Venetia Mine Research Articles

Migration of transition metals and potential for carbon mineralization during acid leaching of processed kimberlite from Venetia diamond mine, South Africa

Carbonation of mafic and ultramafic rocks and mineral wastes provides a permanent way to sequester excess atmospheric CO2. Recent research has shown that this method also offers the potential for enhanced recovery of critical metals from mine tailings. In this study, processed kimberlite from the Venetia diamond mine (South Africa) was used in column acid leaching experiments to assess both its carbonation potential and whether critical metals such as nickel could be recovered during mineral carbonation. Processed kimberlite was treated daily with one pore volume of either deionized water or dilute hydrochloric acid (0.04 M, 0.08 M, 0.12 M and 0.16 M) for 28 days. Iron-rich yellow precipitates consistent with yellow ground formed during the experiments both at the top of the residue columns (corresponding to the inlets of the columns) and within the leachates collected from the bases of columns. The carbonation potential and mobility of transition metals were investigated using a combination of quantitative X-ray diffraction (XRD) using Rietveld refinements, inductively coupled plasma mass spectrometry (ICP-MS), scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDXS), transmission electron microscopy (TEM) coupled with EDXS, and synchrotron-based X-ray fluorescence microscopy (XFM). Our results show that the high proportion of clay minerals (e.g., lizardite, smectite, talc, chlorite) in the processed kimberlite act as the primary source for Mg and transition metals such as Ni; however, calcite dissolution is the main source for Ca. The amount of Mg and Ca extracted from processed kimberlite increases with HCl concentration. If acid leaching of processed kimberlite were used at Venetia, the amount of Mg leached from clay minerals would provide an estimated CO2 offset potential ranging from 2.1 to 15.8 % of the mine's total annual emissions. The leached Ca from silicate dissolution could also provide an estimated CO2 offset potential ranging from 2.1 to 8.1 % of the mine's total annual emissions. However, the amount of CO2 released by calcite dissolution during this process is equivalent to 2.1–14.3 % of total annual CO2 emissions at Venetia., thus resulting in a net estimated CO2 offset potential of 2.1–9.6 % if the Ca released from calcite could not be recarbonated. If all of the Ca could be reprecipitated as calcite, the acid leaching techniques employed in this study could offset 4.2–23.9 % of the Venetia mine's CO2 emissions. Ultimately, greater concentrations and/or amounts of acid may be used to access more of the offset potential of Mg phyllosilicates in kimberlite, but the CO2 released by calcite dissolution must also be won back by recarbonation.

Geochemical modeling of CO2 sequestration in ultramafic mine wastes from Australia, Canada, and South Africa: Implications for carbon accounting and monitoring

Passive carbonation in ultramafic mine wastes results from the spontaneous reaction of gangue minerals with carbon dioxide (CO2) to form stable carbonate minerals that store CO2. Mines can benefit from unintentional CO2 sequestration to offset greenhouse gas (GHG) emissions. Quantitative X-ray diffraction (QXRD) analysis of numerous samples of mine wastes has typically been used to quantify passive carbonation rates. This analysis yields valuable information about carbon sinks; however, extensive mineralogical assessments are technically demanding and cost-prohibitive for routine carbon accounting. Alternatively, we explored using inverse geochemical modeling to estimate passive carbonation rates and monitor CO2 sequestration in active mines. Water chemistry data, tailings mineralogy, and operational information routinely collected by mines were used as inputs for models. The predictive capabilities of the models were tested for Mount Keith nickel mine (Australia), Diavik diamond mine (Canada) – for which rates were previously determined using QXRD. A new site — Venetia diamond mine (South Africa) — was used to illustrate the potential of geochemical modeling for carbon accounting and as a long-term monitoring tool for CO2 sequestration. Mount Keith models predicted passive carbonation rates (3900 g CO2/m2/yr) consistent with previous QXRD assessments (2400 g CO2/m2/yr; 172 samples) using only two water samples. CO2 removal rates for Diavik were found to be impacted by seasonality and to range between ∼375 and 510 g CO2/m2/yr, showing similarities with previous QXRD rates estimates (313–350 g CO2/m2/yr). With the long-term water chemistry records (2009–2018) available for Venetia mine, we predicted calcite as the main CO2 sink and that ∼14,875 t CO2 were stored in the kimberlite residues impoundments (3.5 km2) over 9 years at a rate of ∼470 g CO2/m2/yr. Moreover, long-term monitoring shows a decline in CO2 removal rates at Venetia, possibly due to changes in kimberlite reactivity and current mine waste disposal practices. Overall, inverse modeling proved that it can be a viable alternative or complement to QXRD techniques for carbon accounting in active mines, substantially reducing the need for mine waste sampling and permitting long-term monitoring. Moreover, the model shows how passive carbonation rates change dynamically with the mine production and waste management practices. Simultaneously, the model provides insights into potential mineral reactions operating in tailings and that regulate carbon sequestration mechanisms. For that reason, this accounting technique has an enormous potential to assist in decision-making for implementing enhanced CO2 sequestration strategies (e.g., enhanced rock weathering) and to support carbon sequestration monitoring, validation, and reporting in active mines.

