Field trip guidebook to the Upper Fir carbonatite-hosted Ta-Nb deposit, Blue River area, east-central British Columbia
R.S. Rukhlov, T.C. Chudy, H. Arnold, D. Miller
Carbonatites are igneous rocks containing abundant primary carbonate minerals. These rare rocks generally form in intracratonic settings as part of crustal-scale dome and rift systems. Historically regarded as petrogenetic curiosities, recent interest in strategic metals has led to significant exploration for carbonatites. In the Canadian Cordillera, carbonatites were emplaced episodically, at ca. 810-700, 500, and 360-330 Ma, forming part of the British Columbia alkaline province, which defines a long (at least 1000 km), narrow (ca. 200 km) orogen-parallel belt. The ca. 810-700 Ma and 500 Ma carbonatites were injected during protracted breakup of the supercontinent Rodinia and passive margin development on the western flank of Laurentia. In contrast to these and to most carbonatites globally, the 360-330 Ma carbonatites, such as the Blue River area examples, are unusual. They were emplaced near the continental margin during subduction rather than in the cratonic interior during continent breakup. The Blue River carbonatites include at least 18 carbonatite and 2 alkaline, silica-undersaturated-rock occurrences. This field trip considers the characteristics, magmatic evolution, and mineralization of the Blue River carbonatites as represented by the Upper Fir complex, which hosts one of the largest and best studied Nb-Ta deposits in the Canadian Cordillera. Exploration by Commerce Resources Corporation at the Upper Fir complex established an NI 43-101-compliant resource of 48.4 million tonnes (Indicated) averaging 1,610 ppm Nb2O5 and 197 ppm Ta2O5 plus 5.4 million tonnes (Inferred) averaging 1,760 ppm Nb2O5 and 191 ppm Ta2O5. Participants will examine outcrops and drill-core sections of amphibolite-grade metacarbonatites, related metasomatic rocks, syntectonic pegmatites, enclosing pelites and amphibolites of the Mica Creek assemblage (750-550 Ma), and Mesozoic-Cenozoic Cordilleran structures. We consider the tectonometamorphic overprinting of igneous features in the Upper Fir carbonatites, as recorded by paragenetic relationships, mineral chemistry, recrystallization, and retrograde deformation.
Isotopic evidence and spatial and temporal associations with large igneous provinces containing ultramafic and alkaline silicate rocks indicate derivation of carbonatite magmas from sub-lithospheric mantle plumes. New isotopic and elemental compositions of minerals from Blue River carbonatites and related rocks are indistinguishable from worldwide carbonatites generated by such deep-mantle plumes. The 360-330 Ma Cordilleran examples formed along the western margin of Laurentia while subduction was taking place immediately to the west. Lithospheric extension related to this Late Paleozoic subduction is considered responsible for rifting the continent margin and initiating the Slide Mountain ocean as a back-arc basin. We suggest that this same back-arc extension triggered emplacement of the Blue River carbonatites, which were derived from a long-lived, deep-level mantle plume that was tapped episodically since the Neoproterozoic.
View GeoFile 2018-6 (PDF, 7.9 MB)
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