Publication details
Hydrothermal replacement of amblygonite-montebrasite by blue tourmaline – examples from the Moldanubicum, Czech Republic
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| Year of publication | 2017 |
| Type | Conference abstract |
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| Description | Blue tourmaline (Fe-rich fluor-elbaite > fluor-schorl) reaction rims around amblygonite from LCT pegmatites from Dolní Bory, Puklice and Dobrá Voda were studied. Primary mineral assemblages include K-feldspar, albite, quartz, schorl, muscovite, lepidolite, elbaite and two generations of amblygonite-montebrasite. Replacement reactions with tourmaline were typically observed in amblygonite I adjacent to Li-enriched parts of albite unit. The rims are typically formed by fine-grained mixture of quartz + tourmaline + secondary fluorapatite, clay minerals, and muscovite; the latter frequently mixed with crandallite-group minerals in Dolni Bory. Secondary blue tourmaline (Fe-rich fluor-elbaite > fluor-schorl ) is generally Na- (0.70-0.95 apfu) and F-rich (0.65-0.91 apfu). Contents of Licalc. (0.21-1.05 apfu), Al (6.42-7.92 apfu) and Fe (up to 2.17 apfu) (Fig. 2.) are variable, amounts of Mn > Ca, Ti, and Mg are low to negligible. A complex multi-stage replacement process is apparent. In early subsolidus stage amblygonite was replaced by the assemblage tourmaline + quartz + fluorapatite + muscovite. During the low-T stage fluorapatite and muscovite were partially replaced by clay minerals plus gorceixite and goyazite whereas tourmaline-quartz assemblage is stable. Negligible to none concentrations of Mg and Ti in secondary fluor-elbaite and muscovite suggest that the system was closed to the host rocks during the first stage of hydrothermal alteration. The presence of Ba and Sr in late crandallite minerals indicates an open system during the second stage. Formation of blue tourmaline and associated muscovite and fluorapatite requires the following elements: Na, K, Li, Ca, Al, Fe, Si, B, P, F and H2O. Lithium, Al, P and F are present in the replaced amblygonite. Apparent input of B, alkalis, Ca and H2O in fluids is required. Due to absence of Fe in amblygonite, it also must be present in the fluids. Replacement of garnet by blue tourmaline (Buřival and Novák 2015) is very similar to amblygonite replacement but garnet may provide Fe, and Mn but no Li and F. Consequently, input of B, alkalis, Ca, H2O, Li, Fe (Mn) is assumed during early replacement processes in complex (Li) granitic pegmatites. |