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Abstract

The Ordovician St. Peter Sandstone in the Illinois Basin has undergone complex diagenelic modification involving (1) early K-feldspar, dolomicrospar, and illite precipitation, (2) late quartz, planar and baroque dolospar, anhydrite, calcite, and illite cementation and (3) carbonate-cement and K-feldspar dissolution. In southern Illinois, burial reconstruction in combination with silicate mineral age dates indicate that late-diagenetic cementation in the St. Peter Sandstone occurred during deep burial (˜3,300 m) in Late Pennsylvanian and Early Permian time when major ore-forming (MVT) events were taking place in the region. Maturation kinetics suggest that precipitation temperatures at this depth were ˜140°C, assuming the major heat source was from the basement. The range of burial temperatures predicted for the St. Peter Sandstone (˜65°–140°C) compares closely with the temperatures of hydrothermal ore-forming fluids (˜80°–18O°C) suggesting the fluids involved in diagenesis (i.e., dolomitization and quartz and anhydrite precipitation) may have been part of the same (paleo)hydrologic system that caused MVT mineralization. In the shallow northern part of the basin, fluid inclusion data indicate ihat dolospar precipitated at higher temperatures (110°–115°C) than would be expected in these otherwise low temperature (<50°C) rocks. Provided these values are reliable, the St. Peter Sandstone was affected by a heat source that was not burial related. Hydrothermal fluids associated with the Upper Mississippi Valley District could account for these temperatures. Fluid inclusion and isotopic data indicate that the fluids involved in burial cementation throughout the basin were saline and comparable in composition to the brines responsible for MVT mineralization (˜20 wt% NaCl equivalent). In the absence of igneous activity, warm, topographically driven fluids (i.e., low temperature (<200°C) brines) moving updip from the southern tectonic margin of the basin can explain much of the dolospar and the associated mineral cements in the St. Peter Sandstone. 18O-depleted dolospar concentrated along the La Salle anticline in east-central Illinois suggest that this structural feature was a major conduit for the movement of these hot fluids through the basin. The ultimate source of the fluids may have been the Ouachita fold belt or the Reelfoot rift. There also is some evidence Ihat fluids were expelled from the Arkoma and Black Warrior Basins.

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