Rare Earth Fluorocarbonates in Carbonaceous Shales of the Ishlya Graben (Western Slope of the Southern Urals)

The aim of this work is to characterize rare earth fluorocarbonates, first discovered in carbonaceous shales of the Ishlya graben, and to develop an acceptable genetic model of their formation.

Riftogenic formations are represented by interbedding of terrigenous rocks (carbonaceous shales, siltstones, siltstones) with a volcano-plutonic association (gabbrodolerites, basic effusives with a small amount of pyroclastic material). All rocks are metamorphosed under epidote-amphibolite facies conditions (T = ~ 390–490 °C, P = ~ 2.5–10 kbar).

Bastnaesite-(Ce), hydroxylbastnaesite-(Ce), parisite-(Ce) and synchysite-(Ce) were found in carbonaceous shales, morphologically represented by single xenomorphic segregations, veinlets of various morphologies, microgranular masses in selvages of carbonate and quartz-carbonate veinlets, aggregates filling voids in pyrite crystals and spherulites composed of long-prismatic, needle-shaped crystals. It is shown that the genesis of rare-earth fluorocarbonates is caused by hydrothermal metamorphism with a change in the chemical composition of the fluid (CaCl₂ + NaCl → FeCl₂). In this case, the source of CO₂ was the oxidation of organic matter in carbonaceous shales, and Ca²⁺ was the albitization of plagioclase. Based on the analysis of indicator ratios of rare earth elements in fluorocarbonates from different regions of the world, it has been established that the chemistry of the mineral formation environment and the scale of the substance redistribution processes are of great importance during their formation (in the case of their locality, greater diversity in the chemical composition of the resulting rare earth fluorocarbonates should be expected).

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