Journal of Petrology | Volume 27 | Number 4 | Pages 967-986 | 1986
© Oxford University Press 1986
research-article |
Halogen Geochemistry of the Stillwater and Bushveld Complexes: Evidence for Transport of the Platinum-Group Elements by Cl-Rich Fluids
Department of Geological Sciences, AJ-20, University of Washington Seattle, Washington 98195
Received August 21, 1985; Revised typescript accepted February 4, 1986
ABSTRACT
Compositional data on apatite, phlogopite, and amphibole indicate that the high-temperature hydrothermal fluids which affected the lower portions of the Stillwater and Bushveld Complexes were Cl-rich. Apatites from the platinum-group element (PGE) ore zones from both complexes are enriched in Cl relative to other cumulus and noncumulus apatites in these intrusions and to apatites from the Skaergaard and Kiglapait Intrusions and the Great Dyke. Apatites from all five intrusions can be grouped into three distinct compositional fields: (a) Cumulus apatites are essentially fluorapatites with molar Cl/(Cl+OH+F) <0·03; (b) noncumulus apatites, with the exception of those from the PGE ore zones of the Stillwater and Bushveld Complexes, have Cl/(Cl+OH+F) <0·20; (c) Cl-rich apatites associated with PGE-rich zones have Cl/(Cl+OH+F) between 0·45 and 1·0. The REE content of noncumulus and Cl-rich apatites also show a positive correlation with Cl concentration. It is argued that because Cl is less soluble in silicate melts than F and because melts with extremely high Cl/F ratios are unknown, the Cl-rich apatites equilibrated with Cl-rich hydrothermal fluids exsolved during solidification of the cumulate sequence.
The Cl, F, and OH contents of phlogopites and amphiboles are more variable. Compositional heterogeneity is due to crystal-chemical controls on halogen contents, variation in the halogen content of the original melt/fluid phase and subsolidus re-equilibration during cooling with both surrounding mineral phases and low temperature fluids. However, both the Stillwater and Bushveld phlogopites are enriched in Cl compared to those from the Skaergaard and Kiglapait Intrusions.
The compositions of coexisting minerals from the platinum deposit of Olivine-Bearing Subzone I of the Stillwater Complex are used to compute a fluid composition. The fluid is rich in alkalis and iron as well as HCl, and the solution composition is consistent with fluid compositions deduced for the PGE-bearing secondary hortonolite pipes of the Bushveld Complex. The high (Pt+Pd)/Ir ratios of these deposits are also consistent with a hydrothermal origin, as both Pt and Pd are more soluble in Cl-complexing fluids than Ir.
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