by Nikolai LAVEROV, Academician,

Vadim DISTLER, Dr. Sc. (Geol. & Mineral.), Institute of the Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry, Russian Academy of Sciences,

and Gennady MITROFANOV, Cand. Sc. (Geol. & Mineral.), East Siberian Institute of Geology, Geophysics, and Minerals, Ministry of Natural Resources of the Russian Federation

Metals in the platinum and gold group do not as a rule form deposits where they are present in equal proportions. For example, in the world-famous platinum deposits at Norilsk, Russia, and Bushvelde, South Africa, these metals are associated with nickel, iron, cobalt, and copper sulfides, and their gold content is at least two orders less than that of platinum. In turn, gold is to be found in various other deposits, the majority of which have none of the platinum group metals.

These localization specifics are attributed, above all, to different sources of ore material. In the case of platinum, these are normally deep-lying (subcrustal) basic and hyperbasic magmas, while most gold ore deposits were formed by hydrothermal solutions arising at low depths in the course of magmatic processes within the terrestrial crust.

In the past few years, we have witnessed the discovery and development of large deposits of gold, uranium, copper, lead, zinc, cobalt, etc., some of them containing platinum group metals as well. These ores occur in rift structures, in areas of weak magmatism, where they are localized in ancient rocks rich in organic carbon, the so-called black shale. These rocks are shaped into extensive transregional belts, but the metal ore deposits are only to be found in local fields where some types of metasomatism ^* and ore deposition occur. Some scientists ascribe the concentration of ore material in these rocks to the metamorphism of the black shale beds. In large uranium deposits, however, the ores of this chemical element and also those of gold, nickel, cobalt, and platinum group elements must, probably, have a different origin.

In geologic structure, the Sukhoi Log deposit north of Lake Baikal is similar to deposits localized in black shales. By some estimates, the deposits contain nearly 1,000 tons of gold in the ore bodies concentrated in terrigenous and carbonate terrigenous sedimentary rocks of the Middle and Upper Riphean (about 1 bln years ago). Accumulated during the formation of the continental rift system, these sediments metamorphosed into quartzite-sericite- chlorite shales with sandstone bands. The ore body is rich, from top to bottom, in organic matter (C _org ), the content of which varies from 2-3 to 5-7 mass percent.

The noble metal ore bodies in Sukhoi Log are arranged in laminations and come together with quartz-sericites and carbonate metasomatites, with quartz pockets and veins, magnesia-iron carbonates, and impregnated pyrite. Significantly, mineralized gold or any other metal ores do not occur beyond these meta-

^* Metasomatism-changes in the chemical composition of the original rocks.- Ed.

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morphosed ores. The platinum group is, in turn, located in the zone of weakly metamorphosed rocks, with secondary carbonates, quartz and impregnated sulfides, and also in the maximum gold mineralization zones.

The studies have shown the deposit to have some distinct mineralogical and geochemical specifics. In addition to the principal ore-forming minerals, pyrite and native gold, the researchers have found a large group of minerals, including those that have not occurred before: various native metals, solid metal solutions, intermetallides (chemical metal compounds), sulfides, arsenides, tellurides, sulfo-tellurides, selenides, bismuthides, anti- monides, and sulfosalts. The deposit ores have been found to contain many key elements, each with its own mineral phases. They include Au, Ag-Fe, Ni, Co, Cr, Ti, Pt, Pd, Zn, Cu, Pb-Sn, W, Mo, Th, and Zr, belonging to at least two geochemical associations of metals which, as a rule, never occur together. The first association of Fe, Ni, Co, Cr, Ti, Pt, and Pd is typical of hyper- basic-basic rocks, and the second, including Sn, W, Mo, Th, and Zr, is characteristic of common granitoids.

Of all platinum metals, platinum occurs most frequently always attended by palladium, at a ratio close to 10:1. Other platinoids are more rare occurrences. The platinum-rich areas of the ore bodies have a complex shape, localized in the upper sections of the gold ores and overlying rocks, devoid of commercially viable gold concentrations. In the base part of the ore body, platinum metals are scattered haphazardly, rarely aggregating into large accretions. The maximum mineralization zones of platinum metals and gold are, therefore, shifted relative to each other vertically.

It is extremely difficult, though basically important, to identify the manifestation forms of ores in black shale deposits of the Sukhoi Log type. Although they have been discovered previously in quantities above 1 g/ton in major gold ore deposits as well, such as Muruntau and Kumtor in Central Asia and Natalkino in Russia's northeast, their occurrence forms could not be identified always with certainty.

To look for platinum metal manifestations in Sukhoi Log, the prospectors used precision fractionation of minerals in fine and super-fine jigs, along with specialized spectroscopic techniques. They studied possible bonds between these metals and the organic material of the ore-bearing rocks and sulfide mineral phases and their own mineral carriers. The composition of the carbonaceous material was assayed in samples containing 0.5-2.5 mass percent of C _org . No correlation was found to exist between this parameter and the gross concentrations of noble metals. The organic material was dominated by kerogen (unstructured graphite-like phase), with sone gaseous phase (CO ₂ , CH ₄ , C ₂ H ₆ , H ₂ and N ₂ ), and soluble organic matter. Its principal components, however, were normal saturates, ethyl carbonates, high-molecular hydrocarbons, phthalates, and amides. There is, however, some difference in the concentration and variety of types of soluble organic matter in the gold ore body and its overlying and underlying non-metalliferous rocks.

