Cl-rich amphiboles and micas in rocks of the Middle Riphean Kusa-Kopan complex of mafic layered intrusions (southern Urals, Russia)

High-Cl amphiboles and micas were found in rocks of the Kusa-Kopan complex of the Middle Riphean layered ore-bearing (Ti, Fe, V) mafic intrusions in the Southern Urals, Russia. Chloro-hastingsite (3.30–4.57 wt% Cl) was found in garnet-bearing orthoamphibolite in the Medvedevsky massif. Compositions of Cl-rich (3.26–4.84 wt% Cl) amphibole from borehole # 2 of the Kusinsky massif located to the north, correspond to “potassic-ferro-chloro-pargasite”, in some cases to potassic-chloro-hastingsite. Thus, in addition to the high-K trend in the evolution of Cl-rich amphiboles, a tendency to low-K content was revealed. Apart from amphibole, biotite with Cl content from 1.73 to 2.32 wt% corresponding to the annite-phlogopite series was observed. Calculations showed that Cl content reached 1.4 wt% in the melt/fluid during the crystallization of the high-Cl amphiboles of the Kusa part of the layered intrusion, and was about 6000 ppm during the crystallization of the “chloro-hastingsite” from the Medvedevsky massif. The same effect could be seen in lower-Cl varieties, where the differences reached a factor of three. The Cl content in the initial melt did not exceed 400 ppm, locally increasing up to 1000–4000 ppm. Chlorine-rich amphiboles are the result of a local excess concentration of chlorinated compounds of complex composition at the final stage of the magmatic system development (under subsolidus conditions). The emergence of an open system cannot be ruled out, in which Cl content in the fluid increases over time resulting from an external source, which is enriched in Cl due to the addition of mobilized brines from the host rocks additionally enriched with CO 2 .