- The geologist who mapped South Africa’s gold and diamond fields – Martin Plaut
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- DISCOVER DIAMONDS - HISTORY
The depth of oxidation may be reasonably estimated by adding 25 m to the overburden depth. The geological structure at Lebowa Platinum Mine is not complicated by faulting. According to existing workings, minor faulting is expected to occur, and would consist of dextral and sinistral strike-slip faults, normal and reverse dip-slip faults and faults with more complex combinations of these components.
The geologist who mapped South Africa’s gold and diamond fields – Martin Plaut
Major conjugate joint set orientations were measured from strong macro-lineament features evident from an aeromagnetic survey image Figure 7. Joint sets may result in poor ground conditions for mining but are not considered a geological loss. An airborne magnetic survey has successfully identified three to four swarms of northeast striking dolerite dykes Figure 7. Post-mineralisation dyke occurrences are noted on the Zeekoegat, Middelpunt and Umkoanesstad farms. Current underground workings at Umkoanesstad have intersected dykes up to 10 m wide.
No serious problems were encountered during mining through these features, and no significant displacements were noted to be associated with them. The aeromagnetic response to these features exaggerates the actual width dimension as depicted in Figure 7. Not all dykes have magnetic responses and a few very minor proportion east-west orientated dykes are known to have no magnetic response. The BC stratigraphy is sometimes affected by randomly occurring, late-stage replacement pegmatite bodies. These pegmatite bodies have a range of compositions from highly ultramafic to felsic.
LPM is no exception to the occurrence of these geological features, but is noted to have minimal evidence for the more mafic replacement pegmatoids. Geological losses are incurred as a direct result of geological features which remove or prevent the presence of the mineralisation, and therefore the ability to mine. Geological features which incur geological losses are:. Pegmatoids and dunite replacement of the reef.
Diabase, lamprophyre, and syenite dykes. Structural and alteration phenomena, such as faults, shears, joints, veins, alteration zones. LPM has few major faults, a number of dykes and numerous potholes. Deloitte is satisfied that the geological loss factors currently being applied are appropriate. Deloitte recommends that future studies are conducted at LPM aiming at the quantification of the geological loss associated with bifurcated UG2 reef, as well as geological losses due to potholing below a depth of m cf.
These segregated layers have been the favoured lithologies for the accumulation of economic quantities of PGMs and base metals. Specifically, the Merensky Reef is that portion of the Merensky Pyroxenite that is bounded by a top and bottom chromitite stringer and that has a 4E grade above 1. Higher 4E grades are commonly associated with the chromitite stringer and or a slightly coarser textured pyroxenite. The UG2 almost exclusively attracts the PGMs, with trivial proportions of mineralisation occurring in the immediate hangingwall and footwall lithologies.
The hangingwall chromitite stringers and chromitite layers are known for their minor PGM occurrences. The immediate UG2 footwall pegmatoidal feldspathic pyroxenite is known to host some mineralisation. Also references by Eales et al.
The UG2 mineralisation is comprised mainly of PGM accumulations that are hosted within the chromitite layers and have variable occurrences in the immediate footwall rocks, but very little in the hanging wall rocks. A 95 cm resource cut in most instances allows for the complete extraction of the mineral content.
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In the case of the presence of internal lenses bifurcation of pyroxenite, anorthosite or norite, the resource cut width may have to be increased to ensure that the UG2 is completely extracted. Ensuing sections of this TR document will refer to the UG2 and its diluting footwall and hangingwall constituents making up the 95 cm resource cut as the UG2. The PGM mineralisation occurs in solid solution with sulphides, sulpharsenides, arsenides, bismuthides, tellurides, bismuthotellurides and alloys.
PGM-sulphides, tellurides, and alloys are the main constituents of mineralisation in the UG2. Prill splits were determined as part of the Mineral Resource estimation process described in this technical report. At LPM the mineralisation within the Merensky occurs at both the lower and upper chromitite stringers. Most of the PGMs are associated with the upper chromitite stringer and often extend over wider intervals to below the chromitite stringer.
Mineralisation associated with the lower chromitite stringer at the base of the Merensky is generally over a very narrow interval and is sometimes absent. High PGM grades are often associated with the lower chromitite stringer, but due to its greater separation from the upper stringer, it was not included in the Mineral Resource estimates in this TR. The Merensky has visible base metal sulphides commonly pyrite and pyrhhotite and, as a result, may have viable concentrations of copper and nickel.
