Trace-Metal Bearing Sulfides and Oxides Constrain Mississippi Valley Type Mineralization in the Cambrian St. Lawrence Formation, Western Wisconsin

Session: 37th Annual Undergraduate Research Exhibition Sponsored by Sigma Gamma Epsilon (Posters)

Presenting Author:

Simone de Montigny

Authors:

de Montigny, Simone¹, Diesl, Madeleine², Goldstein, Arin³, Hueper, Samantha⁴, Rougvie, James R.⁵, Zambito, James Joseph⁶

(1) Beloit College, Beloit, WI, USA, (2) Mount Holyoke College, South Hadley, MA, USA, (3) Amherst College, Amherst, MA, USA, (4) Pennsylvania State Univ, Dept of Geosciences, University Park, PA, USA, (5) Beloit College, Beloit, WI, USA, (6) Beloit College, Beloit, WI, USA,

Abstract:

Mississippi Valley Type (MVT) deposits, found throughout southwest Wisconsin and into Illinois, Iowa, and Minnesota, are characterized by iron, lead, zinc, and copper sulfides, barite, and rare cobalt and nickel minerals. This study aims to identify and characterize MVT mineralization in the Cambrian St. Lawrence Formation, an interbedded dolostone and siltstone in Buffalo and Trempealeau counties of western Wisconsin, well north of the MVT mining district. Iron oxide and sulfide-bearing vugs and concretions throughout the St. Lawrence were evaluated using XRD, SEM, ICP-MS, and pXRF analyses. The occurrence of trace metal-bearing MVT sulfides and sulfide weathering product oxides in this unit is significant because the underlying strata are a regional groundwater aquifer.

St. Lawrence outcrop localities and core samples contain scattered iron oxide- and sulfide-bearing vugs and concretions; iron oxides are primarily composed of goethite, and the sulfides are predominantly pyrite. Goethite occurs locally in cubic crystal habits interpreted as pseudomorphs after pyrite in vugs. Trace metal concentrations in these minerals are variable, with some vugs and concretions displaying low trace metal values. Some outcrop and core samples show elevated (>1000 ppm) trace metal concentrations, particularly of lead, zinc, arsenic, copper, and nickel. Copper and barium are present in moderate amounts compared to the other trace metals. Iron oxide and sulfide-filled vugs examined are relatively higher in lead, zinc, arsenic, and nickel as compared with iron oxide and sulfide concretions. This aligns with previous findings that lead and zinc-bearing sulfides are found in vugs and fractures in the MVT mining district. The trace metal composition in both concretions and vugs is indicative of MVT mineralization, and suggests a post-Cambrian origin for these features.

This study extends the known distribution of MVT mineralization to the north of the mining district, and may suggest that MVT mineralization also occurs throughout underlying Cambrian sandstone aquifer units. The presence of elevated trace metals in oxides and sulfides in the St. Lawrence Formation within Buffalo County provides insight into potential impacts on groundwater resources. As oxidation of sulfide minerals occurs, trace metals may be liberated; therefore, redox condition changes due to water table fluctuations are possible sources of groundwater contamination. To mitigate this issue, private well owners should case their wells below the water table to avoid trace metal contaminants.