Yu. A. Kalinin

Nanoparticles of noble metals in the supergene zone

Formation of noble metal nanoparticles is related to various geological processes in the supergene zone. Dispersed mineral phases appear during weathering of rocks with active participation of microorganisms, formation of soil, in aqueous medium and atmosphere. Invisible gold and other noble metals are incorporated into oxides, hydroxides, and sulfides, as well as in dispersed organic and inorganic carbonic matter. Sulfide minerals that occur in bedrocks and ores unaltered by exogenic processes and in cementation zone are among the main concentrators of noble metal nanoparticles.The ability of gold particles to disaggregate is well-known and creates problems in technological and analytical practice. When Au and PGE nanoparticles and clusters occur, these problems are augmented because of their unusual reactions and physicochemical properties. The studied gold, magnetite, titanomagnetite and pyrite microspherules from cementation zone and clay minerals of laterites in Republic of Guinea widen the knowledge of their abundance and inferred formation conditions, in particular, in the contemporary supergene zone. Morphology and composition of micrometer-sized Au mineral spherules were studied with SEM and laser microprobe. The newly formed segregations of secondary gold on the surface of its residual grains were also an object of investigation. The character of such overgrowths is the most indicative for nanoparticles. The newly formed Au particles provide evidence for redistribution of ultradispersed gold during weathering. There are serious prerequisites to state that microorganisms substantially control unusual nano-sized microspherical morphology of gold particles in the supergene zone. This is supported by experiments indicating active absorption of gold by microorganisms and direct evidence for participation of Ralstonia metallidurans bacteria in the formation of peculiar corroded bacteriomorphic surface of gold grains. In addition, the areas enriched in carbon and nitrogen have been detected with SEM on the surface of gold spherules from Guinea. Such organic compounds as serine, alanine, and glycine are identified on their surface with Raman spectroscopy. The experiments have been carried out and new data have been obtained indicating the role of micromycetes in concentration and distribution of noble metals in ferromanganese nodules of the World Ocean. Au and Pt were detected in the system with radioisotopes. It has been established that two forms of gold distribution develop within pseudomorphs of fungi colonies: (1) as pseudomorphic concentrates and (2) dispersed form unrelated to the colony structure. Inhomogeneities in distribution of dispersed platinum are manifested in the form of linear anomalies with elevated concentrations at the margins of the colonies.

The Suzdal gold-sulfide deposit in the black shale of Eastern Kazakhstan

The Suzdal gold-sulfide deposit is situated in the northwestern part of the West Kalba gold belt in Eastern Kazakhstan and belongs to the genetic type of stringer-disseminated mineralized zones hosted in the Lower Carboniferous black-shale volcanic-carbonate-terrigenous sequences. Mineralization is controlled by the NE-trending Suzdal Fault. In the north, the deposit borders on the Early Triassic Semeytau volcanic-plutonic structure. Mineralization is superposed on the Late Paleozoic complex of metadolerite and quartz porphyry dikes. Ore deposition was a long-term process comprising four stages. The first stage was related to deposition of slightly auriferous pyrite syngenetic to host rocks. The second stage is characterized by formation of the first productive (with invisible gold) fine-acicular arsenopyrite mineralization accompanied by sericitization and localized in the tectonic zone. The stockwork ore with pocket-disseminated base-metal mineralization and free microscopic gold of the third stage is hosted in silicified rocks. The ore formation has been completed by quartz-stibnite veins superposed on all preceding types of mineralization. According to Ar/Ar dating of sericite, a chronological gap between the second and the third stages is estimated at 33 Ma. The deposit is an example of polygenetic and multistage mineralization.

