Petrography and geochemistry of hydrothermal breccias hosted in the Schrikkloof Formation (Rooiberg Group), north of Modimolle, South Africa

Abstract

The Schrikkloof Formation, composed of rhyolites, tuffs and hydrothermal breccias, is the youngest rhyolitic volcanic sequence of the Rooiberg Group of the Bushveld Igneous Complex and is known to host polymetallic Sn-F-REE. The aim and objective of this research is to understand whether the hydrothermal brecciation in the upper Schrikkloof Formation is associated with ore forming processes or not. The study area is located approximately 20 km north of Modimolle and a total of 30 rock samples (7 rhyolites, 10 tuffs, 12 hydrothermal breccias and 1 vein) was collected from outcrops. These samples were sent for petrography, XRF (major element analysis), ICPMS (trace element analysis), SEM-EDX analysis (ore mineralogy).

Geochemical results (REE diagram and Rb vs Nb+Y) suggest that the Schrikkloof Formation was derived from melting of the continental crust and was emplaced in an intra-cratonic (within-plate) tectonic setting. The sparsely porphyritic and glassy textures of both rhyolites and tuffs, respectively, in the Schrikkloof Formation indicate a subaerial environment.

The hydrothermal breccias are particularly hosted within tuffs rather than in rhyolites, perhaps because of competency contrast. Hand-specimen and petrography observations suggested that both tuffs and rhyolites underwent varying degrees of hematitisation and sericitisation alterations.

In addition, results also indicated that hydrothermal breccias are predominantly characterised by silica-rich (SiO2 > 70%) clasts cemented by Fe-rich veins (Fe2O3 > 80%). The clasts are mainly composed of quartz and K-feldspar, whereas the cement consists of hematite associated with anglesite (PbSO4) and monazite ((Ce,Nd,La)PO4). The Gresens’ and Grant’s mass balance approach results suggest that the hematitisation process is associated with the overall gain of Pb-Zn-Ce-Nd-La in hydrothermal breccia and hematitised tuffs. In conclusion, hydrothermal breccias are associated with Pb-Zn mineralisation and this may suggest that there is a concealed Pb deposit.