Fanus Viljoen

Implications of the carbon isotope and mineral inclusion record for the formation of diamonds in the mantle underlying a mobile belt: Venetia, South Africa

A total of 199 diamonds from the Venetia kimberlite, South Africa, whose mineral inclusion chemistry had already been measured, were analyzed for their carbon isotopic composition. Silicate inclusions in these diamonds either belong to a peridotitic (P-Type), an eclogitic (E-Type) or transitional, websteritic (W-Type), paragenesis. The carbon isotopic composition of 161 P-Type diamonds ranges from δ13C = −2.23 to −18‰ vs. PDB. The large number of samples available and the wide range in δ13C permitted, for the first time, a detailed analysis of the relationships between P-Type inclusion chemistry and the carbon isotopic composition of the diamond host. The δ13C sampling frequency distribution is multi-modal. Examination of the inclusion chemistry (chromite, olivine, garnet) as a function of the carbon isotope composition mode to which the host belongs, as well as multivariate regression analyses, revealed no correlation between inclusion chemistry and 13C content. The inclusion compositions in diamonds of low13C content are not distinctive. For a given carbon isotopic composition the combination of Ni/Fe and Mg/(Mg + Fe) of olivine inclusions varies systematically along fractionation trends. The composition of the olivine inclusions and the 13C content of their hosts can be interpreted as reflecting similar petrogenetic processes occurring in several mantle environments into which carbon of variable isotopic composition was introduced. The iron/magnesium distribution between coexisting garnets and olivines permits an estimate of their pressure/temperature equilibration conditions. Diamonds whose inclusions were equilibrated at lower temperatures and pressures tend to have, on average, lower 13C contents. The compositions of coexisting olivines and chromites suggest oxygen fugacities between 2.9 and 5.8 log units below the quartz-fayalite-magnetite buffer at 50 kbar, and temperatures between 1280 and 1490°C prevailed during diamond formation. Inclusions from diamonds of lower 13C content do not indicate systematically lower fO2 values during their formation. The fO2/T conditions determined suggest minimal (0.0 to −0.5‰) isotope fractionation between a C-H-O vapor phase (carbon dioxide, carbon monoxide, methane, water, hydrogen) and diamond. The large 13C depletion of some Venetia P-Type diamonds appears to be unrelated to the composition of their inclusions, igneous fractionation trends, oxygen fugacity, and vapor isotope fractionation processes. This conclusion is consistent with evidence deduced from more limited data sets from other kimberlites. Eclogitic diamonds constitute less than 10% of the inclusion bearing Venetia diamonds. The δ13C values of 19 E-Type samples range from −4.39 to −15.6‰. The nature of the relationship between inclusion chemistry and carbon isotopic composition of the hosts parallels that observed for P-Type diamonds. W-Type diamonds occur least frequently; the δ13C range for six samples is −3.74 to −5.91‰. The carbon isotope distribution and inclusion chemistry of Venetia diamonds are akin to those of diamonds from most kimberlites located on the Kaapvaal craton. This indicates that diamond formation in the mantle underlying the Limpopo Mobile Belt followed processes and involved carbon sources that are very similar to those involved in diamond formation in the mantle beneath the Kaapvaal craton.

Application of a Field Emission Mineral Liberation Analyser to the in Situ Study of Platinum-Group Element Mineralisation in the Merensky Reef of the Bushveld Complex, South Africa

Quantifying spatial relationships of minerals of the platinum-group elements in ores in situ is difficult, as petrologists are faced with issues of representative sampling, together with laborious and time-consuming microscopic examination. Although automated mineralogy on SEM-based platforms does provide a solution (Fandrich et al., 2007), this is a time-consuming process on conventional instruments equipped with a thermionic tungsten source and SiLi detectors, when applied to the in-situ examination of rock’s thin sections. The advent of the field emission mineral liberation analyser allows for very rapid, automated acquisition of mineralogical data on ores and successor products. FEI’s 600F mineral liberation analyser (MLA) combines a very bright Schottky field emission source with two high-speed silicon drift (Bruker) energy dispersive X-ray spectrometers, with the result that analysis time for MLA applications is significantly lowered. We present procedures followed, and the results of, platinum-group mineral (PGM) searches, identification and size frequency distributions, as well as the quantification of host rock phases occurring in close association with the PGMs encountered, utilising an FEI 600F MLA. Samples of chromitite stringers in Merensky Reef from the Two Rivers and Marikana platinum mines examined for the present study contain a full suite of PGMs consisting mainly of ferroplatinum (PtFe), cooperite (PtS), braggite (PtPdNiS), maslovite (PtTeBi) and laurite (RuS), as well as other Pt and Pd sulfides, arsenides, tellurides, bismuthides and alloys, which range in size from <4 micron to ~100 micron. Gold is present as electrum (AuAg) and also as a rare AuBiPdTe compound.

Mineralogical Assessment of the Metamorphosed Broken Hill Sulfide Deposit, South Africa: Implications for Processing Complex Orebodies

The Broken Hill Pb–Zn–Cu–Ag ± (Au) deposit is located in northwestern South Africa, ~750 km NNE of Cape Town in the Northern Cape Province of South Africa. In conjunction with the other deposits of the world-class Aggeneys-Gamsberg district (i.e., Broken Hill-Broken Hill Deeps, Swartberg/Black Mountain, Big Syncline, and Gamsberg deposits), the Broken Hill and associated deposits are of particular importance as they represent the only active lead and zinc mining operation in South Africa. Yet, in spite of their economic importance, the mine reports diminished recovery rates compared to similar mines, regularly incurs penalties for high concentrations of deleterious elements and no credit for the small quantities of Au recovered. Therefore, the aims of this investigation are to provide a detailed mineralogical assessment of the sulfide minerals to provide new insight that could aid in improving the ore recovery and potential environmental concerns. A detailed investigation of the Broken Hill deposit highlights the complexity of the sulphide assemblage, which comprises various types of base metal and iron-sulfides/sulfosalts, texturally and chemically modified during subsequent phases of deformation and metamorphism. The high degree of complexity and heterogeneity within the deposit may explain the poor recovery rates. Therefore, mineralogical investigations should be conducted on a regular basis, ensuring efficient and optimal recovery. Detailed mineralogical investigations of the sulfide minerals reveal a high level of complexity with multiple varieties of base metal and iron sulfide, and sulfosalt minerals deposited during several paragenetic stages, which have been confirmed by whole rock and mineral chemical analyses. In spite of these obvious complexities, and analogous to the paragenetic sequence of previous investigations, four paragenetic stages of sulfide deposition have been identified at the Broken Hill deposits. They are: (T 1) Primary (pre-metamorphism); (T 2) early metasomatic (retrograde-metamorphism); (T 3) late metasomatic (retro-/pro-grade-metamorphism); and (T 4) late open-space-fill (retrograde metamorphism). Primary (T 1) sulfides occur within primary magnetite quartzite where they comprise massive to annealed groundmass bodies. Early metasomatic (T 2) sulfides are restricted to remobilized and reconcentrated massive magnetite to magnetite quartzite and embody the bulk of the minable sulfide bodies. In contrast, late metasomatic (T 3) sulfides are characterized by cataclastic sulfide matrix breccias, while the final stage (T 3) occurs as a late open-space-fill in paragenetically younger sulfides, oxides, and rarely silicates. In conclusion, this investigation provides insight into the mineralogical information that can be used to improve ore recovery and mitigate potential environmental concerns. The high degree of complexity and heterogeneity within the deposit may explain the poor recovery rates. Therefore, further mineralogical investigations should be conducted on a regular basis, ensuring efficient and optimal recovery.