Download Mineralogical and geochemical investigations of Critical Zone rocks

Mineralogical and geochemical investigations of Critical Zone
rocks south and north of the Steelpoort fault of the eastern
Bushveld Complex – Petrogenetic implications
C. Gauert1, H. Giebel1,2, M. Schannor3, J.J. Beukes1
1. Dept. of Geology, University of the Free State, Bloemfontein, RSA, [email protected]
2. Inst. f. Geol. Wiss., Martin-Luther-Universität Halle-Wittenberg, Germany, [email protected]
3. Inst. f. Geologie, FreieUniversität Berlin,Germany, [email protected]
ABSTRACT
Critical Zone rocks from the MG and UG2 intervals and the Merensky cyclic unit of various sections of the Eastern
Bushveld Complex were investigated in terms of mineralogy and geochemistry. Variations in thickness and texture
of sections south and north of the Steelpoort fault were compared as well as the density of pegmatoidal pipe
development.
Approaching the Steelpoort fault from south along the parallel running Dwars River fault, the density of
pegmatoids occurrence increases and the magmatic layering shows more disturbances. Immediately north of the
Steelpoort fault, pegmatoid density is high but decreases distally to the fault. It is suggested that the fault acted as a
conduit for late-stage iron-rich liquid intruding the cumulate pile especially within and in proximity to the fault
zone. Disturbance of the cumulus pile could have occurred by magma - floor rock interaction due to turbulent flow
in the fault’s/conduit’s vicinity, together with changes in magma pressure, addition of volatiles by floor rock
assimilation and degassing.
Facies changes in thickness and grade of UG2 and MR units within 100s of metres at Two Rivers Platinum (TRP),
could reflect changes in liquid line of descent, intensity of magma convection, pulses of replenishment,
contamination and volatile content of the magma.
The cumulate thickness between the UG2 and Merensky Reef decreases with increasing distance from the fault in
the northern part of the eastern limb, coinciding with rapid changes in magma chamber bottom morphology.
Therefore, compartmentilization of the northern sector, caused by basement updoming, influences the thickness of
the igneous sequence.
The economic effect caused by the conduit situation is that PGE grade in vicinity of the fault zone decreases (TRP)
or becomes very erratic (e.g. Rhodium Reefs Mine).
KEYWORDS: Critical Zone, Bushveld Complex, Eastern Limb, Steelpoort fault, Two Rivers Platinum
InkabayeAfrica : 9th Annual Workshop 2012, Potsdam - Germany