MAGNETITE TRACE ELEMENT COMPOSITIONS IN ADJACENT MAGNETITITE- AND SILICATE-LAYERS IN THE BUSHVELD COMPLEX

The Upper Zone of the Bushveld Complex contains more than 21 magnetite-rich (>90 vol%) layers hosted within gabbroic and anorthositic layers. Magnetitite layers range in thickness from several cm to >2m and commonly show sharp contacts with silicate layers below and diffuse contacts with silicate layers above. Compared to the magnetitite layers, disseminated magnetite is smaller with more abundant ilmenite exsolution, and is typically interstitial. Several models have been proposed to explain the formation of the magnetitite layers, including changes in oxygen fugacity or pressure, fractional crystallization, and magma mixing.

Twenty-three trace elements in magnetite were analyzed by LA-ICPMS from eight massive magnetitite layers and disseminated magnetite from the adjacent silicate layers. A complex 2 m thick magnetitite layer with intermediate gabbroic and plagioclase-rich partings was also studied.

Magnetite in silicate layers are relatively enriched in V, Ge, Mn, Mo, Sn, and Hf and depleted in Mg and Sc compared to magnetite from magnetitite layers. Abundances of Ni, Cr, Co, Al, Ga, and Zr in silicate-layer magnetite show both relative depletion or enrichment, or are the same as the adjacent magnetitite layer.

There is no consistent relationship between trace element concentration and the silicate rock type or position relative to the adjacent magnetitite layer. Of particular interest is the lack of correlation of V and Cr with stratigraphic position of the silicate layers, as these elements are the most compatible in the magnetite and are expected to have a lower concentration in disseminated magnetite above the magnetitite layers. Vanadium enrichment in all silicate layers agrees with the findings of Klemm et al. (1985) and suggests high oxygen fugacity conditions triggered the rapid crystallization of magnetitite layers, while disseminated magnetite crystallized in equilibrium conditions. Inconsistencies in trace element concentrations in disseminated magnetite above and below adjacent magnetitite layers suggest equilibration with melt that has experienced various degrees of fractionation.