A MASS BALANCE OF THE PLATINUM-GROUP ELEMENTS FOR THE ENTIRE BUSHVELD COMPLEX

We have determined PGE, Ni, Cu, S, Cr and a range of incompatible element concentrations for a complete section of the Rustenburg Layered Suite; the layered intrusion part of the Bushveld Complex.Compared to the composition of the marginal rocks the weighted average of the RLS is enriched in Cr, Ni, Ir, Rh and depleted in incompatible elements such as Sm.Pt, Pd and Au are similar in the marginal rocks and the RLS.

 

	Sm	S	Cu	Ni	Ir	Rh	Pt	Pd	Au	Cr
	ppm	%	ppm	ppm	ppb	ppb	ppb	ppb	ppb	ppm
Upper Zone	2.57	0.915	524	174	0.17	0.21	1.7	1.6	2.5	195
Main Zone	0.62	0.011	24	100	0.10	0.20	6.7	3.5	0.6	107
Critical Zone	0.31	0.007	21	533	3.32	6.61	52.0	17.4	3.5	6825
Lower Zone	0.34	0.013	19	908	1.47	1.90	13.6	10.0	3.6	5727
All of the RLS	1.00	0.233	146	322	0.87	1.53	14.0	6.4	2.0	2157
Boninitic	2.5	0.08	58	300	0.35	1.44	18	11	3.1	1000
Tholeiitic	3.5	0.03	62	128	0.20	0.4	10	5	1.	300

There are two possible reasons for theenrichment of compatible elements and depletion of incompatible elements in the RLS (e.g. Eales 2002).A) Either the marginal rocks do not represent the composition of the magma that filled the chamber and that the magma that filled the chamber was a crystal slurry making it more ultramafic than the marginal rocks;or B) a fractionated magma equal or greater in volume to the RLS was originally present.It is possible that the granophyres and granites of the Bushveld Complex could represent the fractionated magma.

A second point worth noting is that PGE/Sm ratios ofthe whole of the Lower and Critical Zones are greater than that of the liquids indicating that the process that concentrated PGE was operating right from the first formed cumulates and is not process that suddenly takes place at the Main Zone Critical Zone boundary. 

Eales, H.V. (2002)Min. Mag. 66, 815-833.