Metamorphism of sulfides and gangue quartz at the Degdekan and Gol’tsovsky gold deposits, Magadan oblast

The Degdekan and Gol’tsovsky gold-quartz deposits are located in the southeastern Yana-Kolyma gold belt. The orebodies occur as quartz veins hosted in metaterrigenous rocks and cut by postmineral basic-intermediate dikes. It was established that metamorphism of sulfides and gangue quartz was restricted to a few centimeters off the dike contact. According to sulfide geothermometers, the metamorphic temperatures close to the contact of dikes attained 700°C at the Degdekan deposit and were no higher than 491°C at the Gol’tsovsky deposit. The formation of the forbidden assemblage of quartz and loellingite and its fine-grained texture indicate that the thermal effect on the Degdekan ore was short-term. The prolonged heating of the ore at the Gol’tsovsky deposit gave rise to the aggradation recrystallization of quartz and the formation of equilibrium sulfide aggregates that show only insignificant differences in composition from the primary phases. The average homogenization temperature of primary and pseudosecondary fluid inclusions is 206 ± 40°C in the unmetamorphosed veins and 257 ± 33°C in the metamorphosed veins. The salinity of fluids in the primary and pseudosecondary inclusions in quartz veins of both types varies from 0.5 to 14.0 wt % NaCl equiv. The melting temperature of liquid CO₂ in the carbon dioxide inclusions, ranging from ?57.0 to ?60.8°C, suggests an admixture of CH₄ and/or N₂. The unmetamorphosed quartz veins were formed at a fluid pressure varying from 0.7 to 1.3 kbar, while quartz veins at the contact with dikes crystallized at a pressure of 0.8–1.5 kbar. The results of gas chromatography showed the presence of CO₂ and H₂O, as well as N₂ and CH₄. The average bulk of volatiles contained in the fluid inclusions in quartz from the metamorphosed veins is 1.5–2 times lower than in the unmetamorphosed veins; this proportion is consistent with the occurrence of decrepitated gas inclusions in the heated quartz.     

Ore-bearing fluids of the Eldorado gold deposit (Yenisei Ridge,Russia)

The Eldorado low-sulfide gold-quartz deposit, with gold reserves of more than 60 tons, is located in the damage zone of the Ishimba Fault in the Yenisei Ridge and is hosted by Riphean epidote-amphibolite metamorphic rocks (Sukhoi Pit Group). Orebodies occur in four roughly parallel heavily fractured zones where rocks were subject to metamorphism under stress and heat impacts. They consist of sulfide-bearing schists with veins of gray or milky-white quartz varieties. Gray quartz predominating in gold-bearing orebodies contains graphite and amorphous carbon identified by Raman spectroscopy; the contents of gold and amorphous carbon are in positive correlation. As inferred from thermobarometry, gas chromatography, gas chromatography-mass spectrometry, and Raman spectroscopy of fluid inclusions in sulfides, carbonates, and gray and white quartz, gold mineralization formed under the effect of reduced H₂O-CO₂-HC fluids with temperatures of 180 to 490 °C, salinity of 9 to 22 wt.% NaCl equiv, and pressures of 0.1 to 2.3 kbar. Judging by the presence of 11% mantle helium (³He) in fluid inclusions from quartz and the sulfur isotope composition (7.1-17.4‰ δ³⁴S) of sulfides, ore-bearing fluids ascended from a mantle source along shear zones, where they “boiled”. While the fluids were ascending, the metalliferous S- and N-bearing hydrocarbon (HC) compounds they carried broke down to produce crystalline sulfides, gold, and disseminated graphite and amorphous carbon (the latter imparts the gray color to quartz). Barren veins of milky-white quartz formed from oxidized mainly aqueous fluids with a salinity of < 15 wt.% NaCl equiv at 150-350 °C. Chloride brines (> 30 wt.% NaCl equiv) at 150-260 °C impregnated the gold-bearing quartz veins and produced the lower strata of the hydrothermal-granitoid section. The gold mineralization (795-710 Ma) was roughly coeval to local high-temperature stress metamorphism (836-745 Ma) and intrusion of the Kalama multiphase complex (880-752 Ma).     

Fluid+melt intrusions in lechatelierite from the Popigai suevites: A product of dynamic interaction melts and fluids during the shock melting of the target gneiss

