Chrome ore mine Locality – Chromite operations at Dwarsrivier Mine form part of the Chrome Division of Assmang Limited. The mine is situated on the farm Dwarsrivier 372KT, approximately 30 kilometres from Steelpoort and 60 kilometres from Lydenburg, in Mpumalanga province in South Africa. Located at longitude 30°05’00″E/latitude 24°59’00”S, Assmang purchased the farm from Gold Fields Limited,
together with all surface and mineral rights in October 1998.
Chrome ore mine History – Neighbouring properties to the north and south of Dwarsrivier had existing chrome mining operations at the time of purchase. The Feasibility Study of the plant, tailings dam and designs for the open-cast and underground mines then commenced. After the completion of the Feasibility Study, approval to proceed with the final design and construction work was given in July 1999.
Chromite was obtained from the open-cast mining areas at a rate of approximately 0,9 million tons a year and these areas were mined out within five years. Underground mining commenced in 2005 at a rate of 1,2 million ROM tons a year. Dwarsrivier Mine is specifically geared to deliver high-quality metallurgical-grade chromite to the Machadodorp smelter. In addition, the plant has been designed to produce chemical-grade products.
Mining authorisation – An old-order Mining Licence 21/99 was granted in October 1999. It was granted for the mining of chrome and platinum group metals. An application for the conversion to a new-order mining right was submitted during
October 2007.
Geology – Dwarsrivier Mine is situated in the eastern limb of the Bushveld Complex, which comprises persistent layers of mafic and ultramafic rocks, containing the world’s largest known resources of platinum group metals, chromium and vanadium. The mafic rocks, termed the Rustenburg Layered Suite, are approximately 8 kilometres thick in the eastern lobe, and are divided formally into five zones.
The rocks of the Marginal Zone at the base of the succession consist mainly of pyroxenites with some dunites and harzburgites. Above the Marginal Zone, the Lower Zone comprises mainly pyroxenites, harzburgites and dunite, and is present only in the
northern part of the Eastern Lobe, and only as far south as Steelpoort. The appearance of chromitite layers marks the start of the Critical Zone, economically the most important zone.
The layers are grouped into three sets termed the Lower, Middle and Upper groups. The sixth chromitite seam in the Lower Group (LG6), is an important source of chromite ore and is the orebody being mined at Dwarsrivier Mine. In the Eastern Lobe, in the vicinity of Dwarsrivier, the strike is nearly northsouth, with a dip of approximately 10 degrees towards the west. Average thickness of the LG6 seam is about 1,86 metres in the Dwarsrivier area. Pipe-like dunite intrusions are evident in the area, as well as dolerite dykes that on average strike northeast-south-west. No significant grade variation is evident, especially not vertically in the ore seam. Small, insignificant regional variations do, however, exist.
Mineral Resources and Reserves – Information was obtained from boreholes with 300 to 150-metre grid spacing. Resources were determined with a decreasing level of confidence.
• Measured Resource (150 metres drill grid spacing).
• Indicated Resource (300 metres drill grid spacing).
• Inferred Resource (drill grid spacing greater than 300 metres).
All possible Resources down to a mineable depth of 350 metres below ground level have been considered. Vertical diamond drill holes are used for geological and grade modelling, except where information is needed to clarify large-scale fault planes. The Mineral Resource at Dwarsrivier Mine is based on a total of 232 diamond drill holes that have been used for grade estimation and orebody modelling purposes. The drill core is NQ size and is geologically and geotechnically logged. The collar position of the drill holes is surveyed, but no down hole surveys are done, and the holes are assumed to have minimal deflection.
The chromitite seam is bounded above and below by pyroxenites. As such, the ore horizon is clearly defined. The core is sampled from the top contact downwards at 0,5-metre intervals. The core is split and half is retained as reference material in the core sheds. The other half is crushed and split into representative samples, which are crushed and pulverised for chemical analysis. The samples are analysed fusion/ICP-OES for chrome oxide (Cr2O3), SiO2, FeO, Al2O3, MgO and CaO. Three laboratories, all ISO 17025 accredited for this method, are used. Every tenth sample is analysed in duplicate. SARM 8 and SARM 9 standards, as well as in-house reference material (CRI), are included every 20 to 30 samples in each batch. The density for each sample is measured using a gas pycnometer.
Mineral Resources have been estimated using Ordinary Kriging, where Cr2O3, FeO-, Al2O3, MnO and MgO-contents of the LG6 seam and densities were determined, using block sizes of 50 x 50 x 4 metres.
During mining, a slightly diluted run-of-mine ore inclusive of the “false” hangingwall is fed to the beneficiation plant. In the dense media separation part of the plant, the coarse fraction is upgraded to 40% Cr2O3 with a yield of 80%. In the spiral section of the plant, the finer fraction is upgraded to 44% Cr2O3, and 46% Cr2O3 respectively, for metallurgical-grade fines and chemical-grade fines. A 67% yield is achieved in the spiral circuit.
Year-on-year change – When compared to 2009, the 2010 Mineral Reserves increased by 3,86 million tons to 39,5 million tons (35,64 million tons) and the Mineral Resources increased by 2,72 million tons to 50,60 million tons. The increase in the Mineral Resources was due to the extension of the block model by remodelling of the Indicated Resources. The increase in the
Mineral Reserves is due to the remodelling and incorporation of the “false” hangingwall. The latter caused a decrease in the Cr2O3 grade to 35,75% in the 2010 model.
