MIRONOV

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Dr. Anatoly G. Mironov's Post-Symposium Lectures

 

To schedule a lecture by Dr. Mironov, please contact:

 

Lindsey V. Maness, Jr., Chairman

Resources & Technology Symposia of Colorado, Llc. (RTSC)

12875 West 15th Drive

Golden, CO 80401-3501 USA

Tel: 303-237-6590 E-Mail: lvmaness@china-resources.net

 

Dr. Anatoly G. Mironov, Geochemist with the Russian Academy of Sciences, Siberian Branch, is offering to present these talks in the Denver Metro area in the days following the CEAR 2,000 Symposium. For more information about Dr. Mironov, please check the "ABSTRACTS" portion of the CEAR 2,000 web-site.

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ABSTRACTS of LECTURES

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URANIUM DISTRIBUTION IN BAIKAL SEDIMENTS USING SSNTD FOR PALEOCLIMATE CHANGE

Mironov, A.G.1, Zhmodik, S.M.2, Zhatnuev, N.S.1, and Bugaeva, N.G.1

1Geological Institute of Siberian Branch of RAS

2United Institute of Geology, Geophysics and Mineralogy, SB RAS

Complex study of Baikal lake is being done with the aim of paleoclimate deciphering of Asian continent at present.It has been stated that one of the most sensitive indicators of paleoclimate change are data on trace elements distribution and particularly uranium in floor sediments of Baikal, its age being about 20 Ma. n,f-Radiography has been used for study of local spatial uranium distribution. It enables not only to determine contents of U various forms presence in sediments with high sensitiveness (n10-5 -10-6 wt.%), but to state regularities of their localization in sediments of various composition.

Sediments core has been used from VER-95-2 station, polished sections being made on quartz glass. Synthetic F-flogopite has been used as SSNTD. Polished sections have been irradiated with thermal neutrons of fluence 1 10 16 n/cm2. Tracks were calculated on area of 0,005mm2 particularly for regularly distributed (chemogenic) uranium and detrital uranium minerals. Synthetic glasses with various uranium contents have been used as standards.

Three forms of uranium presence have been found in clayey and diatoms sediments: regular one in clays and diatoms, in microfragments of zircon, apatite, ilmenite and concentration of chemogenic uranium in diatom sediment fragments. Rhythmicity has been stated in U-distribution. Its deciphering can permit restoring very short periods of climate change, considering that sediments accumulation rate is 4-6 ?m/1000 years in Baikal and resolution of n,f-radiography is 10-15 mkm.

The work has been done at RFBR financial support. Grant is N 97-05-96379.

 

EXPERIMENTAL TEST OF RECYCLING MODEL FOR GOLD AND SILVER IN SEA-FLOOR SULFIDE ORES AND GOLD DEPOSITS IN OPHYOLITE BELTS

Anatoly G.Mironov & Nikolay S.Zhatnuev

Buryat Geological Institute, RAS SD, Ulan-Ude

ABSTRACT: The research is aimed at testing the hypothesis of recycling concerning gold and silver sources in connection with gold and silver ore formation in modern see-floor sulfide ore beds of back arc spreading zones and gold deposits in ophiolite belts. It was carried out some experiments to study the gold and silver mobilization, transportation and presipitation during the process of hydrothermal alteration of volcanic glasses of a different composition at the temperature range from 200o to 500oC and pressure of 200, 500 and 1000 bars by using hot seewater under isothermal and thermogradient conditions with the use of radioactive tracer method.

1 INTRODUCTION

The sea-floor sulfide ores of some districts of mid-ocean ridges and especially back arc spreading zones are enriched with gold and silver concentration. It is possible that some sulfide gold deposits in ancient ophiolite belts have the same origin. At present the recycling hyposthesis is used in explaining of the source of the heavy metals (Fe, Cu, Zn) of the see-floor sulfide beds. The main idea of this hypothesis is extracting ore elements from the wall rocks by the hot seawater under it circulation in sea-floor spreading zones. In connection with the discovery of high contents of such elements as gold and silver in sea-floor sulfide ores the problem of their source arises (Hannington et.al., 1986; Sudarikov et al., 1990). The necessity is became to test the recycling hypothesis by experimental methods for explaining of gold and silver accomulating in sea-floor sulfide ores and of the origin of some gold deposits in ophiolite complexes. Many points of this hypothesis are conformed by experimental investigations (for Fe, Cu, Zn) (Bischoff, Dickson, 1975; Hajash, Chandler, 1981). As for gold and silver these data are absent.

2 METHODS OF INVESTIGATION

Some series of experiments of high temperature and pressure in isothermal and thermogradient conditious were carried out using radioactive tracer method. The experiments have been made under various temperature (200o-500oC) and pressure (200-1000 bar) with the use of water and water solutions of 1% H2S, 10% HCl and 3,5% NaCl, oxigen fugacity 10-8 -10-27 bar. Radioisotopes of gold (195Au) or silver (110+110m Ag) with stable isotopes of these elements were placed in basalt, ryolite and andesite glasses with total concentration of gold = 50ppb, silver = 50-100 ppm. The gold and silver distribution was studied by the -autoradiographical method on the nuclear emulsion plates. Polish preparations and thin cross section of glasses after experiments were analysed with the use of electron microprobe analysis. The width of the alteration zones was measured by the microscope with the precision 0,05 mm.

3 RESULTS OF EXPERIMENTS

Five major factors were selected to establish in nature the process of the water-rock interaction: temperature, pressure, composition of solid phase, fluid composition and the regime of oxigen potential within the system. Three zones of alterations were observed in glass samples after experiment: leaching zone, re-cristallization zone and hydrotation zone.

In the zone of maximum alteration (leaching zone) one can observe removal silica, alcaline and the increase of the concentrations of calcium, magnesium, iron and water. The zone of re-cristallization and dehydratation is established in acid glass and is characterized with appearance of fine grain quartz. The hydratation zone is distinguished from original glass with water concentration only.

Autoradiographic studies of glasses after the experiments show that in the experiment regime considered (distilled water, P=1000 bars, buffer NB, T=500, 400, 300, 200oC). One can observe distinct redistribution and removal of gold and silver of various intensity depending on the degree of glass alteration.

Maximum removal of silver and to a less extent of gold is observed out of outward and near-crack zones of intensive alteration at temperatures of 400 and 500oC. Loss of gold and silver from these parts of samples makes 80-95%, especially in glasses of basalt composition. In some samples it is seen that gold removal zones are conformous with the alteration zones at the surface of samples and near cracks. In a number of experiments distinctly seen is the redistribution of gold against crack-formation where there emerge the products of hydrothermal alteration (including clay minerals). In autoradiograms one migh see the spots of intensive blackening which correspond to microsegregations of free (metallic) gold. Silver in these zones is quite often exhibited by oreoles of intensive blackening (with no spots) which evidences of its redepositing seemingly in a sorbed form on the products of hydrothermal alteration of glass.

A similar picture is observed both for rhyolite and andesite glasses with somewhat greater removal of silver (80-90%) in contrast to gold (25-60%).

The intermediate zone (hydration) is characterized for a more complex behaviour of gold and silver conditioned as it will be shown below by the influence of oxygen regime in the system and by pressure. In the experiments with the parameters mentioned (water fluid, P=1000 bars, oxygen buffer NB and +200-500oC) silver is actively removed from the hydration zones of all glasses of various composition (60-90%). Gold is less removed out of hydration zones of basalt glass (50-60%) and is less mobile in those of andesite and rhyolite glasses. In some experiments with non-controlled oxygen regime, gold is preserved in the hydration zone of all glasses (the removal is nomore than 10-20%).

4 CONCLUSIONS

The experiments have showed that in dependence of the rock composition and conditions of the hydrothermal alteration the volcanic rocks may serve as sources of gold and silver. At this depending on the temperature, pressure and oxygen fugacity they may be removed conjoently either separately (chiefly silver), or they might be redistributed in recrystallization zones with partial change of natural oceurence and enrichment of some zones of rock alteration, preconditioning the process of preliminary concentration of metals.

