Mineral layering in the Bushveld Complex; implications of Cr abundances in magnetite from closely spaced magnetitite and intervening silicate-rich layers

Analytical data for the platinum-group elements (PGE) and gold on 17 samples from sulfidemineralized and -unmineralized rocks of the lower portion of the upper zone and two samples from the main zone of the eastern Bushveld Complex are presented. Almost all samples contain less than 100 ppb total PGE, indicating that the sulfide-enriched layers of the upper zone generally contain only negligable amounts of PGE. However, PGE enrichment previously reported from a sulfide-bearing anorthosite below the lower magnetite layer 2 has been confirmed by microscopic investigations and is interpreted as a zone locally enriched by hydrothermal fluids. Increasing PGE concentrations and Pd/Ir and Pt/Pd ratios with stratigraphic height indicate differences in the behavior of the PGE. Incompatibility follows the sequence Ir < Rh < Ru < Pd < Pt, with Pt being approximately eight times more incompatible than Ir. Rocks below and above the main magnetite layer are characterized by two different PGE patterns which are seen as the effect of a magma-mixing event which resulted either from a breakdown of density stratified liquid layers or an influx of a small volume of new magma at the level of the main magnetite layer.Rocks below the main magnetite layer display a chondrite-normalized PGE pattern similar to that of the Picket Pin Pt-Pd deposit in the Stillwater Complex.

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“…8). This melt most likely broke down into a number of density-stratified liquid layers as was also suggested by McCarthy et al (1985).…”

Section: Pge In the Upper Zone Rocks Can Be Related To Fractionation supporting

confidence: 55%

Platinum-group element behavior in the lower part of the upper zone, eastern Bushveld Complex; implications for the formation of the main magnetite layer

Harney

Merkle²,

Gruenewaldt³

1990

Economic Geology

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“…Eales and Cawthorn, 1996;McCarthy et al, 1985). A good example occurs over the depth interval ~1380 to 1390 m, where a ~3 m thick magnetite-rich layer overlain by leucogabbronorite and anorthosite is associated with a gradual upward decline in magnetic susceptibility (Figure 10b).…”

Section: Fractionation Vs Magma Replenishmentmentioning

confidence: 99%

Magmatic stratigraphy in the Bushveld Northern Lobe: continuous geophysical and mineralogical data from the 2950 m Bellevue drillcore

Ashwal

Webb

Knoper

2005

South African Journal of Geology

137

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We present a large database of geophysical, petrological and mineralogical measurements for the ~3000 m Bellevue borehole through the entire Upper Zone (UZ) and about half of the Main Zone (MZ) of the Northern Lobe of the Bushveld Complex, South Africa. Magnetic susceptibilty readings were taken every 2 cm (n = 109,360) and densities were measured on average every 1.7 m (n = 2252). Petrographic data and microprobe analyses (n = 14,160) were obtained for plagioclase, mafic silicates, Fe-Ti oxides, amphibole and biotite in 502 samples throughout the entire sequence of layered mafic cumulates. The Bellevue UZ, as marked by the first appearance of cumulus magnetite, is ~1190 m thick (corrected for mean dip of 17.5º), which is less than UZ thicknesses in the Eastern and Western Lobes. A prominent 4 m thick pyroxenite horizon occurs ~390 m below the UZ-MZ boundary, but we show on the basis of mineralogy that this horizon cannot be correlated with the well known Pyroxenite Marker (PM) of the Eastern and Western Bushveld Complex. If the PM is indeed absent in the Northern Lobe, then a substantial portion, perhaps 500 m of the uppermost MZ may be missing; possible causes include non-deposition (e.g. due to syn-magmatic upwarping or diapirism) or removal (e.g. due to emplacement of UZ magmas). The Bellevue drillcore penetrated only about half of the MZ (total dip-corrected thickness ~1270 m), and the lowermost ~200 m contains unusual olivine-bearing (or troctolitic) horizons that are atypical of MZ rocks elsewhere in the Bushveld Complex. These troctolites have mineral compositions as primitive as those of the upper Critical Zone (CZ), suggesting that they might represent a sliver of CZ rocks dismembered by intrusion of MZ magmas. Alternatively, they may represent an intrusive sill of syn-or post-Bushveld age, or merely a mineralogically unusual horizon in otherwise typical MZ lithologies.Mineral compositions show broad normal fractionation upwards, with plagioclase An78©21, opx (and inverted pigeonite) En 80©26, cpx Mg 86©27, olivine Fo 78©74 (in the lowermost troctolitic horizon) and Fo 59©06 (in the UZ olivine ferrodiorites). There are, however, numerous prominent reversals and discontinuities in mineral compositions, some of which are likely related to magma additions. The extensive dataset of mineral compositions allows the estimation of a new fractionation trend for the Bushveld Complex. On an En-An diagram, the Bushveld trend is shifted toward more An-rich plagioclase at equivalent Mg# of coexisting pyroxenes relative to those for Kiglapait or Skaergaard. This is attributed to the relative paucity in Bushveld of augite, which has a high fractionating power for Ca/Na in evolving liquids.Magnetic susceptibility data clearly reveal the presence of the UZ-MZ boundary. MZ cumulates have susceptibilty values <0.05 SI units, and generally <0.02 SI units. Above the UZ-MZ boundary, susceptibilty varies enormously, from anorthosites (<0.1 SI units) to magnetitites (to almost 5 SI units), and there is excellent correlation bet...

