Paul A. M. Nex

Chromitite formation—a key to understanding processes of platinum enrichment

Abstract The mafic layered suite of the 2050 m.y. old Bushveld Complex hosts a number of substantial platinum-group element (PGE)-bearing chromitite layers, including the UG2, within the Critical Zone, together with thin chromitite stringers of the platinum-bearing Merensky Reef. Until 1982 only the Merensky Reef was mined for platinum, although it has long been known that chromitites also host PGE-bearing minerals (PGM). Three groups of chromitites occur: (i) a Lower Group of up to seven major layers hosted in feldspathic pyroxenite; (ii) a Middle Group with four layers hosted by feldspathic pyroxenite or norite; and (iii) an Upper Group, usually of two chromitite packages, hosted in pyroxenite, norite or anorthosite. There is a systematic chemical variation from the bottom to the top chromitite layer in terms of Cr : Fe ratio and the abundance and proportion of PGE. Detailed studies of 87Sr/86Sr isotope variations undertaken on interstitial plagioclase from chromitites and different silicate host rocks show that the magma from which the chromitites formed (interstitial plagioclase Sri <0.7099) usually differed radically from the resident liquid from which the immediate footwall rocks crystallized (Sri, ca 0.7060-0.7064). These high Sr isotope ratios can only have been produced by sudden and extensive contamination by an extremely radiogenic component. The only viable source for this component in the chamber is the felsitic roof rocks or a granophyric roof-rock melt. It is suggested that such contamination occurred when a new magma influx penetrated the residual liquid and interacted with the overlying roof rock as well as mixing with the resident liquid. It is envisaged in this model that chromite cascaded to the floor together with a small amount of magma adherent to the chromite or entrained within the slurry to produce interstitial silicates with enriched isotopic ratios. The close correspondence of chromitite and PGE enrichment strongly suggests that the contamination process that resulted in chromite formation also triggered precipitation of the PGE. The base of each major chromitite layer marks the point where there was a substantial injection of new magma into the chamber, which resulted in erosion of the cumulate pile, interaction with roof rocks and inflation of the chamber. Thus, the major PGM and chromitite ore deposits of the Bushveld Complex are unconformity-related and are associated with mixing of new magma, coupled to simultaneous contamination by granophyric roof-rock melt. The chromitites represent, therefore, the products of a roof contamination and magma mixing process.

Multiple sulfur isotopes reveal a magmatic origin for the Platreef platinum group element deposit, Bushveld Complex, South Africa

The Platreef ore horizon of the Bushveld Complex, South Africa, is the third largest platinum group element ore deposit in the world, but the origin of its ore remains enigmatic. A complex contact relationship between the igneous and footwall rocks of the Bushveld Complex, coupled with evidence for widespread late-stage hydrothermal processing, obscures the original mineralization history of the Platreef. We constrain the parental magmatic origin of the Platreef by exploiting multiple sulfur isotope contrasts across Bushveld Complex contact zones in order to see through the effects of postmineralization hydrothermal activity. We report S isotope measurements made on samples collected along two profi les through the Platreef into underlying metapelitic and metacarbonate footwall rocks. In both profi les, igneous rocks far from the contact have low Δ 33 S values (average Δ 33 S = 0.15‰), whereas metasedimentary rocks far from the contact have high Δ 33 S values (Δ 33 S to 5.04‰) with a smoothly varying profi le between the two end members. The midpoint in both isotope profi les is displaced into the footwall, defi ning a classic advective-dispersive tracer geometry. This geometry is not present in the associated δ 34 S values. The displacement of the Δ 33 S front suggests fl uid transport and advection of S into the country rocks; this was accompanied by back diffusion of the S isotope tracer into the Platreef. The Platreef magma was apparently S saturated prior to emplacement and, counterintuitively, lost S during the formation of the present Platreef ore horizon.

Spinel-bearing assemblages and P-T-t evolution of the Central Zone of the Damara Orogen, Namibia