REFERENCES

Bischoff, I.L., Dickson, F.W. 1975. Seawater-basalt interaction at 200oC and 500 bars: implication for origin of sea-floor heavy-metal deposits and regulation of seawater chemistry. Earth and Planetary Science Letters, 25: 385-397.

Hajash, A., Chandler, G.W. 1981. An experimental investigation of high-temperature interactions between seawater and ryolite, andesite, basalt and peridotite. Contrib.Mineral.Petrol., 78:240-254.

Hannington, M.D., Peter, I.M., Scott, S.D. 1986. Gold in sea floor polymetallic sulfide deposits. Econ.Geol., v.81:1867-1883.

Sudarikov, S.M., Ashadze, A.M., Stepankova, T.V. et al. 1990. Hydrothermal activity and ore forming in spreading zone of MAR. Doklady Akademii Nauk SSSR, v.311,2:440- 445.

 

GOLD DEPOSITS IN OPHIOLITE BELTS AND RELATED BLACK SHALES OF SOUTHERN SIBERIA

MIRONOV A.G., Geological Institute Ulan-Ude, Russia

ZHMODIK S.M., United Institute of Geology, Geophysics and Mineralogy, Novosibirsk, Russia

In Southern Siberia series of gold deposits is referred to carbonaceous rocks included into ophiolite complexes. Gold mineralization is related to black shale sulfide zones or quartz vein in basic and ultrabasic rocks. The stratiform sulfide bed and high contents of platinum group elements in ores and black shale sections is inherent to this gold deposit type. The most part of these deposits have a number of specific features such as: 1) wide spread stratiform sulfide ores; 2) simultanious presence in ores of sulfide minerals formed under high and low sulfur activity; 3) presence of fine grained sulfide ores with blue opal quartz ovoides (0,5-1,5 cm); 3) very large variation of silver content in the native gold (30-60%) and many others. Paleogeodynamic reconstruction for the Late Riphean and Early Paleozoic complexes of this region showed that marginal seas, backarc basins and backarc basin spreading zone may be distinguished there. To the end of the Early Paleozoic this whole vast oceanic area with ophiolites and volcanic areas was consumed in subduction and tectonic collision zones as a result of accretion of the microcontinent and other tectonic structures to the Siberian craton. Geochemical and isotopic peculiarities of black shales and their ophiolite belts us well as sulfide ores of some gold deposits allow to draw an analogy to the modern sulfide ores from hydrothermal vents of back arc basin spreading zones in the south-west part of the Pacific ocean. The S isotope values in sulfide ores showed a range from +1,5% to 5,1% that implies a contribution of hydrogen sulfide released from sea water sulfate in the deep hydrothermal reaction zone. Pb isotope rations determined in pyrite and galena indicated a narrow interval of very low values 206Pb/204Pb (15.398-15. 817), 207 Pb/ 204Pb (15.020-15.180), 208Pb/ 204Pb (35.800-36.210). This isotope signatures indicates that primary ophiolitic (mantle) crust is the source of lead. The 87 Sr/86 Sr ration of gold-bearing sulfide ores ranging from 0,710 to 0,713 re higher than those of the oceanic basalts but are similar to seawater (0,7091). The 12C values (from -20,0%to -25,5%) of graphite from Zun-Kholba gold ores indicate a biogenic nature of the organic matter in the black shales.

 

LARGE ZUN-KHOLBA GOLD ORE DEPOSIT: PROBLEMS OF STRUCTURE, COMPOSITION AND GENESIS

A.G.Mironov, P.A.Roshchektaev, S.M.Zhmodik, A.A.Kulikov, N,S,Karmanov

Zun-Kholba gold ore deposit, found in 1955, is situated in S-E part of East Sayan, in N-E framing of Garga block. Archean-early-proterozoic rocks of complex of Garga block base, schist-carbonaceous complex of riph-vend, ophiolite complex, Barun-Kholba volcano-plutonic complex and plagiogranites of Sumsunur complex of early-middle-paleozoic age compose geological structure of Kholba ore area. The deposit is localized in schist-carbonaceous rocks, Irkut (Mongoshin) and volcanogene-terrigene rocks of lychir series crushed into folds of high orders and undergone schist formation and mylonitization. Gold-quartz-sulphide mineralization has been presented by mineralized aons and vein-shaped ore bodies with layered and lens-shaped banded, quartz-sulphide massive pyrites and pyrite-polymetallic ores as well as quartz veins and veinlet-impregnated ores. Gold contents in ores vary from 0,6-0,8 g/t (in black schists) to 90-100 g/t (in quartz-sulphide ores). The characteristic features of ore mineralization are vivid prevalence of fine gold, referred mainly to sulphides, heterogeneous gold composition (pirity of some gold grains is from 300 to 950). Many peculiarities of geological structure, mineralogy and ore bodies composition, data on isotope composition of sulphur, lead and carbon enable to refer the deposit to poligenic and polychronic ones with participation of plenty of processes, beginning with sedimentary-hydrothermal sulphide-formation in arc-behind spreading zones and ending with typical quartz veins formation.

 

PRECIOUS METALS IN BLACK SHALES FROM THE TRANSBAYKALIA REGION: GEOCHEMISTRY, MINERALOGY AND ORIGIN

Anatoly G.Mironov, Alexey A.Kulikov, Yury Ch.Ochirov & Alexander S.Yatsenko

Geological Institute, Academy of Science, Ulan-Ude, Russia

Sergey M.Zhmodik

Associated Institute of Geology, Geophysics and Mineralogy, Academy of Science, Novosibirsk, Russia

 

ABSTRACT: Precious metal-rich black shales in the Transbaykalia region (Southern Siberia) occur in different levels of the Upper Proterozoic and Early Paleozoic volcanic-sedimentory deposits. High concentrations of Au (0,5-3,0 ppm) and PGE (0,1-2,2 ppm) are located in black shales with sulfide mineralization and graphite-bearing quartz-sulfide metasomatites. Extreme concentration of Ag (20-53 ppm) is found in sulfur-free carbonaceous slightly crystallization flints. The origin of precious metals anomaly concentrations is connected with exhalative fluids forming chemical sediments and replacement ores in black shales and alkali rocks intrusion.

1 RESULTS OF INVESTIGATIONS

1.1 Geology

Black shales are mostly spread in Northern (the Kotera synclinorium) and Southern Pribaikalye (the East Sayan). In the Kotera synclinorium shales are met in the Riphean deposits composing a trough of 8000-9000 m total thickness. Carbonaceous deposits are spread in a section middle part where they form horizons and benches of 50-400 m thickness. The volcanogene and tufogene rocks of the main composition prevail in a section lower part; the section is completed by carbonate rocks with black shale horizons of 1,5-20 m thickness. The quantity of carbonaceous aleuroshales in the general section of sedimentary deposits is 5-10% in the lower part (the Ukolkit suite), 20-45% in the middle part (the Nyandonin suite) and 15-20% in the upper part of the section (the Barguzin suite)(Mironov et al.1991).

Primary carbonaceous-flinty-clay shales are metamorphosed in a different degree (zonal metamorphism): from the chlorite-sericitic subfacies of green schist facies to epidote-amphibolitic facies. These deposits have the most distinctive composition and are enriched in sulphur (3-5%, to 20%) and organic carbon (2-4%, at most to 19%). The often observed alternation of carbonaceous shale intercalations both with carbonate rocks, and with metasandstones, the presence of cross lamination with the angles of layer slope to 5-10o, moderate rock silica (SiO2 is 55-62 weight %) and the noticeable prevalence of FeO over Fe2O3 (Fe2O3/FeO=0,22-0,55) and Na2O over K2O are characteristic for the Kotera synclinorium carbonaceous deposits.