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Chrome ore mine, chrome mining and processing industry
Chrome ore mine
Chrome ore mine Locality – Chromite operations at Dwarsrivier Mine form part of the Chrome Division of Assmang Limited. The mine is situated on the farm Dwarsrivier 372KT, approximately 30 kilometres from Steelpoort and 60 kilometres from Lydenburg, in Mpumalanga province in South Africa. Located at longitude 30°05’00″E/latitude 24°59’00”S, Assmang purchased the farm from Gold Fields Limited,
together with all surface and mineral rights in October 1998.
Chrome ore mine History – Neighbouring properties to the north and south of Dwarsrivier had existing chrome mining operations at the time of purchase. The Feasibility Study of the plant, tailings dam and designs for the open-cast and underground mines then commenced. After the completion of the Feasibility Study, approval to proceed with the final design and construction work was given in July 1999.
Chromite was obtained from the open-cast mining areas at a rate of approximately 0,9 million tons a year and these areas were mined out within five years. Underground mining commenced in 2005 at a rate of 1,2 million ROM tons a year. Dwarsrivier Mine is specifically geared to deliver high-quality metallurgical-grade chromite to the Machadodorp smelter. In addition, the plant has been designed to produce chemical-grade products.
Mining authorisation – An old-order Mining Licence 21/99 was granted in October 1999. It was granted for the mining of chrome and platinum group metals. An application for the conversion to a new-order mining right was submitted during
October 2007.
Geology – Dwarsrivier Mine is situated in the eastern limb of the Bushveld Complex, which comprises persistent layers of mafic and ultramafic rocks, containing the world’s largest known resources of platinum group metals, chromium and vanadium. The mafic rocks, termed the Rustenburg Layered Suite, are approximately 8 kilometres thick in the eastern lobe, and are divided formally into five zones.
The rocks of the Marginal Zone at the base of the succession consist mainly of pyroxenites with some dunites and harzburgites. Above the Marginal Zone, the Lower Zone comprises mainly pyroxenites, harzburgites and dunite, and is present only in the
northern part of the Eastern Lobe, and only as far south as Steelpoort. The appearance of chromitite layers marks the start of the Critical Zone, economically the most important zone.
The layers are grouped into three sets termed the Lower, Middle and Upper groups. The sixth chromitite seam in the Lower Group (LG6), is an important source of chromite ore and is the orebody being mined at Dwarsrivier Mine. In the Eastern Lobe, in the vicinity of Dwarsrivier, the strike is nearly northsouth, with a dip of approximately 10 degrees towards the west. Average thickness of the LG6 seam is about 1,86 metres in the Dwarsrivier area. Pipe-like dunite intrusions are evident in the area, as well as dolerite dykes that on average strike northeast-south-west. No significant grade variation is evident, especially not vertically in the ore seam. Small, insignificant regional variations do, however, exist.
Mineral Resources and Reserves – Information was obtained from boreholes with 300 to 150-metre grid spacing. Resources were determined with a decreasing level of confidence.
• Measured Resource (150 metres drill grid spacing).
• Indicated Resource (300 metres drill grid spacing).
• Inferred Resource (drill grid spacing greater than 300 metres).
All possible Resources down to a mineable depth of 350 metres below ground level have been considered. Vertical diamond drill holes are used for geological and grade modelling, except where information is needed to clarify large-scale fault planes. The Mineral Resource at Dwarsrivier Mine is based on a total of 232 diamond drill holes that have been used for grade estimation and orebody modelling purposes. The drill core is NQ size and is geologically and geotechnically logged. The collar position of the drill holes is surveyed, but no down hole surveys are done, and the holes are assumed to have minimal deflection.
The chromitite seam is bounded above and below by pyroxenites. As such, the ore horizon is clearly defined. The core is sampled from the top contact downwards at 0,5-metre intervals. The core is split and half is retained as reference material in the core sheds. The other half is crushed and split into representative samples, which are crushed and pulverised for chemical analysis. The samples are analysed fusion/ICP-OES for chrome oxide (Cr2O3), SiO2, FeO, Al2O3, MgO and CaO. Three laboratories, all ISO 17025 accredited for this method, are used. Every tenth sample is analysed in duplicate. SARM 8 and SARM 9 standards, as well as in-house reference material (CRI), are included every 20 to 30 samples in each batch. The density for each sample is measured using a gas pycnometer.
Mineral Resources have been estimated using Ordinary Kriging, where Cr2O3, FeO-, Al2O3, MnO and MgO-contents of the LG6 seam and densities were determined, using block sizes of 50 x 50 x 4 metres.
During mining, a slightly diluted run-of-mine ore inclusive of the “false” hangingwall is fed to the beneficiation plant. In the dense media separation part of the plant, the coarse fraction is upgraded to 40% Cr2O3 with a yield of 80%. In the spiral section of the plant, the finer fraction is upgraded to 44% Cr2O3, and 46% Cr2O3 respectively, for metallurgical-grade fines and chemical-grade fines. A 67% yield is achieved in the spiral circuit.
Year-on-year change – When compared to 2009, the 2010 Mineral Reserves increased by 3,86 million tons to 39,5 million tons (35,64 million tons) and the Mineral Resources increased by 2,72 million tons to 50,60 million tons. The increase in the Mineral Resources was due to the extension of the block model by remodelling of the Indicated Resources. The increase in the
Mineral Reserves is due to the remodelling and incorporation of the “false” hangingwall. The latter caused a decrease in the Cr2O3 grade to 35,75% in the 2010 model.