3D Fabric analysis of Venetia Mine, South Africa: Using structural measurements and implicitly-modelled surfaces for improved pit slope design and risk management

The geometry and structural history of high-grade granite-gneiss units, which host the Venetia diatremes in South Africa, have been modelled in 3D using an implicit, rules-based, conditional geometrical technique. The volume contains a pervasive, dominant foliation, which forms the main plane of strength anisotropy in ongoing design and mining of the Venetia pit, which has a planned final depth of approximately 465 m below surface. The well-developed S2 foliation is a continuous feature that was locally re-oriented during D3 and D4 and localised ‘wrapping’ around competent amphibolite/hornblendite lenses or boudins. These local variations have important implications for pit design and operational risk management, particularly where they dip adversely out of the slope, or occur at the base of bench stacks, or in inter-ramp areas. These structures, when undercut by dips steeper than their friction angle, may result in planar sliding, presenting an operational safety risk. Best practice requires early identification of these occurrences, to make design adjustments where warranted. Lithological contacts, which have been transposed into parallelism with the S2 foliation, and a robust set of structural data collected over the last 10 years, are analysed to quantify and represent the variable orientation of S2 in 3D space. This contribution describes several methods, using leading software packages such as Leapfrog Geo™, Micromine™ and Python, to generate representative S2 form surfaces and anisotropy block models, both of which are used for downstream geotechnical engineering analysis. Outputs are also translated into apparent dip “heat” maps that show the angular interaction between S2 foliation and a pit design surface. These methods augment, and lessen the dependence on, typical 2D wedge-shaped design sectors that are commonly employed in pit design. This approach has important safety and economic benefits. As the rock mass fabric is characterised, it supports quick identification of potentially higher-risk areas (e.g. adverse undercut fabric with greater likelihood of slope instability) in current mine plans, which justifies more detailed analysis and/or additional monitoring controls to ensure safe and efficient mining. It also allows rapid mine design optimisation.

Implementing Integrated Catchment Management in the upper Limpopo River basin: A situational assessment

A three-phase study was initiated as a way to promote Integrated Catchment Management approaches in the Limpopo River basin. This paper presents the situational assessment, which should enable De Beers to understand how their Venetia Mine operations are located within a broader and highly dynamic socio-economic and ecohydrological landscape as it pertains to water risks. The second phase, Risk assessment, aims to develop conservation interventions in the identified areas; the third phase will develop mechanisms for implementing water stewardship schemes to mitigate the shared water risks.Analysis of the social-ecological system (hydrological, climatic, ecological, socio-economic and governance systems) of the Limpopo River basin indicates that the institutional arrangement of the Limpopo River basin is neither simple nor effective. The basin is rapidly approaching closure in the sense that almost all of the available supplies of water have already been allocated to existing water users. If the proposed ecological flow requirements were to be met for all of the tributaries, the basin would be ‘closed’. On-going and projected land use changes and water resources developments in the upper reaches of the basin, coupled with projected rainfall reductions and temperature increases, and allocation of the flows for the ecological reserve, are likely to further reduce downstream river flows. The coupled increase in temperature and decrease in rainfall is of great concern for everyone in the basin, especially the poorer communities, who rely on rain-fed agriculture for their livelihoods. Increased temperatures also lead to increased evaporation from reservoirs and therefore result in a decrease in water availability. This will lead to increased abstraction of groundwater, especially from alluvial aquifers, and consequently an increase in river transmission losses and a decrease in river flows.

