Sosedka pegmatite body at the Malkhan deposit of gem tourmaline, Transbaikalia: Composition, inner structure, and petrogenesis

摘要

It has been shown that composition, inner structure, and genesis of the Sosedka tourmaline-rich pegmatite body were triggered by the totality of sequential of processes during various evolutionary episodes of the Malkhan granite-pegmatite system: (1) heterogenization of the pegmatite melt (first of all, in terms of alkalis and volatile components) in the chamber and en route to the emplacement site and (2) differentiation within the crystallizing pegmatite body, with the former process more significantly affecting the structure of the pegmatite body and the distribution of miaroles in it. When emplaced, the portion of the pegmatite magma that formed the Sosedka vein consisted of compositionally distinct melts, which contained suspended segregations of material of different composition that later gave rise to miaroles. Aqueous silicate colloid liquids could play an important role as a mineral-forming medium filling the miaroles. These liquids are sometimes found as unusual inclusions, resembling melt inclusions, along with usual fluid inclusions, in miarole minerals. Melt crystallization in the pegmatite-hosting chamber also contributed to the formation of the miaroles and resulted in low-temperature residual melts and fluids. These melts and fluids were forced away by the crystallization front and mingled with colloid-fluid bubbles, which were brought by the melt, or were segregated as separate miaroles.