Forming conditions and age of native silver deposits in Anti-Atlas (Morocco)

663 Mercury bearing silver and Ag and Hg intermetal lides are described in primary and oxidized ores from different type ore deposits: Ni–Co–Ag–Bi–U, mer cury, polymetallic, Ag–Sb, Ag–Hg, and others. At the same time, the economic concentrations of these min erals are recorded only in ores of the Ni–Co–Ag–Bi– U and Ag–Hg deposits. The Ag–Hg deposits repre sent a peculiar separate type of silver mineralization, which distinctly differs from other types by the domi nant role of native Hg silver (main ore mineral) and the presence of different sulfides and Ag sulfasalts. There are several ore areas with such mineralization: Anti Atlas in Morocco, Verkhoyansk region in Yaku tia, the Altai–Sayan region, and others. Anti Atlas, which hosts three known deposits of the Ag–Hg type (Imiter, Igoudran, Zgounder) and several ore occur rences including those in the ore field of the Bou Azzer Co deposit [1–3], is most famous among them. The Imiter deposit, development of which started as early as in the 8th century and continues at present, is the largest of these deposits. The Imiter and Igourdan deposits are located among Middle Proterozoic terrigenous rocks repre sented by sandstones, siltstones, and black shales, which are intruded by Neoproterozoic gabbro, diorite, and granodiorite bodies. They are unconformably overlain by the Upper Neoproterozoic effusive sequence composed of andesites, rhyolites, and ign imbrites with dikes and stocks of andesites, dolerites, rhyolites, and granite porphyries. The Neoproterozoic igneous rocks in the ore field of the Imiter deposit host age and type variable mineralization: small occur rences of Mo greisen, Au–Ag and polymetallic Ag bearing, Co–As, and barite ores. The Neoproterozoic rocks are overlain by Cambrian platform strata with post Ordovician dolerite, monzonite, and K microsy enite dikes. The biotite and K feldspar 40Ar–/39Ar ages of microsyenites determined at the Analytical Center of the Institute of Geology and Mineralogy (Novosi birsk) are estimated to be 204.5 ± 2.5 and 199.5 ± 2.4 Ma, respectively (Fig. 1).

Spatial-temporal and genetic relationships between gold and antimony mineralization at gold-sulfide deposits of the Ob-Zaisan folded zone

The Ob-Zaisan folded zone is a fragment of a single structure composed of Paleozoic sedimentary and volcanogenic rocks (mainly black shale), which was formed at the margin of the Siberian continent and features a common set of magmatic complexes and mineral systems. However, there are some differences that determine the specific geological and metallogenic features of the Irtysh-Zaisan and Kolyvan-Tomsk fragments of the Ob-Zaisan folded zone. In the gold deposits of the West Kalba and Kolyvan-Tomsk auriferous belt, the main gold-sulfide mineralization is controlled by zones of shearing and dynamic metamorphism in carbonaceous carbonate-terrigenous rocks. This type of mineralization was formed in tectonic blocks in a compressional setting. Antimony mineralization is characterized by brecciated textures and the vein-like morphology of ore bodies, reflecting extensional tectonics. At some deposits (Zherek, Mirazh, Dalny), Sb mineralization is spatially separated from the main gold-sulfide ores and shows cross-cutting relations to the principal ore-controlling structures. In other gold deposits, stibnite is spatially associated with disseminated gold-sulfide ores and forms mineral assemblages with Ni, Co, Au, Pb, and Fe (Alimbet, Zhanan, Legostaevskoe, Semiluzhenskoe, and Kamenskoe deposits).This study reveals no direct correlation between Au and Sb in gold-sulfide ores of these deposits. SEM analysis indicated the absence of free gold in stibnite veins. However, atomic absorption and electron microprobe analysis indicated the presence of “invisible gold” from a few ppm to several tens of ppm in the stibnite. High gold contents in the gold-sulfide ores overprinted by antimony mineralization (Suzdalskoe, Zhanan, and Legostaevskoe deposits) can be explained by the processes of regeneration and redeposition.The results of microstructural observations, isotope geochronology, studies of mineral assemblages and fluid inclusions in the ores from gold deposits of the Ob-Zaisan folded zone suggest that antimony mineralization was formed at gold-polysulfide stage, which was separated from the early ore pyrite-arsenopyrite stage by a 30 Ma time gap. It can be assumed that the essentially stibnite mineralization was formed at a separate stage and was separated from the gold-polysulfide mineralization by a 7 Ma interval of tectonic activity. Our Ar-Ar data on sericite from ore samples, combined with U-Pb data on zircons from igneous rocks and previous data from the literature show that there were two major stages of ore formation: the Early Permian (282-270 Ma) and the Early Triassic (250-240 Ma). Most researchers suggest that these stages of mineralization can be related to the epochs of intraplate magmatism that led to the formation of the Tarim (280 Ma) and Siberian (250 Ma) large igneous provinces. These global geological events are generally connected with the influence of Tarim and Siberian mantle plumes.

