GEOCHEMICAL EVIDENCE OF A GENETIC LINK BETWEEN BLACK SHALES AND MISSISSIPPI VALLEY-TYPE ORES IN THE OZARK REGION

Organic rich, black shales have long been known to be a source for hydrocarbons. These shales may also be a source for Pb-Zn deposits. Fluids expelled from the shales, both hydrocarbons and mineralizing fluids, are likely traveling along the same fluid pathways, and may even be migrating simultaneously. Mississippi Valley-Type (MVT) Pb-Zn deposits are often found in close proximity to hydrocarbons and their presence may be related. MVT deposits occur along the peripheries of sedimentary basins and are typically hosted within carbonate rocks. The saline fluids responsible for creating the ores are sourced from within the adjacent sedimentary basin, are of low temperature (around 60ºC – 150ºC), and may flow hundreds of km through the basin before precipitating the ores. The hydrocarbons are often concentrated in different areas of the host basins than the sulfide ores; however, ores in some deposits are characterized by hydrocarbon and brine fluid inclusions. The late Permian ores in the Tri-State (TS) and Northern Arkansas (NA) mining districts are hosted by platform carbonate rocks and vary in age from Ordovician and Mississippian for NA and Mississippian for TS. Sphalerite samples and black shales of the Chattanooga Shale and the Fayetteville Shale were collected from strategic locations within the TS and NA mining districts. Mineral separates and whole-rock shale samples were analyzed for their trace element concentrations and Pb isotopic ratios in order to compare and contrast them and evaluate the source of metals. The analyses were conducted on a Thermo Scientific iCAP Q ICP-MS (elemental concentrations) and high-resolution Nu Plasma MC-ICP-MS (isotopic ratios) at the University of Arkansas.

Current results show that there is a genetic link between the organic rich, black shales and nearby MVT deposits. Organic rich, black shales may represent the source for the metals due to their anomalously high metal content in addition to the fact that shales have the most connate fluid associated with sediments before compaction. Very dense sodium-calcium-chloride basinal brines, also called “oil-field” brines due to their occurrence with oil in many basins, carry the mobilized metals. The saline fluids may have been expelled from the Arkoma Basin during the Ouachita orogeny.