Mineralogy and geochemistry of the Erongo Sub-Volcanic Granite-Miarolitic-Pegmatite Complex, Erongo, Namibia

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Marco E. Ciriotti
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Mineralogy and geochemistry of the Erongo Sub-Volcanic Granite-Miarolitic-Pegmatite Complex, Erongo, Namibia

Messaggio da Marco E. Ciriotti » mer 24 ott, 2018 10:41

Referenza:
▪ Falster, A.U., Simmons, W.B., Webber, K.L., Boudreaux, A.P. (2018): Mineralogy and Geochemistry of the Erongo Sub-Volcanic Granite-Miarolitic-Pegmatite Complex, Erongo, Namibia. Canadian Mineralogist, 56, 425-449.

Abstract:
The early Cretaceous anorogenic, sub-volcanic Erongo Granite, Namibia, was emplaced into metasedimentary rocks of the Neoproterozoic to Cambrian Damaran Orogeny. The Erongo Granite contains pegmatitic miaroles containing abundant beryllium and boron mineralization and segregations of rounded quartz-tourmaline orbicules (tourmaline nests). This study focuses on the mineralogy of the miarolitic pegmatites and quartz-tourmaline orbicules and the geochemistry of the Erongo Granite. Erongo miarolitic pegmatites are famous for spectacular mineral specimens of schorl, foitite, aquamarine, topaz, fluorite, and jeremejevite. Mineral samples from three areas where artisanal mining exposed miarolitic cavities were examined: Lion's Head (LH), Hohenstein Gorge (HG), and Tubusis (TB). Tourmaline, biotite, and associated minerals were investigated from granite, miarolitic cavities, and quartz-tourmaline orbicules from these three areas. Whole-rock geochemical analyses of granites from the LH and HG locations were also performed. The LH and HG granites have different modal mineralogy, with LH richer in plagioclase and HG richer in biotite and quartz and slightly richer in K-feldspar; both are unusually rich in B for anorogenic granite.
Biotite occurs principally in the granite and is typically absent in miarolitic cavities and quartz-tourmaline orbicules, where the iron-rich phase is tourmaline. The micas range from Mg-siderophyllite to annite in LH granite, are Li-rich siderophyllite in HG granite, and are annite in TB granite, using the classification of Tischendorf et al. (1997). Micas in both the LH and HG tourmaline nests are Li-rich siderophyllite. All mica is Fe dominant at the Y-site, with variable contents of the lesser Y-site occupants. The LH mica has significantly more trivalent iron, suggesting that conditions may have been more oxidizing than in the HG location.
Tourmaline from granite ranges from iron-rich schorl with low to intermediate X-site vacancies to lower-iron schorl. Tourmaline from miarolitic cavities is dominantly schorl and fluor-schorl. The H2O content of schorl determined by LOI is very similar to H2O calculated by stoichiometry for the EMP analyses (3.23av calc versus 3.35av LOI), thus we infer that the oxygen content of the V-site in schorl is low. Prismatic schorl crystals from some miarolitic pockets are etched and altered to foitite at their ends. Rare color-zoned tourmaline was found in millimeter-size crystals in one pocket. Beryl, topaz, fluorite, and jeremejevite show little chemical variation throughout the complex.
Whole rock geochemical analyses of the granites show both LH and HG to be peraluminous, sub-alkaline, and relatively low in HFSE. Chondrite-normalized REE plots show both granites to be only slightly enriched in LREE. Plots are relatively flat with a strong negative Eu anomaly. Relative to continental crust, Erongo Granite samples are depleted in Ba, Sr, and Ti and relatively enriched in Zr, Nb, Th, U, Y, and REE. There are, however, distinct chemical differences between the two locations. The LH granite is more calc-alkaline and more enriched in LREE; the HG granite is more tholeitic and enriched in Rb, Cs, HREE, and Y with a much flatter CN REE plot with a stronger negative Eu anomaly. Tectonic discrimination diagrams (Pearce et al. 1984) show that the LH and HG granites do not plot strictly in the WPG field but have a mixed geochemical signature; the LH granites plot in the VAG-syn-COLG field whereas HG granites plot along the syn-COLG and WPG boundary. We interpret that this mixed geochemical signature results from substantial contamination from the metasedimentary rocks of the Damara orogen altering the geochemistry from a more typical A-type granite toward the VAG-syn-COLG field and introducing B to the melt that eventually became the Erongo Granite and produced the boron-rich miarolitic pegmatites.
The geochemical signature of the Damara orogen is most evident in the quartz-tourmaline orbicules and miarolitic cavities, where volatile and incompatible elements were sufficiently concentrated via fractional crystallization to cause exsolution of a second fluid phase, crystallization of tourmaline, and the development of pegmatitic textural and mineralogical changes.
Marco E. Ciriotti

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