Carbonatites are rare magmatic rocks composed of over 50 vol.% of carbonate minerals. They typically occur in a stable cratonic setting or in a rift-related environment and are often accompanied by varying amounts of ultramafic and alkaline rocks. Carbonatites commonly reveal enrichment in REE, Nb, P, and HFSE and are therefore considered as the major deposit-type for such commodities. Despite their economic importance, our knowledge of the genesis of these unusual rocks is scarce. Consensus exits about their origin in the upper or asthenospheric mantle via very low degrees of partial melting, but the wide range in compositions and mineralogy of individual complexes precludes a simple petrogenetic model.
The Blue River carbonatites (BRC) are the subject of my Ph.D. study and are situated within the Monashee Mountains of the Omineca Belt, south-eastern British Columbia, Canada. They intruded the metamorphosed clastic sediments and amphibolites of the Late Proterozoic (Hadrynian) Horsethief Creek group as lenses and sills of variable size. Beside high Nb2O5 (up to 1400 ppm) and phosphorus contents (~3.3 wt.% P2O5), the BRC are characterized by unusually high tantalum concentrations of ~ 250 ppm Ta2O5 (on average).
My study aims to explain the origin and magmatic evolution of the carbonatitic magma and the processes that lead to tantalum enrichment in these rocks, with particular focus on the Upper Fir occurrence. Detailed mineralogical investigations of the major ore phases (pyrochlore and ferrocolumbite) together with fluid inclusion studies of apatite are the most promising approaches. Information about the character of the source of the carbonatitic magma should be obtained from strontium isotopes and whole rock geochemistry. Depending on the progress and results of the above studies, oxygen and carbon isotopes might be included as well as geochronological investigations on zircon and monazite.
The BRC, together with numerous other carbonatites, occur in the so-called Carbonatite Belt of British Columbia, which parallels the Canadian Cordillera in the Omineca Crystalline Belt. The accumulation of potential REE and tantalum deposits requires a detailed and profound investigation and understanding of the carbonatite-alkaline rock systems in order to develop a successful exploration strategy. Results of this project will also contribute to the incomplete general knowledge about the genesis of this rare and peculiar type of magmatic rock.