Sulfur Isotope Variations in the Elk Creek Carbonatite Complex, Southeastern Nebraska, USA

The Elk Creek Carbonatite Complex (ECCC) comprises eight different units of carbonatite and silicate rocks, including a magnetite-dolomite carbonatite that hosts the largest niobium deposit in the United States. This study focused on interpreting the first set of sulfur isotopes from this system to decipher between magmatic and secondary processes. Using sulfur isotope variations I was able to discriminate between hypogene and supergene mineralization, and measure (1) formation temperatures (2) possible sulfur source (3) degassing effects on isotopic composition, and (4) post-magmatic alteration. The groundwork for mapping and interpreting iron oxide alteration is also laid out in this thesis. Hypogene mineralization displays isotopic fractionation between sulfate and sulfide minerals similar to other carbonatites worldwide. The δ34S values for primary mineralization range from +1.77‰ to +11.06‰ for sulfates and -10.85‰ to +2.31‰ for sulfides. Sulfide-sulfate mineral pairs yielded equilibrium formation temperatures ranging from 660°C to 730°C. In addition to temperature, δ34S values for bulk sulfur (δ34SΣS) were determined. The calculated range for δ34SΣS is -2.23‰ to -1.64‰, which is representative of a mantle source. Degassing during magma evolution altered sulfide-sulfur isotope composition. As a result of H2S outgassing, several pyrite samples were enriched in 34S having δ34S values of +0.6‰, +2.26‰, and +2.31‰. These values are more enriched in 34S compared to hypogene sulfide. Supergene sulfate has δ34S values ranging from -11.89‰ to -4.31‰. These values are significantly depleted compared to hypogene sulfates. In addition to δ34S values, 87Sr/86Sr values were used as evidence that supergene alteration occurred. Three anomalously high 87Sr/86Sr values have values similar to Pennsylvanian seawater and sedimentary rocks. Iron oxide alteration is prevalent in the ECCC, and could be a potential issue for future mining projects. Thick iron oxide veins could be a serious problem for the stability of an underground mine. When altered, by hematite or limonite, the carbonatite rock becomes friable and brittle. Identifying and mapping these veins would be of utmost importance to mine construction. Advisor: Richard M. Kettle