Kimberlite wall-rock fragmentation processes: Venetia K08 pipe development

Current kimberlite pipe development models strongly advocate a downward growth process with the pipe cutting down onto its feeder dyke by means of volcanic explosions. Evidence is presented from the K08 kimberlite pipe in Venetia Mine, South Africa, which suggests that some pipes or sub-components of pipes develop upwards. The K08 pipe in pit exposure comprises >90 vol.% chaotic mega-breccia of country rock clasts (gneiss and schist) and <10 vol.% coherent kimberlite. Sub-horizontal breccia layers, tens of metres thick, are defined by lithic clast size variations and contain zones of shearing and secondary fragmentation. Textural studies of the breccias and fractal statistics on clast size distributions are used to characterize sheared and non-sheared breccia zones and to deduce a fragmentation mechanism. Breccia statistics are compared directly with the statistics of fragmented rock produced from mining processes in order to support interpretations. Results are consistent with an initial stage of brecciation formed by upward-moving collapse of an explosively pre-conditioned hanging wall into a sub-terranean volcanic excavation. Our analysis suggests that the pre-conditioning is most likely to have been caused by explosions, either phreatic or phreatomagmatic in nature, with a total energy output of 2.7 × 109 kJ (656 t of TNT). A second stage of fragmentation is interpreted as shearing of the breccia caused by multiple late kimberlite intrusions and possible bulk movement of material in the pipe conduit related to adjacent volcanism in the K02 pipe.

Emplacement temperatures of pyroclastic and volcaniclastic deposits in kimberlite pipes in southern Africa

Palaeomagnetic techniques for estimating the emplacement temperatures of volcanic deposits have been applied to pyroclastic and volcaniclastic deposits in kimberlite pipes in southern Africa. Lithic clasts were sampled from a variety of lithofacies from three pipes for which the internal geology is well constrained (the Cretaceous A/K1 pipe, Orapa Mine, Botswana, and the Cambrian K1 and K2 pipes, Venetia Mine, South Africa). The sampled deposits included massive and layered vent-filling breccias with varying abundances of lithic inclusions, layered crater-filling pyroclastic deposits, talus breccias and volcaniclastic breccias. Basalt lithic clasts in the layered and massive vent-filling pyroclastic deposits in the A/K1 pipe at Orapa were emplaced at >570°C, in the pyroclastic crater-filling deposits at 200–440°C and in crater-filling talus breccias and volcaniclastic breccias at <180°C. The results from the K1 and K2 pipes at Venetia suggest emplacement temperatures for the vent-filling breccias of 260°C to >560°C, although the interpretation of these results is hampered by the presence of Mesozoic magnetic overprints. These temperatures are comparable to the estimated emplacement temperatures of other kimberlite deposits and fall within the proposed stability field for common interstitial matrix mineral assemblages within vent-filling volcaniclastic kimberlites. The temperatures are also comparable to those obtained for pyroclastic deposits in other, silicic, volcanic systems. Because the lithic content of the studied deposits is 10–30%, the initial bulk temperature of the pyroclastic mixture of cold lithic clasts and juvenile kimberlite magma could have been 300–400°C hotter than the palaeomagnetic estimates. Together with the discovery of welded and agglutinated juvenile pyroclasts in some pyroclastic kimberlites, the palaeomagnetic results indicate that there are examples of kimberlites where phreatomagmatism did not play a major role in the generation of the pyroclastic deposits. This study indicates that palaeomagnetic methods can successfully distinguish differences in the emplacement temperatures of different kimberlite facies.

The structural, metamorphic and temporal evolution of the country rocks surrounding Venetia Mine, Limpopo Belt, South Africa: Evidence for a single palaeoproterozoic tectono-metamorphic event with implications for a tectonic model

This paper presents the results of a detailed and fully integrated pressure–temperature–time-deformation (P–T–t-D) study of the country rocks around Venetia Mine in the Central Zone of the Limpopo Belt, South Africa. Detailed structural mapping around Venetia Mine delineates four deformation events (D1–D4). Relict S1 comprises quartzofeldspathic bands in biotite gneiss and amphibolite. S2, defined by biotite in biotite schist, gneissic banding in biotite gneiss, long axes of quartzofeldspathic augen, and the long axes of amphibolite lenses/boudins, is axial planar to F2. F3 occurs predominantly at the contacts between biotite gneiss and biotite schist, forming open to closed, upright to inclined, E-W- to ENE-trending, shallowly plunging folds. D4, which was constrictional-prolate in nature, refolded S1, S2, F2 and F3. F4 folds and an L4 mineral lineation, defined predominantly by sillimanite in metapelitic schist, plunge moderately NE to NNE and overprint all previous fabrics. Poles to refolded S2 foliations show a characteristic great circle distribution. In turn, the pole to this great circle coincides with the orientation of L4. Such patterns are also found in the Avoca, Bellevue and Ha-Tshansi sheath folds in the Central Zone, albeit that the plunge of the lineations at Venetia are predominantly NE-wards, rather than SW-wards. The moderately NE- to NNE-plunging F4 sheath folds and L4 lineations accord with the NE-SW trend of sheath folds and associated mineral elongation lineations observed elsewhere in the Central Zone. However, PbSL dating of syntectonic garnets constrain the minimum and maximum age of D2 and D4 structures respectively to c. 2037–2040 +/− 22Ma. The metamorphism developed in metapelitic lithologies is characterized by peak amphibolite facies conditions of ∼6.5kbar and 680°C. Pseudosection modelling of growth zoning in garnet provides evidence of a prograde pressure and temperature increase from ∼5kbar at 600°C to ∼6–6.5kbar at 650–680°C, which is interpreted to be a consequence of tectonic thickening. Collectively, the integrated P–T–t-D data unequivocally demonstrates that the country rocks around Venetia experienced a structural–metamorphic event in the palaeoproterozoic. Coupled with published data we suggest a simple two-fold tectonic model, which involves the Central Zone as separate terrane docking with the Kaapvaal during the Neoarchean and later, during the Palaeoproterozoic this Central Zone-Kaapvaal amalgam collides with the Zimbabwe Craton.

