Halogen Geochemistry of the Stillwater and Bushveld Complexes: Evidence for Transport of the Platinum-Group Elements by Cl-Rich Fluids

Compositional data on apatite, phlogopite, and amphibole indicatethat the high-temperature hydrothermal fluids which affectedthe lower portions of the Stillwater and Bushveld Complexeswere Cl-rich. Apatites from the platinum-group element (PGE)ore zones from both complexes are enriched in Cl relative toother cumulus and noncumulus apatites in these intrusions andto apatites from the Skaergaard and Kiglapait Intrusions andthe Great Dyke. Apatites from all five intrusions can be groupedinto three distinct compositional fields: (a) Cumulus apatitesare essentially fluorapatites with molar Cl/(Cl+OH+F) <0?03;(b) noncumulus apatites, with the exception of those from thePGE ore zones of the Stillwater and Bushveld Complexes, haveCl/(Cl+OH+F) <0?20; (c) Cl-rich apatites associated withPGE-rich zones have Cl/(Cl+OH+F) between 0?45 and 1?0. The REEcontent of noncumulus and Cl-rich apatites also show a positivecorrelation with Cl concentration. It is argued that becauseCl is less soluble in silicate melts than F and because meltswith extremely high Cl/F ratios are unknown, the Cl-rich apatitesequilibrated with Cl-rich hydrothermal fluids exsolved duringsolidification of the cumulate sequence. The Cl, F, and OH contents of phlogopites and amphiboles aremore variable. Compositional heterogeneity is due to crystal-chemicalcontrols on halogen contents, variation in the halogen contentof the original melt/fluid phase and subsolidus re-equilibrationduring cooling with both surrounding mineral phases and lowtemperature fluids. However, both the Stillwater and Bushveldphlogopites are enriched in Cl compared to those from the Skaergaardand Kiglapait Intrusions. The compositions of coexisting minerals from the platinum depositof Olivine-Bearing Subzone I of the Stillwater Complex are usedto compute a fluid composition. The fluid is rich in alkalisand iron as well as HCl, and the solution composition is consistentwith fluid compositions deduced for the PGE-bearing secondaryhortonolite pipes of the Bushveld Complex. The high (Pt+Pd)/Irratios of these deposits are also consistent with a hydrothermalorigin, as both Pt and Pd are more soluble in Cl-complexingfluids than Ir.     

Petrology and Mineral Compositions of the Middle Banded Series of the Stillwater Complex, Montana

The two olivine-bearing zones of the Middle Banded series ofthe Stillwater Complex are characterized by an increase in thenumber of cumulus minerals with height. In each, anorthositeand anorthositic troctolite dominate the lower part whereasolivine gabbro and gabbronorite form much of the upper portions.Electron microprobe analyses of cumulus minerals indicate littleor no variation of average mineral compositions with height.In addition, no significant lateral variations in cumulus mineralcompositions occur along 8 km of section. Plagioclase from throughoutthese zpnes shows the complex, reverse and oscillatory zonationpatterns also seen in plagioclase from the thick (>500 m)anorthosites that sandwich the zones. The data suggest thatthe entire Middle Banded series is genetically related and thereforerequires models for the origin of the thick anorthosites toalso explain the olivine-bearing rocks between them. However,textural features such as discordant troctolites, pegmatoids,slump structures and variably developed mineral laminations,and chemical features such as zonation in clinopyroxene producedby intergranular exchange with orthopyroxene and large within-samplevariations in the mg-number of olivine in low-olivine troctolites,indicate that significant modification of these rocks by postcumulusprocesses has taken place, thereby obscuring evidence of theirgenesis. KEY WORDS: Stillwater Complex; Middle Banded series; layered intrusions; mineral chemistry; postcumulus processes ^*Corresponding author. Telephone: (919) 681-8169. Fax: (919) 684-5833. e-mail: wpm{at}roguegeo.dukeedu     

Carbon Distribution in the Stiliwater Complex and Evolution of Vapor During Crystallization of Stillwater and Bushveld Magmas

The occurrence and distribution of carbon in the StillwaterComplex have been investigated. In mineralized troctolite andassociated rocks of olivine-bearing zone I (OB I), carbon ispresent as graphitic material and calcite. The assemblage forsterite-antigorite-calcite-graphiteand the petro graphic relations indicate equilibration of thecarbon-rich phases during serpentinization. Typical OB I troctolitecontains 500–1100 ppm wt. carbon, 40–70% of whichis in calcite, whereas troctolite from higher stratigraphicpositions generally contains <400 ppm carbon. Due to themetamorphism, it is not possible to deduce the extent to whichenrichment of carbon in the ore zone is inherited from magmaticprocesses. In contrast, there is good evidence for magmaticgraphite in parts of the Bushveld Complex. The C-O-H-Cl system has been investigated for conditions ofStillwater and Bushveld crystalliz ation. In alkali-poor fluidsover a wide range of igneous and metamorphic conditions, theimportant chlorine species are HCl and CH₃Cl The addition ofchlorine to a C-O-H fluid in equilibrium with graphite leadsto a quantitative increase in HCl+CH₃Cl and corresponding decreasein H₂O contents, and, when Cl/H exceeds 1, to a CO₂+CO-richfluid with little H₂O Similarly, in more reduced fluids, CH₄contents are depressed by the formation of CH₃Cl. From consideration of volatile solubilities and abundances inmafic magmas and the nature of the C-O-H-Cl system, it is hypothesizedthat the first fluid to exsolve from Bushveld and Stillwaterintercumulus melt was composed of a mixture of CO₂ CO, and HClwith minor amounts of sulfur species and H₂O A model is developedfor the evolution of such a fluid with cooling. The model assumesthat graphite began to precipitate from the fluid at supersolidustemperature and that the system cooled down a T-f_o2 path parallelto and >2 log units below that of the Ni-NiO oxygen buffer.Upon the appearance of graphite, the fluid evolved to a morehydrogen-rich composition by graphite precipitation and lossof oxygen to the surrounding silicate-oxide assemblage. Coolingof fluid to 25?C below the first appearance of graphite resultedin reduction in the fluid mass by >70%, thus concentratingchlorine, sulfur and other residual species in the intercumulusfluid and melt. The model explains the presence of chlor-apatiteand the enrichment of graphite in the Bushveld Critical Zoneand predicts that chlor-apatite-bearing Stillwater rocks weresimilarly enriched in graphite during crystallization.     

