--> ABSTRACT: Fluid/Rock Interactions and Fluid Migration in the Reelfoot Rift System, by Timothy J. Keller, Kevin L. Shelton, Jay M. Gregg; #91020 (1995).

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Fluid/Rock Interactions and Fluid Migration in the Reelfoot Rift System

Timothy J. Keller, Kevin L. Shelton, Jay M. Gregg

Pulses of hydrothermal basinal brines flowing through Cambro-Ordovician rocks in southern Missouri formed the large Mississippi Valley-type (MVT) deposits of the region and extensively altered large volumes of rock. These brines are thought to have emanated from one or more neighboring sedimentary basins. The potential role of the Reelfoot Rift as a source and/or conduit for these fluids, and associated MVT deposits was addressed in this study. The cathodoluminescent microstratigraphy and carbon and oxygen isotopes compositions of the rocks from the Reelfoot Rift were compared to those of rocks from the southeast Missouri (SEMO) ore district Observed similarities and/or differences were used to determine the role of the Reelfoot Rift in this hydrothermal system.

Rocks in the Reelfoot Rift contain four dolomite cement types: 1) A multiple generation cement similar to the cement associated with ore mineralization in the SEMO ore district (type I); 2) A simple two generation cement (type II); 3) A layered non-luminescent to dully luminescent cement (type III); 4) A brightly luminescent cement (type IV). The paragenetic sequence of the observed cements indicates that type III cement was deposited first followed by type II and finally type IV. Since type I was not observed in the same samples as the other cements, its paragenetic relationship cannot be determined.

The ^dgrv18O and ^dgrv13C values of cement types I, III, and IV from the Reelfoot Rift range from -9 to -6% (PDB) and -3 to 0% (PDB), respectively, while the isotopic values for type II cement range from -14 to -10% (PDB) and -15 to 5% (PDB), respectively. The ^dgrv18O and ^dgrv13C values of the host dolomite associated with these cements range from -10 to -5% (PDB) and 0 to 3% (PDB), respectively. The ranges of cement types I, III, IV and the host dolomite values generally correspond to the isotopic values of cements and host rocks associated with mineralization in SEMO while type II cement does not.

Although type I cement of the Reelfoot Rift and the SEMO ore district cement contain four similar luminescent zones, the stratigraphic placement of the middle two zones differs. Zones two and three from the Reelfoot Rift correspond to zones three and two respectively in SEMO or district. This difference, as well as the presence of other cement types in the Reelfoot Rift which do not occur in the SEMO ore district, could indicate a hydrothermal system which was isolated from the SEMO ore district. The isotopic similarity and difference provide further evidence that the hydrothermal system of the Reelfoot Rift was either isolated from or more complex than the hydrothermal system of the SEMO ore district.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995