ETD Collection

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    The effects of cut-off grade and block sizes on the net present value for an iron ore deposit
    (2019) Malisa, Moore Theresa
    Optimisation of the Net Present Value (NPV) needs to consider cut-off grade as well as the block model size. This study considered the impact of these on the optimisation of the NPV. The orebody could be mined sub-optimally due to the misunderstanding of the relationship between cut-off grade and block model size. The research was based on an iron ore deposit mined through open-pit mining method in South Africa. The main objectives of the study were: to understand the importance of cut-off grade; to determine the effect of block model sizes on the average grade; to determine the effects of the block sizes and cut-off grades on the NPV, and to determine which block sizes and cut-off grade maximise the NPV. It was found that there were different cut-off grades at different levels of the exploitation of the iron ore deposit. These differences can lead to the deposit not being mined optimally. Therefore, it was important to understand the importance of cut-off grades, hence the need to investigate the effects of cut-off grades. The effect of block sizes on the NPV was included because there was insufficient research on the topic. From the literature review, the cut-off grade was defined as the boundary that separates material that is discarded from the material that is taken further for treatment. The cut-off grade determines whether the material will be considered as waste or ore. If the cut-off grade is too high, more material will be discarded as waste while a lower cut-off grade increases the entire mining capacity. The lower the cut-off grade, the higher the Mineral Reserve. It was shown from the literature that the determination of the cut-off grade is determined by factors such as the price of the commodity, production costs, grade distribution, environmental factors and other factors. The literature review highlighted that a block model is a representation of orebody characteristics, whereby a single cube will have sizes (x, y and z). The single cube will be allocated with grades, volumes, rock types, densities and many more attributes assigned to it depending on what information is required. The block model dimensions should represent the minimum block that could be selectively mined, that is, the smallest selective mining unit. The block model sizes are selected at the initial stages of creating the block model. The block size is also dependent on the sample spacing. The block size should be one-half or one-fourth of the sample spacing. When the selective mining unit is selected it should take into account the excavator that will be used to load the material. The selective mining unit is important since it determines the amount of dilution that will be encountered during mining. The larger the selective mining unit, the more the dilution, which decreases the grades. The methodology that was used to analyse the effects of cut-off grade and block sizes on the NPV was through the use of grade-tonnage curves and the DCF for different block sizes and cut-off grades. NPV is the sum of the DCF’s and the NPV assists in projecting the future revenues in terms of mines that are already in production. The DCF’s for this report were done only for 10 years because it was enough to create data of a high level of confidence. The cut-off grades that were used were 53%,60%,63% and 64% Fe as they covered the definition of the ore for the iron ore deposit. The base block model size 6.25m x 6.25m x10m.The base block-model was re-blocked into sizes: 12.5m x 12.5m x 10m; 25m x 25m x 10m and 50m x 50m x10m. Grade-tonnage curves were created for each block model size including the base 6.25m x 6.25m x 10m block model. The obtained tonnes and an average grade above certain cut-off grade were used to create the DCF in Excel to obtain the NPV. The results showed that an increase in the cut-off grade decreases the tonnes above the cut-off grade while increasing the average grade. The larger the block size, the lower the average grade due to increased dilution. The larger block sizes result in a lower NPV if the effects of mining selectively are not considered. However, if the effects of selective mining are considered, larger block sizes result in an optimised NPV. Some of the conclusions were that small block sizes result in an optimised NPV only if the effects of selective mining are not considered while larger block sizes result in an optimised NPV when the effects of selective mining are considered. The 25m x 25m x 10m which is a larger block model size is not affected by selective mining and it resulted in a higher NPV when compared to the 12.5m x 12.5m x 10m,therefore,it is better to work with larger block model sizes to avoid selective mining. It was recommended that a 60% Fe cut-off grade paired with a 12.5m x 12.5m x 10m block size to be used when the effects of selective mining are not considered since it increases the tonnes above the cut-off grade, thus increasing the LOM and the NPV is optimised. A 60% Fe cut-off grade paired with a 12.5m x 12.5m x 10m block size was also recommended to be used when the effects of selective mining are considered as this optimises the NPV. A 60% Fe cut-off grade paired with a 25m x 25m x 10m block size is recommended since it does not require to be mined selectively.