Abstract The Central Zone of the Damara Orogen in Namibia is dominated by intrusive granitoids and upper amphibolite to granulite-facies metasediments. High-temperature low-pressure mineral assemblages near the Atlantic Coast have not been previously described with spinel symplectites. In semi-pelitic gneisses, the symplectites are composed of hercynitic spinel and Fe-Ti oxides occurring within the cores of zoned porphyroblasts of antiperthite+sillimanite. In pelitic schists, hercynite-sillimanite symplectites occur together with minor quartz in the core of Mg-cordierite porphyroblasts (X Mg = 0.66–0.72). Thermobarometry determines the P-T conditions of spinel symplectites to be 527 ± 112°C and 4 ± 1 kbar. Textural relationships indicate that the symplectite post-dates the metamorphic peak conditions dated by syn-metamorphic granite at 534 ± 7 Ma (published U-Pb zircon age) but are still relatively early within the metamorphic history. Subsequent isobaric regional heating and partial melting occurred in association with biotite breakdown and garnet growth. Thermobarometry gives 700 ± 50°C and 4 ± 1 kbar for this episode. This regional isobaric heating is equated with the emplacement of voluminous granitoid magmas, dated at post-534 ± 7Ma and pre-508 ± 2 Ma (published U-Pb zircon and monazite ages) and generated during decompression caused by the exhumation of the Central Zone. The system then cooled through 550 ± 25°C and 2 ± 1 kbar at 465 ± 5 Ma (published Rb-Sr and Ar 40 /Ar 39 biotite-muscovite ages) on its way to the surface. The occurrence of a later separate thermal overprint as evidenced by spinel symplectites and garnet growth argues in favour of a two-stage metamorphic model. This study confirms the clockwise P-T path for the Central Zone of the Damara Orogen.

Geochemical effects of magma addition: compositional reversals and decoupling of trends in the Main Zone of the western Bushveld Complex

Abstract The Main Zone of the Bushveld Complex, which is ~3-3.5 km thick, comprises a sequence of gabbronorites with minor anorthosites and pyroxenites. The Pyroxenite Marker (PM), a thin orthopyroxenite layer occurring towards the top of the Main Zone in the eastern Bushveld, marks the change from an inverted pigeonite-bearing microgabbronorite below, to a primary orthopyroxene-bearing porphyritic gabbronorite above. In the western Bushveld the PM has only been observed in core material although its surface position can be inferred from the mineralogical and textural changes. Whole-rock geochemistry of surface and core samples from the Brits and Marikana areas, together with mineral compositional data, have been integrated with published analyses to elucidate the magmatic processes that occurred during the addition of new magma into the chamber, just below the level of the PM. Modal and major-element data show that most lithologies lie close to the plagioclase-two pyroxene cotectic. However, there are three units below the PM in which distinct modal layering is developed. Changes in geochemical trends for trace-element abundances in both whole-rock and mineral-separate data occur at ~150 m below the PM, below a layered package, the Hexrivier Unit. However, there is a displacement of at least 40 m between the beginning of the reversals in An content in plagioclase and in Mg# in pyroxene which occur at 124 m and 80 m below the PM respectively. In addition, a gradual change occurs in the mineral parameter Mg# in opx minus An in plagioclase, with the plagioclase becoming more primitive relative to the pyroxene. This decrease and the decoupling of geochemical trends has not been noted in the Main Zone of the Bushveld Complex before. The decoupling between plagioclase and pyroxene compositional reversals is not easily explained by modal effects, an influx of phenocrysts in the new magma, or by infiltration metasomatism. Instead we propose that new magma pulses first entered the chamber some 150 m below the PM and that this magma had a composition that crystallized more primitive plagioclase but similar pyroxene to the magma residing in the chamber. The chamber was intruded by progressively more magma that had a more primitive composition particularly in terms of pyroxene composition. This model can explain the decoupling between plagioclase and pyroxene compositional trends. Continued mixing between resident magma and the new influxes occurred over an interval of ~150-200 m. Above the PM fractional crystallization processes dominated and continued into the Upper Zone.

Petrology, geochemistry and uranium mineralisation of post-collisional magmatism around Goanikontes, southern Central Zone, Damaran Orogen, Namibia

