Seifert, AV; Zacek, V; Vrana, S; Pecina, V; Zacharias, J; Zwaan, JC
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+ or -actinolite+ or -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 degrees 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.% K (sub 2) O, 3-4 wt.% fluorine, 0.12-0.74 wt.% Li (sub 2) O, 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. (super 40) K/ (super 40) Ar 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. Emerald samples from the Kafubu area range in intensity of colour from light to dark green. The colour is unevenly distributed and often exhibits zoning. The emeralds have many inclusions and are moderately fractured. Electron microprobe analyses of Kafubu emeralds (n = 97) show the contents of 0.17-0.59 wt.% Cr (sub 2) O (sub 3) , 0.69-1.21 wt.% FeO (super tot) , 1.66-2.54 wt.% MgO, and 1.72-1.98 wt.% Na (sub 2) O, besides the major components. The emeralds exhibit a distinct dichroism, are inert to long- and short-wave ultraviolet radiation, and commonly show green under the Chelsea filter. UV-VIS spectra show peaks of both divalent and trivalent iron, besides chromium. Niobian rutile is newly identified as an inclusion in emeralds from the Mbuwa mine. For the first time, data on fluid inclusions in beryl/emerald, tourmaline and quartz are provided for these deposits. Three types of fluids were recognized: aqueous-only, aqueous-carbonic, and carbonic. Ambiguous primary inclusions were identified in some beryl/emerald crystals, scarce primary inclusions in tourmaline, and pseudosecondary inclusions in quartz. Several generations of secondary inclusions are common in all minerals considered here. The highest homogenization, i.e. minimum trapping conditions at 400-450 MPa and 360-390 degrees C, were recorded in the quartz of several samples. The majority of the quartz-tourmaline+ or -emerald veins was deposited at lower pressures around 200-400 MPa.
spectra; mineralization; mineral assemblages; mineral resources; chemical composition; basement; Africa; absolute age; Kafubu Zambia; open-pit mining; host rocks; ring silicates; Proterozoic; mineral composition; major elements; mineral deposits, genesis; gems; fluid inclusions; komatiite; Mesoproterozoic; Paleozoic; Miku Mine; mines; Precambrian; faults; geochemistry; upper Precambrian; plutonic rocks; deformation; silicates; textures; granites; mining; metasedimentary rocks; Ordovician; niobium ores; petrography; Zambia; pegmatite; Mbuwa Mine; inclusions; Grizzly Mine; emerald; tantalum ores; ultramafic composition; metallogeny; dates; Kamakanga Mine; East Africa; P-T conditions; mineral exploration; Pioneer Mine; metamorphic rocks; ultraviolet spectra; trace elements; surface mining; metal ores; X-ray fluorescence spectra; K/Ar; veins; igneous rocks; Muva Group; volcanic rocks; stratigraphic units