L. P. Plyusnina

Carbonization and geochemical characteristics of graphite-bearing rocks in the northern Khanka terrane, primorie, Russian Far East

Regional carbonization was examined in Riphean metamorphic complexes in the northern part of the Khanka terrane. The results obtained by various techniques of physicochemical analysis indicate that all petrographic rock varieties of this complex bear elevated concentrations (from 10−4 to 10−6 wt %) of Au and PGE. XRF data were used to describe a wide spectrum of trace elements: Ti, V, Ni, Cr, Pt, Pd, Re, Rh, Os, Ir, Cu, Hg, Au, Ag, Ta, Nb, Sr, Rb, Zr, La, W, Sn, Pb, and Zn. The Rb/Sr-Ba diagram shows the fields of anatectic granite-gneisses, biotite granites, lamprophyres, graphitized crystalline schists, black shales, skarns, and quartz-graphite metasomatic rocks. The C isotopic composition in graphite from the metaigneous rocks (lamprophyres and crystalline schists of the amphibolite facies) corresponds to δ13C from −8.5 to −8.7‰, which suggests that the carbon could be of endogenic provenance. The carbon isotopic composition of the greenschist-facies black shales corresponds to δ13C from −19.9 to −26.6‰, as is typical of organogenic carbon. The concentrations of precious metals in the rocks are, on average, one order of magnitude lower than in the graphitized crystalline schists. The origin of the precious-metal ore mineralization was likely genetically related to the regional carbonization process.

The Noble Metal Distribution in the Black Shales of the Degdekan Gold Deposit in Northeast Russia

The mineral and chemical composition of the carbon-bearing rocks of the Late Permian Pionerskaya Formation containing the Degdekan gold deposit has been studied. The bulk contents of Au, Ag, Pt, and Pd in the black shales and their light, sulfide, and electromagnetic fractions were determined by electrothermal atomization. The mineral composition and the phase analysis of the rocks were studied using a scanning electron microscope. Gold is present as fine xenomorphic grains of high fineness with an Fe admixture of up to 4 at %, as well as intergrowths of kustelite and electrum. The Au and Pt contents in the black shales and ores vary in a wide range (g/t): Au 0.01–13.12, Pt 0.001–1.34. The highest Au contents (up to 1748 g/t) were noted in the sulfide fraction. The Pt-bearing phases were not found, whereas a Pt content of about 0.61 wt % was determined using an electron microscope in a carbonaceous matrix. The initial rocks have a steady and low Pt content (less than 0.007 g/t). A stable even Au distribution in the studied rocks was established within 1.14–2.46 g/t. The chemical analysis of the soluble fraction of the carbonaceous matter extracted from the black shales showed the presence of Au 0.375, Ag 3.68, Pt 0.147, and Pd 0.052 g/t. It has been concluded that the carbon-bearing rocks of the Pionerskaya Formation play a resource role in the accumulation of noble metals, whereas economic concentrations of the latters are formed in the course of the superimposed metamorphic-hydrothermal processes.

Noble Metals in Carbon-Rich Metamorphic Rocks of the Khanka Terrane, Primorie

Carbon-rich metamorphic rocks of Riphean age in the northern part of the Khanka terrane were first analyzed for concentrations of noble metals (Au, Ag, Pt, Pd, Ir, Os, and Ru). According to the data of various physicochemical analytical techniques, the Au and Pt concentrations broadly vary: from 0.01 to 52 ppm for Pt and from 0.1 to 30 ppm for Au. Various techniques of sampling and analysis variably affect the losses of these metals because of difficulties in the decomposition of metal-carbon chemical bonds. The carbon isotopic composition (13C from −8.5 to −8.7‰) of the graphitized amphibolite-facies rocks widespread in the core of the Ruzhino paleodiapir suggest that their carbon is of mantle provenance. The Early Cambrian metaterrigenous rocks metamorphosed to the greenschist facies have 13C from −19.9 to −26.6‰, which testifies to its organic origin. The elevated concentrations of noble metals in these rocks suggest that the sources of carbon and metals were polygenetic and that the ore-forming system evolved over a long time span.

