Karelianite and vanadian phlogopite from the Merelani Hills gem zoisite deposits, Tanzania

• Published in: Canadian mineralogist Vol. 46; no. 5; pp. 1183 - 1194
• Main Authors: Giuliani, Gaston, Ohnenstetter, Daniel, Palhol, Fabien, Feneyrol, Julien, Boutroy, Emilie, de Boissezon, Hélène, Lhomme, Thérèse
• Format: Journal Article
• Language: English
• Published: Mineralogical Association of Canada 01-10-2008
• Subjects: Africa; East Africa; epidote group; eskolaite; fluid phase; gems; gneisses; graphite; karelianite; Merelani Hills; metamorphic rocks; mica group; mineral assemblages; Mineralogy; native elements; orthosilicates; phlogopite; Precambrian; Proterozoic; sheet silicates; silicates; sorosilicates; Tanzania; tanzanite; upper Precambrian; zoisite; Merelani; karelianite; graphitic gneiss; Mozambique Belt; Tanzania; vanadian phlogopite; tanzanite
• ISSN: 0008-4476; 1499-1276
• DOI: 10.3749/canmin.46.5.1183

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 δ13C equal to -24.0 per mil, 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.