Groundwater with high salinity is widespread in different climatic and geologic environments of the world. The formation of its chemical composition, however, is still debatable. The chemical composition of groundwater has been studied in 19 springs of the Tuva depression. In this area, hydrocarbonate, sulfate, and chloride waters with different cation compositions discharge. Their TDS value varies mainly from 1 to 6 g/L, reaching 315 g/L at only one locality. The chemical composition of the studied waters is reflective of the geostructural, hydrogeologic, landscape, and geochemical conditions. The main processes determining the chemical composition of the waters are their interaction with aluminosilicate minerals, dissolution of gypsum and halite, evaporation, and oxidation of sulfide minerals.

Mafic-ultramafic massifs of Tuva territories are located both within linear belts tracing deep faults and as "disordered" areas associated with some belts sufferedthe later fold-block dislocations. Sometimes fragments of mafic-ultramafic bodies are present in granitoidal intrusive asxenogenic blocks of various sizes. Areal location of ultramafic massifs is particularly observed in the east of Tuva within the Khamsarynsky structural-formation zone as well as in the structures of the Tuvinian-Mongolian middle massif. Geochemical features of the ultramafic massi frock variety studies are continued for obtaining convincing conclusions on their formational affiliation. Ultramafic massifs of Tuva have isolated single data in terms of the geochemical characteristics which requires a purposeful work in this direction. The present paper summarizes for the first time petro-geochemical characteristics of dunites of the Nizhnetarlashkynsky ultramafic massif and fragments of individual picritic dikes among them.

Presented are the results from determining the principal ion composition of the waters and the composition of chemical elements, including rare and rare-earth elements) in the waters of three salt lakes: Shara-Nur, Dus-Khol and Bai-Khol, on the territory of the drainless Ubsunur depression. Common to these lakes are their small depth, and their location in the bottoms of basins without outflow, between small bald mountains in the zone of dry climate with abrupt temperature fluctuations. More likely these lakes have their origins in the remnants of ancient water basins. Accumulation of salts in the lakes is associated with continental salinization processes.

Tuvino-Mongolian massif is held to be one of largest massifs in a structure of the Central-Asian Fold Belt fringing the Siberian Platform on the south and south-west. Such massifs with the crystalline basement are considered as microcontinents - fragments of continental massifs, included in the supercontinent Rodinia and attached to structures of surroundings of the Siberian Platform in the course of the Paleoasian ocean development. New information on development of endogenous processes in the Tuvino-Mongolian massif has been obtained from recent investigations. Features of geological setting of metamorphic complexes (Naryn, Moren, and Erzin complexes) have been studied. The Naryn complex is made of essential carbonate, terrigenous-carbonate, and terrigenous piles of the Balygtyglchem, Chartys, Naryn, and Chinchilig formations. These rocks are attributed to deposits of epicontinental shelf seas. The Moren complex includes volcanic and sedimentary rocks metamorphosed in the conditions of the amphibolite facies and formed in the environment of rifting structures of passive continental margins. The Erzin complex is made of biotite and garnet-biotite gneisses, intense migmatitized, with relicts of the granulitic facies. Their protoliths were sedimentary rocks typical of basins of passive continental margins. Hypersthene and bipyroxene crystalline schists of the Lower Erzin tectonic plate are attributed to the Erzin complex too. The initial rocks of the schists can be considered as fragments of paleoisland-arc and paleoocean formations. U-Pb dates (SHRIMP) of detrital zircons from metaterrigenous rocks of the Moren complex are 750 M.a. and of the Erzin complex - 800 and 900 M.a. This determines a lower age boundary of rocks accumulation.

Platinum group minerals (PGM) are rather widespread as admixture in gold placer deposits in Tuva. The present paper reports new data on PGM in the Kundus gold placer confined to the Kaakhem ophiolite belt. The minerals are mainly represented by solid solutions of the Os-Ir-Ru system. They make up rims of sulfoarsenides, sulfides, and arsenides of the platinum group elements (PGE) developed after primary minerals. PGMs of this placer always contain traces of Pd (0.33–1.58 wt %), Cu (0.29–0.50 wt %), and As (0.03–2.17 wt %), as well as Ni and Sb (within the detection limit). Typomorphic features of minerals along with the set of main elements and isomorphic trace-elements in the major and secondary mineral species, suggest that sources for the studied placer was represented by the Alpine-type ultramafics and associated chromitites. We cannot also rule out that PGM mineralization was influenced later intrusions that promoted the formation of rims of sulfoarsenides, sulfides, and arsenides of PGE. The PGM rims are marked by the S and As isomorphism, which characterizes the composition of mixtures rather than independent mineral types (end members of isomorphic series). In one case, minerals are represented by the isomorphous mixture of sulfoarsenides with a limited role of sulfides; in another case, by arsenides with a limited role of sulfoarsenides.

This paper presents the results of the first comprehensive geological, petrographic, mineralogical, geochemical, and geochronological studies of ultramafic and mafic magmatism in the poorly explored southwestern Tuva (evidence from the Birdag and Khayalyg massifs). These massifs intrude ortho- and paraschists, which are presumably Middle Proterozoic. Amphibole gabbros in the Khayalyg massifs contain numerous relict host-rock xenoliths. The massifs are dominated by meso- and leucocratic amphibole gabbros with minor ultramafic (serpentinous plagioclase harzburgites and plagioclase lherzolites) and transitional (wehrlites, olivine clinopyroxenites, hornblendites, melanocratic olivine gabbronorites) rocks. The serpentinous plagioclase harzburgites and plagioclase lherzolites occur in the amphibole gabbros of the Birdag massif as small lenticular bodies. The latter are interpreted as xenoliths of ultramafic restites of earlier protrusion rather than mafic-melt differentiates. The wehrlites, olivine clinopyroxenites, hornblendites, and melanocratic olivine gabbronorites forming the outer zones of the xenogenic bodies of ultramafic restites are considered hybrid rocks. They resulted from the contact reactions of mafic melts and their fluids with the xenogenic bodies of ultramafic restites, which were feldspathized during this interaction. In the gabbros from both massifs, the chondrite-normalized content of MREE and HREE is lower and that of LREE is higher than those in N-MORB. The plagioclase peridotites, wehrlites, and olivine clinopyroxenites forming xenogenic bodies among the amphibole gabbros of the Birdag massif are richer in REE (especially LREE) than the ultramafic restites in ophiolite associations. This is because they were infiltrated by fluids enriched in these elements during their alteration under the influence of later mafic melts. The studies suggest that the Birdag and Khayalyg ultramafic–mafic massifs result from the spatial coexistence of (1) more ancient small protrusions of ultramafic restites, which occurred as allochthons among Middle Proterozoic(?) metamorphic rocks, (2) later gabbroic intrusions (from 494 ± 16 to 450–447.4 ± 5 Ma), and (3) hybrid transitional rocks (wehrlites, olivine clinopyroxenites, hornblendites, melanocratic olivine gabbros) making up contact-reaction zones along their boundaries.

Issue of the influence of eolian processes on the formation of gold placers in the Tuva region is considered. It has been revealed that the formation of placers is related not only to hydrodynamic processes but also to eolian processes, which are reflected in the formation of relief, redistribution of metal, and reworking of the useful component. Study of typomorphic features of the eolian gold and regularities of its redistribution revealed the possibility of discovery of a new type of placer gold (eolian and heterogeneous placers).