Comparison of mineralised and unmineralised A-Type granitic systems in Southern Africa
Vonopartis, Leonidas Christos
The Palaeoproterozoic Lebowa Granite Suite is the largest A-Type granitic system on Earth, and it is associated with one of the largest tin provinces in Africa. Tin mineralisation occurs as both endo-and exogranitic styles, within the roof facies of the Lebowa Granite Suite and the surrounding rocks of the Pretoria Group and overlying Rooiberg Group volcanics. The Zaaiplaats tin field is historically the second largest tin producer in South Africa and comprises the hydrothermally altered and stanniferous Bobbejaankop and Lease granites. Cassiterite in the Zaaiplaats tin field occurs as low-grade disseminations and within high-grade swarms of tourmaline-quartz hydrothermal pipes and greisenised lenticular ore-bodies. The formation and mineralisation of these granites and hydrothermal features have been topics of debate since their discovery in the early 1900s. Thus, the formation of the Zaaiplaats tin field was re-investigated using a modern integrated approach, incorporating: field mapping; petrography; multi-spectral remote sensing; whole-rock geochemistry; and mineral chemistry. The formation of the Zaaiplaats tin field granites is a culmination of several sequential and interdependent factors. The extensive fractionation of the Nebo granite allowed the concentration of an F, B, Cl, Fe and incompatible element-rich magmatic-hydrothermal fluid towards the granitic cupola. This accumulation of fluid resulted in the alteration of the roof facies and developed the chloritised, greisenised, tourmalinised and partially mineralised Bobbejaankop granite. The continued accumulation and saturation of these magmatic-hydrothermal fluids caused the sub-solidus alteration of the roof of the Bobbejaankop granite. This resulted in the formation of the texturally variably and mineralised Lease microgranite. The sequence of petrological events is demonstrated using incompatible element fractionation proxies such as TEDI, Zr/Hf and Rayleigh Fractionation modelling, which indicate an extensive degree of granitic fractionation from the Nebo to Bobbejaankop then Lease granite. Moreover, the fluid-sensitive ratios of Nb/Ta and Y/Ho highlight an increasing degree of F-rich magmatic-hydrothermal fluid interaction from the Bobbejaankop to the Lease granite. In addition, multi-spectral remote sensing, in conjunction with pXRF mapping, highlights the increased accumulation of magmatic-hydrothermal fluids towards the roof of the granitic system. This interpreted increase in hydrothermal fluid accumulation is validated by an increase in chloritisation and greisenisation in the predicted locations. This integrated mapping approach proved effective in identifying regions of increased alteration, which are commonly associated with disseminated cassiterite mineralisation. Therefore, the synergetic use of remote sensing and pXRF mapping is proposed as a useful tool in exploration. Tourmaline, cassiterite and quartz mineral chemistry provided novel insights into the magmatic to magmatic-hydrothermal evolution of the Zaaiplaats tin field. The tourmaline mineral chemistry reveals two compositionally different tourmalinisation events: an initial schorl–foitite evolution in the Bobbejaankop granite; and a later-stage and more Fe3+-rich, Fe3+-rich schorl–schorl–foitite tourmaline in the Lease granite and hydrothermal pipes. The petrographic associations and compositions of cassiterite revealed a predominantly hydrothermal origin for the ore, and an association with the late-stage Fe3+-rich tourmalinisation. Therefore, the composition of tourmaline can be used to vector potential endogranitic tourmaline-associated cassiterite mineralisation. Trace elemental proportions and their ratios (Al/Ti and Ge/Ti) in quartz reveal an increasing degree of granitic fractionation and influence of the magmatic-hydrothermal fluid. The trace elemental signature of the quartz from the hydrothermal pipes is distinct, in comparison to the magmatic and magmatic-hydrothermal signatures of the quartz within the granites. The trace elements-in-quartz demonstrate an ability to identify the sequence of granitic evolution and infer the point of fluid-saturation. This allows the identification and targeting of the most fractionated and hydrothermally influenced lithologies to guide exploration. A comparison of the Zaaiplaats tin field with other Africa A-Type granite suites, in particular the Gaborone Granite Suite, highlights multiple factors that influence metallogenic potential. A whole-rock geochemical review of the selected granite suites, together with a mica and chlorite comparison of the Zaaiplaats and Gaborone granites, reveal differences between mineralised and barren A-Type granites. The mica compositions from mineralised suites exhibit a Fe-Li trend, whereas barren granitoids tend to evolve along a more Mg-rich trend. Moreover, a useful mica–chlorite ternary diagram, K2Ox10 –Al2O3–(FeO+TiO2+MgO+MnO), is proposed to geochemically illustrate the degree of chloritisation and subsequent muscovitisation of sheet silicates. A sequence of events in the formation of an A-Type granite is proposed that promotes metallogenic potential: (i) the generation of a melt from an enriched mantle source; (ii) high temperature melting beneath an Archean craton; (iii) extensive and prolonged fractionation of a granitoid; (iv) the formation of Fe-rich trioctahedral micas and their evolution along a Fe-Li compositional trend;(v) the concentration and saturation of a metal-, F-and Cl-rich magmatic-hydrothermal fluid; and (vi) mineralisation that is induced during late-stage greisenisation by the acidic, saline and metal-bearing magmatic-hydrothermal fluid.
A dissertation submitted to the Faculty of Science, University of the Witwatersrand, in fulfilment of the requirements for the degree of Doctor of Philosophy, 2021