The Stillwater complex in Montana and the Bushveld complex in South Africa are among the few intrusions that contain extensive stratigraphic sections in which the halogen-bearing minerals are unusually rich in chlorine. This is in contrast to the majority of layered igneous complexes that crystallized from basaltic parent liquids (e.g., the Skaergaard intrusion in Greenland, the Munni Munni and the Windimurra complexes in Western Australia and the Great Dyke in Zimbabwe) in which the halogen-bearing minerals are distinctly fluorian. Because of the wide variation in halogen concentrations in these two intrusions, they are ideal intrusions in which to study possible causes for these variations and their potential for economic metal transport.

This study has three major objectives:

1) Halogen variations in the Bushveld complex and related rocks: Halogen geochemistry below the platiniferous Merensky Reef of the Bushveld complex is still poorly known, as are halogen contents of the associated marginal sills. We propose to more fully sample these rocks to investigate the compositions of halogen-bearing minerals using electron microprobe. The associated sills are of particular interest as their compositions may preserve evidence of the original halogen concentrations of Bushveld magma(s). Modern magmas tend towards higher Cl/F ratios with higher overall volatile concentrations, suggesting that the parent magmas of the Stillwater and Bushveld complexes were "wetter" than commonly assumed. If so, this has important consequences for crystallization behavior and the transport of ore elements.

2) Cl isotopic characteristics: Preliminary Cl isotopes on a handful of Stillwater biotite suggest these fluids were derived from a crustal source (i.e., relatively light Cl) rather than degassed mantle as is characteristic of MORB (relatively heavy Cl). We propose to analyze and compare stable Cl isotopic characteristics with existing data for Sr, O and H isotopic trends for these two intrusions (particularly the Bushveld complex), their country rocks and associated contemporaneous sills and dikes. Along with petrographic and field evidence, this data should allow us to constrain models suggesting that high Cl is due to a mantle fluxing event, contamination or metamorphic fluid infiltration from the country rocks. Comparison with Cl-poor intrusions and other particular magma types (esp. boninites) will also test if Cl isotopes are distinct and if they have similarities with magmas believed to have formed by volatile fluxing of mantle source regions.

3) PGE-Halogen correlation: Evolution of Cl-rich fluids from intercumulus liquids could be a potentially useful transport mechanism for the platinum-group elements (PGE). It has been suggested that the cumulates below the principle ore zones in these intrusions were the source for the PGE in the deposits. Hence, we want to determine to what extent of background PGE (to be analyzed by ICP-MS) , S , base metals and Cl are correlated. These data will be combined with petrographic and geochemical studies, augmented with quantitative modeling of degassing, PGE transport and mineral-liquid-fluid equilibria, to test models of PGE transport and concentration.

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