MINERALOGICAL COMPOSITION OF DIABASE AND ALTERED DOLOMITE NEAR ANNAPOLIS, MISSOURI

Dolomites of Cambrian age are found throughout the St. Francois Mountains of southeast Missouri and often lie unconformably on top of the Precambrian basement rocks due to the paleo-topography during deposition. In some areas, there is distinct evidence of alteration and mineralization within the dolomites. One such location is found near Annapolis where the dolomite has been silicified and nearly all of the original dolomitic material replaced with quartz. A diabase dike is in close proximity to the silicified zone of the dolomite and may have provided the conduit for fluids producing the alteration. The purpose of this study was to examine samples from the relatively unaltered dolomite, the silicified dolomite, and the diabase in order to establish the mineral changes associated with the alteration. Several samples were collected from the area and organized from the more highly altered rocks to the least altered rocks. The rocks were analyzed in hand sample, thin section, and using x-ray diffraction. In the diabase, plagioclase was abundant in both hand sample and thin section. The XRD results confirmed the presence of plagioclase as labradorite and supported additional thin section data indicating magnetite. The highly altered dolomites exhibited original sedimentary laminations and varied in color from light purple to pink to cream to dark red including distinct veins. In thin section, this sample was very fine grained and the minerals that appeared to be present were magnetite and quartz. After XRD analysis it was concluded that quartz was present and the mineral tolovkite was present as well. The other four samples were dolomite that showed little alteration in hand sample but a slight variation in hardness. These contained laminations about 1-2 mm thick that varied in color from dark red to cream. All of these contained cream veins and within these were white crystals. The XRD results showed that dolomite, ankerite, and minrecordite were present in the dolomites.