PETROLOGY OF THE NON-FELSIC ROCKS ASSOCIATED WITH THE SILVERMINES GRANITE

Previous studies of a 1.3 meter-wide diabase dike, intruding the Silvermines granite along the St. Francois River in SE Missouri, identified plagioclase and magnetite as the primary minerals with lesser amounts of ilmenite, chlorite, serpentine, quartz, calcite, and pyrite. An unusual texture was also identified in the diabase consisting of small spherical structures rimmed with magnetite crystals surrounding finer-grained calcite. In this study, we analyzed 16 samples collected from eight non-felsic dikes related to the Silvermines dike swarm. The mineralogy of the dikes was consistent through most samples, with carbonate (8-48%), plagioclase (8-58%), olivine (tr-10%), magnetite (6-80%), pyrite and alteration minerals including sericite, occurring most commonly. In addition to the mineralogy, two unusual textures were identified. The first consisted of aligned plagioclase crystals in a matrix of approximately 80% magnetite. This texture could be interpreted as magmatic magnetite related to the emplacement of the dike swarm. The second texture included magnetite-rimmed carbonate-rich spherules similar to those identified in previous work. These spherules were found both in the 1.3 meter Silvermines dike and in two other dikes in the dike swarm. One sample contained what appears to be a magnetite spherule. Nine samples included quartz crystals with abundance ranging from a trace to 50%. Quartz crystals may be xenocrysts formed as a result of magma contamination by granite debris during emplacement of the dikes. The dikes of the Silvermines area are mafic, have been subsequently altered, perhaps by mineralizing fluids, and in some cases, magma may have encountered the proper conditions to evolve to a magnetite-rich composition.