Genetic features of transparent scapolites of the muzkol metamorphic series, Central Pamir

Background. the genetic features of transparent scapolite of the muzkol metamorphic series (Pr₁), Central Pamir, are studied. The geological position of its deposits and occurrences is confined to albitites and granite pegmatites of the Sarydzhilga formation. Albitites with cavities containing transparent scapolite are represented by the Chernogorskoe industrial deposit and the Sirenevoe small deposit. Granite pegmatites with miarolitic cavities containing transparent scapolite are represented by Ledensi, Perevalnoe, and Verkhnee (with an uncertain industrial potential) occurrences. The genetic features of albitites and granite pegmatites with transparent scapolite are considered. The chemical features of transparent scapolite in both albitites (from 5 to 18 % of meionite minal) and granite pegmatites (from 21 to 29 % of meionite minal) were determined. These differences are also reflected in the IR spectra of scapolite samples in the range of 700–800 cm^-1. Two generations of scapolite — hydrothermal-m etasomatic and hydrothermal — were established in albitites and granitic pegmatites. The scapolite of the latter generation was shown to exhibit the highest potential for jewelry application. The source of the fluid substance (Na, Cl, etc.) that caused the crystallization of scapolite was the marbles of the Sarydzhilga formation, i. e., proto-euporite formations. The formation of transparent scapolite is associated with the regressive stage of zonal metamorphism.

Objectives. (1) To establish the geological structure of mineralization areas with transparent scapolite. (2) To determine the chemical composition of transparent scapolite by X-ray spectral local microanalysis and IR spectroscopy. (3) To review the authors’ and literature data on fluid inclusions. (4) to identify the genetic features of transparent scapolite from the rocks of the muzkol metamorphic series.

Materials and methods. Samples for the study were collected during fieldwork in 2018, 2021, and 2022. In total, 10 scapolite crystals were studied. The chemical composition was examined by X-ray spectral microanalysis using an JCXA-733 device by JEOL using a Si(Li)-energy dispersive spectrometer and an INCA Energy 350 system (Oxford Instruments) at an accelerating voltage of U = 20 kV and a probe current of I = 1 nA (analyst L. A. Pautov, Fersman Mineralogical Museum of the Russian Academy of Sciences).

Results. a location map of scapolite deposits and occurrences and a geological plan of the chernogorskoye deposit were compiled. The chemical compositions of scapolite contained in albitites and granite pegmatites were determined. Fluid inclusions in scapolite samples were analyzed. The IR spectroscopy of jewelry-level scapolite was carried out. The genetic features of transparent scapolite from the Muzkol-R angkul anticlinorium were established.

Conclusion. Two generations of scapolite were identified in albitites and granite pegmatites of the Muzkol metamorphic series. The first was formed at a temperature of 580–450 °C and a pressure of 370–130 MPa; the second — at a temperature of 400–200 °C and a pressure of about 75 MPa. Scapolite samples from albitites and granitic pegmatites differ in chemical composition and IR spectra. Jewelry-level scapolite in albitites and granite pegmatites is characterized by the meionite minal content of 5–18 % and 21–29 %, respectively. The analysis of the IR spectra of scapolite samples at a wavelength of 700–800 cm^-1 showed that the wave numbers of the absorption bands and the ratio of their intensities can be considered as typomorphic properties of transparent scapolite. The formation of transparent scapolite is largely associated with the composition of mineral-f orming solutions and gases, manifested in fluid inclusions represented by chloride brines, as well as liquid and gaseous cO₂. Their source was the carbonate rocks of the Sarydzhilga formation, for which proto-evaporite sedimentation was typical.

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