The paper presents data on lechatelierite form suevites of the Daldyn Formation in the Popigai astrobleme. Some of the lechatelierite samples show a complicated structure and contain block of diaplectic quartz glass and dynamic “intrusions” of glasses of types I, II, and III. The glasses of types I and II abound in fluid inclusions and display evidence of partial homogenization with lechatelierite. The glasses of type III are clearly separated from all other glasses but show evidence of dynamic interaction with them in the molten state. Fluid inclusions in the glasses of types I and II are syngenetic but have notably different densities from those of completely liquid or gaseous inclusions at 20°C. As is indicated by cryometric data, the liquid phase of the inclusions is aqueous solution of low salinity (5–8 wt % NaCl_equiv). The bulk petrochemistry of the glasses of type I characterizes them as highly silicic (96.04 wt % SiO₂ on average), with elevated K and Na concentrations (Na₂O + K₂O = 0.72 wt % on average), with 0.73 wt % Al₂O₃ (on average) and analytical totals 1.97 wt % less than 100%. The glasses of type II are also rich in SiO₂ (91.51 wt % SiO₂ on average) but contain a broader spectrum of concentrations of major oxides (totaling 5.53 wt % on average) and deficient analytical totals (by 2.96 wt % on average). The glasses of type III are completely equal to impactites produced by melting gneisses of the Popigai astrobleme. The glasses of type I are interpreted to be the intrusion products of the “early” highly mobile and H₂O-rich fluid+melt mixtures, whose protolithic material was K-Na feldspars of the target rocks. The derivation of these melts was associated with the capturing of much silica and water at a highly mobile behavior of K and Na and an inert behavior of Al. The glasses of type II were produced by the extensive mixing of silica and water at the limited involvement of apogneiss melts, and these glasses are sometimes deficient in Al. The glasses of type III are usual mixed apogneiss melts. Excess silica in the glasses of types I and II and their richness in water and deficiency in Al suggest impact anatexis and the selective separation of components during their derivation; the parental fluid-melt mixtures of these glasses were derived from such “hydrous” varieties of the target gneisses as diaphthorized and fractured rocks. The evolution and partial vitrification of lechatelierite and the glasses of types I and II proceeded under residual shock pressures, as follows from data on the dense (from ∼0.5 to 1 g/cm³) aqueous inclusions in these glasses, which suggest that the inclusions were captured in the glasses under pressures from ∼0.8 to 3.3 GPa. It follows that our lechatelierite samples have a complex multistage genesis, and their quenching facilitated the preservation of “intrusions” of various stages of shock melting, including the products of the “early” impact anatexis of the gneisses with the selective separation of components at the active participation of water.     

Glaciation and ~ 770 Ma Ediacara (?) Fossils from the Lesser Karatau Microcontinent,Kazakhstan

The Cambrian explosion, c. 530–515 Ma heralded the arrival of a diverse assembly of multicellular life including the first hard-shelled organisms. Fossils found in Cambrian strata represent the ancestors of most modern animal phyla. In contrast to the apparent explosiveness seen in the Cambrian fossil record, studies of molecular biology hint that the diversification observed in Cambrian strata was rooted in ancestry extending back into the Ediacaran (635–542 Ma). Fossil evidence for this mostly cryptic phase of evolution is derived from the soft-bodied fossils of the Ediacaran biota found throughout the world and bilaterian embryos found in the Doushantuo lagerstätte in South China. The first appearance of Ediacara fauna is thought to have followed the last of the ~ 750–635 Ma Neoproterozoic glacial episodes by 20–30 million years. In this paper, we present evidence for the oldest discovery of the ‘Ediacara’ discoidal fossils Nimbia occlusa and Aspidella terranovica (?) that predate the early Cryogenian glaciations by more than fifty million years. There is considerable disagreement over the significance of discoidal Ediacaran fossils, but our findings may support earlier suggestions that metazoan life has roots extending deeper into the Proterozoic Eon. We also confirm the presence of a Late Cryogenian (e.g. “Marinoan”) glaciation on the Lesser Karatau microcontinent including dropstones and striated clasts within the glacial strata.     

Composition of the Ordovician lithospheric mantle: Evidence from the study of peridotite xenoliths from camptonite of the Sangilen highland,central Asian fold belt

Doklady Earth Sciences -     

The age of camptonite dikes of the Agardag alkali-basalt complex (western Sangilen): results of Ar/Ar and U/Pb dating

A camptonite dike swarm (Agardag alkali-basalt complex) in the western part of the Sangilen Upland abounds in mantle xenoliths. Mineralogical, petrographic, and petrochemical studies show that the dikes are composed of lamprophyres of two groups, basic and ultrabasic. Ar/Ar dating of amphibole and phlogopite megacrysts gives an intrusion age for the dikes of 443.0 ± 1.3 Ma. ²⁰⁶Pb/²³⁸U dating of zircon from a glomeroporphyritic intergrowth in camptonite from one of the dikes yielded a core age of 489.0 ± 5.4 Ma. This corresponds to the time of formation of the Chzhargalanta granite–leucogranite complex (489.4 ± 2.6 Ma). The ²⁰⁶Pb/²³⁸U age of the zircon rim is 444.0 ± 7.5 Ma. The ages obtained by Ar/Ar dating of amphibole and biotite megacrysts and by U/Pb dating of the magmatic rim of zircon crystal from the camptonite coincide within the dating error, which indicates that the camptonite dikes formed in the Late Ordovician. These dikes are the oldest-known example of mantle-derived xenoliths in mafic volcanic rocks from an off-craton setting. These are samples of the Upper Ordovician lithospheric mantle.