Geology of a complex kimberlite pipe (K2 pipe, Venetia Mine, South Africa): insights into conduit processes during explosive ultrabasic eruptions

K2 is a steep-sided kimberlite pipe with a complex internal geology. Geological mapping, logging of drillcore and petrographic studies indicate that it comprises layered breccias and pyroclastic rocks of various grain sizes, lithic contents and internal structures. The pipe comprises two geologically distinct parts: K2 West is a layered sequence of juvenile- and lithic-rich breccias, which dip 20–45° inwards, and K2 East consists of a steep-sided pipe-like body filled with massive volcaniclastic kimberlite nested within the K2 pipe. The layered sequence in K2 West is present to > 900 m below present surface and is interpreted as a sequence of pyroclastic rocks generated by explosive eruptions and mass-wasting breccias generated by rock fall and sector collapse of the pipe walls: both processes occurred in tandem during the infill of the pipe. Several breccia lobes extend across the pipe and are truncated by the steep contact with K2 East. Dense pyroclastic rocks within the layered sequence are interpreted as welded deposits. K2 East represents a conduit that was blasted through the layered breccia sequence at a late stage in the eruption. This phase may have involved fluidisation of trapped pyroclasts, with loss of fine particles and comminution of coarse clasts. We conclude that the K2 kimberlite pipe was emplaced in several distinct stages that consisted of an initial explosive enlargement, followed by alternating phases of accumulation and ejection.

Mechanically disrupted and chemically weakened zones in segmented dike systems cause vent localization: Evidence from kimberlite volcanic systems

Deformation and alteration zones along kimberlite dikes hold clues as to how point-source volcanic vents can localize along sheet-like intrusions. Brittle deformation zones occur in host rock adjacent to kimberlite intrusions of the Swartruggens Kimberlite Dike Swarm, South Africa. Deformation includes local fracturing and brecciation and is associated with relay zones between offset dike segments. Breccia zones indicate dilation and hydraulic fracturing, and some zones along the dikes were affected by chemical corrosion, forming fresh cores surrounded by onion-skin concentric shells of altered rock. The alteration was caused by either exsolved magmatic volatiles moving in advance of the magma through the fracture, or by hydrothermal fluids. Consideration of the time scales needed for chemical corrosion of host rock requires intrusions to stall at depth prior to transport to higher crustal levels. Highly disrupted offsets could be preferred locations for explosive activity and initial vent formation as dikes approach the surface. A kimberlite pipe forms after magma breaks through to the surface; the altered zones are reamed out and fresh cores in spheroidally altered rock are incorporated into the pipe fill along with more angular country-rock material, as observed in layered volcanic breccias in kimberlite pipes at the Venetia Mine, South Africa. This model may have wider implications for the localization of conduits along dikes in other volcanic systems. Dike segmentation provides weak zones where hydrothermal fluids and magmatic volatiles can be preferentially channeled. Chemical corrosion can further weaken these zones, which may then become the locus for initial phreatic and phreatomagmatic explosions, creating shallow vents that can then channel magma to the surface during eruption.

