Evaluation of the Carbonate Resources of Southern Minnesota

Abstract

The Paleozoic carbonate deposits in southeastern Minnesota, consisting of several formations ranging from limestone to dolomite, constitute an important mineral resource of the state. They are quarried quite extensively for aggregate, aglime, and dimension stone, but new uses being developed for carbonate rocks often require greater adherence to specifications for chemical composition than is now required. Because no comprehensive study of chemical compositions of the several formations was available and because this information must be provided to potential producers and consumers for expansion of the industry, this evaluation by the Natural Resources Research Institute (NRRI) was approved for funding by the Minerals Coordinating Committee (MCC) through the Department of Natural Resources (DNR). The duration of the project was from October 1, 1987, to June 30, 1989, and the amount of funding was $168,300. Because no sample materials from previous studies were available for additional chemical analyses, most samples for this evaluation were taken by channel sampling, diamond drilling, or sampling split core from two drill holes that are stored by the DNR. The Minnesota Geological Survey (MGS) provided county maps with possible sampling sites keyed to references and provided consultation and background information on the various carbonate formations and their general characteristics. Channel sampling sites were selected to provide reasonable geographic distributions of the formations and maximum stratigraphic sections of fresh rock. Sample sites were briefly described and continuous samples taken with hammer and chisel over the sections. Channel samples were taken at 45 locations over 1423 vertical feet to provide 196 assay intervals. Diamond drill core sampling was undertaken to provide samples of formations at locations not well exposed for channel sampling and where high vertical faces were not otherwise accessible. The 17 holes at 15 locations totaled 1724 vertical feet and provided 300 assay sample intervals. The MGS supervised drilling, collected and logged core, and provided detailed stratigraphic descriptions. The core was split and one half is stored by the DNR in Hibbing. All samples were crushed, pulverized, and analyzed at the Coleraine Research Laboratory (CRL). Chemical analyses were run on 615 samples representing 3774 feet, and differential thermal analyses (OTA) were run on 307 samples representing 2236 vertical feet. Most samples assayed for this evaluation are classed as dolomites with an encouraging number containing more than 90.0% CaC03 + MgC03 over intervals of more than 20 feet. Most of these locations would require selective mining practices and/or beneficiation to produce moderate or high purity carbonates. Formations of the Devonian System were sampled only in Mower County where both the Cedar Valley and the Spillville Formations trended to higher purity from west to east. A high purity unit of the Stewartville Formation, approximately 50 feet thick, is present in Olmsted and Fillmore Counties. Many Prosser Formation sections are impure dolomites to dolomitic limestones, although several 15- to 45-foot thick moderately pure units do occur. The only true limestones (less than 10% of the carbonates as dolomite) noted in the Prosser Formation were sampled in Fillmore County. Most Platteville sections sampled are classed as impure dolomites· to dolomitic limestones often with relatively high sulfur contents. Only in Fillmore County was the entire Platteville section classed as limestone, and it was also impure. Most Shakopee Formation samples were impure dolomites, but one 30-foot section in northwestern Scott County was exceptionally pure with 95.4% CaC03 + MgC03• The Oneota Dolomite is one of the most prominent and extensively quarried formations in the project area. In the West from Mankato northward, it is classed as impure dolomite with very few thin units that are not impure. However, in the eastern counties, the Oneota includes many 20- to 80-foot sections that are moderately and even highly pure. Additional evaluations of Minnesota carbonate rocks should include benefication testing to develop methods of producing carbonates to meet consumer specifications for new applications. Selective mining, Munsell color determinations, and relationships between calcining characteristics and differential thermal analyses should be further investigated.