It is of great importance to understand the origin of UG2 chromitite reefs and reasons why some chromitite reefs contain relatively high contents of platinum group elements（PGEs： Os, Ir, Ru, Rh,Pt, Pd） or highly si...It is of great importance to understand the origin of UG2 chromitite reefs and reasons why some chromitite reefs contain relatively high contents of platinum group elements（PGEs： Os, Ir, Ru, Rh,Pt, Pd） or highly siderophile elements（HSEs： Au, Re, PGE）. This paper documents sulphide-silicate assemblages enclosed in chromite grains from the UG2 chromitite. These are formed as a result of crystallisation of sulphide and silicate melts that are trapped during chromite crystallisation. The inclusions display negative crystal shapes ranging from several micrometres to 100 μm in size.Interstitial sulphide assemblages lack pyrrhotite and consist of chalcopyrite, pentlandite and some pyrite. The electron microprobe data of these sulphides show that the pentlandite grains present in some of the sulphide inclusions have a significantly higher iron（Fe） and lower nickel（Ni） content than the pentlandite in the rock matrix. Pyrite and chalcopyrite show no difference. The contrast in composition between inter-cumulus plagioclase（An_（68）） and plagioclase enclosed in chromite（An_（13））, as well as the presence of quartz, is consistent with the existence of a felsic melt at the time of chromite saturation.Detailed studies of HSE distribution in the sulphides and chromite were conducted by LA-ICP-MS（laser ablation-inductively coupled plasma-mass spectrometry）, which showed the following.（Ⅰ） Chromite contained no detectable HSE in solid solution.（Ⅱ） HSE distribution in sulphide assemblages interstitial to chromite was variable. In general, Pd, Rh, Ru and Ir occurred dominantly in pentlandite, whereas Os,Pt and Au were detected only in matrix sulphide grains and were clearly associated with Bi and Te.（Ⅲ）In the sulphide inclusions,（a） pyrrhotite did not contain any significant amount of HSE,（b） chalcopyrite contained only some Rh compared to the other sulphides,（c） pentlandite was the main host for Pd,（d）pyrite contained most of the Ru, Os, Ir and Re,（e） Pt and Rh were closely associated with Bi forming a continuous rim between pyrite and pentlandite and（f） no Au was detected. These results show that the use of ArF excimer laser to produce high-resolution trace element maps provides information that cannot be obtained by conventional（spot） LA-ICP-MS analysis or trace element maps that use relatively large beam diameters.展开更多
The formation of anorthosites in layered intrusions has remained one of petrology＇s most enduring enigmas. We have studied a sequence of layered chromitite, pyroxenite, norite and anorthosite overlying the UG2 chromi...The formation of anorthosites in layered intrusions has remained one of petrology＇s most enduring enigmas. We have studied a sequence of layered chromitite, pyroxenite, norite and anorthosite overlying the UG2 chromitite in the Upper Critical Zone of the eastern Bushveld Complex at the Smokey Hills platinum mine. Layers show very strong medium to large scale lateral continuity, but abundant small scale irregularities and transgressive relationships. Particularly notable are irregular masses and seams of anorthosite that have intrusive relationships to their host rocks. An anorthosite layer locally transgresses several 10 s of metres into its footwall, forming what is referred to as a ＂pothole＂ in the Bushveld Complex. It is proposed that the anorthosites formed from plagioclase-rich crystal mushes that originally accumulated at or near the top of the cumulate pile. The slurries were mobilised during tectonism induced by chamber subsidence, a model that bears some similarity to that generally proposed for oceanic mass flows. The anorthosite slurries locally collapsed into pull-apart structures and injected their host rocks. The final step was down-dip drainage of Fe-rich intercumulus liquid, leaving behind anorthosite adcumulates.展开更多
The Merensky Reef hosts one of the largest PGE resources globally.It has been exploited for nearly 100 years,yet its origin remains unresolved.In the present study,we characterised eight samples of the reef at four lo...The Merensky Reef hosts one of the largest PGE resources globally.It has been exploited for nearly 100 years,yet its origin remains unresolved.In the present study,we characterised eight samples of the reef at four localities in the western Bushveld Complex using micro-X-ray fluorescence and field emission scanning electron microscopy.Our results indicate that the Merensky Reef formed through a range of diverse processes.Textures exhibited by chromite grains at the base of the reef are consistent with supercooling and in situ growth.The local thickening of the Merensky chromitite layers within troughs in the floor rocks is most readily explained by granular flow.Annealing and deformation textures in pyroxenes of the Merensky pegmatoid bear testament to recrystallisation and deformation.The footwall rocks to the reef contain disseminations of PGE rich sulphides as well as olivine grains with peritectic reaction rims along their upper margins suggesting reactive downward flow of silicate and sulphide melts.Olivine-hosted melt inclusions containing Cl-rich apatite,sodic plagioclase,and phlogopite suggest the presence of highly evolved,volatile-rich melts.Pervasive reverse zonation of cumulus plagioclase in the footwall of the reef indicates dissolution or partial melting of plagioclase,possibly triggered by flux of heat,acidic fluids,or hydrous melt.Together,these data suggest that the reef formed through a combination of magmatic,hydrodynamic and hydromagmatic processes.展开更多