Character of gold mineralization in the late Cenozoic alluvium of the Bauntov area (Vitim highland, Transbaikal region)

Study of gold potential of the Late Cenozoic alluvium in the Bauntov area is based on the Taloi basin and Sivo sector (China River basin). Primary placer-forming gold is mainly represented by endogenous varieties. In contrast to alluvium of streams in the Taloi basin, Middle-Upper Quaternary alluvium of the Sivo sector (China River basin) is enriched in fine and tiny gold (smaller than 0.25 mm). The paper presents data testifying to a low Au content in the silt fraction of slimes from boreholes drilled in the Cenozoic alluvium (alluvium-deluvium) from transverse valleys in the Taloi basin and, hence, to an significant manifestation of the chemogenic concentration of this metal therein. Issue of the existence of specific conditions favorable for the formation of hydrogenic gold in the northern high-latitude zones requires further studies. The results show that lithochemical gold aureoles have an important prospecting significance. They also suggest the practical significance of accumulations of fine and tiny gold in the syn-stratal alluvium and alluvium-deluvium of large ore placer areas.

Disseminated gold–sulfide mineralization at the Zhaima deposit, eastern Kazakhstan

The Zhaima gold–sulfide deposit is located in the northwestern part of the West Kalba gold belt in eastern Kazakhstan. The mineralization is hosted in Lower Carboniferous volcanic and carbonate rocks formed under conditions of marginal-sea and island-arc volcanic activity. The paper considers the mineralogy and geochemistry of primary gold–sulfide ore and Au-bearing weathering crusts. Au-bearing arsenopyrite–pyrite mineralization formed during only one productive stage. Disseminated, stringer–disseminated, and massive rocks are enriched in Ti, Cr, V, Cu, and Ni, which correspond to the mafic profile of basement. The main ores minerals are represented by finely acicular arsenopyrite containing Au (up to few tens of ppm) and cubic and pentagonal dodecahedral pyrite with sporadic submicroscopic inclusions of native gold. The sulfur isotopic composition of sulfides is close to that of the meteoritic standard (δ34S =–0.2 to +0.2). The 40Ar/39Ar age of three sericite samples from ore veinlets corresponds to the Early Permian: 279 ± 3.3, 275.6 ± 2.9, and 272.2 ± 2.9 Ma. The mantle source of sulfur, ore geochemistry, and spatial compatibility of mineralization with basic dikes allow us to speak about the existence of deep fluid–magmatic systems apparently conjugate with the Tarim plume.

Antimony Mineralization at the Gold Deposits of the Ob‐Zaysan Folded Zone (Southern Siberia and Western Kazakhstan).

The Ob-Zaysan folded zone is a fragment of a single marginal-continental structure of the Siberian continent, made up of Paleozoic sedimentary and volcanogenic rocks (mainly black shale) and characterized by a common set of magmatic complexes and mineral systems. However, there are some differences which determine the geological and metallogenic specific features of the Irtysh-Zaysan and Kolyvan-Tomsk areas of the Ob-Zaysan folded zone. In the gold fields of these areas the main gold-sulfide mineralization (orogenic gold style) is controlled by sheared zones and is formed under tectonic compression in long-developing block structures.

Forecasting and prospecting criteria of Au-bearing weathering mantles in southern Siberia, Russia

The geological and genetic model of gold concentration at different stages of weathering has been developed on the basis of long-standing geological and geochemical research of Au-bearing weathering mantles in the main ore fields of Siberia. This model makes it possible to substantiate forecasting and prospecting criteria of gold deposits hosted in linear karst zones of weathering, to delineate prospective areas in southern Siberia, and to estimate their resource potential.