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    An integrated geological and geophysical study of the Deblin Copper Mine are in Kombat South, Southern Otavi mountain land, Northern Namibia
    (2018) Shilunga, Josia Tulongeni
    The Deblin Copper Mine is located at the transition between the Swakop rift-basin and the carbonate platform of the Otavi Mountain Land. Copper and zinc mineralization occurs mainly in carbonate host rocks close to a major thrust fault underlain by bimodal metavolcanics with mineralized quartz-calcite veins. Copper sulphides of chalcopyrite and bornite occur as disseminated and vein-type mineralization associated with sphalerite and pyrite. The area is characterized by outcrops of carbonates and limited metavolcanics. Wall rock alterations include sericitization and chloritization of metavolcanics, as well as silicification and dolomitization of carbonates. Vein type mineralization is characterized by a relatively narrow range of sulphur isotopes (δ34S = -4.9‰ to +6.0‰) compared to that of disseminated sulphides (δ34S= -9.99‰ to +3.63‰). The two sulphur isotope ranges suggest that both magmatic and sedimentary sulphur sources were involved in the genesis of the deposit. Geochemical and petrological data suggest that the metavolcanic rocks in the area formed as alkali basalts in an oceanic island geotectonic setting. Magnetic data revealed concealed faults which may have acted as conduits for mineralizing hydrothermal fluids. The deposit shows ore forming processes associated with both VMS and MVT-type mineralization
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    A comparison between two calcite-rich deposits in palaeoproterozoic dolomitic units of the Transvaal supergroup: beestekraal, North West province and lime acres, Northern Cape
    (2018) Ntibane, Thabile Malecia
    The Malmani and Campbellrand Subgroups have small zones of calcite that did not undergo dolomitisation when it took place within the Transvaal Supergroup. To get the calcite rich zonations of limestone the Malmani and Campbellrand Subgroups may have avoided dolomitisation and dedolomitisation during the geological events of the Transvaal Supergroup. This project integrated geological mapping, petrology, microprobe analysis, mineral identification, bulk geochemistry and point geochemistry in an attempt to understand and compare the calcite rich limestone zonations that occur in the Malmani and Campbellrand Subgroups. The calcite rich limestone either preexisted the deformation of the Transvaal Supergroup or resulted from dedolomitisation of the deformed Transvaal Supergroup. The Campbellrand Subgroup of the Griqualand West Basin is composed of shallow shelf carbonates that were deposited under shallow subtidal conditions, whereas the Malmani Subgroup has the Oaktree Formation which is generally dolomitic, except in the Crocodile River fragment. The sedimentary structures on the Lime Acres Member are interbedded by fine grained limestone, chert and dolomitic matrix. The Lime Acres Member has two NS fault which have high grade limestone between them and on the far western side of the faults, dolomite ore deposit lies on the eastern side. The limestone structures and textures which are found on the western side of the fault 2 (F2) and eastern side of the fault 1 (F1) in Lime Acres were well preserved. The dip is at about 3⁰ -7⁰ to the west. The Oaktree Formation in the Malmani Subgroup hosts the carbonate deposit at Beestekraal which comprises dolomitic limestone with chert-rich and chert-poor zones within the limestone. The deposit dips to the west at an angle of about 33⁰ and strikes NS. There are four faults that cut across the deposit and these are striking WE. The faults form normal faults that are fairly steep. Folding is noted in the hanging wall where the WE faults that cut across the deposit. The Campbellrand Subgroup deposit is interbedded with limestone and dolomites. The dolomite is coarse grained and can be classified as replacement rock that formed by diagenetic replacement of the older limestones, while the limestone is fine-crystalline and can be classified as primary limestone i.e. not dedolomitised. The dolomite crystals show twinning and cross-cuttings of cleavage planes under a microscope and has proven to have Mg/Ca value of 0.8 -0.9 with a shallow dip. The characteristics that affect each site are different as Lime Acres has preserved sedimentary structures such as stylolite and stromatolites, whereas such structures are lacking at Beestekraal, where there is a dominance of calcite rich veinlets. The calcite veinlets have upgraded the dolomites at Beestekraal, while the chert downgrades the deposit. Faults, calcite veinlets and quartz inclusions were post depositional of the limestone and dolomite deposit. No evidence was found that the faults could have eroded, downgrade or upgraded the limestone zones. The faults fissures are filled with reddish-clay at Lime Acres and deformed chert at Beestekraal. Re-entrance angles, twinning, fenestrae and cross-cutting cleavage planes were noted in both electron microprobe analysis and petrology analysis. These features are consistent with dolomite structures. There is evidence of three geological events that have occurred on both site where; • Deposition of the magnesium-calcite occurred; iv • Some silica and calcite veinlets were introduced by fluids, the fluid introduction put pressure on the deposit which resulted in compaction. Re-entrance angles formed during compaction and left the prominent calcite veinlets. The calcite veins only appear in Beestekraal; • Faulting occurred post the formation earlier compaction evensts. The faulting caused compression of the existing structures and resulted in wavy stylolites at Lime Acres, deformation along the faults and mud fissures being introduced to the joints. There was uniformity between the X-Ray Fluorescence, X-Ray Diffraction, and electron microprobe analysis results. Both sites have similar mineral composition which are rich in calcium with little deformation. The study has found that the mining areas have somehow not been altered during the dolomitasation phase that affected the Transvaal Supergroup rocks. The pockets of high calcium deposits can be classified as primary limestone, which was somehow preserved during the dolomitisation phase. Post depositional alterations had two different effects on the sites. At Lime Acres the chert, which filled the contacts between the different bedding, was compressed due to high pressure from the overlying material and faulting. The traces of chert have been noted as stylolite and small traceable carbon partings/bands within the limestone and dolomites. At Beestekraal there is infill of iron rich and chert material, which occurs within the cleavage planes and contacts. Both sites have significantly increased calcium content with low magnesium content in production zones, having little impurities from the chert and dolomites that overlie the limestone. The Mg/Ca ratios showed that for all zones used for cement and lime production the Mg/Ca ratios are below 0.6 and 0.1 respectively. Mg/Ca ratios closer to 1 and above are not economically viable for lime and cement production. The low Mg/Ca ratios indicate that the dolomites are more of magnesium calcite content and of primary depositional environment than secondary.