Geology of Ore Deposits

In order to resolve the controversial issue of the origin of fluid(s) and their evolution in magmatic-fluid mineral-forming systems, a comprehensive study of fluid inclusions (FI) and the isotopic (δ¹⁸O, δ²H) composition of granitoid minerals and ore bodies of the Tigrinoye greisen tin-tungsten deposit was carried out. Microthermometric study of FI in quartz and topaz along with study of their gas and bulk chemical composition showed that the homogenization temperature and salinity of the aqueous fluid during the formation of ore veins and greisens comprise intervals from ~400 °C to ~200 °C, and from 3.5 to 7.5 wt.%-eq. NaCl with a noticeable decrease in the salinity of the fluid as T falls. Estimates of ore crystallization conditions based on the FI study data were T = 560–230 °C, P ≈ 1500–2000 bar, log[fO₂] near the fayalite-magnetite-quartz buffer. Data on the bulk composition of the fluid and thermodynamic calculations let us suggest that the late magmatic high-temperature fluid trapped by inclusions in topaz, enriched in K and Sn, was an active participant in the greisenization process, and the fluid trapped by FI in quartz was enriched in Na, depleted in Sn, and corresponds to equilibrium with feldspars of granitoids at low temperatures. The Sn content in FI varies from 3 to 3000 ppm, while in FI in quartz it is very low (3–26 ppm, with an average of 13 ± 9 ppm), and in topaz it is high. Rhyolite porphyries are characterized by high δ¹⁸O values, indicating the sedimentary nature of their protolith: δ¹⁸O(Qz) = 11.4–11.6‰, δ¹⁸O(Fsp) = 8.9–11.0‰, δ¹⁸O of micas from 6.0‰ for the Bolshoi Stock to 9.5–10.1‰ for the remaining intrusion phases. The δ¹⁸O values of rhyolite-porphyry minerals indicate short-term external influence of fluid, and the δ²H values of equilibrium fluid (≈–110…–130‰) indicate intensive degassing of acidic melts. The ratios of δ¹⁸O values in Qz-Znw and Qz-hornfels pairs reflect the pulsed nature of ore formation, generally corresponding to the previously identified stages. The temperature of the oxygen isotope equilibrium in the Qz-Znw pair of ore veins and greisens is T = 400–560 °C, and the δ¹⁸O and δ²H values of the equilibrium fluid correspond to a magmatic source (8.5–11‰ and –63…–86‰, respectively). The δ²H values of total hydrogen captured by quartz in ore veins and greisens (–80…–120‰) indicate a genetic connection between the fluid and the products of rhyolite melt degassing, while the δ¹⁸O values indicate a local equilibrium between the fluid and the host rocks, which was achieved during its filtration from degassing zones to ore deposition zones.

On the example of one of the massifs – Ingursky, formed within the North Mongolian – West Transbaikal polychronic rift system, the conditions that contributed to the formation of rare metal mineralization in it were considered. The basis was the mineral-petro-geochemical studies of the ore-bearing Ingursky and the nearby ore-free Sherbakhtinsky massifs. The totality of the compositions of these arrays forms a general sequence, which can be considered as an evolutionary series that arose during the differentiation of the magma of the syenite composition. The two main factors that led to the formation of rare-metal (Be, Ta, Nb, Th, U, HREE) mineralization in the pegmatites of the Ingursky massif, the first is the high fluid saturation of the alkaline-granitoid melts of the massif, which contributed to their deep differentiation with the accumulation of ore mineralization in the late derivatives of pegmatite and pneumatolytic processes. The second is a rather long ~ 6 million years thermal history of rocks of the massif, at sufficiently high temperatures, about 900–500 °C, which contributed to the accumulation and redistribution of ore elements at the final stages of the magmatic process.

