Mineralogy, petrology, geochemistry and origin of the Elk Creek Carbonatite, Nebraska

The Elk Creek Carbonatite consists of carbonate and silicate phases. The carbonate phase consists of massive carbonatite and carbonatite breccia. The massive carbonatite is composed of dolomite, apatite, opaque minerals, pyrochlore, and secondary quartz, barite and sulfides. The fragments in the breccia are single mineral grains and rock fragments, in a groundmass of fine dolomite, phlogopite, chlorite, barite and opaque minerals. The silicate phase consists of altered basalt, altered lamprophyre and altered syenite. The basalt has sharp contacts with the carbonatite, and usually retains a porphyritic texture. Phenocrysts are replaced by fine-grained dolomite, but retain euhedral shapes consistent with olivine and pyroxene. The matrix is composed of dolomite, phlogopite, chlorite, apatite and opaque minerals. The lamprophyre, a minette, has gradational contacts with the carbonatite, and is composed of biotite, orthoclase, apatite, opaque minerals, and secondary minerals (phlogopite, dolomite, chlorite, barite and quartz). The syenite is interlayered with the lamprophyre, and composed of orthoclase, biotite, opaque minerals, and secondary minerals (dolomite, clay minerals, barite and quartz). The massive carbonatite has major and trace element contents similar to an average magnesiocarbonatite, and is enriched in MgO, CaO, CO$\sb2$, Sr, Nb and Ba. The altered silicate rocks contain high CaO, MgO and CO$\sb2$, and relatively low SiO$\sb2$. They are enriched in Rb, Ga, Zr and Hf, but depleted in Sr. The massive carbonatite contains lower total REE than the average magnesiocarbonatite. The lamprophyre contains the highest REE; the basalt the lowest; and the syenite contains similar REE contents to the massive carbonatite. All rocks have similar REE distribution patterns with enrichment in LREE, while the silicate phase in the basalt has a pattern with a gentle slope. The Sr isotopes suggest a mantle source of the carbonatite. The distribution coefficients of some trace and rare earth elements between the carbonatite and the silicate rocks, and the spatial relationship of the rocks suggest that the syenite and the lamprophyre might be immiscibly comagmatic with the carbonatite, while the basalt is not