Platinum- and gold-bearing rodingites of the ust'-dep ophiolite block (middle amur region)

The Ust’-Dep ophiolite block, a part of the Selemdzha‐Zeya Belt, occupies an area of 350 km 2 and incorporates apoharzburgite serpentinite massifs intruded by numerous (>60) dikes of diabases and granitoids up to 100 m thick and more than 1 km long. Ophiolitic outcrops of this block make up a SW- to NW-trending band extending from the right bank of the Zeya River to the Gar River basin. Only small fragments of ophiolite massifs are preserved in spurs of the Tukuringr Ridge (Fig. 1). The Ust’-Dep and Gar protrusions in this area include dislocation zones with superimposed metasomatic alterations (development of listvenites and rodingites with gold mineralization). The majority of rodingite and listvenite occurrences are confined to outcrops of Lower Paleozoic‐Middle Cretaceous granitoid intrusions and dikes. Listvenites usually considered the source of placer gold are scrutinized in [1, 2]. However, data on the gold potential of rodingites in the Ust’-Dep block are lacking, although they are similar to rodingites in the Zolotaya Gora deposit (southern Urals) that was previously proposed as a holotype of the gold‐rodingite association [3]. Their similarity is manifested in the development of rodingites among listvenitized serpentinites. Therefore, the gold mineralization of Uralian rodingites is attributed to a younger process of listvenitization [4].

Nature of Graphitization and Noble Metal Mineralization in Metamorphic Rocks of the Northern Khanka Terrane, Primorye

Elevated contents of noble metals (NM) have been established in the Riphean-Cambrian graphite-bearing complexes of the northern Khanka Terrane, which metamorphosed under conditions of greenschist to granulite facies. At the previously known graphite deposits of the Turgenevo-Tamga group, NM comprise (ppm): Pt (0.04–62.13), Au (0.021–26), Ag (0.56–4.41), Pd (0.003–5.67), Ru (0.007–0.2), Rh (0.001–0.74), Ir (0.002–0.55), and Os (0.011–0.09). Analyses of graphitized rocks carried out with various methods (IMS, INAA, AAS, AES, fire assay) reveal a wide scatter of the results related to the specifics of sample preparation, in particular, due to a significant loss of NM by thermal oxidation decomposition. Analysis of a low-soluble graphite residue obtained by treatment of graphitized rocks allowed us to establish genetic links between NM mineralization and carbonic alteration of various igneous, granulite- and amphibolitefacies metamorphic rocks, which occur over a vast area. The nonuniform distribution of graphite and NM in rocks, their fine dispersivity, and compositional variability of NM indicate that their origin is related largely to endogenic processes with the participation of deep reduced fluids. In greenschist-facies rocks, fluorine, bromine, and iodine are associated both with ore minerals and graphite, providing evidence for transport of NM by halogene- and carbon-bearing fluids. The inhomogeneous distribution of metals in graphite, microglobular structure, and carbon isotopic composition are the guides for its gas-condensate crystallization. At the same time, thermal analysis and Raman spectroscopy show that graphite formed by metamorphism of carbonaceous matter contained in sedimentary rocks also occurs. It is concluded that the predominant mass of NM is of fluid-magmatic origin with the participation of exogenic and metamorphic sources of metals.

Graphite of the Turgenevskoe and Tamginskoe deposits of the Lesozavodsk area in the Primor’e region

The regional carbonization of the Riphean metamorphic complexes is discussed using as an example the Tamginskoe and Turgenevskoe graphite deposits located in the northern part of the Khanka terrane. It is shown that the noble metal mineralization associates closely with the graphitization. Isotopic, X-ray, and thermal analyses and Raman spectroscopy were first used for investigating the structural state of the graphite with defining its two varieties. The first of them is represented by nanocrystalline fluidogenic graphite that was formed during gas condensate crystallization from deep-seated reduced ore-bearing fluid. The second variety (large-flake graphite) represents a product of metamorphic recrystallization of carbonaceous terrigenous protoliths. The recrystallization was accompanied by the granitization of the sedimentary protolith, mobilization, and the transfer of the carbonaceous and ore matter of the host rocks. It is inferred that the graphitization associated with noble metal mineralization is a polygenic process. The graphite of the first generation associates closely with amorphous diamond-like carbon. This unexpected find may bear genetic information useful for geological and geochemical reconstructions.

