Project: The petrogenesis of the Fir carbonatite system, east-central British Columbia
In my Ph.D. project I am investigating the petrogenesis of the Blue River carbonatites (BRC) that are situated within the Monashee Mountains of the Omineca Belt, south-eastern British Columbia. They intruded metamorphosed clastic sediments and amphibolites of the Late Proterozoic (Hadrynian) Horsethief Creek group and now form variably deformed and folded lenses and sills of up to 80 meter in thickness.
Carbonatite-alkaline rock complexes are known to host a range of commodities and they attracted much notice in the past four years because of the strong demand for rare earths elements. The main focus of my studies lies on the Fir carbonatite system which stands out in particular for its high Ta content (weighted average of 200 ppm Ta) which is unmatched by any other carbonatite occurrence in the Canadian Carbonatite belt or even worldwide, at least with respect to the consistency of grade.
The accumulation of particular elements in a carbonatite is controlled by a variety of magmatic processes and the tectonic setting in which their parental melts are generated. Contrary to the widely known textbook opinion, which chiefly describes a central-type complex of carbonatites and associated ultramafic-alkaline rocks that intruded along crustal-scale lineaments within a consolidated craton (e.g., Winter, 2001), a steadily growing number of different tectonic settings has been discovered to host carbonatite complexes (e.g., Chakhmouradian et al., 2008). Further diversification of our carbonatite understanding provided a mineralogic-petrogenetic classification which is based on the distinction of several different clans, all described by a particular mineralogy/mineralization (e.g., Mitchell, 2005). This diversity obviously precludes a generalization of the petrogenesis of carbonatites which is particularly true with regard to the geochemistry and the mineral chemistry (for latter see Reguir et al., 2011). It is thus necessary to investigate each occurrence on its own merits in order to establish potential links or indicators for the enrichment of strategically important metals.
My petrogenetic-mineralogic investigations encompass (1) the petrography of all units potentially related to the Fir carbonatite system including various types of fenites and carbonatites, (2) a fabric analysis of the carbonatite textures, (3) (limited) whole rock geochemistry of various carbonatite units and fenites, and (4) the mineral chemistry of rock-forming and accessory phases with an emphasis on the Nb-Ta mineralization.
The high degree of deformation in the Canadian Carbonatite belt certainly adds more complexity to the matter and has been addressed in the first part of the Ph.D. project. Through petrographic investigations focusing on the (micro-)fabric evolution a general sequence of events has been inferred. The oldest texture reflects a high-grade metamorphic event which transformed the carbonatite into a gneissic rock. This texture has been overprinted by retrograde shearing and folding which resulted in strongly mylonitized rocks. The mylonitic shear zones are a very important piece of information that can help unravel the complex deformational history of this carbonatite system which is also a requirement for a realistic deposit model.
The petrographic findings have been followed up by geochemical investigations, which focused on mineralogically different units (e.g., calcite vs. dolomite carbonatite) but included also the textural aspect (e.g., gneissic vs. foliated). One interesting outcome of this investigation is that mylonitic shear zones are preferentially developed in a dolomite carbonatite unit with a particular geochemical composition and mineralization, which is not easily detectable in hand specimen.
Mineralogical investigations including microprobe analyses revealed that the differences in the major carbonatite units are also reflected at the accessory level as evidenced by variations in the composition of the Nb-Ta phases and their paragenesis. This knowledge allows a discrimination of various ore-type zones at the deposit scale by using specific geochemical signatures and it allows to evaluate these variations with regard to the magmatic evolution.
The results from the different parts of this Ph.D. project are already a valuable contribution to the continuous improvement of the exploration process and resource development including metallurgy, since they directly relate to aspects such as structural modelling of the deposit, ore-type distribution and ore composition. Another important aspect is the potential alteration accompanying high-grade deformation and mylonitization, which directly relates to the ore distribution, especially in deposits in the Canadian Carbonatite belt. These results are also very encouraging from the academic point of view and implicate further detailed petrogenetic studies. The goal is to elaborate the mineralogical characteristics and a potential clan association of this carbonatite system in order to propose a genetic model and improve future exploration for strategic metals.
Roundup 2013 Poster: The Petrography, Geochemistry and Mineral Chemistry of the Fir Carbonatite System, east-central British Columbia: An Update of Current Knowledge with Implications for Deposit Modelling