Compositions of pegmatoids beneath the J-M Reef of the Stillwater Complex, Montana, U.S.A.

Abstract

Pegmatoids sporadically occur in the cumulates of the Ultramafic series and Norite zone I and Gabbronorite zone I the Lower Banded series below the level of the platiniferous J-M Reef of the Stillwater Complex, but are less common in Norite zone II and Gabbronorite zone II above the reef. The pegmatoids are composed of the same minerals (plagioclase, orthopyroxene and clinopyroxene) as the host cumulates and in approximately the same proportions, but with grains that may be in excess of several tens of centimeters in length. Whole-rock compositions show the pegmatoids to be modestly enriched in lithophile incompatible major and trace elements relative to the host cumulates. Whereas average host cumulates of Norite zone I and Gabbronorite zone I have a whole-rock molar Mg (Mg + Fe) ratio of 81 ± 1.5 and a normative An content of 85 ± 1.5%, almost all pegmatoid samples have lower values with Mg (Mg + Fe) ratios as low as 71 and normative An content as low as 58%. These results are confirmed by electron microprobe analyses. Ratios of soluble/insoluble lithophile trace elements (e.g., Ba/Zr) show inconsistent trends. Excess sulfide in some pegmatoids strongly affects chalcophile-element abundance. If it is accepted that the pegmatoids define pods where late-stage volatile-rich fluids were locally enriched, or channel-ways by which fluids migrated up through the cumulate sequence, then it is clear that such fluids differ from the host assemblages only in that the fluids were in equilibrium with a slightly more evolved bulk-rock compositions. Calculated fluid compositions are relatively rich in Fe and Na and poor in Ca and Mg, and are consistent with the compositional features of the pegmatoids. The lack of significant difference in the ratios of soluble/insoluble lithophile trace elements between the pegmatoids and the host cumulates suggests that it could be difficult to detect geochemical signatures for the former presence of igneous fluid in nonpegmatoidal cumulate rocks using these elements alone.