1.2 Geochemistry and mineralogy

The detailed geochemical researches of the Kotera synclinorium black shales permitted to determine quantitative characteristics for many microelements and the general tendencies of their behaviour during the formation of the Kotera synclinorium sediments. The heightened contents of uranium (the coefficient of accumulation, Ca=2-5), molybdenum (Ca=2-4),vanadium(Ca=2-5), silver (Ca=3-8), copper (Ca=2-10), nickel (Ca=5-15),zirconium (Ca=2-7) and the lower contents of thorium (Ca=0,5), potassium (Ca=0,7), phosphorus (Ca=0,3) and manganese (Ca=0,7) are characteristic for the black shale horizons of the section upper and middle parts. The accumulation of these elements is observed from below upwards in the horizons of carbon-bearing rocks. Au, Pt and Pd average contents vary from 5-7 ppb to 25-35 ppb in some horizons. The following groups of elements having a strong correlation bond are revealed according to the data of correlative and factorial analyses, as: Cr-V-Mo-U, Co-Ni-Cu-Zn and U-Cu-Ag are for carbonaceous-flinty slate; U-K-Au and Au-K-Cr-Y-Zr are for carbonaceous-clay shales and U-Ag-Ba-V-Y, Ni-V-Mo and Au-K-Ni-Co for carbonate carbonaceous shales.

The high contents of gold and platinoids are marked out at several levels in the Kotera Synclinorium section middle and upper parts. The detailed study of the structure and matter composition of the shales composing these horizons has shown that the following three types of carbonaceous deposits can be singled out which have the heightened concentrations of Au and EPG, as:

(1) carbonaceous-quartz-micaceous shales with the inset sulfide mineralization (2-7%) of pyrite and pyrrotite having thin inclusions of sphalerite and chalcopyrite. Thin gold (3-30 mkm) of 415-762 gold purity and 0,4-0,8 ppm total content is singled out in the shales of this type.

(2) zones of densely inset and continuous sulfide mineralization forming thin bodies (0,2-0,7 m) concordant with lamination. The number of sulfide minerals (pyrite, pyrrotite, sphalerite and chalcopyrite) reaches 25-50%. Thin gold of 0,5-1,0 ppm concentration is also singled out here which is localized mainly in sulfide minerals.

(3) zones of quartz-sulphide streaking in black shale horizons confined, as a rule, to the dikes of plagiogranitic composition. Quartz and sulfides (pyrite, rarely pyrrhotite, sphalerite and chalcopyrite compose short veins and lenses which are often boudinaged. Both thin gold (gold purity of 550-630), and coarse one (600-640) having the general content of 1,5-3 ppm are also met here.

High concentrations of platinum and palladium (0,1-2,2 ppm) are met in the black shales of the first and the third types in the Kotera synclinorium three locations. Platinum highest content (2,2 ppm) are fixed in the black shale horizon (the 1st type) in association with bituminous limestones in the Nyandonin suite middle part directly over the arkose sandstone key horizon. Palladium maximum content (1,1 ppm) is determined in the zones of silica veined. In a number of cases the heightened concentrations of platinoids are connected with the complex anomalies of uranium, nickel, zonc and copper (0,01-0,1 wt.%). Os and Ir heightened concentrations are revealed in the sulfide-rich horizons of the 2nd type black shales (up to 0,1 ppm).

In spite of the thorough mineralogical study of samples the forms of finding platinoids are not determined. The prevalence of thin particles (3-9 mkm) in a native form is revealed only for palladium.

1.3 Eastern Sayan black shales

The other region where black shales are widely manifested is the south-eastern part of the East Sayan (Dobretsov et al.1989). Carbonaceous rocks are met here at several stratigraphic levels of the Later Riphean and Early Paleozoic periods. Black shales in the Riphean deposits are closely connected with the ophiolitic complex and often occur in the ophiolitic cover foot in the form of patches of from 20-30 m up to 200-400 m thickness among green shales, metabasites and carbonate rocks and sometimes contain fragments and blocks of these rocks. Black shales of this type are carbonaceous-clay in composition, are enriched in K2O at the expense of the wide development of hydromicas and sericite and contain 1,5-5 weight % of a carbonaceous substance which is concentrated in intercalations and isometric piles creating puckering shale texture together with micaceous minerals. Sulphidization zones are developed locally in the form of stratiformed bodies composed by pyrite and pyrrhotite mainly and by sphalerite and chalcopyrite rarely (Zhmodik et al.,1985).

The Lower Paleozoic black shales are closely associated with sandstones and dolomites (the Dabanzhalgin suite). Carbonaceous-flinty black shales and black flints form horizons and lenses of 0,3-200 m thickness. These rocks are of carbonaceous-chlorite-sericite-quartzy composition with SiO2 high content (70-95 weight%) and Al2O3 and Corg moderate quantities (1,5-2%). Organic carbon contents reach 19% in some horizons.

Black shales of this type possess an original geochemical characteristics. The high contents of Au (12-42 ppb), U (8-72 g/t), Mo (50-120 ppm), V (100-800 ppm) and Ag (0,1-3 ppm) are characteristic for carbonaceous-flinty rocks. The most typical assotiations of elements marked out on the basis of correlative and factorial analyses are U-Mo-V-Ag-Corg, Au-U and Ni-Co-Cu-Sn. Pt (x=17,5 ppb) and Pd (x=18,9 ppb) average contents are considerably higher than the ones in the Riphean black shalrs (Pt=1,3 ppb and Pd=3,4 ppb).

Some horizons of carbonaceous-flinty rocks are enriched in uranium (0,01-0,1 wt.%), molybdenum (0,1-0,2 wt.%), vanadium (0,1-0,5 wt.%) and silver (20-53 ppm). Sulphur small quantity or its absence is characteristic. Extreme concentration of Ag is found in sulfur-free carbonaceous slightly cristallization flint. High concentration of Au, Pt, Pd (0,1-1,5 ppm) occur only in carbon-quartz-sulfide metasomatites with graphite. Such concentrations of precious metals are more typical for graphit-bearing metasomatites with uranium-thorium mineralization of Botogol graphite deposits. The study of carbon isotopes revealed that the 13C values both in black shale (13C=-20--27%) and graphite bearing metasomatite (13C=-16--22%) indicate on the biogenic character of carbon material.

CONCLUSIONS

Precious metal-rich black shales in the Transbaykalia occur in different levels of the Upper Proterozoic and Early Paleozoic deposit. The first level the horizons enriched with sulfides of Fe, Zn, Cu among Riphean volcanic-sedimentary deposit of Nothern Pribaykalia (Kotera synclinorium). Metamorphism of black shales promote to desintegration of primary pirite with an allocation of pirrhotite, chalcopyrite and sphalerite, with an enlargement of gold particles. The second level - the horizons of Riphean black shales associating with ophiolite complex rocks (Eastern Sayan) contain hightened concentrations of Au, Pt, Pd in local bodies of Sulfide deposits. Conceivably such sulfide beds might represent as a relic of sea-floor sulfide ores. The 34S values in pyrite from these sulfide beds ( 34S=-1--6%) indicate the involvement in they formation of mantle sulfur contaminated with sea water sulfur. The third level - the carbonaceous flinds and black shales among low Paleozoic limestoun content the high concentration of Ag with tipical geochemical association of U-Mo-V-Ag. The appearance of gold and platinum mineralization is connected with graphit-bearing metasomatites localized in carbonate rocks and alkalirocks intrusion.The origin of precious metals anomaly concentration is connected with exhalative fluids forming chemical sediments and replacement ores in black shales and alkali rocks intrusions.

REFERENCES

Dobretsov, N.L., Belichenko, V.G., Boos, R.G., et al., 1989. Geology and ores of the East Sayan. Novosibirsk: Nauka, 127p.

Mironov, A.G., Zhmodik, S.M., Yatsenko, A.S., et al., 1991. Geochemistry and formation types of black shales from Northern Baykal region. In book: Geochemical researches and explorations of precious and radioactive elements. Ulan- -Ude, 23-25.

