Giuliani Gaston, Ohnenstetter D., Palhol F., Feneyrol J., Boutroy E., Boissezon de H., Lhomme T. (2008). Karelianite and vanadian phlogopite from the merelani hills gem zoisite deposits, Tanzania. Canadian Mineralogist, 46 (Part 5), p. 1183-1194. ISSN 0008-4476.
Titre du document
Karelianite and vanadian phlogopite from the merelani hills gem zoisite deposits, Tanzania
Giuliani Gaston, Ohnenstetter D., Palhol F., Feneyrol J., Boutroy E., Boissezon de H., Lhomme T.
Source
Canadian Mineralogist, 2008,
46 (Part 5), p. 1183-1194 ISSN 0008-4476
The Merelani deposits of gem-quality vanadian zoisite ("tanzanite") in Tanzania occur in Proterozoic vanadium-rich sedimentary series metamorphosed to the amphibolite facies. The gem-bearing assemblage consists of quartz, sulfides, graphite and "tanzanite" occurring in hydrothermal veins deposited in hydrothermally altered graphitic gneisses, marbles and calc-silicates. Two rare vanadian oxide and silicate minerals crystallize on the margins of pyrite and V-bearing pyrrhotite lenses, which are cross-cut by graphite-"tanzanite"-bearing quartz veins. Karelianite has a nearly pure end-member composition in the karelianite-eskolaite solid-solution series, with 93 to 96 wt% V2O3. Vanadian phlogopite closely associated with karelianite has high MgO (18.5 to 22.8 wt%), F (1.2 to 2.0 wt%) and TiO2 (up to 1.3 wt%). The V2O3 content varies between 4.2 and 10.9 wt%. The protolith of the host rock that contains the mineralization is a black shale; the hydrothermal graphite has a delta C-13 equal to-24.0%, which indicates a sedimentary source rich in organic matter. During hydrothermal alteration, V and Cr were scavenged by fluids from the graphitic gneisses, and fixed by the oxide and silicates. Textural evidence of dissolution of karelianite indicates that vanadium was remobilized by the hydrothermal fluid during the formation of late-stage vanadian zoisite ("tanzanite"), found in veinlets that cross-cut both sulfides and vanadium-bearing minerals.