This paper focused on the results of the Trend Surface Analysis (TSA) of the Rustenburg Layered Suite (RLS) within the three major limbs of the Bushveld Complex. This second part focused on the Eastern Bushveld Comple...This paper focused on the results of the Trend Surface Analysis (TSA) of the Rustenburg Layered Suite (RLS) within the three major limbs of the Bushveld Complex. This second part focused on the Eastern Bushveld Complex and discussed the major trends, geometry and age relationship of the various structures within the Complex. The trend surface analysis of the Eastern Bushveld reveals that most of the residual positive structures occur as isolated closures with dome shape and are consistent with the location of the diapiric structures previously identified by geophysical and field mapping techniques.展开更多
The focus of this paper is on determination of the geometry and stratigraphic contact pattern of the Rustenburg Layered Suite (RLS) in the Northern Bushveld Complex area using available borehole data and trend surface...The focus of this paper is on determination of the geometry and stratigraphic contact pattern of the Rustenburg Layered Suite (RLS) in the Northern Bushveld Complex area using available borehole data and trend surface analysis technique. This technique was used to analyse over one hundred borehole log data in the Northern Bushveld Complex in order to describe the geometric pattern and trends of the RLS rocks. The results demonstrate the usefulness of this technique in identifying structural features. Regional trends of each of the stratigraphic units reveal the presence of regional structures that were not obvious at the surface. This first part of the paper focused on the Northern Bushveld Complex, while the second and the third part focused on the eastern and western Bushveld limbs respectively.展开更多
The geological setting of the Bushveld Complex,the world’s largest layered igneous body containing most of the planet’s chromium,Platinum Group Elements(PGEs)and vanadium resources,is outlined.The complex is situate...The geological setting of the Bushveld Complex,the world’s largest layered igneous body containing most of the planet’s chromium,Platinum Group Elements(PGEs)and vanadium resources,is outlined.The complex is situated in a central position on the ancient Kaapvaal Craton of South Africa,with the mineralisation contained in three large arcuate mafic to ultramafic Limbs,comprised of rocks of the Rustenburg Layered Suite(RLS).The Limbs are overlain by co-magmatic felsic rocks while the floor consists mainly of sediments of the Transvaal Supergroup.展开更多
基金granted by Laurentian UniversityNational Key R&D Program of China(Grant No.2016YFC0600207)
文摘It is of great importance to understand the origin of UG2 chromitite reefs and reasons why some chromitite reefs contain relatively high contents of platinum group elements（PGEs： Os, Ir, Ru, Rh,Pt, Pd） or highly siderophile elements（HSEs： Au, Re, PGE）. This paper documents sulphide-silicate assemblages enclosed in chromite grains from the UG2 chromitite. These are formed as a result of crystallisation of sulphide and silicate melts that are trapped during chromite crystallisation. The inclusions display negative crystal shapes ranging from several micrometres to 100 μm in size.Interstitial sulphide assemblages lack pyrrhotite and consist of chalcopyrite, pentlandite and some pyrite. The electron microprobe data of these sulphides show that the pentlandite grains present in some of the sulphide inclusions have a significantly higher iron（Fe） and lower nickel（Ni） content than the pentlandite in the rock matrix. Pyrite and chalcopyrite show no difference. The contrast in composition between inter-cumulus plagioclase（An_（68）） and plagioclase enclosed in chromite（An_（13））, as well as the presence of quartz, is consistent with the existence of a felsic melt at the time of chromite saturation.Detailed studies of HSE distribution in the sulphides and chromite were conducted by LA-ICP-MS（laser ablation-inductively coupled plasma-mass spectrometry）, which showed the following.（Ⅰ） Chromite contained no detectable HSE in solid solution.（Ⅱ） HSE distribution in sulphide assemblages interstitial to chromite was variable. In general, Pd, Rh, Ru and Ir occurred dominantly in pentlandite, whereas Os,Pt and Au were detected only in matrix sulphide grains and were clearly associated with Bi and Te.（Ⅲ）In the sulphide inclusions,（a） pyrrhotite did not contain any significant amount of HSE,（b） chalcopyrite contained only some Rh compared to the other sulphides,（c） pentlandite was the main host for Pd,（d）pyrite contained most of the Ru, Os, Ir and Re,（e） Pt and Rh were closely associated with Bi forming a continuous rim between pyrite and pentlandite and（f） no Au was detected. These results show that the use of ArF excimer laser to produce high-resolution trace element maps provides information that cannot be obtained by conventional（spot） LA-ICP-MS analysis or trace element maps that use relatively large beam diameters.