Zhmodik, S.M., Mironov, A.G., Nemirovskaya, N.A. et al., 1985. Geochemical model of gold - raremetal polygenetic deposits formation in black shales. In book: Genetic models of endogenic ore formation. Novosibirsk,v 2.

 

THE ROLE OF OCEANIC CRUST FRAGMENT IN GOLD ORE DEPOSITS FORMATION

 

A.G.Mironov, S.M.Zmodik

Positional connection of many gold ore deposits with ophiolite belts, in the boundaries of which black schist deposits, occurring either at the lower part of ophiolite covers or inside them among volcanogene and volcanogene-sedimentary series are developed, is widely known. Geochemical studies of basic-ultrabasite rocks and carbonaceous deposits have often shown higher gold contents (n10- 6 -10-5 mass.%) caused probably by regional peculiarities of ophiolite complexes, original enrichment carbonaceous deposits and presence of sulphidization zones.

Geodynamic reconstructions of such rock complexes have shown that they are relics of oceanic crust obduced into continental subdued stratas. Oceanic crust has subduced to a great extent and has not shown in accretional blocks.However, both types of geodynamic development of oceanic crust can have direct relation to gold ore deposits formation.

1) Sulphide of oceanic bed often have high (to industrial) gold and silver contents in modern back-arc spreading zones. Paleotectonic reconstructions of ophiolite complexes, with which gold ore deposits are connected testify to analogy in their formation with modern of marginal seas. Mineralogical, geochemical and isotopic composition of sulphide ores of a number of Sayan-Baikal region deposits testify to the presence of just such type of gold mineralization.

2) Subsequent processes of accretion and granitization will bring to transformation of original sulphide ores and rocks of ophiolite and black schist complexes with higher gold contents. Maximum changes are connected first of all with processes of granitization, when these complexes were gold sources for formation of rich and contrasting gold ore deposits.

3) Mechanism of partial melting of subduced oceanic plate with melting of granitoids of andesite type has been realized, with which gold ore and gold-silver deposits of continental margins are more often associated, in case of subduction of parts of oceanic crust enriched in gold.

MAIN GEODYNAMIC SETTING OF GOLD ORE DEPOSITS FORMATION IN SOUTH SIBERIA FOLD BELTS

 

A.G.Mironov

ABSTRACT:

In the fold belts of Southern Siberia, the predominant localization of gold ore deposits in particular tectonic terranes and synaccretionary sequences has been established. It enables to speak about main geodynamic settings of gold ore deposits formation i.e. the oceanic, island-arc, collisional and interplate ones. A particular attention has been paid to the oceanic and island-arc settings playing a principal role in deposit formation under other conditions.

Analogies have been established between processes of sulfide and gold formation in ancient and modern settings of marginal seas, island arcs and back-arc spreading zones from the example of East Sayan gold deposits. It has been shown that the oceanic (transitional) crust and particularly its parts enriched in gold (carbonaceous sediments, sulfide deposits), play a decisive role in the formation of large group gold ore deposits, having both an evident association with ultramafite-mafite rocks of ophiolite association, and island-arc granitoids, widely spread in fold belts. A model for the mechanism of formation of gold ore deposits associated with the granitoids intrusions and batholiths in various geodynamic settings, has been developed.

 

GEOCHEMISTRY OF GOLD, PLATINUM AND PALLADIUM IN BLACK SHALES AND MAGMATIC CARBONACEOUS ROCKS

 

Mironov, A.G., Kulikov, A.A.,Mironov,V.A., Geological Institute of SB RAS, Ulan-Ude, Russia; Zhmodik, S.M., United Institute of Geology, Geophysics and Mineralogy of SB RAS, Novosibirsk, Russia; Sapoznikov, D.Y., Vernadsky Institute of Geochemistry RAS, Moscow, Russia

Carbonaceous rocks of various genesis occur in one of Siberian platform (East Sayan) folded framing, that permits to compare carbonaceous matter effect on concentration and distribution of precious metals.

Carbonaceous deposits (black shales) occur at several PR3-PZ1 stratigraphical stratas levels as well as at the base of ophiolite cover in volcanogenic horizons. Three types of black shales are determined: carbonaceous-siliceous, carbonaceous-micaceous and carbonaceous-sulfide-micaceous (Py + Po = 5-30%). The most high background contents of Pt and Pd are characteristic for carbonaceous-siliceous shales (x Pt = 15,6 ppb, x Pd = 24,6 ppb). Gold predominantly concentrates in sulfide horizons of black shale deposits, locating in lower plate of ophiolite cover (25-1560 ppb). Many features point to that they are old sulfide edifices of back arc spreading zones, thrushed together with ultrabasic and basic members of ophiolite complex.

Hyperbasites, composing the upper ophiolite plate are as well locally carbonized. Two types of carbonatization are distinguished: (a) areal like fine veinlets and (b) linear like dyke-shaped tectonites (to 3-5 m thick). Dunites-harzburgites are as a rule carbonized, more seldom-serpentinites, Corg contents reaching 8-9 wt.%. Accessory minerals like ilmenite, chalcopyrite, sphalerite, galena, spinel, garnet, zircon are widely developed in them. Native gold (CAu = 15-520 ppb), sperrylite (CPt = 4,0-53,0 ppb) and diamond (0,1-0,8 mm in size) also occur. Carbon isotopic composition of extracted kerogen is in the range of 8,85% to -- 16,30 promille.

Carbonized albitites (replacing dykes) developing on dykes of basic and middle composition occur in boundaries of the same ophiolites. As a rule, they are porphyry rock of actinolite-albite composition with biotite, arfvedsonite and albitized plagiclase in phenocrysts, saturated with carbonaceous matter and are therefore black in colour. Content of carbonaceous matter reaches 5,2 wt.%. Most albitites are prevalingly replaced by dispersed carbonaceous particles. Phenocrysts of albite or aegirine-augite are carbonized only in margins or along cracks within crystals. Actinolite developing in ground-mass does not contain dispersed carbon. Unlike carbonized hyperbasites, carbonized albitites contain higher amounts of Zr, Y, Ba, TR. Pt, Pd and Au contents are close to ones in hyperbasites. Silver siguificantly dominates (to 75 ppm) with presence of native forms (grains to 100 mkm in size). Carbon isotopic composition is like from albitites is like d13C = -10,5 -14,0. promille.

Dispersed carbonaceous matter is present in marginal facies of stock of plagiogranites intruding serpentinites and carbonized harzburgites. Carbonaceous matter associates with biotite flakes, apatite, calcite, potassium feldspar, epidote, locating in guartz cracks and veinlets cross-cutting all minerals. Massif is composed by beresitized plagiogranites withiut dispersed carbon, but with pyrrhotite instead of pyrite in accessory mineralization in central parts. Corg content is not high here (0,1-0,5 wt.%) and d13C is close to that in black shales (-18,9-23,5% ). Pt and Pd contents correspond here to clarks of these concentrations (10-75 ppb). Gold ore bodies of quartz-sulfide composition, also containing dispersed carbonaceous matter have been known in this massif.

And finally, dispersed carbonaceous matter and large bodies of graphite occur in massif of nepheline syenites, breaking bituminous limestones. Widely known Botogol (Aliber) graphite deposit locates here. By interrelations of nepheline syenites with mother rocks and carbon isotopic composition of graphite bodies and bituminous limestones it has been determined that ball-like ore bodies of graphite in nepheline syenites formed at magmatic stage of alkaline intrusion development at participation of reduced fluid and active interaction of intrusion and bituminous limestones. Detailed studies of geochemistry and mineral graphite bodies enabled to discover higher contents of Au (15-450 ppb) and Pt (2-460 ppb) in them. In addition, gold occurs like microparticles (5-25 mkm) in graphite, and platinum is likely sorbed on graphitic matter.

