TY  - THES
T1  - Platinum-group element distribution in base-metal sulfides of the Merensky Reef and UG2 from the eastern and western Bushveld Complex, South Africa
A1  - Osbahr,Inga
Y1  - 2012/10/11
N2  - Base-metal sulfides (BMS) in magmatic Ni-Cu-PGE deposits are important carriers of platinum-group elements (PGE). The distribution and concentrations of PGE in pentlandite, pyrrhotite, chalcopyrite and pyrite were determined in samples from the mineralized proportion of four Merensky Reef and two UG2 intersections from the eastern and western Bushveld Complex. Electron microprobe analysis was used for major elements, and in situ LA-ICP-MS for trace elements (PGE, Ag and Au). Whole-rock trace element analyses were performed on representative samples to obtain mineralogical balances. 

Both Pt and Pd in Merensky Reef samples from the western Bushveld show a ?top loaded? mineralization, mainly concentrated in the upper chromitite stringer and its immediate vicinity. Samples from the eastern Bushveld reveal more complex distribution patterns, since the concentration maxima are located in the lower and upper chromitite stringer area.
In UG2 samples of the eastern and western Bushveld, the highest Pd and Pt concentrations are located in the lower and upper portion of the UG2 chromitite. 

In situ LA-ICP-MS analyses of PGE in sulfides reveal that pentlandite carries the highest and most distinctly elevated PGE contents, whereas pyrrhotite and chalcopyrite contain very low PGE concentrations. Pentlandite is the principal host of Pd and Rh in the ores of the Merensky Reef and UG2, being incorporated in the crystal lattice. Palladium and Rh concentrations in pentlandite reach up to 700 ppm and 130 ppm, respectively, in the Merensky Reef samples from the eastern Bushveld, and up to 1750 ppm Pd and 1000 ppm Rh in those from the western Bushveld. Only traces of Pt are found in the BMS. Pyrrhotite contains significant, though generally lower amounts of Ru, Os and Ir, but hardly any Pd or Rh. Chalcopyrite contains most of the Ag but carries extremely low PGE concentrations. 
In the UG2 samples, Pd and Rh in pentlandite reach up to 400 ppm and 200 ppm, respectively, in the western Bushveld and up to 1000 ppm Pd and 140 ppm Rh in the eastern Bushveld Complex. Significant amounts of Ir (150 ppm) and Ru (175 ppm) were found in the UG2 samples. Pyrrhotite contains significant amounts of Rh (100 ppm), Os (70 ppm) and Ru (200 ppm). Chalcopyrite contains very low PGE concentrations, although maximum Pt concentrations of 20 ppm were detected. 

Mass balance calculations performed on the Merensky Reef samples reveal that in general, pentlandite in the feldspathic pyroxenite and the pegmatoidal pyroxenite hosts up to 100 % of the Pd and Rh and smaller amounts (10-40 %) of the Os, Ir and Ru. Pyrrhotite and chalcopyrite usually contain less than 10 % of the whole-rock PGE. The remaining PGE concentrations, and especially most of the Pt (up to 100 %), are present in the form of 
discrete platinum-group minerals (PGM) such as cooperite/braggite, sperrylite, moncheite and isoferroplatinum. 
In the UG2 of the western Bushveld pentlandite hosts 5 % of the whole-rock Pd and 15 % of the whole-rock Rh while in the eastern Bushveld it is 40 % of the Pd and 30 % of the Rh. Less than 20 % of whole-rock Os, Ir and Ru is incorporated in the pentlandite of the UG2. Similar to the Merensky Reef, in the UG2 the remaining PGE and almost 100 % of the Pt are present in the form of discrete platinum-group minerals.    

Distribution patterns of whole-rock Cu, Ni and S versus whole-rock Pd and Pt commonly show distinct offsets in the Merensky Reef. The general sequence of ?offset patterns? of PGE and BMS maxima, in order from bottom to top, is Pd in pentlandite ? Pd/Pt in whole-rock ? (Cu, Ni, S) in whole-rock, and is quite obvious in most of our samples. Occasionally, only partially similar or more complex trends are also present.

Generally, however, the highest Pd concentrations in pentlandite appear to be related to the earliest, volumetrically rather small sulfide liquids, found at the base of the Merensky Reef sequence. A possible explanation for the offset patterns may be Rayleigh fractionation.

KW  - Merensky Reef
KW  - Bushveld Komplex
KW  - Buntmetallsulfide
KW  - LA-ICP-MS
CY  - Erlangen
PB  - Universitätsbibliothek der Universität Erlangen-Nürnberg
AD  - Universitätsstraße. 4, 91054 Erlangen
L2  - http://www.opus.ub.uni-erlangen.de/opus/volltexte/2012/3542
ER  -