Abstract The Goanikontes area lies within the southern Central Zone of the northeast trending branch of the Damara Orogen. The cover succession around Goanikontes, which comprises Etusis to Chuos Formation metasediments, is in tectonic contact with older pre-Damaran basement rocks. The area can be divided into three structural domains with basement in the east, a northeast-plunging anticline of Damaran metasediments to the west and a high strain zone up to 1 km wide that separates them and truncates the anticline. The high strain zone has provided an important focus for the emplacement of sheeted granites adjacent to the basement-cover contact. Goanikontes is also one of several significant uranium anomalies within the Damaran Orogen, and the excellent 3D exposure of the Swakop river has provided evidence for an interpretation of the tectono-metamorphic setting of magmatism and mineralisation. The granitoids have been divided into equigranular granites and sheeted leucogranites. The sequence of emplacement of the equigranular granites is from red syenogranite (534±7 Ma) to later grey monzogranite (517±7 Ma) with foliated basement-hosted granite of probable time equivalence to the syenogranites. Each type can be distinguished on major- and trace-element geochemistry. The later sheeted leucogranites, which are volumetrically the most important, intrude both basement and cover rocks. These have been divided into six distinct types based on field characteristics and structural setting backed by geochemical data and fluid extraction analyses. The earliest type A are irregular in form, boudinaged and folded by D 3 , and geochemically distinct with notably low HFSE; type Bs are white, weakly foliated, folded by D 3 , garnetiferous and highly peraluminous; type C are tourmaline-bearing, occasionally boudinaged and exhibit the typical sheet-form within the cover rocks. Of the post-D 3 sheets, type D, which is restricted to the high strain zone is characterised by smoky quartz, high radioactivity and often by visible betafite or beta-uranophane; type E, the dominant type within the high strain zone contains prominent oxidation haloes and type F is red in colour, coarsely pegmatitic and has the highest concentration of alkalis. The uranium-mineralised type D sheets have consistently higher fluid and CO 2 content than other sheeted leucogranites. The close correlation between sheet type and uranium abundance, supported by linear trends on LIL plots suggests that the distribution of radiogenic elements is primarily magmatic with more recent meteoric re-distribution, rather than due to substantial hydrothermal modification

The structural setting of mineralisation on Tweefontein Hill, northern limb of the Bushveld Complex, South Africa

Abstract Tweefontein Hill is situated in the northern limb of the Bushveld Complex where there is a significant change in strike of the Platreef and Transvaal Supergroup footwall rocks. Discontinuous massive sulphide mineralisation attributed to gravitational settling of a magmatic sulphide liquid occurs within this structure. Detailed mapping and structural investigations show that there are two pre-Bushveld ductile deformation events which have resulted in a major south-west plunging fold at Tweefontein Hill. This fold structure is a primary control on the distribution of massive sulphide mineralisation. Subsequent brittle deformation can be correlated with regional structures and has resulted in focusing hydrothermal mineralisation with remobilised platinum group elements into north-south and north-north-east trending faults.

Fluid extraction from quartz in sheeted leucogranites as a monitor to styles of uranium mineralization: an example from the Rössing area, Namibia

The Rössing Mine area in Namibia contains uranium mineralization primarily within crustally derived post-collisional sheeted leucogranites. Adjacent to the mine in the SH area, a small 1-km long granite cupola of coalesced sheets is also characterized by elevated U levels. In the Rössing pit, in the SJ area, U is hosted by uraninite and secondary U minerals including beta-uranophane. Conversely, in the SH area, U is hosted principally by the pyrochlore-group mineral, betafite. Manometric determination of the proportions of H2O, CO2 and non-condensables (mostly methane) has been carried out on inclusions within quartz from leucogranite samples from both the SH and SJ anomalies. This method has been able to distinguish between leucogranites of the SH and SJ areas which has not been possible with thermometric fluid inclusion studies or by major and trace element geochemical investigations. Fluid geochemistry of sheeted leucogranites from the SH area exhibit lower absolute H2O contents, lower H2O/CO2 ratios and low total fluids compared to that from the SJ area. Fluid geochemistry and large ion lithophile (LIL) element data suggest that the occurrence of the SJ anomaly as a major uranium deposit is linked to the high H2O and total fluid contents. This study illustrates the usefulness of this technique to distinguish between economic mineralization and the sub-economic SH anomaly.

Minerals and Mining in South Africa

Mining in southern Africa has a long pre-colonial history. Haematite mining in Swaziland can be traced back more than 40,000 years, iron smelting dates from 400 AD or earlier, and copper from 900 AD. The most iconic evidence for pre-colonial mining are the gold artefacts from Mapungubwe dated between 1220 and 1300 AD. In 1681, early Dutch colonists became aware of the copper in the Northern Cape and so began the colonial interest in the metals of South Africa. Since then, diamonds were discovered in 1866 with several large stones still being found. Gold was discovered in Barberton in 1884, then in the Johannesburg goldfield in 1886. Both continue to contribute important sources of revenue to the South African economy. Production of platinum metals began from the Merensky Reef in 1926, leading to South Africa now producing 78% of the world’s platinum. Other world-class deposits of the modern era include iron from Sishen and manganese from Kuruman.

Lamprophyric dykes in the Bushveld Complex: the lithospheric mantle and its metallogenic bearing on the Bushveld large igneous province

In Southern Africa, the geodynamic setting and source(s) of magmas that fed the Bushveld Complex remain unresolved – not least the controls on the metallogenic signature of this large igneous provi...