Comparison of pure carbonaceous matter, extracted from black shales, carbonized hyperbasites, albitites and graphite ore bodies showed their significant variability. By data of roentgenostructural and thermal analyses, Carbonaceous matter represents itself kerogen (black shales), graphitoid (albitites), disordered graphite (hyperbasites) and crystalline graphite (Botogol deposit).

Distribution of precious and accessory elements is typical for carbonaceous matter (table 1). Higher contents of Ir is typical for kerogen of ultrabasic rocks and Ag, TR, Zr is characteristic for kerogen of acid rocks.

Geological and geochemical data testify to deep (mantle) origin of hyperbasites carbonatization rather on site of ophiolites restite complexes formation.

GEOCHEMISTRY OF GOLD, PLATINUM AND PALLADIUM IN

 

 

Table 1. Concentration of precious metals in kerogen from carbonaceous rocks (ppb)

Sample No.

Ru

Pt

Os

Ir

Au

Ag

Tn-44

19,0

11,5

12,3

16,4

44,5

1590

Tn-40

4,2

5,5

1,2

0,8

50,0

10700

Tn-46

11,5

2,0

4,5

4,5

25,0

4500

Tn-48

13,2

85,0

5,5

4,8

1790,0

75000

Ub-3

-

-

-

-

37,0

7100

S-4618

-

-

-

-

24,0

4300

Note: Tn-44 - kerogen from carbonaceous hyperbasites;

Tn-40, Tn-46, Tn-48 - kerogen from carbonaceous albitites;

Ub-3, S-4618 - kerogen from carbonaceous-micaceous-siliceous shales.

Neutron activational analysis by D.Y.Sapozhnikov.

 

PLATINUM-GROUP MINERALS FROM ALLUVIUM NEAR EASTERN SAYAN OPHIOLITE (SOUTHERN SIBERIA, RUSSIA)

 

Mironov, A.G., Ochirov, Y.C., Karmanov, N.S., Geological Institute, SB RAS, Ulan-Ude, Russia

 

New show of platinum-group minerals (PGM) has been found in placer (alluvium) of middle Kitoi river (Eastern Sayan), which tributaries drain ultrabasic plates of ophiolite cover. 32 grains of platinum group minerals have been extracted from electromagnetic fraction of heavy concentrate. They have not been discovered in magnetic and heavy unmagnetic fractions. Grains are weakly rounded. Their size is 0,3 to 0,8 mm. They are silver-white colores sometimes with grey margin. Grains analysis has been done with electron microprobe MAR-3 (3 mkm in diameter), V=24 kV, J = 40 mA.

A majority of grains is represented by platinum-bearing rutheniridosmin (wt.%: Ru - 11,0-29,5; Os - 30,3-50,9; Ir - 28,0-39,0; Rh - 0,3-1,8; Fe - 0,2-0,8 (tabl.1). Inclusions (rounded, hexagonal and rectangular forms), 3-15 mcm in size, have been found in most grains as well as rims and margins (to 4-10 mcm) around rutheniridosmin grains. Rounded inclusions (to 15 mcm) are composed by alloys complex in composition (wt.%: Pt - 35,8; Ru - 17,6; Os - 11,3; Ir - 28,3; Rh - 4,2; Ni - 0,23; Fe - 0,98). Margin near this inclusion (2-3 mcm) has composition of sulpho-arsenide plate (wt.%: Pt - 12,8; Ru - 7,4; Os - 4,6; Ir - 39,8; Cu - 0,4; Rh - 0,7; As - 25,9; S - 9,9). Sometimes rounded inclusions (15 mcm) consist of two parts grey and light-grey in colour. Grey part is arsenide of nicel and rhodium (wt.%: Rh - 40,2; Pd - 1,5; Ni - 23,2; As - 30,26); light-grey part is cuprous platinum (wt.%: Pt - 71,7; Cu - 14,6; Sb - 5,3; Ir - 0,8; Os - 0,6; Ru - 0,1; Ni - 3,9; Fe - 3,4).

Crystalline inclusions (10x20 mcm) of rectangular form bright white in colour are composed by platoosmoiridium (Ir - 60,4; Os - 26,1; Ru - 5,1; Rh - 1,8; Pt - 6,1; Ni - 0,3; Fe - 1,1). Hexagonal grains composition is close to polyxene (15 mcm) (wt.%: Pt - 77,8-81,3; Fe - 7,2-10,1; Ir - 0,5-4,2; Rh - 0,3-1,0; Ni - 0,2-2,0; Os - 0,43-0,46).

Rims and margins are developed along coarse grains rings. They are from 2-3 to 20 mkm wide and iridium and platinum sulphoarsenide is like in wt.%: Ir - 40,1; Pt - 9,8; Rn - 5,8; Os - 2,4; Rh - 0,7; Fe - 0,07; As - 25,8; S - 10,0) as well as arsenide of Os, Ir and Ru is like that in wt.%: Os - 5,1; Ir - 29,8; Ru - 7,7; Rh - 0,18; Cu - 0,48; Sb - 0,26; Ni - 0,41; As - 11,36). Arsenides of osmium, iridium and ruthenium also occur (wt.%: Os - 38,3; Ir - 31,6; Ru - 3,0; Cu - 0,3; As - 1,0; S - 25,0).

Interrelations among platinum group minerals and between inclusions and margins as well testify to their formation occurred on many testify to their formation ocurred on many stages and in the following order: polyxene - cupruous platinum - platoosmo - iridium - rutheniridosmin-platinum arsenides - platinum sulphoarsenides.

Accompanying minerals of heavy concentrate are represented (%): by garnet (andradite-grossular) - 8; magnetite - 70; martite - 11; awaruite - 8; pyrite - 1. Weak roundness is characteristic for all minerals, PGM as well. Large amount of awaruite (~8%) in heavy concentrate and weak developing of chromitite in it (particular grains) are quite specific. Awaruite occurs like coarse, kidney-shaped grains (to 1,5 mkm), more rarely-like weakly rounded crystals and has composition (wt.%): Ni - 76,5; Fe - 22,8.

Rocks and ores with such set of minerals are spread among plates of ophiolite complex. These are rodingites and zones with pyrite-magnetite and sulfide (pyrite) mineralization in ultrabasic and basic rocks. Lens-shaped bodies with pyrite-magnetite mineralization closely associate with rodingites-garnetbearing rocks developing as a rule, on continent of ultrabasic and basic rocks. Rocks and ores of this type have gold miniralization. They have not been studied for PGM.

Another type perspective for PGM mineralizations is associated with zones of veinlet and impregnated Ni-containing sulfide mineralization in ultrabasic rocks (serpentinites). Ore mineralization is composed by pyrrhotite chalcopyrite, bornite, pentlandite, nicelite and awaruite. Proceeding from association of platinum group allivial minerals with awaruite, presence of this association in zones of Ni-containing sulfide mineralization without chrome-spinellids may be expected.

Thus, due to association of alluvial platinum group minerals with garnett and awaruite, we may suggest presence of platinum-osmium-iridium native and sulphoarsenide mineralization due to processes of rodingitization and hydrothermal changes of ultrabasic rocks.

  MIRTAB1

Table 1. EPG and associated elements in minerals from the Kitoy river alluvium

No.