Post‐emplacement serpentinization and related hydrothermal metamorphism in a kimberlite from Venetia, South Africa

AbstractThe common serpentine–diopside matrix assemblage in volcaniclastic kimberlite (VK) at the Venetia Mine, South Africa is ascribed to a secondary origin, because of post‐emplacement serpentinization and associated hydrothermal metamorphism. Volcaniclastic deposits with 20–30% porosity infill kimberlite pipes in the waning stages of kimberlite eruptions. Olivine macrocrysts are typically rimmed by talc and are pseudomorphed by lizardite, with minor magnetite. The fine matrix consists of mixtures of lizardite, chlorite, smectite, brucite, calcite, titanite and andradite, an assemblage which either pseudomorphed microcrysts or in‐filled voids. Locally we recognize microcryst pseudomorphs rich in sub‐microscopic mixtures of lizardite with smectite, and other microcryst pseudomorphs and void‐filling matrix rich in chlorite and lizardite. Interstitial lizardite and associated phyllosilicates (brucite, smectite and chlorite) crystallized progressively from meteoric or hydrothermally derived pore waters, and Si4+ and Mg2+ released into the fluid phase during serpentinization of olivine macrocrysts. Radial‐fibrous fringes of diopside microlites around crystals display void‐filling textures because of unrestricted growth into pore spaces. Secondary diopside is attributed to Si4+, Mg2+ and Ca2+ cations released into the fluid phase by interaction with olivine, calcite and plagioclase in siliceous xenoliths. The paucity of primary, fine‐grained groundmass phases resistant to alteration, for example, perovskite and spinel, precludes an origin for the intergrain matrix as altered interstitial ash, glass or a late‐stage kimberlite melt. Isovolumetric replacement of olivine results in a volume increase of 60% so that pore spaces in the original deposit can be easily filled up with serpentine. The source of Al3+ to form chlorite and smectite is attributed to alteration of plagioclase in xenoliths which comprise 20–30 vol.% of the deposit. Titanite, hydro‐andradite and second‐generation diopside precipitate as hydrothermal minerals from calcium‐bearing serpentinizing fluids in replacement reactions and as void‐filling minerals. Consideration of mineral equilibria in the CaO‐MgO‐SiO2‐H2O‐CO2 system constrains the common matrix assemblage of lizardite and diopside in XCO2)–T space. At 300 bar, the assemblage is stable only at temperatures below 370 °C and XCO2 &lt; 0.01. This upper limit on temperature is well below the plausible solidus of ultrabasic magmas. Furthermore, the requirement of trace CO2 in the fluid phase implies a post‐emplacement external source rather than ‘autometamorphism’ from kimberlite‐derived fluids, because of high PCO2 commonly inferred for kimberlite magmas.

Geological control on slope failure mechanisms in the open pit at the Venetia Mine

Research Article| September 01, 2003 Geological control on slope failure mechanisms in the open pit at the Venetia Mine Wayne Peter Barnett Wayne Peter Barnett De Beers Consolidated Mines Limited, Premier Mine, P.O. Box X1015, Cullinan 1000, South Africa, e-mail: wayne.barnett@debeersgroup.com Search for other works by this author on: GSW Google Scholar South African Journal of Geology (2003) 106 (2-3): 149–164. https://doi.org/10.2113/106.2-3.149 Article history first online: 07 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Wayne Peter Barnett; Geological control on slope failure mechanisms in the open pit at the Venetia Mine. South African Journal of Geology 2003;; 106 (2-3): 149–164. doi: https://doi.org/10.2113/106.2-3.149 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySouth African Journal of Geology Search Advanced Search Abstract The Limpopo Belt is a complex accretionary terrane that has undergone numerous deformational events. Tectonically juxtaposed lithology, open to isoclinal folding, cross-cutting and re-activated shear zones, and closely interlayered metamorphic gneisses and schists make pit slope design and maintenance risky. Pit slope design effects the stripping ratio and the “bottom-line” profitability of a mine. The geological model is the basis on which a pit design starts. At Venetia Mine the model is a tight, northward verging syncline that plunges shallowly towards the east-northeast. The geology has been modeled three-dimensionally using GEMCOM software. The jointing patterns have been studied and hypothetically related to the geology. The synform fold model implies three major pit slope design sectors, the (a) southern limb, (b) fold hinge zone and (c) northern limb of the fold. The southern limb experiences predominantly planar failure, a problem that has resulted in a reduction in the pit slope angle from 51° to 37° and 44° in two of the southern domains respectively. The northern limb undergoes bench-scale toppling and wedge failure. The hinge zone suffers only from local wedge failure. Bench-scale folding and brittle faulting have created more local problems. Some faults create large slope-scale wedge failures. These geological variations and the relative orientation/position of the excavation necessitated the definition of a total of 11 geotechnical domains, each with an individual pit slope design. The improved understanding of the geology and its impact on the rockmass behaviour will lead to improved blasting practices and steeper slope angles. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Hydrogeology of the Venetia Diamond Mine, South Africa

