O. V. Avchenko

Experience of Modeling the Garnet + Orthopyroxene + Spinel + Plagioclase Reaction by the Method of Thermodynamic Potential Minimization

Relative to the method of phase correspondence, physicochemical modeling of mineral associations based on the method of thermodynamic potential minimization and consideration of solid solutions offers new opportunities [1]. Although the thermodynamic database of end members and the models of mineral solid solutions are not sufficiently accurate, the high potential of the method based on minimization for the solution of several petrological issues is evident. The present paper shows an implication of this method for thermodynamic analysis of the garnet reaction structure, for which oxygen pressure and fugacity cannot be estimated by conventional thermobarometry. Solid inclusions in alkaline basalts of the Konfetka paleovolcano in Primorye [2] contain fragments of fractured garnet crystals replaced by orthopyroxene‐ spinel‐plagioclase symplectite along fractures and the periphery (figure). Bright orange stilpnomelane is developed locally in the reaction rim. The symplectite has a specific composition (Table 1). Orthopyroxene shows appreciable variations in the Al 2 O 3 content: a lower concentration (2.9‐3.4 wt %) at the contact with garnet than at the contact with spinel and plagioclase (9.5‐10.3 wt %). Plagioclase is also heterogeneous. Its composition varies from An 84 to An 60 . At the same time, the Fe mole fraction of orthopyroxene and spinel is nearly constant. However, the spinel composition is characterized by a lower degree of oxidation ( Fe 3+ / Σ Fe = 8.1‐11.8 at %, Table 1). The compositional heterogeneity of minerals emphasizes the nonequilibrium state of the symplectite rim, at least, with respect to the Al 2 O 3 potential. Elucidation of PT constraints of the garnet replacement by the orthopyroxene‐spinel‐plagioclase symplectite can provide insight into the genesis of garnet inclusions. The equilibrium temperature of the orthopyroxene‐spinel system can be estimated approximately by the Liermann‐Ganguly method [3]:

New data on the composition of jordanite–geocronite Pb–Sb–As sulfosalts at the Berezitovoe deposit (Upper Amur Region, Russia)

New data on the composition of jordanite–geocronite Pb–Sb–As sulfosalts in the vein–disseminated gold ore at the Berezitovoe deposit were provided. Within the studied quartz–sulfide sample, some grains of Pb–Sb–As sulfosalts, uniform in composition and azonal, are drastically different from each other in As and Sb contents forming a quasi-continuous range of solid compounds from jordanite to geocronite. Jordanite–geocronite minerals at the Berezitovoe deposit (relative to the Darasun deposit) are characterized by a higher Pb concentration with a low polymetal total. It is assumed that such typomorphic features of the composition of Pb–Sb–As sulfosalts are indicative of specific formation conditions of gold mineralization in polymetallic ores at the Berezitovoe deposit.

Genesis of garnet-bearing rocks at the Berezitovoe deposit, Upper Amur Region, Russia

The geological position, composition of mineral assemblages, and typomorphism of major minerals from garnet-bearing rocks at the Berezitovoe gold-base-metal deposit in the Upper Amur Region have been studied in detail. These are ore-bearing metasomatic rocks and metamorphosed porphyritic dikes. The garnet-bearing metasomatic rocks reveal zoning, which is caused by various degrees of metasomatic transformation of the Paleozoic porphyritic granodiorite that hosts the ore zone. The metasomatic replacement of granodiorite was accompanied by loss of Na, Ca, Ba, Sr and gain of K, Mn, and Rb. Garnet-bearing metamorphosed intermediate dikes occur within the metasomatic zone. The PT conditions of metamorphism and metasomatism are similar and estimated at 3.9 kbar and 500°C from various mineral equilibria. The results of physicochemical simulation of garnet-bearing mineral assemblages carried out by minimizing the Gibbs free energy and the geological data show that garnet-bearing mineral assemblages arose at the Berezitovoe deposit as a result of local high-temperature thermal metamorphism of previously formed low-temperature metasomatic rocks close in composition to classic beresite. In this connection, we propose considering garnet-bearing metasomatic rocks as high-temperature metamorphosed beresites.