Pt

Ru

Os

Cu

Sb

Ir

Rh

Pd

Ni

Fe

As

S

S

1

2.12

16.92

46.02

-

-

32.06

1.78

-

-

0.15

-

-

99.05

2

-

11.56

50.94

-

-

37.59

0.26

-

-

0.15

-

-

100.50

3

1.60

16.40

41.42

-

0.1

39.00

0.74

-

0.11

0.39

-

-

99.76

4

2.79

21.27

40.58

-

-

33.43

1.17

-

-

0.25

-

-

99.48

5

3.10

10.20

51.47

-

-

36.26

0.65

-

-

0.19

-

-

101.87

6

5.85

22.29

40.01

-

-

31.05

1.15

-

-

-

-

-

100.35

7

1.70

13.37

46.14

-

-

38.91

0.39

-

-

0.26

-

-

100.77

8

1.69

16.51

43.59

-

-

37.97

0.50

-

-

0.19

-

-

100.45

9

1.64

15.18

47.56

-

-

34.92

1.05

-

-

0.30

-

-

100.65

10

0.72

15.17

46.04

-

-

36.88

0.64

-

-

0.34

-

-

99.07

11

2.26

33.91

31.96

-

-

28.03

0.74

-

-

0.18

-

-

97.08

12

1.73

14.71

45.92

-

-

38.05

0.37

-

-

0.18

-

-

100.96

13

1.64

15.66

45.61

-

-

38.30

0.70

-

-

0.31

-

-

102.22

14

1.98

16.62

43.40

-

-

36.45

0.56

-

-

0.25

-

-

99.26

15

2.77

29.55

30.33

-

-

35.48

1.33

-

-

0.26

-

-

99.72

16

6.08

5.10

26.13

-

-

60.45

1.85

-

0.28

1.08

-

-

100.97

17

35.80

17.60

11.33

-

-

28.30

4.16

-

0.23

0.98

-

-

98.44

18

71.73

0.11

0.64

14.58

5.29

0.84

0.34

-

3.95

3.37

0.45

-

99.32

19

81.31

-

0.46

 

-

4.18

1.04

-

1.99

10.07

-

-

99.05

20

9.79

5.81

2.38

0.44

-

40.09

0.73

-

-

0.07

25.80

9.96

95.07

21

-

7.10

51.10

0.48

0.26

29.82

0.18

-

0.41

-

 

11.36

100.70

22

-

3.02

38.30

0.33

-

31.63

-

-

-

0.09

1.05

24.99

99.41

23

-

-

-

-

-

-

40.19

1.55

23.24

-

30.26

-

95.24

24

-

-

-

-

-

-

-

-

76.48

22.78

-

-

99.26

25

-

-

-

99.05

-

-

-

-

-

-

-

-

99.05

Note: 1-14 - rutheniridosmin; 15-17 - platosmoiridium; 18 - cuproplatinum; 19 - polyxene; 20 - arsenosulfide of platinoids; 21-22 - sulfide of osmium and iridium; 23 - arsenide of rhodium and nickel; 24 - awaruite; 25 - native copper.

The analysis was performed with electron microprobe analyzer MAR-3 by Analyst N.S. Karmanov.

 

 

 

Autoradiography and neutron-activation radiography in geochemical investigation of the environment

 

A.G.Mironov

Buryat Geological Institute of Siberian Branch of RAS

Sakhyanova str.,6, Ulan-Ude, Russia, 670047

 

Different kinds of the autoradiography are widely used in geochemical investigations for a micromapping and concentration study of a number of elements in minerals, rocks and environment.

It should be noted that the autoradiography as a method was developed and widely used in investigations of the regularities of natural radioactive element (NRE) distributions in rocks and ores. I.Joliot-Curie (1946) also considered the possibility of the use of nuclear emulsions for the study of the rock radioactivity. The first autoradiographies were used for the study of the localization of radium and uranium in granites and sedimentary rocks. Later the method was improved and at present obtaine high resolution and sensitivity owing to the use of special nuclear emulsions and optical microscopy.

The method of autoradiography was widely used in such fields of science and technology as biology, medicine, metallurgy, electronics a.o., after obtaining artificial radioisotopes. In the geological investigation main attention was paid to the autoradiography of natural elements. The radioactive tracer method in combination with the autoradiography technique began to be used in geochemistry due to the wide introduction of the experiment into the geological science.

At present, in geochemistry the autoradiographic studies are under way in the following lines:

- autoradiography of natural radioisotopes (U, Th, Ra);

- autoradiography of synthetic radionuclides obtained when irradiating the samples of rocks in reactors, accelerators or isotope irradiation facilities;

- autoradiography of synthetic radioisotopes introduced into the system while modelling the geological processes.

The autoradiography of the natural radioactive elements includes: a) Macroautoradiography; b) Microautoradiography; c) Neutron-Induced Autoradiography.

Macroautoradiography .( 7a 0, 7b 0, 7g 0-radiography of the radiators amount) is used for the study of the distribution nature of the common radioactivity of a rock sample created by the macro- and microinclusions of the radioactive minerals. This method of autoradiography is connected with an exposure of the polished surfaces of the samples rocks or ores with the X-ray films during 10 to 70 days according to the sample radioactivity. In a number of cases we observe not only radioactive element distributions but also texture or structure peculiarietes of rocks.

Microautoradiography .( 7a 0-track radiography) enables to study the distribution of the NRE in the microobjects with comparatively small contents of uranium and thorium. In this case the autoradiogram is studied under the microscope with the magnification 150-300 that due to the fact exposure of 7a 0-particles is doesn't produce common blackening but produces isolated traces or tracks. For such autoradiography is usually used a special thick-layered emulsion for the nuclear investigations (A-2, 50 mkm thick) and solid state nuclear track detectors (CR-39, CN-85, LR-115) which is insensitive to the other kinds of radiation as well as the polymer films or some other solid state nuclear track detectors. As the energies of 7a 0-particles of uranium and thorium are different, there is a possibility to distinguish between them by the track length. The sensetivity of such autoradiography can reach to 10 5-3 0 mass.% with the exposure of 50-70 days.

This autoradiography is very effectiv for "hot particles" discovery in soils and plants.

Neutron-Induced Autoradiography .(n, f-radiography) is widely used in the geochemical investigation for the selective study of the local distribution of uranium and thorium in the rocks and minerals with a wide range of their concentrations. The maximum sensitivity of the determination can reach to 10 5-10 0wt.% - for uranium, and 10 5-6 0wt.% - for thorium. The method based on the nuclear fission by thermal ( 5235 0U) and - fast neutrons ( 5232 0Th) and registration fission tracks in the detectors, among which the polymer films, synthetic fluorine-phlogopite and glass are most simple and commonly used. High sensitivity and resolving power, possibility of study with the significant magnification (100-300) is a great advantage of this method. This was responsible for the fact that the Neutron-Induced Autoradiography has recently become the main method for study the uranium and thorium distribution in rocks and ores.

We have conducted investigation of the local distribution of the clark contents of uranium in the carbonacerous rocks and observed its re-concentration under the action of different metamorphic and hydrothermal-metamorphic processes. Lavsan (20 7m 0m), CR-39 and fluorine-phlogopite were used as detectors, the irradiation in the reactor was carried out by a neutron fluence of 10 515 0-10 514 n 5. 0cm 5-2 0; etching of the detectors under standard conditions.

Comparison of 7a 0- and f-autoradiography enabled to establish the peculiarities of various NRE distribution and reveal the miscoincidence of 7a 0- and f-images in some plots of rocks and ores. It seems to be associated with partial removal of uranium decay products and particularly of radium.

Neutron activation-induced beta radiography . in the geochemical investigation of environment has been developed in last years. K.Googman and G.Thompson (1947) irradiated polished rock slabs and measured distributions of sodium, iron, tungsten e.o. using X-ray film. Subsequently the method hasn't come into wide use mainly because of its bad selection power. Now this technique based on neutron irradiation of polished rock samples and beta-autoradiography using nuclear-track emulsion is used for determining and micromapping of such elements as Al, Si, Ca, Ti, Cr, Mn, As, Sb, REE, Co, Ag, Au, W. If one knows the composition of minerals ore rocks, the periods of half life of their radioisotopes and the effective nuclear cross section, one can select the optimum conditions to yield autoradiography images of the distribution of one an amount of definite elements by changing the irradiation time, the its cooling time and exposure.

The most intresting results have been obtained for gold. It is for first time in geochemical studies that we have succeeded in establishing the nature of thin-dispersed gold in quartz and sulfide ores.

