Features of carbonate speleothems from the Belaya Cave (Zabaykalsky Krai)

. The research gives the first data on mineralogical studies of carbonate speleothems in the karst cave named Belaya. Dolomite-calcite, calcite-magnesite and hydromagnesite compositions of secondary carbonate sediments in various grottoes of the cave were determined by thermal analysis (TG-DSC) and X-ray diffraction (XRD). Creating a mineralogical reserve or park with the Belaya Cave as its core is suggested.


Introduction
The Belaya Cave is located in the Priargunsky district of Zabaykalsky Krai, 6.5 km northeast of the Margutsek railway station and 35 km northwest of the city of Krasnokamensk; orohydrographically, the cave is located on the left riverbank of the Urulyunguy (left tributary of the Argun River of the Amur basin) on the southeastern slopes of the spurs of the Pochekuy ridge which is a continuation of the Margutsek ridge and stretching parallel to the Klichkinsky ridge (figure 1).The entrance to the cave is located at the altitude of about 850 m above sea level; its length is 448 m, depth (amplitude) is 40.3 m, volume is 29378 m 3 .GPS coordinates: N50°26'09.46",E117°58'15.93".
The cave was discovered in 2000 by the miners of the Quartz artel which at that time was working on the Nachirsky section of the Garsonuy fluorite deposit.Passing the exploration adit, the group entered a large natural cavity which now is regarded as the uppermost level of the cave (the Great Hall); the lower (and most beautiful) levels remained unknown.For several years, dozens of people visited the cave until the leaders of Quartz decided it was unsafe and ordered the miners to blow up the entrance.In 2020, the GoBike.ruteam found and cleared the blocked entrance and began exploring the lower levels of the cave named Belaya (literally, 'White' in Russian) due to the pure white color of the carbonate outgrowths in most grottoes.Together with the Dolgan caving club (the village of Bagdarin, Republic of Buryatia), the cavers expanded the previously impassable paths and discovered grottoes and halls unique in their beauty.In August-October 2020, an instrumental topographic survey was performed, and a 3D model of the cave was built.

The main part
Since the cave is located within the Garsonuy fluorite deposit, its geology is characterized by the following.The ore field, including occurrences of fluorite (50 km 2 ), is located in the northern part of the Urulyunguy residual massif, composed mainly of Precambrian sedimentary rocks which appeared during the geosynclinal development stage of Eastern Zabaykalsky Krai.Ore bodies are distributed over the area of 9 km 2 and localized in sedimentary terrigenous-carbonate and carbonate-siliceous-terrigenous strata of the Nortui and Byrka formations (Vendian-Riphean), broken by stock-like intrusions of the Gazimursky granitoid complex (Early Paleozoic) and by dykes of granite-porphyry and aplites.The field consists of seven isolated areas (Garsonuysky, Darinsky, Yuzhno-Savinsky, Olimpiysky, Nachirsky, Zimoveysky and Mansky) controlled by disturbances in the Tsagan-Zolotuisky submeridional fault zone.The deposit belongs to the carbonate-quartz-fluorite mineral type; the main minerals are quartz, calcite, fluorite; the secondary are kaolinite, montmorillonite, halloysite, zeolite (stilbite); impurities are pyrite, marcasite, galena, and baryte.Three mineral types of ores are identified -essentially fluorite (60-70% fluorite), quartz-fluorite (up to 65% quartz), and quartz-carbonate-fluorite (about 40% quartz and 5-48% carbonates) [1].
Near the cave, there is Klichka -a geological (paleontological) natural Riphean monument; it's section is mostly composed of sugar-like marbles with single tubes of Udokania replaced by albite, calcite, zeolite, and fluorite.Rare bioherms of cyanobacteria Conophyton, Stratiferа biostromes and Collenia nodules occur.There is an interlayer of black limestones with a complex disharmonious texture, with numerous separate Udokania rods and their intergrowths in two, three or four in the form of 'rosettes', 'bushes' or 'branches'.The section contains two interlayers of black horizontally bedded limestones with fragments of thalli of green algae (Dasycladales) [2].
Dark gray to black carbonaceous limestones, as well as thin-layered calcareous dolomites with rare horizons of black shales, prevail in the Nachirsky site where the Belaya Cave is located.This site is also characterized by the development of multicolored and honey-yellow fluorite and banded wax-like chalcedony-like quartz-jasper (excellent ornamental raw materials) in ore veins [3].
In January 2021, one of the authors of the present research visited the cave to obtain a scientific collection of secondary mineral formations from different halls and grottoes.Figure 2 gives examples of selected carbonate speleothems.Besides, clay deposits and carbonate granules were collected from the bottom of the Flat Hall and the Fairytale Grotto.In other grottoes of the cave, thermal analysis revealed a heterogeneous composition of carbonate sediments, including stalactites where dolomite is also present in addition to calcite.Pure calcite demonstrates one massive endothermic peak connected to the dissociation of CaCO3 (figure 3 b).The obtained thermograms show either a triple wide endothermic peak (figure 4 a) where the first two peaks correspond to the decomposition of dolomite and the third is connected to the dissociation of calcite [4], or a broken endopic also indicating a mixed composition of carbonate speleotems (figure 4 b).Thermal analysis also revealed hydromagnesite (sample BC-21/05/1) and calcitemagnesite (sample BC-21/12) carbonate associations in the white bulk from the Fairytale Grotto (figure 5 a).In the Hall of the Black Waterfall, also known as Clay, snow-white crystallictites growing on clay substrate were investigated, and the analyzes showed that in one case they consist of dolomite (sample BC-21/06), and in the other case a mineral association (sample BC-21/07) of dolomite and calcite is observed (figure 5 b).

Results
The presented results of the study of carbonate speleothems from the Belaya Cave indicate their similarity to those from other caves in terms of mineral composition and formation conditions [5].Further mineralogical and geochemical studies of the Belaya Cave and other karst caves in Zabaykalsky Krai will provide an opportunity to discover the conditions which helped speleothems form in the past.This will provide recommendations for preserving these unusual mineral formations in the modern conditions of increased anthropogenic load, namely, the tourist flow and mining activities.The authors of the present research propose to create a mineralogical park (geopark) or a nature reserve on this territory, where the Belaya Cave will serve as the main attraction.