Lead Isotope Geochemistry of Mississippi Valley-Type Pb-Zn Deposits of the Ozark Region, U.S. Midcontinent: Constraints on the Origin of Ore Metals

Abstract:

The Ozark region of the U.S. midcontinent is one of the world’s most important provinces of MVT mineralization, hosting world-class ore deposits. The ores in the Tri-State (TS) and Northern Arkansas (NA) districts, dominated by sphalerite, are mainly hosted by platform carbonate rocks and vary in age from Ordovician and Mississippian for NA and Mississippian for TS. The deposits are considered to have formed from a regional hydrothermal flow system consisting of sedimentary brines discharged from the Arkoma basin and adjacent platform during the Late Pennsylvanian to Early Permian Ouachita orogeny.

New MC-ICP-MS Pb isotope analyses of sphalerites are presented in order to compare and contrast the isotopic signature of ores from the NA and TS districts with those from other MVT districts from central and eastern US and trace metal sources. The Pb isotope ratios of ores from the TS District (²⁰⁸Pb/²⁰⁴Pb between 40.7443 and 41.2626; ²⁰⁷Pb/²⁰⁴Pb between 15.8633 and 15.9571; ²⁰⁶Pb/²⁰⁴Pb between 21.8373 and 22.1956) plot in an area that is superimposed on the Pb isotope field defined by samples from the Central Missouri District, suggesting similar metal sources. The sphalerites are less radiogenic than samples from the Upper Mississippi Valley District, but more radiogenic than samples from any other MVT district.

Sphalerites from the NA District have lower Pb isotope values (²⁰⁸Pb/²⁰⁴Pb between 39.4633 and 40.8863; ²⁰⁷Pb/²⁰⁴Pb between 15.8216 and 15.9176; ²⁰⁶Pb/²⁰⁴Pb between 20.2396 and 21.6438) than the TS District ores; they plot below the field defined by samples from the Illinois-Kentucky district and overlap the field defined by ores from the Southeast Missouri (Viburnum and Old Lead Belt) district, implying similar metal sources. Current data suggest that basement of Grenvillian age (1 – 1.2 by), thought to be present in Arkansas, to the south of the Viburnum Trend, may be a likely source of the radiogenic Pb component. Pb data from ores in the NA and the Southeast Missouri districts are collinear with data from the TS and Central Missouri districts.