Cervandonite-(Ce) in ores of the Berezitovoe deposit: Second finding in the world

The supposedly second finding of rare arsenosilicate cervandonite-(Ce) in the world is characterized. The mineral was recognized in the ore-bearing metasomatic rocks of the Berezitovoe gold-base metal deposit (Upper Priamurye, Russian Far East) in association with quartz, biotite, muscovite, orthoclase, garnet (almandine-spessartine), tourmaline, basic plagioclase, and sulfides. The cervandonite is represented by optically homogeneous and heterogeneous aggregates with visible crystals from 10 fum to 0.1–0.3 mm in size. Based on the microprobe analysis, the average chemical composition of the homogeneous cervandonite-(Ce) aggregates is as follows (wt %): Ce2O3 - 13.00, La2O3 - 5.70, Nd2O3 - 5.20, Pr2O3 - 1.41, Y2O3 - 0.77, Sm2O3 - 0.77, Eu2O3 - 0.23, Gd2O3 - 0.54, Dy2O3 - 0.31, ThO2 - 1.12, UO2 - 0.30, TiO2 - 12.86, Al2O3 - 9.24, Fe2O3 - 8.93, FeO - 2.68, CaO - 0.14, SiO2 - 19.98, As2O3 - 16.19. The comparative study of the cervandonite-(Ce) from the Berezitovoe deposit and the analogous minerals from the Alpine mica gneiss of Mt. Pizzo Cervandone (Central Alps) showed that the former mineral can be assigned to a new variety of cervandonite-(Ce) in terms of its compositional features. This variety is characterized by an ordered stoichiometric composition corresponding to the simpler theoretical formula (Ce,Nd,La)(Fe3+, Fe2+, Ti4+, Al)3 (Si2As3+)3O12.

New U-Pb isotopic data on the age of metamorphic and igneous rocks of the western margin of the Selenga-Stanovoi orogenic belt

New data on the isotopic age of zircons from metamorphic and igneous rocks are given for the Berezitovoe and Kirovskoe deposits located in the eastern margin of the Selenga-Stanovoi orogenic belt. The zircons were studied with the LA-ICP-MS method in the State Key Laboratory of Continental Dynamics, Northwestern University, Sian, Shansi province, People’s Republic of China. The results allowed us to reveal three age epochs in formation of the eastern part of the Selenga-Stanovoi orogenic belt (1.87 Ga; 380–330 Ma; 138–125 Ma), which have significant importance for understanding of the geological structure of the south-eastern framework of the North Asian craton.

Physicochemical Crystallization Conditions of Al-F Sphene in Metasomatic Rocks with Ore Mineralization at the Berezitovoe Deposit

Al-F sphene (grothite) was found in mineralized rocks at the Berezitovoe Deposit in the Russian Far East. The paper is devoted to the mineral assemblages and composition of the mineral and its thermodynamic crystallization conditions. The average Al and F concentrations (p.f.u., microprobe data) in the grothite are 0.45 and 0.42 in sample 1374, 0.32 and 0.32 in sample 1306, and 0.35 and 0.33 in sample 96. Grothite was found in the rocks in association with chlorite, ilmenite (pyrophanite), and magnetite, and this mineral assemblage was obviously overprinted on the primary garnet-biotite assemblages. We estimated the temperature of grothite crystallization at 400–500°C. With regard for available experimental data on the mineral equilibrium between Al-F sphene, fluid, and anorthite, a tool is proposed for evaluating F concentrations in fluids by the equilibrium of Al-F sphene with plagioclase, rutile, and F-bearing aqueous fluid. Our model simulations indicate that the maximum F concentration in fluid during the crystallization of Al-F sphene richest in F at the temperatures and pressures of metasomatic rocks at the Berezitovoe deposit could reach 300–500 mg per kg of the aqueous solution. The level of F concentration in the fluid during the crystallization of Al-F sphene at the deposit is comparable with the F concentration in fluid during the development of greisens and rare-metal pegmatites, but these high F concentrations were reached only during the final evolutionary stages of the deposit.

Magnetite-Ilmenite Equilibria in Archean Enderbites from the Sutam Complex (Aldan Shield)

