SOLUTION-COLLAPSE BRECCIA DEVELOPMENT AND PORE-FILLING, LOWER ORDOVICIAN KNOX GROUP, TENNESSEE-KENTUCKY

Widespread solution-collapse breccia networks, initiated by a ~10-My period of subaerial exposure following deposition of the Lower Ordovician Knox Group carbonates, extend throughout the Appalachians and into the continental interior of North America. These networks host strata-bound zinc and related MVT mineralization precipitated from basinal brines generally linked to late Paleozoic orogenic events. A regional understanding of porosity development and cementation of Knox Group carbonates is of economic importance as well as a monitor of regional fluid flow. Knox strata are exposed in the imbricate thrust fault belts in eastern Tennessee, where they dip from 10 to 40°. The essentially horizontal Knox Group carbonates in central Tennessee and southern Kentucky are covered by a minimum of 90 m of younger strata, but exploration core and mines provide regional samples. In areas where exposure-related subsurface solution was the dominant process, the amount of collapse is considerable with either strata-bound breccia zones or local “break-through” breccia bodies that affect 100m or more of upper Knox strata and may extend upward into Middle Ordovician units. The upper part of the late breccia systems had high interblock and fracture porosity that has been partially cemented by carbonate, sulfide, and related minerals. Breccias commonly are cemented by carbonate minerals with varying amounts of remaining porosity; only localized areas have commercially significant amounts of sulfide minerals. Breccia characterization (enhanced by X-ray computed tomography) and regional cement microstratigraphy (quantified by LA-ICP-MS geochemistry), support a varied history of breccia formation and cementation in the region.