Summary:
Emerald mineralisation in the Kafubu area, including the Kagem deposits, belongs to a group referred to as “schist hosted emeralds”, relating to the interaction of Be-bearing fluids relating to pegmatite dykes or granitic rocks, with Cr -rich mafic and ultramafic schists or weakly metamorphosed ultramafic rocks.
Kagem’s Chama, Libwente, Fibolele and Chibolele deposits form part of a semi-regional scale tight-isoclinal fold system, which trends northeast or east-northeast, ranging in dip from near flat-lying to up to 60° to the southeast or south-southeast, and is locally offset by a series of predominantly north-northwest striking structures.
The suite of PEG dykes and quartz-tourmaline veins that intrude the stratigraphic succession throughout the Kagem deposits occupy a range of trends, both concordant and discordant to the local stratigraphy. At Chama and Chibolele, most discordant dykes strike north or northnorthwest, dipping at around 50° to sub-vertical towards the east or east-northeast. The discordant dykes and veins at Libwente and Fibolele occupy the same trend set, striking north-northwest, but with a steeper, typically sub-vertical dip.
The emerald mineralisation is hosted by the ultramafic TMS unit, with three main styles of mineralisation recognised:
• discordant reaction zone (“RZ”) material adjacent to the PEG and quartz-tourmaline vein contacts;
• concordant RZ material concentrated along the footwall and rarely the hanging wall contacts of the TMS unit; and
• discordant RZ hosted by brittle structures within the TMS unit distal to the PEG and quartz-tourmaline veins.
The RZ is laterally discontinuous and varies in thickness from a few centimetres to more than 2 m.
Mineralisation
Emerald and beryl mineralisation in the Kafubu area, including the Kagem deposits, belongs to a group referred to as ‘schist-hosted emeralds’, in which emeralds occur predominantly in phlogopite or other types of schists. The origin of schist-hosted emerald and beryl deposits is controversial, but is known to require specific geological conditions in which beryllium bearing fluids interact with chromium bearing host rocks. The most established model for emerald and beryl mineralisation in the Kafubu area involves the interaction of Be-bearing fluids relating to pegmatoid dykes or granitic rocks, with Cr-rich mafic and ultramafic schists or unmetamorphosed ultramafic rocks. Other models for schist-hosted emerald and beryl mineralisation propose syn- to post-tectonic growth of beryl in metasomatised ultramafic rock adjacent to Be-bearing PEG during regional metamorphism.
Both the concordant and discordant RZ are laterally discontinuous and vary in thickness from a few centimetres to more than 2 m. All three styles of RZ are mineralogically similar, being composed of phlogopite-biotite-tourmaline aggregates, which are highly soft and friable, providing a protective buffer ideal for the preservation of beryl and emerald crystals. The RZ typically contain beryl mineralisation, of which a variable fraction may be emerald, depending on the chemistry of the TMS. Chemical analyses of phlogopite-rich RZs from emerald and beryl deposits throughout the Kafubu area, indicate that the transformation of ultramafic units into phlogopite schist involves a major influx of K, Al, F, Li and Rb, localised enrichment of Be, dilution of Cr and Ni, and removal of Ca and Si.
Emeralds are a member of the beryl group of minerals which have the chemical formula Be3Al2(SiO3)6 and which show a strong prismatic habit and an imperfect (0001) cleavage perpendicular to the long axis of the crystal (basal pinacoid). They have a hardness of 7.5 to 8.0 and a specific gravity of 2.65 to 2.80. Emerald is the deep green translucent variety of beryl and results from the substitution of Cr, ferrous iron, and in some cases, traces of V, for Al in the crystal lattice.
Kafubu area beryls are typically white to yellowish to bluish white, while the emeralds have a moderate to strong green colouration due to low to moderate levels of Cr2O3 in the range 0.11 wt% to 0.77 wt%. The Kafubu emeralds are characterised by a wide range of trace element contents, typically with moderate levels of Mg and Na, and a moderate to high Fe content. The gemstones have enriched trace element levels, most notably of Cs and Li, but also of K, Rb, Ti, Sc, Mn, Ni, and Zn. Vanadium content is low.
The Kafubu emeralds have relatively high specific gravity (2.69 – 2.77) and refractive index values, especially relatively to emeralds from Colombia. Beryl and emerald mineralisation in the Kafubu area typically forms as subhedral to euhedral hexagonal crystals that often grow in aggregates of multiple gemstones.