SUBCONTINENTAL SCALE FLUID TRANSPORT OF THE SULFIDE COMPONENT OF MISSISSIPPI VALLEY-TYPE ORES

The midcontinent of the U.S. hosts Zn-Pb mineralization in Paleozoic carbonate rocks. These ‘Mississippi Valley-Type’ districts together contain hundreds of millions of tons of sulfide ore minerals and pyrite. Transport of the Zn, Pb, and other metals was by a hydrothermal flow system that extended 100’s of kilometers driven by late Paleozoic Ouachita tectonism. The source of sulfide in the ores is, however, problematic, as is the site of sulfide generation; both local (district scale) and distant sources have been proposed.

An isotopic and petrographic study on over 400 pyrite-bearing ore fluid aquifer samples was conducted to determine the origin of the sulfide sulfur. Samples were from 31 core holes within an area of 25,000 Km². This ‘far-field’ pyrite is a product of the passage of ore-forming fluids because: 1) the pyrite is intergrown with ore-stage vug-filling dolomite and 2) lead isotope compositions of trace lead in the epigenetic pyrite are similar to ores. The far-field sulfur isotopic data show a pronounced maximum in the range –0 to –6 ‰, which overlaps the values for ore sulfides of the geographically dispersed Tri-State, northern Arkansas, and central Missouri Districts. This isotopically light component occurs throughout the entire Ozark region in huge quantities. The enormous mass and isotopic homogeneity of this sulfide argue for formation from a gigantic source. The most likely candidate is thermal decomposition of sedimentary pyrite to form pyrrhotite + H₂S at T>300 °C in clastic sediments of the adjacent Arkoma basin. There is a secondary maximum in the data in the range +6 to +18‰. This range overlaps the main stage ore sulfides of the Lead Belts of southeast Missouri. The isotopically heavy sulfide occurs over a large region but only in an aquifer that fed into the Lead Belts. This heavy sulfide formed by thermochemical reduction of sulfate migrating in this aquifer. The results of this study show that production of ore sulfide was by two distinct abiologic mechanisms and transport was by migrating hydrothermal fluids.