Practically it was pursued in the following way. All ore bodies were sampled at different horizons of gold deposit (over 350 samples), from wich polished sections on quartz glass were prepared. In dependence of gold concentration the radiation was realized by a neutron flux 10 515 0-10 517 0n/cm 52 0. From each sample from 7 to 5 autoradiograms were obtained with various exposures. The most picture of gold distribution was observed in 2-4 exposures. The blackened spots from gold on the autoradiogram were well seen against sulfide minerals.

As a result of ore study by use of autoradiography is a drastically predominating tendency of gold fixing in ore bodies enriched with sulfides. The main mineral-bearer is pyrite where gold is concentrated as microinclusions (tear-shaped) or along the cracks in crystals. Gold often forms the zones of enrichment around spheric occurences of quartz or on the border between pyrite and polymetalic ores.

This technique is very useful under pollution studies in different part of plants with such elements as Au, Ag, As, Sb, W, Cd, S, Cr,Co a.o. Here you can see the autoradiogram of gold distribution in water moss "Fontinalis Gothica". Spots of dark correspond to microparticles of gold in moss shoots.

Besides the above elements exist light elements (as Li, Be, B) that after neutron irradiation became as alpha-radiators with detecting on SSNTD. The sensetivity of the existing procedures for B, Li and Be account for about 10 5-7 0wt.%, 10 5-5 0wt.%, 10 5-3 0wt.% respectively.

_The other kind of the artificial radioisotopes _autoradiography .is related to the experimental geochemistry and petrology. Using the possibility of modelling of those or other geological processes, we can control and study these processes by way of introduction of the radioactive tracer into the system. The advantage of such procedure is high selection and sensitivity, the absence of preventing radioisotopes. This method is very close to the classical method of the "tracers", however in many cases it has a specificity, connected with the modelling of the multicomponent systems of the rock- and ore-formations. For the geochemical studies a radioisotope can be introduced into a melt, can be plotted in the form of cover on a sample, can be added into a solution, a mixture a.o. The main technique of detecting is the 7b 0-autiradiography on a nuclear emulsion.

The technique has only come into use but new results have already been obtained which allow for example to determine the CO 42 solubility in the magmatic melts (on 514 0C), to estimate the Sm, Ce, Ni concentrations coefficient in the fractionation of the basaltic melts.

The Institutes of geochemistry and earth's crust of the Siberian Department of the Russian Academy of Sciences and we carried out a wide range of the experimental research for Au, Ag, Ni, T behaviour studies in the different geochemical processes. The character of gold distribution in the sulfides was studied using the 5195 0Au autoradiography. In particular, we have established the regularity of gold presence in the sulfide crystal lattice in the direct relation to the degree of metallization of the covalent bond of the sulfide minerals.

Firstly it was clearly shown that gold and silver removes from the basalt glasses when the latter undergo high temperature hydrotation by ocean water, that is great importance for the specification and further improvement of the ore formation theory.

The nickel distribution coefficient between pyrrhotite and magnetite was established with the help of the autoradiography of 563 0Ni. The use of tritium gave the possibility to trace the water distribution character in the thermogradient field of magmatic melt.

The autoradiography is carried out in the films and plattes for the nuclear investigation (MR, K-106, AF-3). The quantity of a radioisotope amount to 3,7-10x10 54 0Bq for 1 g of substance. Such radioactivity does not influence the character of the reactions and does not favour the formation of the ions and free radicals, distorning the environment. The duration of the exposures was 3-10 days.

Thus the presented data shows the great variety of the problems, which can be solved with the use of autoradiography in the geochemical investigations. One can hope that thanks to the evident advantages of the method autoradiography will use widely promote progress in Earth sciences.

 

DETERMINATION OF GOLD AND URANIUM MINERALIZATION IN BLACK SCHISTS AND SULFIDE ORES USING RADIOGRAPHY COMPLEX

 

A.G. Mironov, S.M. Zhmodic and Y.C.Ochirov

Buryat Geological institute, Ulan-Ude, Russia

Associated Institute of Geology Geophysics and Mineralogy, Novosibirsk, Russia

 

Abstract:

The autoradiography technique complex was realized for micromapping and determination of gold and uranium mineral particles in black shale and sulfide ores of gold deposit. Alpha radiography, fission track radiography and beta radiography were used for the gold and uranium distribution study in the natural and synthetic minerals and ores. New data on local spatial distribution of uranium and gold for both elements (gold-uranium deposits) and gold-bearing deposit of quartz-gold-sulfide-formation have been obtained.

Key Words: Micromapping, gold and uranium, autoradiography complex, black shales, gold ores.

 

INTRODUCTION

Autoradiographic methods have lately found wide use when studying distribution of not only natural radioactive elements but also a number of stable elements with the use of activation radiography. Of most interest are such elements as gold and forming unique types of complex deposits. The existing means of autoradiography enable to succeed in determining the local distribution of these elements with sensitivity of 0,01-1 ppm. However for one element (uranium) these are clarke contents for practically all classes of rocks whereas for gold these are industrial contents. A certain mentioning of the possibility of autoradiographic study of clarke contents of gold is to be ascertained but most probably it is erroneous (?rmolaev et al.,1986). This limitations on the possibility of complex study of their distribution by use of autoradiography in a wide range of concentrations but it enables to make such studies separately, for each element.

 

METHODS

In particular, for uranium such studies were performed in a wide range of contents by use of a complex of autoradiographic methods with using solid state nuclear track detectors (SSNTD) and nuclear emulsions. Macroautoradiography was used for obtaining a general picture of uranium distribution and study of the nature of its redistribution in black shales with high contents of this element (10-100 ppm). The studies were made on both hotoemulsions (films AF-3, RM-3) and SSNTD (CN-85) with an exposition of 30 days. In the latter case only alpha-emission was noted. Owing to the contineous exposition one can get a microimage of track distribution on film CN-85 and at the time to study some details at great magnification. As it is seen on microautoradiograms given, even this simple means of autoradiography enables to obtain a new information on uranium distribution in schist with its high concentration. The best detector for n,f-radiography is fluorine-phlogopite because of its good resistance to high density flow of neutrons and optimal track shapes. It is indispensable when studying uranium distribution in rocks with its low level concentration.

 

RESULTS OF BLACK SHALE AND GOLD-URANIUM DEPOSIT STUDY

For black shale study we used lawson which is quite appropriate for low fluence of neutrons which is quite sufficient for high uranium concentrations. The picture obtained considerably refine the data on redistribution of uranium in black shales and reveal its distinct redistribution together with the substance into veins and bend microfolds (Fig.1). Complex use of autoradiography when studying gold and uranium occurences gave good results. Ore development is located in early middle Proterosoic carbonate formations (marbles) with horizons of schists and made up of a series of quartz and quartz-feldspar veins with ankerite and sulfides. Gold-uranioum mineralization occurs in quartz-ankerite veins and represented as crystals or nodular aggregates of uranitite and pitchblende of size to 1 cm. Uranitite forms small (2 to 10 mcm) isometric crystals localized in ankerite or micrograined aggregates of pitchblende. With the use of alpha- and fission-radiography the dissemination of uranium in sulfide and rockforming minerals was studied. By use of CR-39 and CN-85 high quality autoradiograms of alpha-emitters distribution are obtained. Comparison of alpha- and f-radiography enabled to establish the peculiarities of various NRE distribution and reveal the miscoincidence of alpha- and f-radiography in some plots of rocks and ores.

Gold is found in most cases in uraninite and pitchblende as it is found out according to the activation autoradiography data. As is known these elements are activated quite intensely, however the selection of radiation and exposure regime (24 hourth after radiation, during 30 min) enabled to obtain the points of intensive blackening from gold against uraninite. Gold as microinclusions (5-30 mkm) is localized in fractures in pitchblende and along the borders of uraninite grains, rarely in quartz and pyrite.