Venetia Mine is located in the northern part of the Limpopo Province. The mean annual rainfall for the area is 344mm and the mean annual evaporation is in the order of 2650mm/annum. Geologically, Venetia Mine is situated in the Central Zone of the Limpopo Belt where a large variety of rock types are developed. These include quartzites, dolomitic marbles, magnetite rich quartzites, amphibolites, gneisses and schist’s. During the hydrogeological investigation a number of boreholes were drilled and were subjected to numerous conventional and unconventional aquifer tests. The country rock of the mine is hydraulically tight but has a large storage capacity for groundwater. Faults and fractures within the country rock enhance the hydraulic conductivity in places and serve as drainage lines into the open pit. The kimberlite may be divided into two significant horizontal hydrogeological units, the so-called “overshot zone” created by drilling and blasting and the undisturbed kimberlite. The “overshot zone” has a high density of artificial fractures and is between 2m and 5m thick. Test pumping of the in-situ kimberlite of the Venetia Diamond Mine has revealed unusually high hydraulic values. Post closure, evaporation might exceed the inflow of groundwater into the pit. It will, therefore, either be dry except for runoff from precipitation or develop into a brine lake where accumulation of salts will take place in the pit. The drawdown of the groundwater table will continue indefinitely because evaporation exceeds recharge. A pit lake model is planned to simulate post closure conditions.

Mylonitization and decomposition of Garnet: Evidence for rapid deformation and entrainment of Mantle Garnet-Harzburgite by Kimberlite Magma, K1 Pipe, Venetia Mine, South Africa

Sheared and unsheared xenoliths of garnet-harzburgite from the K1 Venetia kimberlite pipe (South Africa) are composed primarily of olivine and orthopyroxene (partially replaced by serpentine, magnetite and chlorite) and abundant porphyroblasts of garnet with kelyphitic rims of orthopyroxene, clinopyroxene and spinel. Mylonitized, orthopyroxene defines a mineral elongation lineation whereas bent orthopyroxene and sigmoidal garnet with wings of orthopyroxene define consistent senses of motion within individual xenoliths. The kelyphite surrounding both deformed and undeformed garnet grains is not deformed and appears to have resulted from the inversion of garnet and olivine to orthopyroxene, clinopyroxene and spinel, corresponding to the isograd separating garnet peridotite from spinel peridotite. Subsequent hydration around rims and along fractures within xenoliths resulted in the formation of serpentine, magnetite and chlorite from olivine and orthopyroxene. This hydration apparently resulted from reaction with the entraining kimberlite magma. The preservation of mylonite rather than its annealing and recrystallization to coarse-grained rocks at high temperature strain free mantle conditions requires quenching shortly after mylonite formation. This preservation taken with the presence of undeformed kelyphite around garnet grains implies that mylonitization and entrainment of garnet-harzburgite into kimberlite magma and kelyphite formation occurred during very rapid magma ascent.

Procedures used to produce a digitized geological mapping database of the area around the Venetia Kimberlite pipes, Limpopo Belt, South Africa

A digitized geological mapping database, viewable and printable as maps at various scales, was produced for the area surrounding the Venetia kimberlite pipes using GPS techniques in the field to determine latitude and longitude positions on the WGS-84 map datum and ellipsoid. These latitude and longitude data, collected as degrees and decimal degrees using a GARMIN GPS-45 and GPS-III navigators, were subsequently converted to the local Venetia Mine grid using Trimble software. The Venetia Mine grid coordinates were linked with rock type attributes and structural data using a Microsoft Access database and imported into GEMCOM software with which the map was drawn. Generally the maps consist of two layers, one with the location data and lithological and structural attributes shown as colored dots or symbols and a second with the polygons outlining the various lithological units. Other overlays with geographical or geophysical information may be added. GEMCOM software was used because the data could then be utilized by the GEMS data management system on the Venetia Mine and field notes may be directly accessed by right clicking on any data point shown on the map.

Measured and calculated compressional wave velocities of crustal and upper mantle rocks in the Central Zone of the Limpopo Belt, South Africa - implications for lithospheric structure

Experimental compressional wave velocity (Vp) measurements were performed on felsic metasedimentary and igneous lithologies belonging to the Beit Bridge Group, and other amphibolite facies rocks, found in and around the Venetia Kimberlite pipes, which intrude the Central Zone of the Limpopo Belt. Xenoliths of eclogite, granulite, gabbronorite and garnet-quartz, which represent the major crustal and upper mantle lithologies found in the kimberlite host rock were too small for experimental measurements. Compressional wave velocities for these samples were calculated, based on their modal mineralogy and end member mineral compositions. Compressional wave velocities were corrected for in-situ conditions based on estimated equilibration temperatures and pressures, and the resulting velocities were compared with the seismic refraction model of the Limpopo Belt, in order to construct a lithologic column of the crust and upper mantle below the Venetia Mine.

