U-Th-Pb Systematics in Dolomites and Whole-Rock Dolostones from Burlington-Keokuk Formations, Iowa, Illinois, and Missouri

John A. Hoff, Gilbert N. Hanson, William J. Meyers

Although the U-Th-Pb system is a powerful geochemical tracer, there has been little application to carbonate diagenetic systems, primarily due to difficulties in analyzing trace quantities of lead. Procedures developed in this study have allowed routine analysis of 20 mg samples with lead concentrations of 1 ppm. These procedures used trace element and isotopic data on high-purity dolomite separates and whole-rock dolostones from the Burlington-Keokuk formations (Osagian).

Lead concentrations in the whole-rock dolostones (those analyzed were > 90% dolomite) ranged from 3 to 10 ppm. Although dolomite is the dominant mineral present in these whole rocks, in certain samples a significant proportion of the lead resides in other phases. To place constraints on the origin of lead in the dolomites, it has been necessary to separate carefully the dolomite from the whole rocks prior to analysis.

The present isotopic composition of lead in the earliest generation of dolomite is similar to that reported for other sedimentary rocks in the Mid-Continent region and initial isotopic compositions (calculated at 340 Ma) approximate model u = 8 (²³⁸U/²⁰⁴Pb) and k = 4 (²³²Th/²³⁸U) growth curves. At present, the source of lead in this generation of dolomite cannot be strictly identified. Possible sources include the precursor carbonates and surrounding sedimentary units, as well as basement granites. However, the relatively high ²⁰⁷Pb/²⁰⁴Pb and low ²⁰⁶Pb/²⁰⁴Pb of initial lead in these dolomites suggests that they do not contain a significant component of MVT-like lead found in lead-zinc deposits surr unding the Illinois basin.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.