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THE ROLE OF MAGMA MIXING IN THE GENESIS OF PGE MINERALIZATION IN THE BUSHVELD COMPLEX: THERMODYNAMIC CALCULATIONS AND NEW INTERPRETATIONS—A DISCUSSION

R Grant Cawthorn

Department of Geology, University of the Witwatersrand, PO Wits, 2050, South Africa

E-mail, 065rgc@cosmos.wits.ac.za

The first 20% of the full text of this article appears below.

Sir: The concept of magma mixing producing sulfide mineralization, especially any enriched in platinum-group elements such as the Merensky reef, has become well entrenched. Naldrett and von Gruenewaldt (1989) presented calculations for the saturation limit of sulfur in the presumed parental magma of the Bushveld Complex and derivative liquids, based on well-established phase relationships for that magma. As with most phase boundaries, it is curved with a convexity, such that mixing between two points straddling the curved boundary could lead to a mixed magma that was sulfur-oversaturated. A revised version was presented by Li et al. (2001a), plotting data from the Bushveld Complex on a Zr versus S diagram.

However, whatever two compositional axes are used in a binary diagram, they cannot completely define the sulfur solubility limits for multicomponent magmas. Such solubility limits on a binary diagram are only valid for the very specific compositions for which the curve was constructed, but will not be applicable if different compositions are projected onto the diagram. In both models it is assumed that the magma evolution trend (and hence sulfur solubility) defined by fractionation is exactly the same as that produced during magma mixing. The first issue discussed here is why such an assumption is incorrect.

It was also suggested by Li et al. (2001a) that during norite fractionation the magma may become sulfur undersaturated. This interpretation of their phase diagram also requires reexamination. Finally, the actual composition of the magmas calculated by Li et al. (2001a) must be questioned.

In an erratum (Li et al., 2001b) published in response to a draft of this manuscript, these errors were recognized. However, the brief retraction does not explain the reasons why such models, calculations, and interpretations are not applicable.

There are differences between magma series produced by fractionation and . . . [Full Text of this Article]

This article has been cited by other articles:

				A. J. Naldrett, A. Wilson, J. Kinnaird, and G. Chunnett PGE Tenor and Metal Ratios within and below the Merensky Reef, Bushveld Complex: Implications for its Genesis J. Petrology, April 20, 2009; (2009) egp015v1. [Abstract] [Full Text] [PDF]

				H. M. Prichard, C. R. Neary, P. C. Fisher, and M. J. O'Hara PGE-rich Podiform Chromitites in the Al 'Ays Ophiolite Complex, Saudi Arabia: An Example of Critical Mantle Melting to Extract and Concentrate PGE Economic Geology, November 1, 2008; 103(7): 1507 - 1529. [Abstract] [Full Text] [PDF]

				A. WILSON and G. CHUNNETT Trace Element and Platinum Group Element Distributions and the Genesis of the Merensky Reef, Western Bushveld Complex, South Africa J. Petrology, December 1, 2006; 47(12): 2369 - 2403. [Abstract] [Full Text] [PDF]