The petrology and petrogenesis of the Ren carbonatite sill and fenites, southeastern British Columbia, Canada

• Main Author: Ya'acoby, Avee
• Language: English
• Published: University of British Columbia 2014
• Online Access: http://hdl.handle.net/2429/46653

This thesis explores in detail the petrology and petrogenesis of the Ren carbonatite sill and associated fenites, located in the Monashee mountain range of British Columbia. The carbonatite body and fenites have been significantly deformed and metamorphosed, which has provided a unique petrological research opportunity, since only a few other carbonatite occurrences worldwide have been described from highly metamorphosed orogenic settings. This work aims to address knowledge gaps pertaining to the petrology, petrogenesis and economic exploration of comparable carbonatites in similar geotectonic settings. The effects of amphibolite facies regional metamorphism and progressive deformation are apparent throughout the carbonatite body and adjacent fenites. Many of the rocks display remobilized, passively mixed components, boudinaged structures, and rheomorphic bands, as well as foliation and porphyroblastic textures. Recrystallization of minerals at peak metamorphic conditions (580–730 °C) is indicated primarily in undifferentiated calcite for which metamorphic solvus temperatures (~690 °C) were derived. Other minerals related to metamorphic recrystallization include rims of monazite-(Ce) around earlier fluorapatite, interstitial REE-silicates, and Ca- and Mg-amphibole forming late in the paragenetic sequence, after primary silicates. Despite the metamorphic overprint and alteration, many petrological features pertaining to the igneous paragenetic record have been preserved, such as textures of primary minerals, compositional trends in phlogopite, clinopyroxene and amphibole, and whole-rock geochemistry of rock units. Solvus temperatures (~760 °C) of calcite, higher than peak metamorphic conditions, highlight preservation of the igneous component. The carbonatite is inferred to be a product of primitive mantle melts(s) that did not undergo significant fractionation processes, and intruded the crustal environment relatively undifferentiated. The whole-rock compositional trend of the fenites and partially fenitized host rocks suggests sodic-potassic alteration of the country rock during emplacement of the carbonatite sill. The Nb-Ta and REE mineralizations of the Ren occurrence are both insufficient for economic extraction. Nevertheless, rare and new mineral candidates, (Fe,OH)-analogue to västmanlandite-(Ce) and (Mg)-analogue to biraite-(Ce), discovered in the deposit by the author, emphasize its petrological and mineralogical significance.