Abstract

Two small bodies of Precambrian basic intrusive rock in southeastern Missouri show uninterrupted differentiation of olivine diabase that produced ferrogabbro. Olivine diabase, foliated gabbro and coarse gabbro constitute the major textural rock types. Gravitational settling of plagioclase, olivine and Fe-Ti oxides occurred in both intrusives and iron enrichment is a conspicuous feature. Plagioclase ranges from An 58 to An 35 and olivine varies from Fa 36 to Fa 57 . Because these rocks were intruded into volcanic rocks at a substantial depth and were slowly cooled in an apparently closed system, they afford an opportunity to trace the differentiation trends exhibited by the disseminated sulfides. Pyrrhotite, chalcopyrite, pentlandite, sphalerite and cubanite are the most abundant sulfides deposited initially as unmixed sulfide solid solutions which were essentially coprecipitates with the major silicates. Paragenesis and abundance of primary sulfides are directly related to the differentiation parameters shown by the silicates. The extreme fractionation of these intrusives did not lead to a large concentration of any of the sulfides at any stage during consolidation.

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