A. Rozendaal

Timing and kinematics of the Colenso Fault: The Early Paleozoic shift from collisional to extensional tectonics in the Pan-African Saldania Belt, South Africa

The Colenso fault is a major northwest to southeast trending fault zone in the Pan-African Saldania Belt of the Western Cape Province in South Africa that is spatially closely associated with granitoids of the ~550 to 510 Ma Cape Granite Suite. Most of these granites were previously considered to be largely post-tectonic intrusions, but structural data presented in this study demonstrate the synkinematic emplacement of granitoids into, and along, the Colenso Fault. The kinematic analyses of shear zones and granite fabrics together with previously published and new geochronological data are combined to provide constraints on the complex kinematic history of the fault and the tectonic evolution of the hitherto poorly understood Saldania Belt. Early, strongly gneissose granitoids of the composite Darling batholith (547 ± 6 Ma) were emplaced during sinistral strike-slip movement along the Colenso fault. Both the timing of emplacement and penetrative deformation of the Darling batholith suggest an intrusion of the pluton during the main Pan-African collisional event in the Saldania Belt. The younger Trekoskraal granite intrudes synkinematically into dextral strike-slip faults related to deformation along the Colenso fault. Single-zircon ages from synkinematic aplites constrain the timing of dextral strike-slip shearing to 539 ± 4 Ma. The emplacement of the late-kinematic Cape Columbine granite during dextral strike-slip faulting indicates that dextral strike-slip kinematics along the Colenso fault continued at least until ~520 Ma. These results point to a reversal of strike-slip motion along the Colenso fault at ~540 Ma that coincides with the onset of uplift of rocks of the Saldania Belt. The final exhumation of the belt at ~515 to 520 Ma is marked by the near-surface emplacement of the last phases of the Cape Granite Suite, related subaerial volcanism, sedimentation of the coarse-clastic, fault-bounded Klipheuwel Group, and the overlying fluvial to shallow-marine sequence of the Mid-Cambrian Cape Supergroup. The temporal and spatial overlap between igneous activity and rift-type sedimentation indicates that a substantial part of the Cape Granite Suite was emplaced in an overall transtensional and/or extensional setting. During this time, the voluminous plutonism of the Cape Granite Suite most likely represented a significant heat input that also contributed to a thermal weakening of the crust. In view of the Early Paleozoic extensional setting suggested here, we interpret Ar-Ar mineral ages of ~500 Ma and post-orogenic plutonism that are widely documented from Pan-African belts throughout southwestern Africa to reflect a thermal event related to crustal thinning and associated mantle upwelling that follows the main phase of Pan-African collisional tectonics.

The tin zone: sediment-hosted hydrothermal tin mineralization at Rooiberg, South Africa

The Rooiberg tin field, also known as the Rooiberg Fragment, is located within the western lobe of the Bushveld Complex. The fragment is triangular-shaped, consists of early Proterozoic Transvaal Sequence volcano-sedimentary rocks, and is surrounded by granitoid intrusives of the Lebowa Granite Suite. Practically all the significant tin deposits are hosted by arkoses, located towards the transition with shaly arkoses at the stratigraphic top of the Boschoffsberg Quartzite Member. This stratabound distribution of individual deposits gave rise to the concept of a regionally developed continuous stanniferous zone. On regional scale, the individual deposits are broadly similar and are collectively classified as replacement and open space-filling type. Particular styles of mineralization such as tourmaline pockets/orbicules, disseminated cassiterite, steep and flat dipping sulphidic fractures and hydrothermal carbonate breccias appear to dominate each mine. Petrological and geochemical investigations of four mineralized centres, indicated the presence of a 500–600 m thick stratified zone of pervasively altered clastic sedimentary rocks of possible arkosic precursor composition. Alteration displays a distinct zonal distribution consisting of a grey-green sodic (albitized) foot wall, an approximately 80 m transitional sericitized-tourmalinized sulphidic tin zone and red hematitic potassic hanging wall. The observed zonal pattern and localization of economically significant cassiterite, is explained by the evolutionary path of magmatic stanniferous hydrothermal fluids genetically related to the surrounding acid phase of the Bushveld Complex. Areas of high fracture density, the tectonic focal points, acted as conduits for the ascending reactive fluids. Factors such as fluid-lithostatic pressure equilibration, limited fracture evolution and/or impermeable shaly arkose retarded this ascent. As a result accumulation, lateral spreading of fluid and pervasive alteration occurred over a broad stratified front. Lateral overlapping of these fronts creates the impression of a continuous tin zone. Abundance of geochemical common denominators between the various focal points (mines) supports a shared composition and source for these contemporaneous fluids.