Introduction to recent studies of geology in and around the Venetia kimberlite pipes, Limpopo Belt, South Africa.

Research Article| September 01, 2003 Introduction to recent studies of geology in and around the Venetia kimberlite pipes, Limpopo Belt, South Africa. Jay Barton; Jay Barton Department of Geology, RAU, P.O. Box 524, Auckland Park 2006, South Africa, e-mail: jmb@na.rau.ac.za Search for other works by this author on: GSW Google Scholar Reiner Klemd; Reiner Klemd Institute of Mineralogy, University of Würzburg, Am Hubland, 97074 Würzburg, Germany, e-mail: reiner.klemd@mail.uni-wuerzburg.de Search for other works by this author on: GSW Google Scholar Fred Brown Fred Brown DeBeers Consolidate Mines Ltd., Cornerstone Building, Private Bag X01, Southdale 2135, South Africa, e-mail: fred.brown@debeersgroup.com Search for other works by this author on: GSW Google Scholar Author and Article Information Jay Barton Department of Geology, RAU, P.O. Box 524, Auckland Park 2006, South Africa, e-mail: jmb@na.rau.ac.za Reiner Klemd Institute of Mineralogy, University of Würzburg, Am Hubland, 97074 Würzburg, Germany, e-mail: reiner.klemd@mail.uni-wuerzburg.de Fred Brown DeBeers Consolidate Mines Ltd., Cornerstone Building, Private Bag X01, Southdale 2135, South Africa, e-mail: fred.brown@debeersgroup.com Publisher: Geological Society of South Africa First Online: 07 Mar 2017 Online ISSN: 1996-8590 Print ISSN: 1012-0750 © 2003 Geological Society of South Africa South African Journal of Geology (2003) 106 (2-3): 101–102. https://doi.org/10.2113/106.2-3.101 Article history First Online: 07 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Jay Barton, Reiner Klemd, Fred Brown; Introduction to recent studies of geology in and around the Venetia kimberlite pipes, Limpopo Belt, South Africa.. South African Journal of Geology 2003;; 106 (2-3): 101–102. doi: https://doi.org/10.2113/106.2-3.101 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySouth African Journal of Geology Search Advanced Search Diamond bearing, kimberlite pipes in the Central Zone of the Limpopo Belt, Limpopo Province, South Africa were discovered on the Farm Venetia in the late 1970’s. These ~530 to ~519 Ma pipes constitute a world class ore body, with a historic head grade (since 1992) varying between ~100 and ~135 carats/100 tonnes. They presently produce about ~40% of South Africa’s diamonds and hence ~6% of the total world’s. The position of these pipes within in the Central Zone, an ~2.04 to ~2.0 Ga assembly of various Archean and Proterozoic terranes between the Kaapvaal and Zimbabwe Cratons, apparently defies one of... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Expanding the Geological Model for Finsch Mine

Research Article| December 01, 2002 Expanding the Geological Model for Finsch Mine Wayne Barnett; Wayne Barnett De Beers Consolidated Mines Ltd, Venetia Mine, Messina, 0900 Republic of South Africa, e-mail: wayne.barnett@debeersgroup.com Search for other works by this author on: GSW Google Scholar Christopher Preece Christopher Preece De Beers Consolidated Mines Ltd, Finsch Mine, Lime Acres, 8410 Republic of South Africa, e-mail: christopher.preece@debeersgroup.com Search for other works by this author on: GSW Google Scholar South African Journal of Geology (2002) 105 (4): 381–400. https://doi.org/10.2113/1050381 Article history first online: 07 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Wayne Barnett, Christopher Preece; Expanding the Geological Model for Finsch Mine. South African Journal of Geology 2002;; 105 (4): 381–400. doi: https://doi.org/10.2113/1050381 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySouth African Journal of Geology Search Advanced Search Abstract The geological model (lithology and structure) developed for use in geotechnical engineering projects at Finsch Mine, Northern Cape Province, is described in this paper. The lithology model is presented as a detailed stratigraphic column. This study provides new lithofacies descriptions and suggests formation boundary positions for the upper Campbellrand Subgroup stratigraphy exposed by the open pit and underground tunnel network at the mine. The Klippan Formation lagoonal facies, the Kogelbeen Formation subtidal facies and the Gamohaan Formation microbialite facies are interpreted as a transgression sequence deepening into the basinal banded ironstone formation of the Asbestos Hill Subgroup.The structural model is presented as a three-dimensional computer model, and as a summarized model of the structural domains. The structural model is of a northwest to north-northeast varying strike-slip system with “negative flower structure” geometry. Two phases of open folding have developed one trending east-southeast and the other parallel to the faults. Four sub-vertical joint sets are described. The structures are identified as an attenuated expression of the late-stage events of the Namaqua-Natal orogeny at circa 1Ga. The authors suggest that the latest phase of deformation was caused by oblique collision tectonics expressed initially as compression from the south-southwest producing transtensional north-northwest strike-slips faults with a dextral shear sense. Compression may have then swung to the west-southwest producing fault-parallel folds and a conjugate joint set.The described structures were utilized during kimberlite emplacement, including the early intrusion of northeast striking kimberlite dykes. The mechanics of kimberlite emplacement are not discussed, but this work provides important data for such future research. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

