Fluid and mineral inclusions of charoitite deposit in the Murun alkaline-carbonatite complex, Eastern Siberia, Russia

Abstract

The Murun alkaline‑carbonatite complex in Eastern Siberia, Russia, is known as a unique gem deposit of scenic purple charoitite rocks that consist of up to fifty different minerals, most of which are rare in nature. Charoitites occur in the rocks, which were identified as fenites. Rock-forming minerals of charoitites include charoite, pyroxene, K-feldspar, amphibole, and quartz; various (Ca)-(K)-(Na) silicates often present in large amounts. Apatite, carbonate minerals (mainly strontianite and barytocalcite), baryte, steacyite, dalyite, and various sulfides are typical accessory minerals in charoitites. Several transparent minerals from charoitites were studied in this paper, with special attention given to fluid and mineral inclusions. Fluid inclusions (FIs) are observed in quartz, apatite, tinaksite, dalyite, amphibole, pyroxene, and K-feldspar, including monophase [V (vapor) and L (liquid)], two-phase [L + V (liquid+vapor) and V + L (vapor+liquid)], multicomponent (L + V + S), and solid crystalline inclusions, which are represented by both single grain and composite mineral inclusions. The Raman spectra showed that most two-phase FIs are aqueous with minor N₂ and sometimes contain abundant CH₄ in the vapor phase. Using Raman spectroscopy, the daughter phases in multiphase inclusions were identified as sulfates (baryte and thenardite) and carbonates (witherite and calcite). The following crystalline phases in mineral inclusions were recorded: carbonates (witherite, calcite, barytocalcite, burbankite), sulfates (baryte), sulfides (sphalerite, galena and others), and silicates (quartz, aegirine, amphibole, and some Ca-(K)-(Na) silicate minerals). High-temperature polyphase melt inclusions were found in K-feldspar and tinaksite; however, only partial dissolution was achieved, and total homogenization could not be measured due to decrepitation. The studied fluid inclusions record a complex post-magmatic history, which includes the trapping of an aqueous fluid. The measured total homogenization temperatures (T_h) of FIs in quartz (180–441 °C), apatite (152–364 °C), tinaksite (218–404 °C) and dalyite (264 °C) suggest a secondary origin for all two-phased FIs, while the high-temperature fluid and melt inclusions (T_h > 400–460 °C) recorded in these minerals formed during the magmatic stage. The coexistence of different types of fluid inclusions in transparent minerals associated with charoite, as well as wide ranges of homogenization temperatures, could be evidence of boiling occurring during the formation of charoitites. Fluid evidence and petrographic observations indicate that both magmatic and hydrothermal processes contributed to the formation of the Sirenevyi Kamen charoite deposit.