Distribution and geochemical characteristics of barite and barium-rich rocks associated with the Broken Hill-type Gamsberg Zn-Pb deposit, Namaqua Province, South Africa

Bedded barite forms a distinct component of four Broken Hill-type deposits in the amphibolite-facies metamorphosed Bushmanland sequence of South Africa. In the Gamsberg Zn-Pb deposit, barite has been effectively fractionated from the base metal sulphide facies and concentrated as a separate deposit towards the eastern part of the Gamsberg inselberg. The barite occurs as massive to laminated layers, associated with other oxidised lithologies, such as hematite-quartz rocks and manganiferous iron formations. Geochemical and isotopic evidence favours a non-marine, hydrothermal origin of the Ba and supports a close genetic relationship with the base metal sulphide ores. Fractionation of these hydrothermal components occurred due to redox conditions within the depositional basin, which restricted base metal precipitation to an anoxic basin facies and barite to an oxidised shelf facies. The spatial distribution of barite and other oxidised lithologies indicate that redox transitions occurred immediately before and after deposition of the base metal sulphide ores. Such rapid facies changes are typical for sediment-hosted Zn-Pb deposits, reflecting tectonically induced sub-basin formation and reactivation of feeder conduits, which represent important prerequisites for the formation of these deposits. In addition, the presence of barite in the Bushmanland deposits indicates that the ore-forming fluids were reduced, since Ba cannot be transported in the presence of sulphate. Reduced fluids are sensitive to temperature changes and a decrease in brine temperature most probably resulted in the termination of base metal sulphide deposition. Postdating deposition of the sulphide ores, Fe, Mn and Ba were still supplied to the system and precipitated as manganiferous iron formations and bedded barite lenses that overlie the sulphide horizon. Anomalous concentrations of Ba within the wall rocks of the Aggeneys-Gamsberg deposits indicate that hydrothermal discharge commenced before and continued after peak base metal sulphide deposition. The presence of this Ba halo has implications for exploration of similar deposits in the Namaqua Province.

The quantitative analysis of tungsten ore using X-ray microCT

Volumetric quantification of ore minerals is of interest using non-destructive laboratory X-ray tomography, as it allows high throughput, fast analysis, without any/limited sample preparation. This means traditional chemical analysis can still be performed on the same samples, but good information can be provided in a very short time assisting in exploration, mining and beneficiation decision making as well as sample selection for further chemical analysis. This paper describes a case study in which tungsten WO3/scheelite is quantified in 35mm diameter drill core samples and compared to subsequent traditional chemical analysis for the same samples. The results show a good correlation and indicates that laboratory X-ray CT scanning could replace the more time consuming traditional analytical methods for ore grading purposes in some types of deposits. Different image processing methods are compared for these samples, including an advanced thresholding operation which reduces operator input error. The method should work equally well for other types of ore minerals in which the mineral of interest is the most dense particle in the scan volume, and for which the bulk of the particle sizes are at least 3 times larger than the scan resolution. Performing X-ray microCT scans on drill core samples containing tungsten.Using the microCT data to quantify the grade of tungsten ore non-destructively.Illustrating the advantage of local threshold segmentation over global threshold.Correlating the microCT ore grade results to the industry standard results.

Rare-Earth Element and Thorium Potential of Heavy Mineral Deposits Along the West Coast of South Africa With Special Reference to the Namakwa Sands Deposit

