MINERALOGY AND ORE TEXTURES OF SULFIDES AT THE DECATURVILLE METEORITE IMPACT STRUCTURE IN SOUTH-CENTRAL MISSOURI

The Decaturville structure in central Missouri is circular in plan view, about 6 km in diameter, consists mostly of folded, faulted, and brecciated Paleozoic sedimentary rocks, and has been interpreted to be a meteorite impact structure by recent investigators. The presence of shatter cones, high temperature and pressure quartz, and planar deformation features in quartz support the meteorite impact interpretation.

An ore microscopic investigation of sulfide-rich breccia samples from a sulfide-rich pit in the central uplifted dome at Decaturville was undertaken to determine the mineralogy and textures of the abundant sulfides. The sulfide minerals present, in order of abundance, are marcasite, pyrite, sphalerite, and galena. Pyrite and especially marcasite typically occur in colloform rhythmically banded mineral intergrowths. The mineralogy and textures are characteristic of MVT mineralization. The ratio of sphalerite to galena and presence of jasperoid are similar to the ores of the Tri-State District (Oklahoma, Missouri, Kansas), and are unlike those MVT ores in the Viburnum Trend and elsewhere in the Southeast Missouri Lead District. Although some had speculated that the sulfide-rich rocks at Decaturville could represent a Paleozoic mud volcanoe, the abundance of sphalerite and galena is more like Mississippi Valley-type (MVT) mineralization than what would be expected for a mud volcanoe.

All four sulfides exhibit evidence of intense brecciation. This indicates that the sulfide mineralization did not seek the Decaturville structure as a structurally favorable site for deposition as speculated by some, but rather that the sulfide mineralization was already present at the time of meteorite impact. There is no evidence for multiple periods of brecciation, and all of the observed sulfide textures can be ascribed to a single period of brecciation. Alteration of the sulfides to supergene goethite is common. Although some of the shapes of grains of goethite had been interpreted to represent pseudomorphs after primary magnetite, no grains of the primary iron minerals, magnetite, hematite, ilmenite, nor pyrrhotite were observed.

The brecciation of late Pennsylvanian-Permian Missouri MVT mineralization constrains the age of the Decaturville meteorite impact to post-late Pennsylvanian-Permian.