EVIDENCE AND IMPLICATIONS OF INVASION OF A METAL-ENRICHED FLUID DURING MISSISSIPPI VALLEY-TYPE MINERALIZATION IN THE SOUTHWESTERN OZARK PLATEAU

The Ozark Plateau of the central U.S. hosts some of the world's best representatives of the Mississippi Valley-type (MVT) Zn-Pb deposit class. The region has thus been an important laboratory for testing hypotheses for MVT deposit formation, with results obtained from there likely having broad implications for the entire deposit class. While MVT deposits have long been recognized to be products of some type of sedimentary brines, more specific details of the nature of these brines, particularly the abundances of the ore metals and sulfur, and the manner in which these constituents were transported and precipitated have been more difficult to establish. Recent LA-ICP-MS work on fluid inclusions from the Tri-State and Northern Arkansas districts in the southwestern Ozark Plateau has yielded some important new insights. The results indicate that ore metal contents were low, generally on the order of a few ppm or less, during the gangue phases of mineralization that both preceded and post-dated main-stage sulfide mineralization. In Tri-State during sphalerite deposition, aqueous Pb concentrations rose to at least 35 ppm compared to mean concentrations of about 1 ppm during pink dolomite and quartz deposition. In Northern Arkansas, Pb concentrations during sphalerite deposition rose to even higher values of at least 370 ppm compared to a mean value of about 0.8 ppm during quartz deposition. Changes in Pb concentration are mirrored in both districts by changes in Mg and Br concentration, which are low during quartz deposition and high during sphalerite deposition. In addition, in Tri-State, sphalerite deposition is accompanied by a rise in K. Thus, sphalerite deposition in Northern Arkansas and Tri-State appears to have occurred from a pulse of fluid that differed in composition from the fluid(s) that deposited gangue, perhaps reflecting more highly evaporated seawater. Geochemical modeling shows that sulfide concentrations in the fluid that deposited sphalerite would had to have been at least about two to four orders of magnitude lower than Pb in Tri-State and Northern Arkansas, respectively. Based on mass balance considerations, the results favor a genetic model in which either metals were transported with sulfate and precipitated due to its reduction, or in which the metal-enriched fluid mixed with a sulfide-rich fluid.