Mineralogical Composition of Diabase and Altered Dolostone from the St. Francois Mountains near Annapolis, Missouri, USA

Abstract

Dolostones of Cambrian and Ordovician age are found in the St. Francois Mountains on the Ozark Plateau in southeastern Missouri and often lie unconformably over Precambrian basement rocks due to the paleo-topography. In some areas, there is distinct evidence of alteration and mineralization within the dolostones. One such location is found near Annapolis, Missouri where nearly all of the original dolomitic material is replaced with quartz. A diabase dike is in close proximity to the silicified dolostone and may have provided the conduit for fluids during alteration. The purpose of this study was to examine samples from the relatively unaltered dolostone, the silicified dolostone, and the diabase in order to establish the mineral changes associated with the alteration. Several samples were collected and analyzed in hand sample, in 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 evidence of magnetite as well. The highly altered dolostone exhibited original sedimentary laminations and varied in color. In thin section, the altered dolostone was very fine grained and the minerals that appeared to be present were quartz and an opaque mineral. After XRD analysis it was concluded that quartz was present and a pyrite structured sulfide mineral was present as well. The other four samples were dolomitic with laminations that varied in color from dark red to cream. XRD results indicated that dolomite, ankerite, and minrecordite were present in the dolostone.