PGMs are commonly associated with base metal sulphides and are associated with the silicate and chromite minerals. Several comprehensive reports have been prepared on the mineralogy and metallurgy of the Merensky and UG2 at Lebowa, including those by Malysiak. The 4E PGE grade of the mining cuts is very variable and in the range 5. Base metal recovery is extremely variable but proportional to head grade. Differences in the mineralogical and metallurgical characteristics of the chromitite appear to be related to the immediate footwall type. By comparison, chromitite underlain by pegmatoidal feldspathic pyroxenite contains more PGE-alloys but less potentially recoverable PGMs.
Consequently, this ore type produces the lowest recovery and concentrate grade. Pentlandite is the major BMS with lesser chalcopyrite, pyrrhotite and pyrite. The remaining value minerals are PGE-arsenides, -alloys, -sulpharsenides and gold bearing phases. The PGMs are well liberated. Head grades are relatively high 3.
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The high recoveries are attributed to the high head grade, high liberated PGM component and the low levels of alteration of the reef intersections. The geological exploration and evaluation process involves reconnaissance, planning, diamond drilling, core logging and sampling, trenching and sampling, soil sampling, aeromagnetics, ground magnetics, mapping, processing, interpreting and modelling.
Lebowa Platinum Mine has been the focus of various exploration activities since , with six phases of exploration having been carried out, all involving diamond drilling. Activities have centred on the Merensky, and only since has considerable focus been directed at the UG2. Where the outcrop exists on the Umkoanesstad and Wintersveld farms, it has been mapped. A number of dolerite dykes outcrop in these hills and have also been mapped. During , a trenching program was conducted along the western UG2 outcrop areas on the Zeekoegat farm.
Twenty-six trenches were excavated across this property, resulting in an accurately mapped UG2 outcrop position. At LPM, after reconnaissance and planning, borehole drilling sites are identified using GPS technology and then drilled by a reputable South African contract drilling company. All diamond drilling of recent years has ensured intersections for both the Merensky and UG2 Figure Only boreholes drilled down-dip of the Merensky outcrop have intersected both horizons.
The Merensky and UG2 are separated by some m of intermediate stratigraphy. Deloitte notes that for each borehole with a Merensky and UG2 intersection, three deflections are drilled per parent hole. Core logging is undertaken by qualified geologists at the Driekop Exploration Base located approximately 50 km southeast of LPM where all boreholes and their deflections are accurately logged in terms of lithology, mineralisation, alteration and structure. Logging details are entered directly into laptop computers, making use of the Sable software package designed for this purpose.
DISCOVER DIAMONDS - HISTORY
Geotechnical and structural logging is also carried out by geotechnical staff and structural geologists. During the logging process, the sampling interval through the mineralised succession is determined and individual samples measured, marked-off and numbered according to standards. Sampling is done continuously throughout the sample section. Within the UG2 and Merensky horizons, a maximum of 20 cm sample lengths are taken, but general practise is to keep sampling widths to no less than 15 cm.
Sample lengths of 15 cm are also applied to the hangingwall and footwall lithologies. The top and the bottom contact samples are taken in such a manner that 2 cm of hangingwall and 2 cm of footwall are included. These samples ensure that high grade material at the top and the bottom contacts of the Merensky and UG2 are included.
This facilitates representative modelling of the mineralised horizon and mining widths. The marked core is then submitted to technical staff who proceed to cut the core in half longitudinally. One half of this core is then cleanly broken with a sharp chisel to avoid sample losses at individual sample boundary markings and immediately labelled, bagged and sealed.
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Precautions are taken to avoid any cross contamination between samples. Once the sampling and logging of the boreholes is completed, the remaining core is stored on core racks. The sample intervals and numbers are replicated onto the remaining core surface for reference, and future re-sampling if necessary. The sampling data is fully documented and recorded on site, with records of all sampling maintained.
Typically, the sampler plans sampling at least three days in advance to ensure that working air and water fittings, as well as the required equipment and staff are available. Prior to going underground, the sampler checks the plan for any identified geological disturbances in the area. All on-reef development is sampled. The interval between sample sections is a minimum of 10 m and a maximum of 20 m.