Emerald mineralization in the Kafubu area, Zambia


Authors: Seifert AV, Žáček V, Vrána S, Pecina V, Zachariáš J, Zwaan (Hanco) JC

Published in: Bulletin of Geosciences, volume 79, issue 1; pages: 1 - 40; Received 16 January 2003; Accepted in revised form 18 April 2003;

Keywords: emerald, beryl, quartz-tourmaline veins, phlogopite schists, rare-element pegmatites, komatiite, Muva Supergroup, Kafubu area, Zambia,

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This study provides the first quantitative geochemical, petrological, and mineralogical data on major rock types and minerals in the Kafubu area. Highly magnesian talc-chlorite ± actinolite ± magnetite metabasites hosting emerald mineralization are identified as metamorphosed komatiites. These rocks contain 3000-4000 ppm Cr, and provided the amount of this element necessary for the crystallization of emeralds. Associated amphibolites of andesitic and basaltic-andesitic composition are low in chromium. The estimated equilibration temperatures during regional metamorphism, using amphibole-plagioclase thermometry on several amphibolite samples, gave a narrow interval of 590-630 °C, assuming a pressure of 400-600 MPa. This metamorphic event pre-dates the emerald mineralization and the emplacement of pegmatite dikes. Emerald-bearing phlogopite schists are confined to the contacts of quartz-tourmaline veins and quartz-feldspar pegmatites with magnesian metabasites. The quartz-tourmaline veins and pegmatite dikes are genetically related to a hidden fertile granite pluton. The formation of phlogopite schists from metabasite was associated with the introduction of ca. 8-10 wt.% K2O, 3-4 wt.% fluorine, 0.12-0.74 wt.% Li2O, 1700-2900 ppm Rb, and spikes of beryllium. Quartz-feldspar pegmatites of the Kafubu area, often evolved as albite-dominated types, belong to the rare-element pegmatites of the LCT family (i.e., Li-Cs-Ta), with common beryllium enrichment. The identification of several Nb-Ta minerals (manganocolumbite-manganotantalite, niobian rutile, tantalian rutile, and plumbomicrolite) underlines the geochemical typology of pegmatites. High activities of fluorine (up to 4 wt.% F in phlogopite schists) and boron (in exceptionally abundant tourmaline) probably functioned as solidus/liquidus depressants during evolution of mineralizing fluids and their separation from the parent granite. These fluids deposited quartz-tourmaline veins, and altered the adjacent metabasites into emerald-bearing phlogopite schists. Thus, in the case of the Kafubu emerald area, the classical explanation for the origin of emeralds in schist-related deposits still stands. Data on pegmatite occurrences in active mines and exploration pits, and their extrapolation into the surrounding area, point to the existence of a single, major Kafubu pegmatite field that overlaps the extensive horizons of Cr-rich metabasite. 40K/40Ar dating of muscovite from a muscovite pegmatite and a quartz-tourmaline-muscovite vein accompanying the Be-mineralization records closure of the system within the interval of 452-447 Ma.