TY - JOUR
T1 - Petrogenesis of base metal sulphide assemblages of some peridotites from the Kaapvaal craton (South Africa)
AU - Lorand, Jean Pierre
AU - Grégoire, Michel
PY - 2006/5
Y1 - 2006/5
N2 - Thirty-two peridotite xenoliths from kimberlitic pipes of the Kaapvaal craton were analysed for S and studied in reflected light microscopy and electron microprobe. Correlation between whole-rock S concentrations and sulphide modal abundances has been obscured by kimberlite-related sulphur within the mantle and by low-temperature contamination processes during emplacement. Mantle-derived base metal sulphides (BMS) occur as solitary inclusions (SI) and intergranular blebs. Unfractured SI encloses intergrowths of Ni-poor and Ni-rich monosulphide solid solution (Mss) phases, coexisting with pentlandite (Pn) and Cu-rich sulphides. Textural relationships between Mss phases and Cu-sulphides are consistent with fractional crystallization of Mss from a Cu-Fe-Ni sulphide melt. Pn-rich euhedral SI may have crystallized from a more metal-rich sulphide melt. However, the opaque mineral assemblages of both fractured sulphide inclusions and intergranular BMS point to a progressive desulphurization of Mss, yielding Pn-rich grains, often replaced by Fe-poor heazlewoodite and abundant magnetite, while Cu-sulphides are replaced by native copper. This trend is consistent with reducing conditions generated by low-temperature serpentinization. A residual origin cannot be ruled out for the Mss enclosed in the most refractory peridotites, although their Ni/Fe ratios are too low to be consistent with an equilibration with olivine at magmatic temperatures. Modal abundances of mantle-derived BMS increase in the Fe-enriched metasomatized peridotites. At least two BMS precipitation processes can be recognized: (1) precipitation of Fe-Cu-rich immiscible sulphide melts in phlogopite-(ilmenite-rutile) peridotites and (2) sulphidation reactions from an H 2S-rich fluid phase in phlogopite-K richterite peridotites.
AB - Thirty-two peridotite xenoliths from kimberlitic pipes of the Kaapvaal craton were analysed for S and studied in reflected light microscopy and electron microprobe. Correlation between whole-rock S concentrations and sulphide modal abundances has been obscured by kimberlite-related sulphur within the mantle and by low-temperature contamination processes during emplacement. Mantle-derived base metal sulphides (BMS) occur as solitary inclusions (SI) and intergranular blebs. Unfractured SI encloses intergrowths of Ni-poor and Ni-rich monosulphide solid solution (Mss) phases, coexisting with pentlandite (Pn) and Cu-rich sulphides. Textural relationships between Mss phases and Cu-sulphides are consistent with fractional crystallization of Mss from a Cu-Fe-Ni sulphide melt. Pn-rich euhedral SI may have crystallized from a more metal-rich sulphide melt. However, the opaque mineral assemblages of both fractured sulphide inclusions and intergranular BMS point to a progressive desulphurization of Mss, yielding Pn-rich grains, often replaced by Fe-poor heazlewoodite and abundant magnetite, while Cu-sulphides are replaced by native copper. This trend is consistent with reducing conditions generated by low-temperature serpentinization. A residual origin cannot be ruled out for the Mss enclosed in the most refractory peridotites, although their Ni/Fe ratios are too low to be consistent with an equilibration with olivine at magmatic temperatures. Modal abundances of mantle-derived BMS increase in the Fe-enriched metasomatized peridotites. At least two BMS precipitation processes can be recognized: (1) precipitation of Fe-Cu-rich immiscible sulphide melts in phlogopite-(ilmenite-rutile) peridotites and (2) sulphidation reactions from an H 2S-rich fluid phase in phlogopite-K richterite peridotites.
UR - http://www.scopus.com/inward/record.url?scp=33745373610&partnerID=8YFLogxK
U2 - 10.1007/s00410-006-0074-7
DO - 10.1007/s00410-006-0074-7
M3 - Article
AN - SCOPUS:33745373610
VL - 151
SP - 521
EP - 538
JO - Contributions to Mineralogy and Petrology
JF - Contributions to Mineralogy and Petrology
SN - 0010-7999
IS - 5
ER -