The geology, geochemistry and metallogeny of the Tati Greenstone Belt, Northeastern Botswana

Tombale, Akolang Russia (1992) The geology, geochemistry and metallogeny of the Tati Greenstone Belt, Northeastern Botswana. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The Tati Greenstone Belt of Botswana occurs along the southern margin of the Archean Zimbabwe Craton in the northern contact zone of the Limpopo Belt Detailed mapping and structural analysis indicates that the Tati Greenstone Belt consists of three fault-bounded volcano-plutonic sequences intruded by granitoids. The data further suggest that the belt formed from the accretion of the volcano- plutonic sequences to the Zimbabwe Craton La a manner resembling modem orogenic belts. -- Geochemical data suggest that the Tati Greenstone Belt consists of a komatiite-tholeiite sequence representing a back-arc basin and calc-alkaline volcanic and feldspathic sedimentary rock sequences which overall indicate the existence of an arc environment The geochemical data further indicate the presence of three different magmatic suites within the Lady Mary Group of the Tati Greenstone Belt and these suites have geochemical signatures similar to those of found in modem tectonic environments. The three magmatic suites include LREE-depleted rocks, LREE-enriched rocks and komatiites. The LREE-enriched rocks form the margins of a basinal structure with LREE-depleted and komatiites at the core. The komatiites are postulated to have formed from plume-type magmas generated in deep portions of the Archean mantle. -- Gold mineralization in the Tati Greenstone Belt northeastern Botswana, formed as an integral component of the tectonic evolution of the belt Three main stages of gold mineralization are recognized. The first mineralization stage is related to the late stages of volcanism which formed the volcanic sequences in the belt during this stage hydrothermal fluids circulated through the volcanic structure along fissures, fractures and lithological boundaries, leaching metals and creating extensive carbonate alteration zones. The contact zones between basaltic rocks and synvolcanic gabbroic sills, and inter-pillow areas were the most extensively altered. Metal-bearing fluids, in which the metals were derived during the alteration of the basaltic crust, were discharged along the tops of the volcanic sequences depositing banded iron formations with siliceous, carbonate, oxide and sulphide fades. In particular, sulphide fades banded iron formations in the Tati Greenstone Belt are discontinuous units located at the top of major volcanic sequences, which are either covered by sedimentary rocks including carbonates and minor clastic rocks, or lie directly on the basaltic crust In the waning stages of the hydrothermal activity, lower temperature phases of the hydrothermal fluids precipitated Au, Ag, Sb, As and other related metals in pockets at localized sites. -- The second stage of gold mineralization coincided with the deformation of the belt and the creation of favourable structural sites. During this stage, related to NE to N directed compression and thrusting (D₁), NE to N verging recumbent and overturned folds were formed. Volcanic sequences were thrust onto each other along lithological boundary-parallel faults, which are demarked by banded iron formations and related sedimentary rocks. The second deformation, D₂, was the main foliation-forming deformation event Metamorphic de-watering during this event resulted in large scale foliation-parallel carbonate and quartz-carbonate veining. Carbonate alteration formed during the first stage was extensively replaced by silica during this second stage, creating the silicified alteration zones which accommodated brittle deformation during subsequent deformation events. -- The third stage of gold mineralization was related to D₃ shearing of the volcanic rocks in the belt and major granitoid magmatism. Shearing and faulting created cross stratigraphy faults/shear zones. Banded iron formations and silicified zones formed during the D₂ deformational event accommodated brittle failure during this D₃ shearing event. Secondary fluids penetrated through the lithologies along numerous structures and were heated by granitoid magmas. These fluids remobilized metals from banded iron formations and massive sulphide horizons, redepositing them into brittle fractures. The brittle structures were localised to areas of lithological heterogeneity, such as at contacts between gabbroic sills and basaltic host rocks and in relatively structurally competent rocks such as banded iron formations. The mineral occurrences formed during this stage were low temperature Au-only deposits. -- Gold was produced from at least 70 small mines scattered across the Tati Greenstone Belt Pre-1800 gold production by local tribesmen was also substantial. The various gold occurrences and their characteristics are summarized in a metallogenic map of the belt. Data presented on the metallogenic map include the main commodity produced, metallic associations, the form in which gold occurs, host rock type, and the nature of the structural control.

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