Conditions for the formation of nepheline ores of the Kiya-Shaltyr deposit based on structure analysis of rock-forming minerals (North-Eastern slope of the Kuznetsk Alatau Ridge, Siberia)

Questions of the formation of the structure of the Kiya-Shaltyr deposit of nepheline ores have been raised many times. However, there were quite a few questions about the origin of this deposit. The absolute age of the Sm-Nd method, taking into account the error, showed almost the same age of both urtites and gabbroids at the level of 405–400 Ma. From the contact of the urtite body and the gabbroids, as well as their close interaction, it is quite difficult to determine their formation. In this paper, we show the features of the orientation of nepheline grains, which reflect the features of the introduction of an urtite melt containing a cumulative component of nepheline crystals as an early phase of crystallization. Additionally, the orientations of Петрология / Petrology 34 interstitial clinopyroxene, which reflects the late stage of urtite formation, were studied. The variety of orientations of the rockforming minerals of the urtites of the Kiya-Shaltyr pluton can be interpreted from the perspective of several probable processes of introduction of the foidolite melt. First, the specifics of this melt suggested its porridge-like state, in which there was a sufficiently large number of early nepheline crystals, which could lead to heterogeneity of its geochemical composition. the presence of different types of nepheline and pyroxene orientations can be considered as probable combinations of laminar filling of the intrusive chamber with the subsequent complication of turbulence. The features of the mineralogical composition of the main rock-forming minerals and the features of the geochemical composition of urtites are considered separately. The distribution of rare-earth elements in nepheline cumulates of the urtite type and their residual melts in the form of the pegmatoid phase of ijolites is also shown, which allowed us to determine the lower REE content in urtites in relation to micro-iyolites, pegmatoid ijolites and nepheline syenites, which complete the formation of the Kiya-Shaltyr pluton. This point is one of the confirmations of the cumulative nature of urtites, since nepheline is not a rare earth concentrator, and higher REE concentrations in micro-ijolites and pegmatoid ijolites are due to their accumulation in the residual melt and concentrations in clinopyroxene. The presence of a negative Nb-Ta anomaly and the relative enrichment of Rb, Sr, and U indicate a probable interaction of the plume substance with the previously formed accretion complexes of the subduction zones.