The influence of sulfur on chemisorption of gold by carbonaceous matter at 200–400°C and Ptotal = 1 kbar

pyrite. Experiments were performed in autoclave fur� naces at 200, 300, and 400°С and Ptotal = 1 kbar. Walls of golden ampoules, in which solid phases were loaded, were used as a source of gold. Pyrite and CM were put in separate nonhermetical ampoules. The amount of bidistilled water added was estimated by the P–V–T relation under concrete run conditions. Asphaltene obtained through fractionation of brown coal from the Pavlovsk deposit in Primor’e was used as CM. Preliminary analysis did not register gold in its composition. We used elementary sulfur (10 mg) or pyrite (30 mg) in runs. Run series without sulfur in the C–O–H–Au system was performed for comparison. All three run series on each isotherm were developed and analyzed simultaneously to minimize the influ� ence of experimental error during comparison of the results.

Gold solubility in aqueous-chloride solutions interacting with epidote propylites and epidote-hedenbergite skarns

The interaction of chloride solutions with the mineral assemblages of epidote propylites and hedenbergite skarns with and without sulfides was studied by experimental (at 300 and 400°C) and theoretical (250–400°C) at Ptot = 1 kb simulation. The buffer properties of combined ore-silicate assemblages were estimated. It was shown that the limit of the efficient operation of the buffer within the T-P range studied depends on the pH of starting solutions. It is ≤ 0.01 m HCl in the absence of sulfides and increases up to 0.1 m HCl in the presence of sulfides. It was found that gold solubility in chloride solutions is low owing to their neutralization by interaction with sulfide-free epidote propylites. The appearance of sulfides suppresses this effect and leads to an increase in gold solubility. The bulk solubility of gold is determined by the ore component of buffer mixture, mainly sulfide minerals. Gold precipitation begins at a threshold concentration, which is 0.004 mg/l (at 300°C) for sulfide-free epidote propylites, 0.06 mg/l (at 400°C) for sulfide-free hedenbergite skarns, and up to 0.29 mg/l in the same assemblages in the presence of pyrite.

Modeling of gold mass transfer during the listwanitization and rodingitization using the example of the Ust’-Dep ophiolite complex in the upper Amur territory

Gold mass transfer with chloride and carbonate-chloride solutions was examined at the 300 and 400°C isotherms and Ptot = 1 kbar by means of experimental modeling and theoretical simulations. CO2 was confirmed to suppress Au solubility in fluids. The low Au solubility (mAu < 10−8) determined in the experiments explains the mechanism of its precipitation when serpentinites and listwanites interact with acidic mineralized solutions. Listwanitization, which was genetically related with the emplacement of orogenic granitoids, was determined to have overprinted serpentinites and rodingites and strongly affected Au transport in the oregeochemical system. The characteristics of the metasomatic processes in the Ust’-Dep ophiolites and the gold concentration in the rocks produced by these processes confirm this conclusion.

Pt behavior in the Pt-C-S ± Fe-H2O system at 200–400°C and Ptot = 1 kbar: Experimental results

The effect of sulfur on platinum adsorption on carbonaceous matter (CM) was experimentally studied at 200–400°C and Ptot = 1 kbar. The IR spectra of the experimental products indicate that sulfur accelerates HC condensation and aromatization, but the effect of sulfur on platinum concentrations in the organic fractions is within the analytical uncertainties. SEM images show the development of a multilayer porous carbonaceous film on the walls of the ampoules and platinum in physical contact with carbonaceous matter. The composition of the film varies, depending on its thickness (3–25 μm), within the following limits: 61.06–100 wt % C, 0–33.7 wt % Pt, 0–5.17 wt % O, and 0–0.74 wt % S. The film contains tiny Pt crystals, whose morphology varies with increasing duration of the experiments from nanometer- and micrometer-sized spheroids to subequant, tabular, and wire-like. Depending on their size, the composition of the crystals varies as follows: 23.30–52.45 wt % Pt, 49.57–73.52 wt % C, and 0–4.20 wt % O. According to our SEM data, the kerogen also contains tiny crystalline segregations of carbon aceous platinum whose morphology and composition are analogous to those on the film. The presence of carbon in the tiny platinum crystals deposited from solution can be explained by the background effect of the kerogen of the film and/or by their crystallization from organo-platinum complexes. In our kinetic experiments, local electrochemical reactions produced aggregates of nanometer-sized (60–250 nm) spheroids around larger micrometer-sized (up to 10 μm) spheroids, whose aggregation resulted in larger crystals and their further transformation. The polymorphism, hierarchical aggregation, and compositional variability of the platinum segregations are likely typical of car- bon-bearing systems because of their crystallization from metastable organo-platinum complexes.