The geology, geochemistry and petrology of the metagabbro units of the tati N1-CU sulphide mines and deposits and associated PGEs mineralisation
Date
2019
Authors
Thari, Kabelo Thabiso
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Abstract
Various metagabbro bodies host sulphide mineralisation in the Tati Sulphide Mines and Deposits Complex located in the Tati greenstone belt, southwestern margin of the Zimbabwe craton. The Tati Sulphide Mines and Deposits Complex, referred to here as the TSMDC, is a major metallogenic zone in east Botswana. This study focuses on gaining insight into the geology, whole-rock geochemistry and petrology of the Ni-Cu-PGE- mineralised gabbro units and contact lithologies in order to develop effective exploration targets for the TSMDC. The TSMDC mainly comprises three types of gabbroic units-melanogabbro, mesogabbro and leucogabbro-in contact with a wide range of magmatic rocks that include, granodiorite, porphyritic granodiorite, quartz diorite, tonalite, amphibolite, basalt, dolerite and pegmatites. The gabbro units occur near metasedimentary units that include dolomite, Banded Iron Formation (BIF) and quartzite. Some of the gabbro units host sulphide mineralisation comprising mainly pyrrhotite, pentlandite, chalcopyrite and pyrite, whereas some gabbro units are barren. The gabbro units are generally medium grained and composed mainly of variable propositions of the primary minerals pyroxene (20-30%), plagioclase (30-35%) and quartz (2-7%) and minor secondary minerals that include, chlorite (~2%) and epidote (~1%). The major and trace elements data indicate a suite of variably magmatically evolved gabbro units, which show a wide compositional range in MgO (4-17 wt. %), FeOt (5-20 wt. %), SiO₂ (45-52 wt. %), Al₂O₃ (10-17 wt. %) and CaO (5-15 wt. %). Based on the positive correlations between MgO and FeOtotal, CaO and MgO and Y and Zr, the gabbro units are interpreted as having been generated through a fractional crystallisation process, where pyroxene and plagioclase were the dominant rock-forming minerals. Fractional crystallisation is also evidenced by positive correlation between SiO₂ and Na₂O, + K₂O, and resulted in the formation of an enriched quartz mineralogy in granitoid units of the TSMDC. Generally, gabbro units show chondrite-normalised plots with slight enrichment of LREE relative to depletion of HREE. There is a weak positive Eu anomaly, which suggests plagioclase in combination with pyroxene and sphene controlled the fractionation of the partial melts in the upper crust.
The gabbro units and contact lithologies were emplaced in a continental margin environment prior to and following several deformation events. Three deformation events, D1, D2 and D3 were identified in TSMDC. D1 is ductile-brittle in nature and is characterised by folding (fo1), foliation S1, and NE-SW and NNE-SSW trending faults (F1). D2 resulted in the formation of the Phoenix, Selkirk and Rooikoppie shear zones associated with a regional foliation (S2), lineation (L₂) and en echelon quartz veins (V₂). The third deformation event, D₃, is associated with formation of fracture cleavage and joints that dominate most of the study area. On a regional scale, the N-S and NE-SW trends of crustal lineaments controlled the geometry of the Phoenix, Selkirk mines, Tekwane and Rooikoppie deposits. On a local scale, the structural factors controlling the current position of the Phoenix, Selkirk mines, Tekwane and Rooikoppie deposits are a network of proximal fractures to the crustal lineaments, which provided transport and settling of differentiated magma during interaction with sulphide minerals in the formation of the Ni-Cu-PGEs mineralisation. The integration of the geology, whole rock geochemistry and petrology suggests that various gabbro units host Ni-Cu-PGEs mineralisation and are affected by structural lineaments, which include shear zones in TSMDC. The shear zones acted as conduits for the propagation of Ni-Cu-PGEs mineralisation and hydrothermal fluids. The geometry of the Phoenix, Selkirk mines, Tekwane and Tekwane deposits is coincident with NE and NW trends of the shear zones.
Description
A Dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg in fulfillment of the requirements for the degree of Master of Science
February 2019
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Citation
Thari, Kabelo Thabiso (2019) The geology, geochemistry and petrology of the metagabbro units of the TATI NI-CU sulphide mines and deposits and associated PGEs mineralisation, University of the Witwatersrand, Johannesburg, <http://hdl.handle.net/10539/28061>