Summary:
Geita gold mine (GGM) is hosted in the Geita Greenstone Belt (GGB), which is a northern segment of the Sukumaland Greenstone Belt, located in the northwestern part of the Tanzania Craton and south of Lake Victoria. This Archaean sequence strikes almost east-west, extending for about 80km and is up to 20km wide. The GGB sits dominantly within the Nyanzian Supergroup stratigraphy that is subdivided into the Lower Nyanzian and the Upper Nyanzian Groups.
Deposit type
A simplified stratigraphy of the main igneous rocks in the Geita area is summarised as: Archaean gabbro, basalts, intermediate to acid volcanoclastic sediments, diorites, tonalites-granodiorites, granites and Proterozoic gabbro dykes. Across the Archaean-Proterozoic rocks there is a property-wide paleo-drainage system, which likely flowed towards Lake Victoria. These late sediments likely represent the remnants of a much thicker package that might have covered all the hills exposed today. Both the Archaean-Proterozoic rocks and paleo-alluvials are covered by ferricrete at different levels of induration and evolution, up to 15m thick.
The Geita gold deposits are shear-hosted, Archaean orogenic gold deposits. Within the Geita Gold Mining Limited (GGML) leases, the GGB is subdivided into three major mineralised trends:
- Geita Central Trend, hosting the Nyankanga, Geita Hill and Lone Cone deposits.
- Nyamulilima Trend in the west, hosting the Star and Comet, Ridge 8 and Nyamulilima deposits.
- Matandani-Kukuluma Trend to the northeast, hosting the Matandani and Kukuluma deposits.
The Geita Central Trend contains three major gold deposits occurring along a northeast-southwest mineralised trend: Geita Hill (northeast), Lone Cone and Nyankanga (southwest). Other prospects occur singly: Chipaka in the centre of the GGB, and Kalondwa Hill, P30, Fukiri-Jumanne along a northwest- southeast trending ironstone ridge. Geita Hill, Lone Cone and Nyankanga occur along a moderately northwest-dipping system of reverse faults that have been reactivated several times during subsequent deformation events. The mineralisation is mainly related to diorite and BIF contacts exploited by the shear system. The alteration is restricted within the ore zone and consists of secondary sulphide (mainly pyrite), silica, carbonate and moderate potassic alteration.
The Nyamulilima Trend contains three major gold deposits on an approximately northwest-southeast mineralised trend: Nyamulilima in the northwest (historically named Roberts), Star and Comet, and Ridge 8 in the southeast. Individual deposits occur along a series of north-south trending, steeply dipping, left stepping en-echelon fault zones that cut across the ironstonerich sediments and granite- granodiorite-tonalite intrusions. Mineralisation is preferentially localised along fault zones where they cut the ironstone-granitoid contacts. The mineralisation is associated with secondary pyrite and minor pyrrhotite, silica, carbonate and actinolite alteration.
The Kukuluma Trend contains five gold deposits distributed along an approximately east-west mineralised trend: Area 3 South in the east, Area 3 Central, Area 3 West, Kukuluma and Matandani in the west. The mineralisation is steeply dipping along the contacts of intermediate fine-grained intrusions and magnetite rich chert and ironstone showing a general en-echelon, left stepping geometry. The gold is associated with secondary pyrite, arsenopyrite and minor pyrrhotite. Magnetite, silica, carbonate and amphibole alteration are variably present within the mineralised zone.
Mineralisation style
Deformation in the GGB comprises of early stages of ductile shearing and folding (D1 to D5), with periodic emplacement of large diorite intrusive complexes, sills and dykes. Later stages of deformation (D6 to D8) involved development of brittleductile shear zones, with faults developed in the later stages of deformation, with late emplacement felsic porphyry dykes within the GGB, and granitic intrusions located on the margins of the GGB.
Gold mineralisation occurred late in the tectonic history of the GGB, synchronous with the development of brittle-ductile shear zones (D6). Mineralisation is dominantly sulphide replacement of magnetite- rich layers in ironstone, with local replacement of ferromagnesian phases and magnetite in the diorite intrusions. Primary gold mineralisation is associated with the intersection of the brittle-ductile shear zones and pre-existing fold hinges, with higher-grade concentrations associated with BIF lithologies and with diorite dyke and sill contacts.
Mineralisation characteristics
The mineralisation in the GGB is preferentially hosted within deformation zones developed along the contact of BIF and porphyries of various compositions, and associated with major shear systems. The structures associated with the mineralised system are well-defined, the alteration zone is restricted to the mineralised zone, and quartz veins are rare or missing although silicification is common.