1. South Africa: A case study on the Sandsloot and Overysel farms.
The use of chemostratigraphy and geochemical vectoring as an exploration tool for platinum group metals in the Platreef, Bushveld Igneous Complex,
South Africa: A case study on the Sandsloot and Overysel farms.
T. Mwenze, C. Okujeni, R. Bailie, A-M. Siad A
1. Introduction
2. Aims and Objectives
4. Geochemical classification of rocks
The paucity of geochemical criteria for stratigraphic correlations and defining the styles of mineralisation pose serious problems in locating PGE-rich zones in the Platreef and its metamorphosed-derived floor rocks (Armitage et al., 2002; Kinnaird et al., 2005; Holwell and McDonald, 2006).
The Platreef is a mafic package (comprised mainly of pyroxenites with variable norites, gabbros, peridotites and serpentinite) of ~10 to 400 m in thickness stretching over ~30 km along strike that occurs at the base of the igneous sequence in the northern Bushveld Complex (Fig. 1). This mafic package overlies different floor rocks and is capped by Main Zone rocks of the Bushveld Complex (Fig. 1) (Gain and Mostert, 1982; Harris and Chaumba, 2001; Mitchell and Scoon, 2012).
To identify and distinguish processes (such as magmatic, hydrothermal and/ or post-magmatic) based on mineralogical/ geochemical signatures, which may be useful in stratigraphic correlation and characterizing the nature and styles of PGE mineralisation across the strike and with depth of the Platreef and its metamorphosed floor rock.
To investigate the possible use of geochemical vectoring as a tool to locate the PGE mineralisation within the studied rocks.
The rock classification after Pronost et al. (2008) in Figure 3 below shows two trends:
A magmatic trend defined by the gabbronorites and Platreef pyroxenites.
A metasomatic trend defined by clinopyroxenites and olivine-rich
clinopyroxenites (or simply olivine clinopyroxenites).
3. Methodology
Boreholes OY 482 and SS 330 (Fig. 2), located at the Overysel and Sandsloot farms respectively (Fig. 1), were logged, and a total of 119 quarter cores were sampled for petrographic studies. The elemental contents in the rocks were determined by XRF and ICP-OES analyses and were evaluated using various statistical and mass balance techniques.
Fig. 3 Geochemical classification of rock samples from the studied boreholes, after Pronost et al. (2008).
CP: Clinopyroxenites; OCP: olivine clinopyroxenites.
5. Mass balance results
The results of mass balance calculations to assess elemental exchange between the Platreef (using Platreef pyroxenites data in OY 482) and the dolomite floor rock in SS 330 reveal the following:
Both the clinopyroxenites and olivine clinopyroxenites are highlighted
by gains in SiO2, Al2O3 and Fe2O3 relative to the metadolomites. Also,
there is a gain of MgO in the olivine clinopyroxenites (Fig. 4).
There are significant losses of MgO, CaO and LOI in clinopyroxenites
and of CaO and LOI in the olivine clinopyroxenites (Fig. 4).
Fig. 1 Geological map of the Northern limb of the Bushveld Igneous Complex after Kinnaird and McDonald (2005), modified from Van der Merwe (1978) and Ashwal et al. (2005). The inset map shows the outcrop of the Rustenburg Layered Suite in South Africa after Harris and Chaumba (2001) and the expanded map shows the location of the different farms and the two studied boreholes along the strike of the Platreef, north of Mokopane.
6. Geochemical characterization of studied rocks
Fig. 2 Stratigraphic log of studied boreholes determined from core logging and petrographic observations; i.e. the modal
mineralogical compositions and textural attributes. Location of samples investigated in this study is displayed on
the right side of the log
Two lithogeochemical indices developed [CaO/(MgO+CaO) or Ca/Mg and MgO/(Fe2O3+MgO) or Mg/Fe], based on the geochemical rock classification and mass balance results, are used to characterize the rocks of interest in Figure 5. In the latter, the Platreef pyroxenites, the olivine clinopyroxenites and the clinopyroxenites can be easily distinguished from each other (Fig. 5). The Platreef pyroxenites are characterised by Ca/Mg and Mg/Fe ratios of <0.5 and <0.6, respectively (Fig. 5). The olivine clinopyroxenites are marked by Ca/Mg ratios of <0.4 and Mg/Fe ratios of >0.6, as shown in Figure 5. The clinopyroxenites are marked by Ca/Mg ratios of >0.4 (Fig. 5).
Fig. 5 Bivariate plot showing the lithogeochemical indices developed to discriminate the rocks of interest.
CP: Clinopyroxenites; OCP: olivine clinopyroxenites.
Fig. 4 Median gains and losses pattern for selected oxides in wt % for the clinopyroxenites (CP) in (a) and
olivine clinopyroxenites (OCP) in (b) relative to the metadolomites.
7. BMS mineralisation – lithogeochemical indices
The investigation conducted to assess the inter-relationship
between the base metal sulphides (or BMS; i.e. Cu+Ni) and
the lithogeochemical indices (Ca/Mg and Mg/Fe) revealed
the following trends:
■ The first trend (blue arrow) shows the relationship between BMS and Ca/Mg in the clinopyroxenites which are characterised by an increase in BMS contents associated with a decrease in Ca/Mg ratio (Fig. 6).
■ The second trend (brown arrow) shows the relationship between the BMS and Mg/Fe in olivine clinopyroxenites and Platreef pyroxenites which is characterised by an increase in BMS contents associated with a decrease in Mg/Fe ratio (Fig. 6).
8. Downhole lithogeochemical variations in the studied boreholes
Figure 7 shows the downhole geochemical variation of Ca/Mg, Mg/Fe, Cu+Ni, SO3, PGE index [(Cu/Zn) x Ni/Co)] and Pt+Pd in the studied boreholes. The PGE mineralisation in the clinopyroxenites is associated with zones of low Ca/Mg values, whereas in the Platreef pyroxenites and olivine clinopyroxenites, it is marked by zones of high Mg/Fe values (Fig. 7). The PGE index [(Cu/Zn) x (Ni/Co)] does not correlate consistently with Pt+Pd in the rocks of interest, i.e. Platreef pyroxenites, clinopyroxenites and olivine clinopyroxenites (Fig. 7).
Fig. 6 Ternary diagram illustrating the relationship between the BMS (Cu+Ni) mineralisation and the
lithogeochemical indices. Data for the BMS (Cu+Ni) were divided by 1000 for visualization purpose.
CP: Clinopyroxenites; OCP: olivine clinopyroxenites.
9. Conclusions
The two boreholes studied show contrasting petrographic and geochemical attributes. This dissimilarity is mainly due to the fact that borehole OY 482 comprises Platreef magmatic rocks whereas borehole SS 330 intersected metamorphic/ metasomatic rocks.
The suggested base metal index [(Cu/Zn) x (Ni/Co)] correlates with the Pt+Pd in the BMS-rich zones. This is not always the case in zones of low BMS contents which may reflect changes in the mineralogy of the BMS.
References
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Fig. 7 Downhole lithogeochemical variations in the studied boreholes. Cu+Ni is expressed in ppm, SO3 in wt. % and Pt+Pd in g/t. Ca/Mg= CaO/(MgO+CaO); Mg/Fe=MgO/(Fe2O3+MgO);
PGE index= (Cu/Zn) x (Ni/Co).
Acknowledgements
I would like to thank Inkaba yeAfrica for funding this project and Anglo platinum at Mogalakwena mine (Limpopo) for providing the research materials. My gratitude also goes to my supervisors Prof. C. Okujeni, Dr. R. Bailie and Dr. A-M. Siad.