Evidence for Hot Mississippi Valley--Type Brines in the Reelfoot Rift Complex, South-Central United States, in Late Pennsylvanian-Early Permian

The Ozark region of the United States midcontinent is host for the largest Mississippi Valley­type (MVT) lead-zinc province in the world. This region, located north of the Ouachita foldbelt and covering more than 240,000 km2, includes the world-class MVT districts of Old Lead Belt, Viburnum Trend, and Tri-State, and the smaller Northern Arkansas, Central Missouri, and Southeast Missouri barite districts (fig. 1). Leach and Rowan (1986) presented evidence that brine migration from the Arkoma foredeep (fig. 1), in response to Late Pennsylvanian­Early Permian orogenesis in the Ouachita foldbelt, was responsible for the formation of MVT deposits in the Ozark region. However, we are becoming increasingly convinced that a major component of fluid flow for the lead-rich deposits in the Viburnum Trend and Old Lead Belt in southeast Missouri was from the Reelfoot rift zone (Farr and Land, 1985; Farr, 1989a, 1989b; Erickson and others, 1988; Diehl and others, 1991; Goldhaber and Mosier, 1989; Viets and Leach, 1988, 1990; Horrall and others, 1993). In addition, Shelton, Bauer, and Gregg (1992) suggested the possibility of brine migration from the Illinois basin.

We report herein a reconnaissance fluid inclusion study of epigenetic sparry dolomite from Upper Cambrian and Lower Ordovician strata in two drill cores located in the Reelfoot rift complex, drill cores along the southeastern flank of the Ozark dome and the western flank of the Nashville dome, and samples from a rock quarry in northern Arkansas. In addition, conodont thermal-alteration studies were conducted where possible to provide additional thermal constraints. The fluid inclusion data, combined with conodont thermal-alteration studies, indicate that hot and highly saline brines were present in the Reelfoot rift complex during the Paleozoic. Cathodoluminescent (CL) zoning in sparry dolomite from the Reelfoot rift is similar to zoning in sparry dolomite in the Ozark region that is closely associated with MVT sulfide deposition. If the sparry dolomite studied in this report is temporally related to sparry dolomite in the Ozark region, then these data provide insights into the possible ore-fluid migration with respect to the tectonic history of the rift complex. Because many of our interpretations are based on previous studies of fluid inclusions and sparry dolomite in the Ozark region, we summarize the most important aspects of these earlier reports. In addition, we present a brief overview of the complex geology and tectonics of the Reelfoot rift to provide insight into possible tectonic implications of our data.



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USGS Professional Papers

For more information about this report contact: D. L. Leach

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