Date of this Version
Blessington, M. J., 2014, A Niobium Deposit Hosted By a Magnetite-Dolomite Carbonatite, Elk Creek Carbonatite Complex, Nebraska, USA [Master's thesis]: Lincoln, University of Nebraska, 88p.
The Elk Creek Carbonatite Complex (ECCC) is a large Early Cambrian carbonatite complex that intrudes Precambrian basement rocks in Southeast Nebraska. This dolomitic carbonatite complex includes a magnetite-dolomite rock with accessory barite, ilmenite, rutile, and quartz. This rock is identified by a characteristic enrichment in niobium due to accessory pyrochlore mineralization in the form of disseminations and inclusions in ilmenite and magnetite. Pyrochlore is also present in other carbonatite rocks in the complex as an accessory mineral with sporadic local high-grade intercepts in drill cores.
Carbonatite rock samples are characterized by transmitted-light microscopy, cathodoluminescent microscopy, backscattered electron imaging (BEI), and whole-rock geochemical analysis by XRF and ICP-AES. The typical texture for the magnetite-dolomite carbonatite is a matrix of fine-grained magnetite, ilmenite, barite, and dolomite with clasts of dolomite carbonatite; these clasts can appear as elongate lenticular bodies or angular fragments. Cross-cutting relationships with other carbonatite rocks in the complex show that the emplacement of this rock was an early event in the ECCC. Whole-rock chemical analysis indicates unusual HFSE geochemistry relative to other carbonatite rocks in the ECCC.The geology and chemistry of this rock is also unusual in comparison to carbonatite rocks worldwide. A discreet pulse of reduced, iron-rich carbonatite magma is proposed as the origin for the magnetite-dolomite carbonatite.
Pyrochlore grains in magnetite-dolomite carbonatite and in other carbonatite rocks in the complex are characterized by transmitted-light microscopy, BEI, and microprobe chemistry analysis. Observations drawn from transmitted light microscopy and BEI indicate three different types of pyrochlore present in the Elk Creek Carbonatite Complex: (1) 0.2-2 mm diameter euhedra with oscillatory zoning, (2) unzoned 10-100 μm diameter euhedra, and (3)μm diameter anhedra which occur as inclusions in ilmenite and magnetite. The chemistry of each of the three types of pyrochlore is distinct. Type 1 is enriched in Ta, Type 2 is near-stoichiometric pyrochlore, and Type 3 is enriched in Sr and Ti. The characteristic niobium enrichment in the magnetite-dolomite carbonatite is largely due to the presence of Type 3 pyrochlore. Type 3 pyrochlore formed early in the formation of the magnetite-dolomite carbonatite.
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