RADIOGENIC PB ENRICHMENT IN MISSISSIPPI VALLEY-TYPE ORE DEPOSITS, SOUTHERN OZARKS: CONSTRAINTS BASED ON GEOCHEMICAL STUDIES OF POTENTIAL SOURCE ROCKS

Mississippi Valley–type (MVT) deposits provide a substantial amount of the worldwide Pb and Zn resources. MVT ores are generally located where organic-rich shales can onlap platform carbonate margins along the flanks of sedimentary basins and are often recognized by the presence of hydrocarbon fluid inclusions and interstitial hydrocarbons.

Debate abounds in terms of the source(s) of these metals. The southern Ozark MVT ores are enriched in radiogenic Pb and their Pb isotopic signatures define a linear trend, suggesting mixing of Pb from two distinct end-member components. While one end-member must be highly radiogenic, the other is less so, with Pb isotope ratios equal to, or lower than, the lowest values recorded for ores. Sandstones of the Jackfork Sandstone (Pennsylvanian) and shales of the Chattanooga Shale (Devonian) both appear to represent the lower radiogenic end-member. Cherts from the Cotter Dolomite (Ordovician) and tripolitic chert from the Boone Formation (Mississippian), both hosting some of the ores, contain highly radiogenic Pb and exhibit isotopic ratios comparable to those of ores. However, most cherts have lower 208Pb/204Pb and 207Pb/204Pb for a given 206Pb/204Pb compared to those of ores. These relationships demonstrate that the highly radiogenic end-member cannot be attributed to the host lithologies, suggesting that the source of high ratios likely lies further afield.

An alternative model for the MVT deposits recognizes that organic-rich shales are the only source of Pb. Many organic-rich shales have high U and organic matter content (uraniferous oil shales). Given the array of organic molecules, each with their own thermochemical range, and how metals can be associated with them, the release and deposition of metals typically covers a varying range. Thus, the compositions of the released fluids changes through time, recording the isotopic composition of the released metals through that interval. Mineralization derived from a dynamically evolving fluid may show apparent end members, without the need to call on mixing of fluids from separate sources. Moreover, the slope of the linear trend defined by the Pb isotope ratios of ores corresponds to an age of about 1.2 Ga. Therefore, another explanation for the linear array likely involves the Precambrian basement as supplying the Pb in the southern Ozark MVT deposits.