Fahlore of the epithermal silver-polymetallic Mangazeyskoye deposit (Yakutia, Russia) in terrigenous strata, where it is the main concentrator of silver, has been studied in detail. It has been established that among the minerals of the fahlore group at the Mangazeyskoye deposit, kenoargentotetrahedrite-(Zn) and argentotetrahedrite-(Zn) predominate, while kenoargentotetrahedrite-(Fe), argentotetrahedrite-(Fe) and others occur in smaller quantities; they are found in various mineral associations in close intergrowths with each other, with sulfides (galena, sphalerite, arsenopyrite and boulangerite) and with silver minerals (diaphorite, owyheeite, pyrargyrite, stephanite, miargyrite, freislebenite). According toEPMA data, the chemical composition of the freibergite series at the Mangazeyskoye deposit covers the entire range of compositions by Ag content (from 3 to 8 apfu), forming a solid solution with complete almost continuous isomorphism between the monovalent metals Ag and Cu and complete continuous isomorphism between the divalent metals Fe and Zn. In the freibergite series of fahlore, both at the deposit as a whole and in zonal-heterogeneous aggregates, an inverse correlation was revealed between the Ag and S contents. It was established that the formation of heterogeneity and oscillatory zoning in the aggregates of the Mangazeyskoye deposit fahlore is associated with dissolution-reprecipitation reactions and decomposition/exsolution of the fahlore solid solution. Four generations of fahlore have been identified at the deposit, differing in chemical composition and associated minerals. It has been established that the composition of fahlore depends on the composition of the minerals associated with it: maximum silver concentrations are present in fahlore associated with pyrargyrite and/or miargyrite; in associations with chalcopyrite, fahlore with a high silver content decomposes to form fahlore with a lower silver content; in associations with sphalerite, only fahlore-(Zn) is found. It has been shown that from early mineral associations to late ones (both in terms of the mineral composition of ores and in the chemical composition of fahlore), the ore is enriched in silver by retrograde dissolution-repreciritation reactions The temperatures of deposition of freibergite series minerals at the deposit, estimated using an Ag-fahlore geothermometer, are predominantly in the range of 250–170 °C; these temperatures are consistent with the deposition temperatures obtained by other methods. It is concluded that the fahlore associations were deposited from more alkaline fluids than early quartz; the compositional changes of the fahlore were influenced by the mineral-forming fluid, namely the metal content and the activity of chlorine and sulfur in it; the evolution of the composition of the mineral-forming fluid itself is associated with boiling, mixing and dilution of the fluid and temperature changes accompanied by these events. Kenoargentotetrahedrite-(Zn) and kenoargentotetrahedrite-(Fe) of the Mangazeyskoye deposit are the extreme Ag-, Sb-, Zn- and Fe-members of the sulfur-deficient freibergite series, containing the maximum amounts of silver and divalent metals ever discovered in nature.

The Gorevskoe zinc-lead deposit is located within the Angara-Bolshepit mineragenic zone of the Altai-Sayany province. The ore-host geological section containing polymetallic ores is characterized by a heterogeneous composition of its constituent sedimentary rock differences and is represented locally, at the deposit site, by manganese-containing siderite (50 vol.%), siliceous siderite (30 vol.%), silicite (5 vol.%), dolomite, mudstone, siltstone and marl but regionally – mainly by limestone and meta-siltstone, and by manganese–containing and siliceous siderite. The deposit is located within the zones of regional metamorphism and is characterized by the development of garnet-tremolite-actinolite and cummingtonite-grunerite assemblages. The ores have veined-impregnated, breccia-like, banded textures, rarely massive and relict layered ones; their diverse combination reflects the complex history of the ore formation process. In the ores, there are several generations of the main ore-forming minerals, composing a number of paragenetic mineral associations of different ages: 1) pyrite–pyrrhotite–sphalerite–galena, 2) sphalerite–pyrrhotite–galena with pyrite, 3) quartz–galena with sphalerite, 4) quartz–sphalerite–galena with pyrite and pyrrhotite, 5) pyrite–pyrrhotite–magnetite with sphalerite and galena. As a result of the generalization of data on the study of the isotopic composition of sulfur, carbon and oxygen, it was found that a characteristic feature of all sulfides of ores of the deposit is their significant enrichment with heavy isotope ³⁴S; variations in the isotopic composition of C and O of ore-bearing carbonate rocks are small and close to calcite of marine sediments; carbonate of ores have comparatively light isotopic compositions, especially carbon, which mainly corresponds to the mantle reservoir in terms of isotopic composition. The hydrothermal-sedimentary nature of primary ore accumulations and their subsequent transformation during high-grade regional metamorphism are proved.