The composition of disintegrated titanomagnetites and ilmenites from Archean enderbites of the Sutam Complex of the Aldan shield has been reconstructed. The values of oxygen fugacity and temperature of titanomagnetite‐ilmenite mineral equilibrium at 7.5 kbar general pressure are calculated. In the process of granulite metamorphism oxygen volatility varied within = ‐14.823 ± ‐16.868 at temperatures T = 723 ° C‐910 ° C, corresponding to a line parallel to the quartz‐ferrosillite‐magnetite buffer equilibrium, but about 1.5 orders of above it. The probable values of oxygen volatility for protoliths of Sutam enderbites were not lower than calculated ones. Detailed study of iron‐titanium oxide paragenesis occurring in metamorphic rocks has been given little attention so far. In the meanwhile, a purposeful approach with reconstruction of the primary composition of iron‐titanium oxides may supply valuable data on the temperature and oxidation potential at the time of metamorphism. Study of these minerals is especially important when they occur in ancient magmatic rocks. In this case, hypothesizing a minor change in the oxygen potential during metamorphism makes it possible to obtain the value of the oxidation potential of the magmatic rock protolith. In this way we can assess the oxygen regime of the Earth’s crust during the earliest stages of our planet’s evolution. In our paper this task is being solved on the basis of petrological study of magnetite‐ilmenite equilibria occurring in enderbites of the Sutam metamorphic complex. The Sutam Complex forms a tectonic block of the same name to the south of Aldan shield in the Sutam River basin (Fig. 1). It is distinguished by ultrahigh-pressure conditions of regional metamorphism at depth. It was described first by A.A. Marakushev [1]. According f O2 log f O2 log to [2, 3], the Sutam Complex consists of two strata: the low one involving mainly hypersthene plagiogneisses (enderbites) with minor amounts of garnet, garnet‐ biotite plagiogneisses, and garnet‐sillimanite, cordierite, hypersthene‐sillimanite gneisses, and quartzites. Reconstruction of the primary nature of high-grade rocks of the low and upper strata of the complex [2, 3] revealed that volcanic rocks of calc‐alkaline series, andesites, dacites, and rhyodacites (andesite‐dacite association) dominate in the low strata. Volcanic rocks of the komatiite‐tholeiite series presented by peridotite komatiites and komatiitic and tholeiitic basalts (komatiite‐basalt association) make up an insignificant part of the succession (not more than 10%). The initial composition of the upper strata is reconstructed as intercalation of volcanic rocks of the andesite‐dacite association with greywackes, pelites, volcanoclastics, as well as chemogenic‐sedimentary, sil

Minerals of the Bournonite–Seligmannite Series in Ore of the Beresitovoe Deposit, Upper Amur Region, Russia

New data are presented on segregations and compositions of bournonite-seligmannite series of minerals in gold-bearing veinlets of the Beresitovoe deposit located in the eastern part of the Mongolia–Okhotsk Orogenic Belt. It was established that these minerals represent a discrete series of stoichiometric solid compounds with As formula coefficients varying from 0.2 to 1.2. It is shown that minerals of this series are characteristic of ore deposits that undergo high-grade metamorphism, the P–T conditions of which are sufficient for melting of primary sulfides.

Genesis of the Berezitovoe Gold-Polymetallic Deposit: Evidence from the Isotope Composition of Lead, Oxygen, and Sulfur

1117 The results of the study of stable oxygen, sulfur, and lead isotopes in rocks and ores of the Berezitovoe gold–polymetallic deposit in the Upper Amur allow� ing us to clarify the debatable problems of its genesis are discussed in this paper. The importance of this problem is determined by the fact that the considered deposit is represented by an ore object of a fluid– explosive nature related to poorly studied and special types, which are relatively abundant in adjacent ore areas of East Transbaikalia. The deposit is located in the northwestern part of the Amur area, within the eastern part of the Seleng� ino–Stanovoi Terrane (southeastern frame of the North Asian Craton), in the junction zone with the formations of the northern part of the Mongol– Okhotsk foldbelt. The deposit is represented by meta�

High-alumina titanite in mineral assemblages of the Berezitovy gold-base-metal deposit, Upper Amur Region

Grothite—a rare Al- and F-rich variety of titanite—was identified in two different gold-bearing mineral assemblages of the Berezitovy gold-base-metal deposit, Upper Amur Region, Russian Far East. Grothite is associated with quartz, orthoclase, chlorite, muscovite, tourmaline, almandine-spessartine garnet, ilmenite, pyrophanite, magnetite, fluorapatite, and sulfides. Grothite forms numerous scattered lamellar aggregates 20–100 μm in size with a relatively homogeneous structure. The lamellae grow in chlorite or between chlorite and orthoclase. According to microprobe analyses, variations in major elements of grothite are as follows (wt %): 30.56–34.07 SiO2, 7.91–12.71 Al2O3, 22.83–28.29 TiO2, 23.55–29.21 CaO, 0.52–4.25 FeO, and 2.19–6.16 F. It is suggested that grothite appeared in the gold-bearing mineral assemblages of the Berezitovy deposit due to the specifics of the primary composition of the host rocks and physicochemical conditions of the fluid regime at the final stage of deposit formation.