The Halogen Geochemistry of the Bushveld Complex, Republic of South Africa: Implications for Chalcophile Element Distribution in the Lower and Critical Zones

Halogen-bearing minerals, especially apatite, are minor butubiquitous phases throughout the Bushveld Complex. Interstitialapatite is near end-member chlorapatite below the Merensky reef(Lower and Critical Zones) and has increasingly fluorian compositionswith increasing structural height above the reef (Main and UpperZones). Cl/F variations in biotite are more limited owing tocrystal-chemical controls on halogen substitution, but are alsoconsistent with a decrease in the Cl/F ratio with structuralheight in the complex. A detailed section of the upper LowerZone to the Critical Zone is characterized by an upward decreasein sulfide mode from 0·01–0·1% to trace–0·001%.Cu tends to correlate with other incompatible elements in mostsamples, whereas the platinum-group elements (PGE) can behaveindependently, particularly in the Critical Zone. The decreasein the Cl/F ratio of apatite in the Main Zone is associatedwith a shift to more radiogenic Sr isotopic signature, implyingthat the unusually Cl-rich Lower and Critical Zones are notdue to assimilation of crustal rocks. Nor is the Main Zone moreCl rich where it onlaps the country rocks of the floor, suggestinglittle if any Cl was introduced by infiltrating country rockfluids. Instead, the results are consistent with other studiesthat suggest Bushveld volatile components are largely magmatic.This is also supported by apatite–biotite geothermometry,which gives typical equilibrium temperatures of 750°C. Theincreasingly fluorian apatite with height in the Upper Zonecan be explained by volatile saturation and exsolved a Cl-richvolatile phase. The high Cl/F ratio inferred for the Lower andCritical Zone magma(s) and the evidence for volatile saturationduring crystallization of the Upper Zone indicate the Lowerand Critical Zones magma(s) were unusually volatile rich andcould easily have separated a Cl-rich fluid phase during solidificationof the interstitial liquid. The stratigraphic distribution ofS, Cu and the PGE in the Critical Zone cannot readily be explainedeither by precipitation of sulfide as a cotectic phase or asa function of trapped liquid abundance. Evidence from potholesand the PGE-rich Driekop pipe of the Bushveld Complex implythat migrating Cl-rich fluids mobilized the base and preciousmetal sulfides. We suggest that the distribution of sulfideminerals and the chalcophile elements in the Lower and CriticalZones reflects a general process of vapor refining and chromatographicseparation of these elements during the evolution and migrationof a metalliferous, Cl-rich fluid phase. KEY WORDS: Bushveld Complex; chlorine; platinum-group elements; layered intrusions     

The Lower Zone-Critical Zone Transition of the Bushveld Complex: a Quantitative Textural Study

The Lower Zone–Critical Zone boundary of the BushveldComplex is an intrusion-wide, major stratigraphic transitionfrom ultramafic harzburgite and pyroxenite in the Lower Zoneto increasingly plagioclase-rich pyroxenites and norites inthe Critical Zone. Quantitative textural and compositional datafor 29 samples through this transition show the following: LowerZone orthopyroxene grains are larger, have higher aspect ratios,are better foliated and have a lower trapped liquid componentthan those of the Critical Zone. The larger grain size of theLower Zone results in crystal size distribution plots that arerotated to lower slopes and intercepts relative to those inthe Critical Zone. Although all rocks show differing amountsof foliation, mineral lineations are weak to absent. These dataare consistent with significant compaction-driven recrystallizationin the study section. Numerical modeling of concurrent compactionand crystallization provides a quantitative model of how theLower Zone–Critical Zone transition may have formed: plagioclaseis rare in the Lower Zone because compaction removes interstitialliquid before it reaches plagioclase saturation. However, asthe crystal pile grows, plagioclase saturation is reached inthe interstitial liquid before compaction is complete in moreevolved pyroxenites, producing more abundant but still modestamounts of plagioclase characteristic of the Lower CriticalZone. It is concluded that both the textures and the modal mineralogyare largely controlled by compaction and compaction-driven recrystallization;primary magmatic textures are not preserved. KEY WORDS: Bushveld Complex; compaction; crystal size distributions; crystal aging; igneous textures