David L. Reid

Composition, age and tectonic setting of amphibolites in the central Bushmanland Group, Western Namaqua Province, southern Africa

Abstract The Western Namaqua Province (WNP) of southern Africa is composed of several supracrustal assemblages and intrusive granitoid suites that were formed during the mid-Proterozoic 2.0-1.0 Ga ago. The Bushmanland Group (BG) has been the subject of intense study recently because of its considerable strategic metal potential, hosting several giant stratiform base metal deposits (e.g., Gamsberg and Aggeneys). The BG contains amphibolites which represent metamorphosed tholeiitic basalts with MgO (9-4%) and enriched in Ni relative to modern basalts, a feature reminiscent of Archaean tholeiites. Post depositional mobility of elements is probably widespread and hampers recognition of parental magma composition and source region characteristics. However, the amphibolites have yielded a Sm-Nd isochron age of 1649 ± 90 Ma ( ϵ Nd ( T ) = −0.48 ± 0.89), which is interpreted as the time of extrusion of the basalt precursors, and by inference a minimum sedimentation age for the underlying metalliferous BG sequence. It follows that the BG sequence evolved during an intermediate period in the history of the Namaqua Province, after major crustal stabilisation around 2.0-1.9 Ga and before the last major tectonothermal event around 1.2-1.1 Ga. Results of a parallel Rb-Sr and U-ThPb study of the BG amphibolites indicate strong metamorphic resetting 1.2-1.1 Ga ago, thereby confirming earlier work on the WNP. Characteristics of the source region to the BG basalts have been inspected with MORB normalised trace element abundances, with the salient result being the BG basalts require derivation from a source that experienced variable geochemical modification similar to that proposed for the mantle wedge overlying modern subduction zones. Such a process is currently preferred over crustal contamination, since the Sm-Nd isochron for the BG amphibolites is remarkably linear, but independent confirmation of the Sm-Nd age is still required before this alternative model can be discounted. To reconcile the requirement of subduction related metasomatism in the source to tholeiitic basalts erupted within what appears to be a cratonic sedimentary basin, we suggest that back are continental extension could provide a satisfactory tectonic setting, as such an environment is intrinsically linked to subduction.

Oxygen and carbon isotope patterns in the Dicker Willem carbonatite complex, southern Namibia

Carbon and oxygen isotope data are presented for carbonatites and associated alkali silicate rocks from the Tertiary (49 Ma) Dicker Willem complex in southern Namibia. Carbonatites are grouped into: (1) coarse-grained sovites and associated silicate-oxide-phosphate-rich cumulates; (2) finer-grained alvikite intrusions, showing porphyritic and spinifex textures, comb layering and gravity-settled layers; (3) late-stage dykes, pipe breccias, veins and druses. The early sovites carry many inclusions of silicate rocks (ijolites, syenites). The most primitive carbon and oxygen isotope compositions are found in phenocrysts from calcite-phyric microsovite, bulk s6vites and interstitial carbonate in the ijolites, with δ13C (−5‰ vs. PDB) and δ18O ( +7 to +9‰ vs. SMOW ). Oxygen isotope fractionation between cumulus pyroxene, magnetite and biotite in the sovites yields near magmatic temperatures of 600–900°C. Carbonates in some cumulates yield magmatic temperatures, but commonly show evidence of secondary alteration. Phenocrysts in dolomite-phyric alvikite are slightly enriched in 13C (average δ13C=−3.6‰) and 180 (average δ18O=+9.9‰) relative to primitive ratios, but taken together with data for phyric calcite define a linear trend of increasing δ13C with δ18O and can be modelled as being the product of combined carbonate-silicate-oxide-phosphate fractionation of a parent sovite. Groundmass carbonate in the porphyritic alvikites, as well as the bulk alvikites, all show variable degrees of 18O enrichment relative to the phenocrysts, and reflect partial recrystallization of carbonate in the presence of low-temperature hydrous fluids.

Iron-rich ultramafic pegmatite replacement bodies within the Upper Critical Zone, Rustenburg Layered Suite, Northam Platinum Mine, South Africa