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“…This value varies between 520 and 710 and is strongly dependent on the amount of cumulus magnetite. A large interval of KD values for Cr in magnetite ( 100-620 ) has been reported by Lindstrom ( 1976 ) on the basis of experimental data and a value of 200 has been adopted by McCarthy et al (1985) in the modelling of magnetite in the Bushveld intrusion. On the other hand, Duchesne and Hertogen (1988) used major and trace elements to constrain the composition of the parental magma to MCU III and IV and suggested that a fine grained (chilled) monzonorite found at Tjorn has the appropriate composition.…”

Section: Geochemical Modellingmentioning

confidence: 98%

Magma influx and mixing in the Bjerkreim-Sokndal layered intrusion, South Norway: evidence from the boundary between two megacyclic units at Storeknuten

Jensen

Nielsen

Duchesne

et al. 1993

Lithos

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. Magma influx and mixing in the Bjerkreim-Sokndal layered intrusion, South Norway: evidence from the boundary between two megacyclic units at Storeknuten. Lithos, 29:311-325.The Bjerkreim-Sokndal layered intrusion belongs to the Proterozoic anorthositic province in the Rogaland area of southern Norway. The northwestern part of the intrusion comprises a ca. 6 km-thick Layered Series made up of megacyclic units (MCU) arranged in a syncline; each megacyclic unit reflects the influx of fresh magma into the chamber. The boundary between megacyclic units III and IV has been studied in detail at Storeknuten on the southern flank of the syncline. The megacyclic units can be subdivided into a series of cumulate stratigraphic zones; the interval from the top of zone IIIe to the base of zone IVd is exposed in the Storeknuten area. Modally layered plagioclase-hypersthene-ilmenite-magnetite-augite-apatite cumulates belonging to zone IIIe are overlain by 30 m of massive plagioclase-rich rocks (commonly containing ilmenite and/or hypersthene) constituting zone IVa. The entry of cumulus olivine defines the base of zone IVb (dominantly plagioclase-olivine-ilmenite cumulates) which is about 100 m thick. Many of the olivines are partly or completely replaced by Ca-poor pyroxene/Fe-Ti oxide symplectites. This massive leucotroctolitic zone is overlain by modally layered, laminated plagioclasehypersthene-ilmenite cumulates of zone IVc. The successive entry of magnetite, apatite (accompanied by Ca-rich pyroxene) and inverted pigeonite defines zones IVd, e and f respectively. The entry of Kfeldspar (accompanied by Fe-rich olivine) defines the base of a jotunitic transition zone which passes upwards into mangerites and quartz mangerites.There is a compositional regression through zone IVa. The upper part of zone IIIe has Ca-poor pyroxene with about En68, plagioclase with An44-48 and a Sr-isotope ratio of about 0.7062, while the base of zone IVb has olivine with Fo75 together with En78, An53 and 0.7050 respectively. Similar reversals are shown by the minor element compositions of plagioclase and Fe-Ti oxides. Sr-isotope ratios increase systematically up through zone IVb (reaching 0.7058 in zone IVd) while An% and Sr in plagioclase and Ni and Cr in Fe-Ti oxides decrease. Olivine compositions vary unsystematically and are believed to have changed their Fe:Mg ratios as a result of trapped liquid shift.The magma residing in the chamber when the influx at the base of megacyclic unit IV took place was compositionally zoned, and assimilation of gneissic country rock at the roof had resulted in the St-isotope ratio increasing up through the magma column. The new magma had a Sr-isotope ratio of about 0.7050 while the resident magma had a ratio of 0.7062 at the floor, increasing upwards. The new magma mixed with the basal layer(s) of the compositionally zoned resident magma and crystallization of this hybrid magma during influx and mixing produced the compositional regression in zone IVa. When magma influx

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Mineral layering in the Bushveld Complex; implications of Cr abundances in magnetite from closely spaced magnetitite and intervening silicate-rich layers

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Platinum-group element behavior in the lower part of the upper zone, eastern Bushveld Complex; implications for the formation of the main magnetite layer

Platinum-group element behavior in the lower part of the upper zone, eastern Bushveld Complex; implications for the formation of the main magnetite layer

Magmatic stratigraphy in the Bushveld Northern Lobe: continuous geophysical and mineralogical data from the 2950 m Bellevue drillcore

Magma influx and mixing in the Bjerkreim-Sokndal layered intrusion, South Norway: evidence from the boundary between two megacyclic units at Storeknuten

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