The research, led by Rames Chauke of the Council for Geoscience in Pretoria, focuses on two diamond drill cores: BHDD 003, drilled to 350 metres at the Boschhoek Prospect, and 648 KS/1, a 195-metre core from the historic Mutue Fides-Stavoren Tin Field. Twenty-five samples were logged, thin-sectioned and analysed for major and trace elements using X-ray fluorescence and inductively coupled plasma mass spectrometry. The cores captured four granite facies in vertical succession: the coarse-grained Nebo granite at the base, the brick-red Bobbejaankop granite, the fine-grained Lease granite, and the microgranitic to granophyric Klipkloof granite capping the sequence.

Petrography already told part of the story. The Nebo granite is a leucocratic, equigranular rock dominated by perthitic alkali feldspar and quartz, with biotite and hornblende filling the gaps between grains. Its feldspars show pervasive sericite alteration and its mafic minerals are chloritised, evidence that hot fluids percolated through the rock long after it crystallised. The Bobbejaankop granite is more intensely transformed: plagioclase is heavily sericitised, magmatic biotite and hornblende are chloritised, and some feldspars carry a brown hematite coating attributed to fluid-rock interaction. The Bobbejaankop grades downward into the porphyritic Lease granite, while the granophyric Klipkloof variety caps it at the top of the core.

Geochemically, all four facies belong to the same clan. They are ferroan, calcic to alkali-calcic, peraluminous A-type granites that formed within an intracratonic setting, consistent with the within-plate granite field on tectonic discrimination diagrams. Silica contents range from about 70 to 77 weight percent, calcium, magnesium and phosphorus are strikingly low, and the Fe-index and Shand index confirm their ferroan, strongly peraluminous character. In other words, the Bushveld granites are chemically hot, dry, iron-rich melts of the kind typically generated in continental interiors rather than at subduction zones.

The real drama emerges in the trace elements. Rubidium concentrations climb from an average of 158 parts per million in the Nebo granite to 407 parts per million in the Bobbejaankop-like granites, while barium and strontium collapse from averages of 776 and 41 parts per million to just 108 and 6 parts per million. Because rubidium prefers the melt while barium and strontium are locked into early crystallising minerals, these shifts are the classic signature of progressive fractional crystallisation. The rubidium/strontium ratio jumps from 2 to 11 in the Nebo granite to a remarkable 44 to 229 in the Bobbejaankop-like facies, marking them as far more evolved.

A purpose-built discriminator, the trace element differentiation index defined as barium multiplied by strontium/rubidium, drives the point home. Values below 10 indicate highly fractionated, magmatic-hydrothermally influenced granite. The Nebo granite scores between 70 and 399, whereas the Bobbejaankop, Klipkloof and Lease granites plummet to between 0.1 and 4. The potassium/rubidium ratio tells a parallel tale: least differentiated granites typically exceed 170, but the Bobbejaankop-like facies fall to between 69 and 117, a level that reflects both extreme fractionation and the fingerprint of aqueous fluids stripping alkalis from the rock after it solidified.

Element pairs that normally travel together provided the smoking gun for hydrothermal interference. Yttrium and holmium, and zirconium and hafnium, share such similar charge and radius that their ratios should remain essentially constant in a closed magmatic system. Yet the Bobbejaankop-like granites plot away from this so-called CHARAC field, and their zirconium/hafnium ratios drop below roughly 25, a threshold widely used as a fertility indicator for tin-tungsten-molybdenum mineralisation. Niobium/tantalum ratios fall from about 17 in the Nebo granite to about 10 in the altered facies, corroborating the transition from purely magmatic to magmatic-hydrothermal conditions.

Rare earth element patterns add texture to the story. The Nebo granite displays the smooth, left-tilted profile typical of A-type granites, enriched in light rare earth elements with moderate negative europium anomalies between 0.40 and 0.57. The Bobbejaankop-like granites are far more erratic, with europium anomalies as deep as 0.06 and three distinct heavy rare earth patterns that map neatly onto the three facies identified in the core. Normalised lanthanum/ytterbium ratios, together with a review of published oxygen isotope data showing invariable quartz values across the eastern limb, indicate that all these facies are cogenetic, likely derived from the differentiation of mantle-derived alkaline mafic magma related to the Rustenburg Layered Suite, with their chemical diversity driven by fractionation and the subsolidus removal of alkalis by hydrothermal fluids.

The practical payoff is an exploration tool. Because the Bobbejaankop, Klipkloof and Lease granites carry lower zirconium/hafnium ratios and stronger hydrothermal signatures, they represent a higher fertility index than the Nebo granite and are more likely to host endogranitic polymetallic deposits. The Lebowa Granite Suite is unusual in preserving a complete magmatic-hydrothermal mineralisation cycle, from orthomagmatic tin-molybdenum-tungsten through mesothermal lead-zinc-copper-silver assemblages to epithermal iron-fluorite-uranium-rare earth systems, a sequence unmatched by comparable granite provinces elsewhere in the world.

For a complex whose mafic rocks already host the world’s largest platinum reserves, the granites have long played second fiddle. This study shows they deserve equal billing: the same mantle-derived magmatic engine that built the Rustenburg Layered Suite also cooked up the fertile granites above it, and simple element ratios measured in a drill core can now flag which parts of that granite blanket are worth drilling next. As exploration pushes into deeper and subtler targets, chemical fingerprints like these may prove the difference between a barren hole and the next Vergenoeg or Zaaiplaats.

Subject of Research: Geology and geochemical variations within the eastern Lebowa Granite Suite, Bushveld Igneous Complex, South Africa: insights from fractionation and hydrothermal interference

Article Title: Geology and geochemical variations within the eastern Lebowa Granite Suite, Bushveld Igneous Complex, South Africa: insights from fractionation and hydrothermal interference

Article References: Chauke, R. (2025). Geology and geochemical variations within the eastern Lebowa Granite Suite, Bushveld Igneous Complex, South Africa: insights from fractionation and hydrothermal interference. Frontiers in Geochemistry, 3, Article 1640841. https://doi.org/10.3389/fgeoc.2025.1640841

Image Credits: AI Generated

DOI: 10.3389/fgeoc.2025.1640841

Keywords: Geology, geochemical, variations, within, eastern, Lebowa, Granite, Suite, Bushveld, Igneous, Complex, South