MINERALOGY AND PETROLOGY OF DIABASE INTRUDING THE SILVERMINES GRANITE IN SOUTHEAST MISSOURI

A 1.3 m-wide diabase dike intrudes the Silvermines granite along the St. Francois River in Southeast Missouri. Thin section analyses of samples collected across the dike indicate that the primary mineral is plagioclase (~60%) with an abundance of opaque minerals ranging from 17-19%. X-ray diffraction analysis of the diabase provided additional information about the mineral composition. The plagioclase composition ranged from oligoclase to andesine and opaque minerals were primarily magnetite and ilmenite. Other minerals occurring within the diabase included chlorite, serpentine, quartz, calcite, and silver-bearing pyrite. Textures exhibited in the diabase are variable and unusual. Variable textures include changes in grain size from the chill zone to the interior of the dike from very fine to coarse. Arrangement of the plagioclase laths also varies with some areas having random orientation and others with nucleation centers resembling a daisystone texture. The most unusual texture observed in the diabase is the presence of small spherical structures. These spheres are rimmed with euhedral magnetite crystals and have interiors with a finer-grained texture than the surrounding matrix. Approximately 30% of these spherical structures contain fine-grained calcite at the core. Laser ablation within the spherical structures and acid testing of a polished surface confirmed the presence of calcite. Chemical compositions within the other spherical structures were determined by laser ablation and analysis using IC-PMS and gave highly variable results. The other spherical structures were igneous in composition but enriched in Ti compared to the general composition as expected with higher amounts of ilmenite. There are three possible explanations for the presence of these spherical structures. First, the spheres may represent droplets of an immiscible liquid during the later stages of cooling. Secondly, the spheres may represent xenoliths from the surrounding rock with significantly lower melting temperatures that melted and were preserved as droplets. Finally, the spheres may represent secondary mineralization of void spaces by hydrothermal fluids. Other similar structures have been identified in lamprophyric dikes as ocelli but differ in their composition and texture.