Mineralogy of the lower part of the upper zone, eastern Bushveld Complex : implications for ore-forming processes

Abstract

English: The lower part of the upper zone in the eastern Bushveld Complex was subjected to a detailed mineralogical investigation, aimed at gaining a better understanding of processes that led to the formation of the main magnetite layer (MML), sulfide-enriched horizons and possible concentrations of platinum-group elements (PGE). The most prominent rock types in the investigated stratigraphic sequence are anorthosites, containing variable amounts of magnetite, which are interlayered with gabbronorites, leuco-gabbronorites and magnetite layers. Elevated sulfide concentrations are commonly associated with magnetite layers. The highest values were found in an anorthosite directly below the MML, and the sulfide assemblage typically contains pyrrhotite, chalcopyrite, cubanite and pentlandite in decreasing order of abundance. PGE are not enriched in specific stratigraphic horizons, as in those known from the critical zone. However, a local concentration of Pt and Pd has been encountered in the sulfide-bearing anorthositic footwall of lower magnetite layer 2. Several Pt-Pd-Bi-Te minerals were found, namely michenerite, moncheite, merenskyite and froodite, and textural as well as compositional evidence indicates that these platinum-group minerals are of hydrothermal origin. Post-magmatic hydrothermal processes also affected the mineralogy and composition of sulfides above the MML. Pentlandite, in particular, reflects such hydrothermal activity and displays varying, but considerably elevated, Co concentrations. Changes in the Sr content and the Sr/Al 2o3 ratio of plagioclase, the whole-rock PGE concentration pattern and possibly also the composition of pyroxene at the level of the MML show that the MML formed in response to a magma mixing event between the resident and a less differentiated magma. Magma mixing resulted either from a breakdown of density stratified liquid layers or an influx of a small volume of new magma (up to 30 % of the hybrid). In addition to the formation of the MML, magma mixing is also believed to have caused the segregation of sulfides associated with that layer. It is concluded that the evolution of the upper zone does not reflect simple differentiation, but that it was interrupted by magma mixing events which occurred at the level of the MML and possibly also at other levels during crystallization of the upper zone.

Afrikaans: Die onderste gedeelte van die bosone in die oostelike Bosveldkom-pleks is mineralogies noukeurig ondersoek met die doel om 'n beter begrip te vorm van die prosesse wat aanleiding gegee het tot die ontstaan van die hoofmagnetietlaag (HML), sulfied-verrykte lae en konsentrasies van die platinum-groep elemente (PGE). Die mees algemene gesteente tipes in die stratigrafiese suksessiewat ondersoek is, is anortosiete met varierende hoeveelhede magnetiet, wat tussengelaagd is met gabbronoriet, leukogabbronoriet en magnetietlae. Verhoogde sulfiedkonsentrasies is meestal geassosieer met magnetietlae. Die hoogste konsentrasies kom in 'n anortosietlaag direk onderkant die HML voor. Die mees algemene sulfiedassosiasie bevat, in volgorde van afnemende hoeveelheid, pirrhotiet, chalkopiriet, kubaniet en pentlandiet. Die PGE is nie in enige van die sulfiedgemineraliseerde lae verryk nie. Lokale verryking van Pt en Pd is egter teegekom in die sulfied-draende anortosiet net onder die laer magnetietlaag 2. Verskeie Pt-Pd-Bi-Te minerale is teegekom, te wete micheneriet, moncheiet, merenskyiet en froodiet. Tekstuur en samestelling dui daarop dat hierdie platinumminerale van hidrotermale oorsprong is. Na-magmatiese hidrotermale prosesse het ook die sulfied-mineralogie en samestelling bokant die HML geaffekteer. Veral pentlandiet reflekteer sodanige hidrotermale aktiwiteit en toon varierende, maar aansienlik verhoogde, Co konsentrasies. Veranderings in die Sr-inhoud en die Sr/Al 2o3 verhouding van plagioklaas, die heelgesteente PGE konsentrasiepatroon en moontlik ook die samestelling van pirokseen by die posisie van die HML dui daarop dat hierdie laag moontlik ontstaan het as gevolg van magmavermenging tussen die kristalliserende magma en 'n minder gedifferensieerde een. Magmavermenging kan of die gevolg wees van die ineenstorting van digtheidsgestratifiseerde vloeistoflae, of die gevolg van die invloei van 'n klein volume nuwe magma (tot 30 % van die hibriede produk). Bykomstig tot die ontstaan van die HML, word magmavermenging ook beskou as die oorsaak te wees vir die segregasie van sulfiede geassosieer met die laag. Daar word tot die slotsom gekom dat die ontstaan van die bosone nie die gevolg van 'n eenvoudige differensiasie is nie, maar dat dit onderbreek was deur episodes van magmavermenging soos byvoorbeeld by die HML en moontlik ook op ander tye tydens kristallisasie van die bosone.