323-4 Tourmaline in Meta-Evaporites: Perspectives from Namibian Tourmalinites

See more from this Division: Topical Sessions
See more from this Session: Discovering Petrologic Truth in Minerals II: In Honor of Bernard W. Evans

Wednesday, 8 October 2008: 2:15 PM
George R. Brown Convention Center, 351AD

Darrell Henry, Geology and Geophysics, Louisiana State University, Baton Rouge, LA, Haiting Sun, Dept. of Geology and Geophysics, Louisiana State Univ, Baton Rouge, LA, John F. Slack, MS-954 National Ctr, US Geol Survey, Reston, VA and Barbara Dutrow, Dept. of Geology and Geophysics, Louisiana State University, Baton Rouge, LA
Abstract:

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 O2-. 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.

See more from this Division: Topical Sessions
See more from this Session: Discovering Petrologic Truth in Minerals II: In Honor of Bernard W. Evans

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