An infrared investigation of inclusion-bearing diamonds from the Venetia kimberlite, Northern Province, South Africa : implications for diamonds from craton-margin settings

The Venetia kimberlites in the Northern Province of South Africa sampled diamonds from the lithosphere underlying the Central Zone of the Limpopo Belt. Given the general correlation of diamond-bearing kimberlites with old stable cratons, this tectonic setting is somewhat anomalous and, therefore, it is desirable to characterise the diamonds in terms of their infrared characteristics. A suite of diamonds of known paragenesis from the Venetia mine spans a large range of nitrogen concentrations from less than the detection limit to 1,355 ppm. Diamond nitrogen contents are, on average, higher in the eclogitic diamond population relative to the websteritic and peridotitic diamonds. Nitrogen aggregation states are variable, ranging from almost pure type IaA diamond (poorly aggregated nitrogen) to pure type IaB diamond (highly aggregated nitrogen). On a nitrogen aggregation diagram two distinct groups can be identified based on nitrogen content and nitrogen aggregation state. These are a minor population of diamonds with nitrogen contents generally higher than 500 ppm and nitrogen aggregation states of less than 40% IaB, and another, dominant population that is characterised by higher and more variable nitrogen aggregation. The unusually aggregated nature of the majority of the diamonds analysed is unique to Venetia relative to other intrusives on the Kaapvaal–Kalahari craton, but is similar to aggregation states observed for diamonds from other craton margin or adjacent mobile belt settings such as the Argyle lamproite and the George Creek kimberlite. This could be a consequence of diamond mantle residence at mantle temperatures higher than the norm for other kimberlites from the interior of cratons. Deformation of the mantle, associated with dynamic processes such as orogenesis or subduction, might also be responsible for accelerating the rate of nitrogen aggregation in these diamonds. Low numbers of diamonds with degradation of platelets at the Venetia kimberlite, relative to diamonds from the Argyle lamproite, indicate that deformation was at a significantly lower level. The comparatively low value of diamonds from Argyle (at approximately US$8/carat) as opposed to Venetia (US$90/carat) is in large part because of the very high abundance of brown diamonds at Argyle. Therefore, it is apparent that deformational history of the mantle in which the diamonds were resident prior to or during sampling by the host may have an important role to play in the profitability of a primary diamond deposit. The apparently consistent association of diamonds with unusually aggregated nitrogen with kimberlites, or lamproites intruded into craton margin or mobile belt settings suggests that it may be possible to recognise such contributory sources in alluvial diamond deposits, through the study of the infrared characteristics of the diamonds. Electronic supplementary material to this paper can be obtained by using the Springer Link server located at http://dx.doi.org/10.1007/s00410-002-0385-2

Evaluation of the Hicom 120 mill at Venetia Mine

The Hicom 120 mill is a high intensity, compact, grinding mill of novel design. This technology has been under evaluation by the De Beers Group for a number of years. Based on positive results from pilot plant testwork, an industrial evaluation programme was embarked upon at Venetia Mine from August 1999 to November 1999. The testwork programme was designed to produce reliable data for the mill's comminution characteristics, particle residence time distributions, and engineering soundness. The testwork was conducted with −25 +4 mm and −8 +4 mm kimberlitic ores of varying hardness. The Hicom mill's response was sensitive to the ore hardness, with throughputs ranging between 4 t/h – 8 t/h, production of −1 mm ranging between 18%–46%, and specific energy requirements ranging between 17–66 kWh/t of −1 mm material produced, for −25 +4 mm feeds. The particle residence time distribution tests showed that the load segregates into a particle size order, and the results indicate that the top-size material (25 mm) has a low probability of exiting the mill. The mechanical reliability of the mill was good, and the unit can be considered to be very robust. The liner wear rate was unexpectedly high, with liners lasting approximately 20–80 hours depending on the ore hardness and feed size.