The west coast of South Africa is well known for its Cenozoic unconsolidated marine and aeolian placer deposits. The gem-quality diamond deposits have been effectively depleted; however, the significant heavy mineral concentrations are actively exploited at Namakwa Sands. This world-class mineral sand has an inferred resource of some 900 million tons at a grade of ~10% total heavy minerals (THMs) from which high-grade concentrates of zircon, rutile and leucoxene are extracted and titanium slag and pig iron are produced. Gangue minerals consist of garnet, pyroxene, kyanite, amphibole, apatite and monazite. Several other resources including satellite deposits proximal and distal to the mine area, extensive diamond mine dumps and present-day beach placers demonstrate the vast heavy mineral potential along the west coast. The increased global demand for the rare-earth elements (REEs) uranium and thorium has initiated interest in the economic potential of gangue minerals from these placer deposits. The present quantitative mineral distribution and mineral chemistry study has shown that monazite, impure zircon, leucoxene and garnet are common constituents that host significant concentrations of the REEs uranium and thorium. The total estimated resources in the Namakwa Sands deposit amount to 250 kt REEs, 4.7 kt uranium and 10.5 kt thorium. Monazite is the most significant contributor to the total thorium and light REE resource, whereas zircon is a heavy REE and U-enriched. Garnet and leucoxene show low concentrations of these elements and make a minor contribution to the total resources. Results from the Namakwa Sands deposit suggest that the west coast of South Africa as a whole presents a vast untapped resource for these elements. Considering the anticipated increasing demand for rare earths and possibly thorium, these resources could be produced as a low-cost by-product from existing and future ilmenite–zircon–rutile producers, thereby increasing the in situ value of the placers and ultimate revenue.

Trace and rare earth element chemistry of garnet and apatite as discriminant for Broken Hill-Type mineralization, Namaqua Province, South Africa

This paper reports trace and rare earth element data of garnet and apatite from the Broken Hill-type Gamsberg Zn-Pb deposit and several occurrences of Fe-rich but base metal sulphidepoor metahydrothermal rocks in the Namaqua Province of South Africa. Garnet and apatite within ore-bearing assemblages at Gamsberg are characterised by a prominent positive Eu anomaly, which is not developed in metahydrothermal host rocks at Gamsberg and the investigated ferrugious rocks in the Namaqua Province. The positive Eu anomaly is interpreted as a premetamorphic signature related to the initial ore-forming fluids. It indicates that relatively hot (200–250‰C) and reduced metal-rich brines were responsible for the emplacement of the ores. This distinct ore-related signature may serve as geochemical discriminant in the exploration for Broken Hill-type deposits.

Wall rock alteration and lithogeochemical haloes associated with the sediment-hosted Rosh Pinah Zn-Pb-Ag deposit in the Pan African Gariep Belt, southwestern Namibia

Limitations on the life-of-mine of the sediment-hosted Rosh Pinah deposit urged exploration for extensions to existing ore and similar new deposits in the Gariep Belt. Identification and quantification of hydrothermal wall rock alteration and the extent of dispersion haloes of ore indicator elements were considered supportive to this cause. The results from detailed lithogeochemical traverses on the C Mine and Eastern Orefield, coupled with mineralogical studies and the interpretation of a large (n > 400) multi-element data set of the mine area, showed that the extent of wall rock alteration and associated element dispersion is limited. Barium has shown the most extensive dispersion in argillaceous, arkosic and dolomitic-arkosic wall rocks with anomalous values evident up to 100m into the footwall and 50m into the hanging wall of the ore zone. Alteration of detrital K-feldspar clasts by Ba-rich hydrothermal fluids provided a visual, qualitative index, which exceeds the limits of the elemental halo by 10–20m. Zinc and Pb, as well as Fe and Mn, were considerably less effective as indicator elements. Indices devised for the various lithologies of the Rosh Pinah deposit, such as the Barium Alteration Index and Combined Index, as well as modifications to previously proposed indices are based on variations in Fe(t), MnO, MgO, Al2O3, SiO2, BaO, and K2O. The indices are aimed at minimizing the influence of sedimentologically induced variation in chemistry and maximizing those superimposed by hydrothermal alteration. These quantitative parameters allowed the successful distinction between unaffected and hydrothermally altered wall rocks, as well as an indication of proximity to the ore zone. The results confirmed the limited extent of alteration demonstrated by single element distribution, but indicated that a slightly broader zone has been affected. The limited alteration of feldspathic, dolomitic and siliciclastic sediments is a function of the physico-chemical properties and evolution of the mineralizing fluids. It indicates that the fluids were near neutral to weakly acidic, whereas high concentrations of Ba in the ore and wall rocks required the fluids to be reduced (H2S-dominant). These physico-chemical properties of the fluids resulted in rapid precipitation of the metal load in response to a variety of processes such as cooling, increases in pH, dilution or addition of H2S. Consequently, mineralization has proven to be distinctly concentrated within a favourable horizon, with only limited vertical metal dispersion.