

Tourmaline from meta-evaporitic tourmalinites of the Duruchaus Formation of central Namibia reveals a common compositional trend that is found in a number of meta-evaporite localities. The Neoproterozoic continental rift metasediments of the Duruchaus Formation of the Damara Orogenic Belt in Namibia contain well-preserved meta-evaporitic features. Within the clearly evaporitic metasediments are a series of conformable tourmalinites that are used to investigate the crystal-chemical character of tourmaline developed in this type of lithologic setting. The tourmalinites are considered to develop as a consequence of diagenetic or authigenic replacement of a borate-rich mineral layer originally precipitated in an alkaline lake and as epigenetic replacement associated with boron-rich fluids diffused through the sedimentary rock package The meta-evaporitic tourmalines are generally magnesian (Mg~2 apfu), moderately-to-highly depleted in Al, enriched in Fe³⁺ and calculated W site O^2-. They typically follow this trend along a join between “oxy-dravite” [Na(MgAl₂)(MgAl₅)(Si₆O₁₈)(BO₃)₃(OH)₃(O)] and povondraite [Na(Fe³⁺₃)(Fe³⁺₄Mg₂)(Si₆O₁₈)(BO₃)₃(OH)₃(O)] – the O-P trend. Similar trends have been found in the meta-evaporites at Alto Chapare (Bolivia), Challenger Dome (Gulf of Mexico) and Liaoning (China). The O-P trend is attributed to the influence of oxidizing, highly saline, boron-bearing fluids that are associated with these lithologies. In the Namibian tourmalines there are some deviations from this trend, and they are considered to be a consequence of subsequent overprints related to sulfate-silicate interactions and/or influx of compositionally distinct reactive fluid. The O-P trend has also been noted in tourmalines from the hydrothermal alteration zones of Cu-Mo-Au deposits and may be a fundamental fingerprint that reflects high salinity, boron-bearing and oxidizing aqueous fluids.