文摘The formation of anorthosites in layered intrusions has remained one of petrology＇s most enduring enigmas. We have studied a sequence of layered chromitite, pyroxenite, norite and anorthosite overlying the UG2 chromitite in the Upper Critical Zone of the eastern Bushveld Complex at the Smokey Hills platinum mine. Layers show very strong medium to large scale lateral continuity, but abundant small scale irregularities and transgressive relationships. Particularly notable are irregular masses and seams of anorthosite that have intrusive relationships to their host rocks. An anorthosite layer locally transgresses several 10 s of metres into its footwall, forming what is referred to as a ＂pothole＂ in the Bushveld Complex. It is proposed that the anorthosites formed from plagioclase-rich crystal mushes that originally accumulated at or near the top of the cumulate pile. The slurries were mobilised during tectonism induced by chamber subsidence, a model that bears some similarity to that generally proposed for oceanic mass flows. The anorthosite slurries locally collapsed into pull-apart structures and injected their host rocks. The final step was down-dip drainage of Fe-rich intercumulus liquid, leaving behind anorthosite adcumulates.
文摘The Merensky Reef hosts one of the largest PGE resources globally.It has been exploited for nearly 100 years,yet its origin remains unresolved.In the present study,we characterised eight samples of the reef at four localities in the western Bushveld Complex using micro-X-ray fluorescence and field emission scanning electron microscopy.Our results indicate that the Merensky Reef formed through a range of diverse processes.Textures exhibited by chromite grains at the base of the reef are consistent with supercooling and in situ growth.The local thickening of the Merensky chromitite layers within troughs in the floor rocks is most readily explained by granular flow.Annealing and deformation textures in pyroxenes of the Merensky pegmatoid bear testament to recrystallisation and deformation.The footwall rocks to the reef contain disseminations of PGE rich sulphides as well as olivine grains with peritectic reaction rims along their upper margins suggesting reactive downward flow of silicate and sulphide melts.Olivine-hosted melt inclusions containing Cl-rich apatite,sodic plagioclase,and phlogopite suggest the presence of highly evolved,volatile-rich melts.Pervasive reverse zonation of cumulus plagioclase in the footwall of the reef indicates dissolution or partial melting of plagioclase,possibly triggered by flux of heat,acidic fluids,or hydrous melt.Together,these data suggest that the reef formed through a combination of magmatic,hydrodynamic and hydromagmatic processes.
文摘This paper focused on the results of the Trend Surface Analysis (TSA) of the Rustenburg Layered Suite (RLS) within the three major limbs of the Bushveld Complex. This second part focused on the Eastern Bushveld Complex and discussed the major trends, geometry and age relationship of the various structures within the Complex. The trend surface analysis of the Eastern Bushveld reveals that most of the residual positive structures occur as isolated closures with dome shape and are consistent with the location of the diapiric structures previously identified by geophysical and field mapping techniques.
文摘The focus of this paper is on determination of the geometry and stratigraphic contact pattern of the Rustenburg Layered Suite (RLS) in the Northern Bushveld Complex area using available borehole data and trend surface analysis technique. This technique was used to analyse over one hundred borehole log data in the Northern Bushveld Complex in order to describe the geometric pattern and trends of the RLS rocks. The results demonstrate the usefulness of this technique in identifying structural features. Regional trends of each of the stratigraphic units reveal the presence of regional structures that were not obvious at the surface. This first part of the paper focused on the Northern Bushveld Complex, while the second and the third part focused on the eastern and western Bushveld limbs respectively.
文摘The geological setting of the Bushveld Complex,the world’s largest layered igneous body containing most of the planet’s chromium,Platinum Group Elements(PGEs)and vanadium resources,is outlined.The complex is situated in a central position on the ancient Kaapvaal Craton of South Africa,with the mineralisation contained in three large arcuate mafic to ultramafic Limbs,comprised of rocks of the Rustenburg Layered Suite(RLS).The Limbs are overlain by co-magmatic felsic rocks while the floor consists mainly of sediments of the Transvaal Supergroup.