Platinum metals were not discovered in the soluble organic matter, which only contained mercury, arsenic, and silicon compounds. In some fractions of insoluble organic material, however, the aggregate content of gold and platinum reaches 10 g/ton. Gold is probably present in fine native forms sorbed by the organic material surface. Most likely, platinum metals occur in this form as well.

Their mineral carriers were concentrated by subjecting ore samples to gravity washing, the greatest effect

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being achieved for fine grains (0.06 mm or less). The concentrates examined in a scanning electron microscope provided with a Link- 10,000 energy dispersion attachment showed loose grains of platinum metal minerals or their accretions with sulfides, or, more rarely, with ore-forming minerals. The grains measured 0.5-10 microns being predominantly native platinum and metallic solid solutions of the Pt-Cu-Fe system.

The samples studied showed platinum with low content of copper and iron and some phases rich in copper (probably those associated with tulaminite) and iron (close in composition to isoferroplatinum and tetraferroplatinum). Platinum diaroenide, or sperrilite, is contained in loose grains. Individual grains of palladium phases in the cotulskite and merenskite series were also detected, as well as solid solutions of palladium and silver with bismuth.

The Sukhoi Log ores contain a relatively large number of native metal phases, the most significant of which is native gold, whose composition varies within a wide range: from virtually pure gold Au _99.0 Ag _1.0 to more frequent Au _95.0 Ag _5.0 to Au _89.0 Ag _11.0 , and more rare phases, from Au _85.0 Ag _15.0 to Au _75.0 Ag _25.0 and also silver with admixtures of gold (which contained some mercury and copper impurities). In addition to gold and platinum, the researchers found a large group of a metal mix rarely occurring in nature - iron, nickel, chromium, aluminum, titanium, tungsten, lead, tin, and copper. These metals were also contained in two- and three-component alloys (solid metal solutions), sulfides, arsenides, tellurides, etc. The most characteristic effusion forms of native metals and their solid solutions included mutual accretions, inclusions in silicates and sometimes in pyrite. Native chromium and tungsten were found in accretions with a chlorine-rich phase.

A key factor in establishing the genesis of the ores is the presence of native metals in mineral inclusions formed at the main ore formation stage (associated with the bulk of gold). In particular, accretions of tin and lead and native aluminum and solid iron- nickel solution are present in inclusions as iron sulfides associated with gold. The latter was found to contain ingrowths of solid nickel- antimony, nickel-tin, and tellurium-bismuth solutions. Platinum and solid solutions of the Pt-Cu-Fe system form inclusions in pyrite occurring alongside gold. The ore structure is evidence that platinum mineralization and a large group of native metals were preceded by gold deposition.

Analysis of the mineralization sequence on the results of specialized observations has enabled the researchers to compile a general evolution chart of ore deposition in the Sukhoi Log deposit. The chart is based on the conceptions about the growing sulfur activity from an early ore deposition stage to later stages involving changes in the reduction-oxidation conditions. The native mineral formation stage in this chart is matched by the stability of the low- sulfur mineral association comprising iron and nickel sulfides with a low sulfur content (ferruginous pyrrhotite and pentlandite) and some tin, zinc, and copper monosulfides, and complex sulfides of the same elements. A much later stage saw the effusion of the bulk of iron disulfide, which was accompanied by deposition of lower grade gold and compounds of copper, nickel, iron, gold and silver of other classes. Notably, the forms of nearly all elements and ores range from native to solid metal solutions and intermetallides, and then on to various simple and complex sulfides and other related compounds.

Studies of gas-liquid inclusions in quartz and carbonates associated with the ore minerals of Sukhoi Log were carried out to assess the temperature conditions of ore deposition. They have shown that the initial temperature of the fluids, from which minerals of the main gold ore stage were deposited was at least 310 ⁰ C. The preceding platinum metals were, most probably, formed at higher temperatures (450 to 310 ⁰ C).

Thus, genetic models that can be developed to stimulate the formation of gold deposits within black shales containing platinum mineralization are to take account of the nature of the ore forming process in respect of the enclosing rocks. Of no less importance was the addition of other elements to the ore formation zone from high-temperatures solutions. Some role could be played by the carbon of the enclosing rocks as a geochemical barrier to ore components.

Development of native metals within the ore bodies shows that they were deposited in highly reducing conditions arising, most probably, in a process associated with the hydrothermal combustion of mildly metamorphosed organic carbon.

The discovery and exploration of the platinum mineralization at the Sukhoi Log deposit can probably be examined in several aspects. First, realistic conditions have been created to increase significantly the general value of the ores the deposit contains. Second, many ore deposits in black shales have not been studied to trace any platinum mineralization or have been explored without the use of modern technologies and analytical techniques. Additional studies of such deposits using precision techniques are, therefore, a priority for modern geology concerned with ore deposits.

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