The Elk Creek Carbonatite Complex, Nebraska (USA)

Abstract:

The Elk Creek carbonatite complex (ECCC) is a large Early Cambrian carbonatite-alkaline syenite complex located in SE Nebraska (USA). The carbonatite and related rocks are buried by more than 200 m of Pennsylvanian marine sedimentary rocks and Quaternary till. The pre-Pennsylvanian sub-crop is crudely circular in plan-view and exceeds 30 km2, making it one of the larger carbonatite complexes in North America. The rocks of the complex were intruded in Precambrian granite and gneiss on the eastern margin of the Mid-Continent rift where it has been offset by one of a series of southeasterly trending structures. The primary rock type in the ECCC is dolomite carbonatite. The dolomite carbonatite ranges from fine-grained flow-banded dolomite to a coarse-grained rock comprising large prismatic dolomite crystals. The central portion of the complex comprises a pipe-like intrusion of magnetite dolomite carbonatite and magnetite dolomite carbonatite breccia. Magnetite dolomite carbonatite is typically fine-grained and contains angular or rounded elongate fragments of dolomite carbonatite. Fragments of magnetite dolomite carbonatite are also included in dolomite carbonatite and other carbonatite rocks in the complex. Emplacement of a discreet pulse of reduced, iron-rich carbonatite magma was, therefore, a likely early event in the evolution of the ECCC. The magnetite is altered locally to hematite and other iron oxides. The oxidation ranges from a dusting of hematite to pervasive alteration to hematite and ferric oxyhydroxides and occurs to depths as much as 630 m below the modern land surface. Other volumetrically important rock types include apatite dolomite carbonatite and barite dolomite carbonatite. Both of these rock types are localized largely along fractures, occur later in the intrusive sequence, and may reflect exsolution of phosphates and sulfates with decreasing temperatures. The magnetite dolomite carbonatite hosts significant pyrochlore mineralization. Microprobe analyses of pyrochlore can be used to characterize three generations of mineralization. The first generation comprises coarse Ta-rich pyrochlore. The second and third generations are near stoichiometric Nb-pyrochlore and Ti-enriched pyrochlore. Mineral grain size decreases from the first to the third generation.