Information Circular 41. Geochemical Investigation of Minor and Trace Elements in the Acid-Insoluble Residues of Lower Paleozoic Carbonate and Related Strata, Southeastern Minnesota-The Data Base

Abstract

The Upper Mississippi Valley mining district in Iowa, Wisconsin, and Illinois is one of several world-class lead-zinc mineral districts in developed Paleozoic strata of the northern Midcontinent. Although Mississippi Valley-type deposits vary considerably from district to district, they are defined as being predominantly sphalerite-galena replacement and vein deposits-including vug and breccia fillings-in carbonate host rocks. In general, they are restricted to certain formations; as such they are peneconformable but clearly crosscutting. Therefore, Mississippi Valley-type mineral deposits are epigenetic and stratabound, but not stratiform. Minor occurrences of base-metal sulfides also are present in calcareous and, to a lesser extent, in shaly and sandy rocks in a broad zone surrounding the main Upper Mississippi Valley lead-zinc district. These outlying occurrences are of special interest because similarities in form, distinctive mineral paragenesis, and sulfur and lead isotope systematics imply that they were cogenetic with mineralization in the main district. The outlying occurrences may represent remnants of fluid pathways associated with mineralization in the main district. To evaluate the extent of mineralization in southeastern Minnesota, we initiated a geochemical study that focused on the minor-and trace-element content of insoluble residues in carbonate rocks, using samples from drill holes and operating quarries throughout southeastern Minnesota (Fig. 2; Tables 4-52 in appendix). Regional geochemical studies of this kind have revealed, even in rocks that appear to be barren of sulfide ores, a suite of metals characteristic of Mississippi Valley-type mineral deposits. In particular, this suite includes Pb, Zn, As, Cu, Ni, Ca, Ag, and Mo. Geochemical analysis of insoluble residues from Paleozoic carbonate rocks has become an integral part of the assessment of mineral resources in the northern Midcontinent (Erickson and others, 1981, 1983; Mosier and Motooka, 1983; Viets and others, 1983). Insoluble residues are the materials remaining after calcium carbonate has been dissolved in a aqueous solution of 5:1 hydrochloric acid. Measurements of the minor-and trace- element composition of the residues provide a rapid, yet sensitive means of identifying regional ground-water flow patterns of metal-bearing brines. The method also appears to have some applicability in identifying previously unrecognized areas with Mississippi Valley-type lead- zinc deposits and, by extension, providing clues to possible locations of mineral deposits in southeastern Minnesota.