Abstract Discordant veins, pipes and occasionally subconcordant sheets of iron-rich ultramafic pegmatite disrupt the layered cumulate sequence of the Upper Critical Zone, Rustenburg Layered Suite, Bushveld Complex. These pegmatite bodies have been studied where they replace the Merensky Reef footwall at Northam Platinum Mine, situated in the Swartklip Facies of the western lobe of the Rustenburg Layered Suite. Composed chiefly of ferroaugite and fayalitic olivine, the pegmatites appear to be formed by the preferential replacement of plagioclase-rich cumulates within the layered sequence. Fe-Ti oxides, sulphide (pyrrhotite and chalcopyrite) and plagioclase also occur in variable quantities. Differentiation within the pegmatite is observed where it has spread laterally beneath the impervious Merensky chromitite layer, with the development of subparallel cm-scale layers of massive magnetitite, massive sulphide and sulphide pegmatite. While some Fe-rich mobile phase must have been responsible for the pegmatites, it is concluded that the pegmatite bulk composition does not represent the original liquid. Furthermore the mode of occurrence precludes the injection of a crystal mush. Rather it is argued, mainly on geochemical and isotopic grounds, that Fe-rich residual melts derived from the Upper Zone in the downward crosscutting gap areas migrated laterally and upwards into the adjacent Upper Critical Zone. Variable reaction with the layered cumulates produced the anastomosing pegmatite bodies.

Ore textures and possible sulphide partial melting at Broken Hill, Aggeneys, South Africa I: Petrography

The Broken Hill-type Ph-Zn-Cu-Ag massive sulphide mineralisation at Aggeneys in the Northern Cape Province, South Africa has been subjected to peak metamorphic temperatures ranging between 670 to 690°C and pressures between 3.4 and 4.5 kbar. Due to the Ag and Bi rich nature of the ore, these conditions, based on previous experimental studies, may have been sufficient to allow partial melting of the sulphides to occur. Such features as (1) concentrations of low melting point chalcophile elements (LMCE), which include the metals Pb, Zn, Ag, Ga, As, Se, Cd, In, Sb, Te, Hg, Tl and Bi, (2) sulphide inclusions containing a number of different LMCE-bearing minerals within high temperature metamorphic gangue minerals, (3) low interfacial angles between two LMCE-bearing sulphides, such as galena and sphalerite, and LMCE-bearing sulphides and resistate minerals, particularly silicates, (4) LMCE-bearing sulphides filling fractures within metamorphic silicate minerals, (5) coarse recrystallised pegmatite bodies of galena and chalcopyrite crosscutting the regional foliation in the ore body and (6) the presence of a transgressive Mn selvage around the orebodies have all been recognised at Aggeneys and may be indicators of sulphide partial melting. The abundance of pyrrhotite in the orebodies, particularly in the lower ore body (LOB), may, in part, be due to partial melting of pre-metamorphic pyrite during peak metamorphism. Some euhedral pyrite in the LOB may similarly have been derived from desulphidation of pyrrhotite with falling temperatures. The Aggeneys ores also shows other typical features of metamorphosed sulphide ores, such as recrystallisation, remobilisation, deformation and annealing, many of which have contributed to obscuring previous textural features. The extent of sulphide partial melting is likely to have been small, and its effects obscured by later deformation, remobilisation and recrystallisation. It may, however, have played a small role in locally upgrading the ore. This study examines the petrography of the sulphides and their ore textures. Analytical techniques to examine the composition of the sulphides and of multiphase sulphide inclusions are to be undertaken. More extensive studies using numerous techniques are also required in order to clarify this topic.

Timing of post-Karoo alkaline volcanism in southern Namibia

New radiometric age data are reported for alkaline centres in southern Namibia, and are discussed together with published age data in terms of models put forward to account for post-Karoo (Mesozoic–Recent) alkaline magmatism within the African plate. Agreement between K–Ar and Rb–Sr ages indicate emplacement of the Dicker Willem carbonatite in southern Namibia at 49 ± 1 Ma. Alkaline rocks associated with the Gross Brukkaros volcano show a discordant radiometric age pattern, but the best estimate for the age of this complex is 77 ± 2 Ma, similar to that obtained for the neighbouring Gibeon carbonatite-kimberlite province. The Dicker Willem carbonatite is therefore younger than the Luderitz alkaline province (133 ± 2 Ma), and the Gross Brukkaros volcano, but is older than the Klinghardt phonolite field (29–37 Ma). The new age data argue against a distinct periodicity in alkaline igneous activity in southern Africa, thereby ruling out possible controls by episodic marginal upwarping of the subcontinent. Although the available age data do not appear to be consistent with the passage of one or even two hotspots under southern Namibia, it is argued that the surface expression of hotspots under continents may be so large and overlapping that within-plate magmatism attributed to these thermal anomalies need not necessarily be confined to narrow linear belts or show an age progression. The role of hotspots in continental alkaline magmatism is most likely one of melt generation, while local crustal structure probably controls the distribution and timing of eruption. Major tectonic boundaries in the Precambrian basement underlying southern Namibia seem to have controlled the development of Tertiary alkaline centres in that region.

