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Trace Metal Geochemistry and Mineralogy of Diagenesis in the Cambrian Jordan Formation, Western Wisconsin

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

Presenting Author:

Annabelle Durbin

Authors:

Durbin, Annabelle¹, Lim, Arahan², Strand, Hallie³, Rougvie, James R.⁴, Zambito, James Joseph⁵

(1) Macalester College, Saint Paul, MN, USA, (2) Univ of California Berkeley, Earth and Planetary Sci, Berkeley, CA, USA, (3) Columbia University, New York City, NY, USA, (4) Beloit College, Beloit, WI, USA, (5) Beloit College, Beloit, WI, USA,

Abstract:

The Cambrian Jordan Formation is a quartz sandstone deposited in a nearshore marine setting, and is present throughout the Upper Mississippi River Valley where it is the youngest Cambrian unit. This study investigates post-depositional diagenesis within the formation by characterizing its mineralogy and geochemistry in western Wisconsin, Buffalo and Trempealeau counties. Using XRD, pXRF, and ICP-MS, we examined Liesegang-like banding, vug-like cavities, nodules, and related features to identify mineral phases and associated trace metals to develop a paragenetic sequence for diagenetic features.

The primary iron oxide present in the Liesegang-like banding, vug-like cavities, nodules, and related features is goethite. In the vug-like cavities it is present as cubic pseudomorphs, most likely after pyrite. While the mineralogy is similar, trace metal composition differs between these features, with the Liesegang-like banding containing locally elevated amounts of manganese (up to 1.8 wt.%). The pseudomorphs contain elevated (>1000 ppm) levels of zinc as well as small amounts of sulfur, which likely came from the original sulfide. Iron oxide nodules show elevated zinc and nickel concentrations. An iron oxide-filled burrow-like feature also contains elevated levels of zinc.

The presence of trace metals (zinc and sulfur) in pseudomorphs and iron oxides is indicative of a Mississippi Valley Type (MVT) mineralization. While the Liesegang-like banding contains elevated manganese, it lacks MVT trace metals, suggesting a different origin. These observations document several diagenetic events. The working hypothesis is that iron oxide nodules and pseudomorphs formed originally as sulfides from MVT-related mineralization and later oxidized. Based on cross-cutting relationships, silica and carbonate cementation events resulted in well-cemented lenses and concretions that occurred post-MVT mineralization. Field observations show Liesegang-like banding deflecting around calcite/silica-cemented concretions, indicating that the banding formed after the concretions.

By studying the geochemistry and mineralogy of the Jordan Formation in western Wisconsin, evidence of trace metal-bearing MVT minerals was found well north of the MVT mining district. However, their occurrence in the Jordan Formation is rare and their trace metal concentrations are relatively low, posing minimal environmental risk from natural or human processes.