G. G. Likhoidov

Noble metals in graphitized rocks of the Khanka terrain (Primorye): Evidence from analytical results based on decomposition by the fluoroxidizing method

Development of the geology, geochemistry, and analytical chemistry of noble metals (NM) in the past 30 years has provided new insights into the nature of unconventional complex gold and platinum group metal (PGM) deposits. In particular, such deposits confined to black shales were discovered in Caledonides of the Central Asian orogenic belt, such as the well-known Muruntau, Kumtor, Zun-Kholba, Irokinda, Sukhoi Log, Nezhdaninskoe, Natalka, and others. In the past 10‐15 years, commercial PGM concentrations were detected in ores of these deposits [1‐4 and others]. The PGMs in these deposits are traditionally assigned to the carbonaceous matter (CM) of rocks. However, it turned out that they are likely (and primarily) associated with sulfides. Geochemical aspects of PGM concentration in rocks (and ores) and issues of their reliable determination have attracted the main attention in discussions devoted to the problem of PGM occurrence in black shales in the recent period of more than 20 years [4, 5, and others]. The Khankai massif is located at the center of the Central Asian orogenic belt in the Russian Far East. The NM mineralization (Au + PGM) was recently discovered in graphitized rocks [6] of the well-known graphite deposits in the Lesozavodsk region of Primorye [7]. Relative to the typical black shales mentioned above, these rocks are almost devoid of sulfides [8]. However, this interesting feature is provoked apparently by technical difficulties in the analysis of PGMs. For example, analysis of Au and Pt by glow discharge ion mass spectrometry (GDI MS) [9] yielded 3‐30 and 4‐52 g/t, respectively. Duplicate analysis of the same samples by atomic coupled plasma atomic emission spectrometry (ICP AES) and atomic absorption spectrometry by thermoelectric atomization with prior extract concentration (EC TEA) yielded, on average, two orders of lower Au concentrations, while the presence of Pt was not confirmed at all [10]. However, such contradictory results are common for the analysis of black shales due to both poor representativeness of the samples analyzed and losses of the NM during the chemical preparation of samples and their transfer to solutions, as was demonstrated for the Natalka deposit [5, 11, and others].

Experimental modeling of platinum behavior under hydrothermal conditions (300–500°C and 1 kbar)

The behavior of Pt was studied in the Pt-Fe-S-Cl-H2O, Pt-Fe(Ni)-As-S-Cl-H2O, and Pt-Ni-As-Cl-H2O systems. Kinetic experiments showed that the addition of As and S to the system changes the character of Pt complexing and results in a decrease in the bulk Pt content in the solutions. The intermediate complexes that formed during this process disproportionated to produce cooperite and sperrylite. Under the experimental P-T-μi conditions, the hydrothermal mobility of Pt was mainly provided by its hydrosulfide complexes with a definite participation of chloride complexes. The presence of Ni in the system lowers the redox potential and Pt solubility and prevents the formation of Pt phases, while Ni sulfides and arsenides crystallize copiously. The behavior of Pt and Au in hydrothermal systems and mechanisms of hydrothermal formation of noble metal minerals were considered.

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.

Experimental study of platinum interaction with carbonaceous matter and sulfur at 200–400°C and 1 kbar

1051 The potential of platinum mineralization in car bonaceous rocks (black shales) attracted the attention of geologists after the discovery of a high concentration of PGEs in ores of gold and copper deposits [1–3]. The PGEs in these rocks are characterized by diversity: native metals, intermetallics, sulfides, arsenides, and presumably metal–organic compounds [2, 4]. The existence of platinum–hydrocarbon complexes was supported by experimental studies [5] and later by finds in sublimates of Kudryavyi Volcano [6].