Geochemistry of Karroo dolerite sills in the Calvinia district, Western Cape Province, South Africa

Two Karroo dolerite sills display chemical and mineralogical variation compatible with cumulus enrichment. The Blaauwkrans sill is an olivine tholeiite and contains a central zone slightly enriched in olivine, plagioclase and clinopyroxene. The thicker Hangnest sill is a quartz tholeiite and shows evidence of crystal settling and has a lower zone enriched in cumulus orthopyroxene and plagioclase.The two sills differ quite markedly in their trace element compositions, with the Hangnest magma enriched by a factor of two in LIL elements (Rb, Ba, Nb, Zr, Y) relative to the Blaauwkrans magma. The Hangnest magma contained extremely low Ni contents (3–5 ppm), whereas the Blaauwkrans magma contained higher but more “normal” Ni (100–110ppm). Such contrasting trace element compositions preclude any simple genetic relationship between the two Karroo magmas but they may be related either through a common parent or are derivatives from separate parental magmas.

Chemical and Textural Re-equilibration in the UG2 Chromitite Layer of the Bushveld Complex, South Africa

Variations of mineral chemistry and whole-rock compositions were studied in detail, at millimetre to centimetre intervals, in two vertical drill core profiles through the platiniferous UG2 chromitite layer in the western and eastern limbs of the Bushveld Complex, South Africa. Analytical methods included electron microprobe and LA-ICP-MS analyses of the main rock-forming minerals, orthopyroxene, plagioclase and interstitial clinopyroxene. One profile was also studied by synchrotronsource XRF. Statistical analysis of crystal size distribution of chromite was also performed at different levels in the chromitite layer and in adjacent silicate rocks. The results provide new evidence for chemical and textural late magmatic re-equilibration in the UG2 layer and in the silicate rocks at the contact zones. The chromite crystal size distributions imply extensive coarsening of that mineral within the main chromitite seam, which has erased any textural evidence of primary deposition features such as recharge or mechanical sorting of crystals, if those features originally existed. The mineral compositions in chromitite differ from those in adjacent silicate rocks, in general agreement with predictions of chemical re-equilibration with evolved, residual melt (the trapped liquid shift effect). In detail, the geochemical data imply, however, that the conventional trapped liquid shift model has shortcomings, due to the effects of material transport driven by chemical gradients between modally contrasting layers of crystal mush undergoing reequilibration reactions. In the presence of such gradients, selective open-system conditions may hold for alkalis and hydrogen because of their higher diffusion rates in silicate melts. Differential mobility of components in the interstitial melt can also sharpen the original modal layering by causing minerals to crystallise in one layer and dissolve in another. Detailed trace element profiles by synchrotron XRF reveal an uneven vertical distribution of incompatible elements which implies that the permeability of the chromitite layer may have been significant, even at the latest stages of interstitial crystallization.

The Richtersveld: An Ancient Rocky Wilderness

The Richtersveld , in northwest South Africa, is located at the transition from the coastal plain to the elevated interior plateau and adjacent to the Orange River . Despite this positioning, the primary landscape features of the Richtersveld reflect the much more humid conditions of the Cretaceous period 100 million years ago. Subsequent aridification of the southern African west coast during the Cenozoic resulted in lower landscape denudation rates that left the topography of the Richtersveld largely unaffected, except for periodic changes in climate when increased run-off incised the lower Orange River through the Great Escarpment and into the continental interior. Sea-level fluctuations during the Cenozoic also contributed to river incision, as well as the development of river terraces and marine benches that host economically important diamond placers . Three landscape terrains can be defined along a west–east profile, inland from the coast. The western Richtersveld forms the coastal plain that was cut to near sea level across all lithologies, irrespective of composition and hardness, and is covered by alluvial debris derived from the escarpment and aeolian sands from the coast. The central and eastern terrains form a linear corridor of high topographic relief and dissection that characterises the Great Escarpment. The great antiquity and long-term preservation of the Richtersveld landscape reflects its long geologic and climatic history and is today protected as a wilderness region.

Research on Proterozoic zinc deposits in Namibia and South Africa

The Lower Orange River region of Southern Africa forms the border between southern Namibia and the neighbouring Northern Cape Province of South Africa. Within these diffuse boundaries are contained a diverse range of base metal (Copper-Lead-Zinc) deposits that have produced many millions of tons of ore since organised mining first commenced in the 1850s. New deposits still continue to be discovered and developed today, with the new millennium being heralded by the first production from the Skorpion zinc mine in Namibia, a new shaft and decline at Aggeneys to exploit the Broken Hill deep orebody, and the blending of ore from Swartberg and Gamsberg into the Aggeneys run of mine. As a consequence of new research results and timely reviews being presented at the …