And last of all a few words on the use of autoradiography for study of gold distribution. Many works devoted to gold autoradi?graphy have lately appeared (Flitsian et al.,1990, Potts,1984). These works show the principalpossibility of obtaining autoradiograms of gold in various mineral associations. An attempt has been made to systematically study the by-horizon distribution of gold in one of the deposits in various types of ore bodies and ores and making up a map of gold of the whole deposit. Along with activation autoradiography, microprobe analysis and spark mass-spectrometry for the study of gold composition and sampling was used. The deposit of sulfide formation with prevailing quantity of thin and dispersing gold (1-20 mkm) located in pyrites and less in quartz and other sulfides. The autoradiographic method appeared to be practically the only one giving the information of the character of distribution and forms of occurence of gold in ores (Fig.2). Owing to this it was made possible to obtain new information on the origin and conditions for formation of gold-bearing deposits. Practically it was pursued in the following way all ore bodies were sampled at different horizons of the deposit (over 350 samples).

From which polished sections on quartz glass were prepared. In dependence of gold concentration the radiation was realized by a flow of neutrons fluence of 10 515 0-10 517 0 n/cm2. From each sample from 5 to 7 autoradiograms were obtained with various exposure. The most picture of gold distribution was observed in 2-4 exposures. The blackened spots from gold were well seen against sulfide minerals (pyrite, chalcopyrite, galenite, sphalerite, arsenopyrite). There were few chief interfering impurites - arsenic and antimony in ores.

 

Fig.1. Micrographs of a sample section (a) and corresponding U-235 fission track distribution in lowson (b) of a black shale sample (the reconcentration of uranium in carbon-bearing cracks)

 

CONCLUSION

As a result of ore study by use of autoradiography a drastically predominating tendency of gold fixing in ore bodies enriched with sulfides. The main mineral-bearer is pyrite where gold is concentrated as microinclusions (tear-shaped) or along the cracks in crystals. Gold often forms the zones of enrichment around spheric occurences of quartz or on the border between pyrite and polymetalic ores. Thus made the atlas of distribution and forms occurence of gold enabled to get the picture of gold distribution throughout the entire deposit and at different deapth. Thus new data on local distribution of uranium and gold for both elements (for gold-uranium occurence) and separately in black shales formations and gold-bearing deposits of quartz-gold-sulfide formation have been obtained.

 

Fig.2 Distribution of Au and As in typical quartz-pyrite ore. A- picture of thin and polish section; B autoradiogram of Au, S and As; B`- autoradiogram of Au and S.

 

ACKNOWLEDGMENTS

The research described in this publication was made possible in part by Grants N 93-05-9650 and N 93-05-14019 from the Russian Foundation of the Fundamental Researches.

 

REFERENCES

Ermolaev, N.P., Uljin, M.I., Flitsian, E.S., et al.(1986). Two types of gold re-distributions in black shale sulfide ores. Geologia rudnikh mestorozhdenyi, v.28, N 1, 48-57.

Flitsian E.S.(1990) Development of nuclear phisics methods in geology-geochemical investigation local analysis. In : Autoradiographical method in scientific researches. M.: Nauka, 71-81. 3.

Potts Ph.I(1984). Neutron activation-induced beta autoradiography as a technique for locating minor phases in thin section: application to rare earth element and platinum-group element mineral analysis. Econ.Geol., 79, 738-747.

 

EXPERIMENTAL INVESTIGATION OF GLASS-FLUID INTERACTION USING NUCLEAR TRACERS

A.G. Mironov and N. Zhatnuev

Buryat Geological Institute, Ulan-Ude, Russia

 

Experimental investigations of tracer element behaviour at water-rock interaction are very important under nature processes modelling. However it is difficalt to do for such elements as gold and silver having very low contents in rocks and ores.

Some series of experiments of fluid-rock interaction have been carried out using radioactive tracer method. Radioisotopes of gold (Au-195) and silver (Ag-110m+110) were placed in basalt, granite and andesite glasses with total concentration of gold - 50 ppb, silver - 50-100 ppm. The experimental conditions: T=200-500oC, P=200-1000 bar, t=5-40 days, oxigen fugitive 10-8-10-27 bar, water and water solution of NaCl, HCl, H2S. The gold and silver distribution was studied by the autoradiography method on the nuclear emulsion platte.

In process of water-glass and solution-glass interaction the hydratation, re-crystallization and leach zones were formed. Gold and silver were leached from hydratation zone and were re-concentrated in re-crystallization zone. The high mobility from riolite, basalt and andesite glasses is more tipical for silver (60-95%), whereas the gold was leached from glasses with increase of the oxigen fugitive in system. The gold and silver leached are re-concentrated in the re-crystallization zone as a microinclusions of native gold and silver.

A conclusion was made that volcanic rocks can be a source of a precious metals under hydrothermal alteration. For all this, the gold can be mobilized together with silver, each taken separately, can be re-concentrated in the alterated rocks and can be coused for a preliminarily stage of ore deposits formation.

 

EXPERIMENTAL STUDY OF WATER BEHAVIOUR IN MELT AND MELT-ROCK INTERACTION USING TRITIUM AUTORADIOGRAPHY

A.G. Mironov, S. Zhatnuev and M. Epelbaum

Buryat Geological Institute, Ulan-Ude, Russia; Institute of Experimental Mineralogy, Chernogolovka, Russia

 

New possibilities of the natural systems modelling are discovered by using a radioisotope tracers method. Partially, the water solubility in basaltic and granitic melts and it behaviour under water-bearing melt-rock interaction have been studied with use of tritium autoradi ography (tritium bearing water). The tritium was used as a tracer in the water (3HHO+H2O) with quantitative 10-15 MBc/g. The experiment conditions: T=900-1000oC, P=2 kbar, duration - 3-5 hours. The glass samples radio activity was measured by the autoradiographical method on the nuclear emulsion plate (MR) with the resolution about 10-20 micron and exposition - 5-20 days. The beforehand melting glasses with known water concentration were used: granite glass - 2,8-4,5 mas%,basalt glass - 3,9 mas%.

The autoradiographic data obtaned have shown that water solubility at the pressure 2 kbar in basalt melt is higher than in obsidian and the temperature dependence has different trend: in basalt melt solubility increases with temperature whereas in obsidian - decreases. It was found that the water solubility is more higher in mixing zone of granite and basalt melts and water-bearing melt-rock interaction zone. The edgings of basalt glass (10-20 micron) around the partial melting plagioclase crystals are very enriched with water while the contact of the pyroxene crystals and water-bearing basalt glass did not reveal such kind of edging.

The H2O-saturated melt-glass interaction was carried out in experiments under T=825oC, P=2100-2300 bar with tritium beta radiography. The velocity of the water diffusion from granite H2O-saturated melt into the host glass of the granite and basalt composition was investigated. The effective water diffusion coefficients for granite glass (3,1x10-8 cm2/s) and for basalt glasse (1,5x10-9 cm2/s) were found. The reaction zone on the melt-glass interaction contact with less water concentration is observed.

 

THE ORIGIN OF GRANITOIDES RELATED GOLD DEPOSITS

A.G. Mironov

Buryat Geological Institute, Ulan-Ude, Russia

Plutongene gold-bearing deposits are characterized by the association with granite batholites of variegated composition. Their most peculiar feature is great variety of morphological types, various position in relation to granitoid massifs and blended substance composition. Gold mineralization is found more often in scarns, quartz and quartz-sulfide veins and streak in granitoids or country rocks. Intrusious of granitoides of tholeite, andesite and latite geochemical types as well as palingenetic calc-alkali granitoids might be regarded as potentially gold-bearing granitoides. The main factors responsible for adjacency of gold mineralization to quite certain granitoid massifs are their high water-content, presence of fluid components (particularly chlorine) and gold sources. These conditions are fully realized when forming granitoids of island-arched type (geochemical types - tholeiite and andesite) as a result of partial melting of subducted oceanic crust having in its composition water-containing salts, highened contents of gold, sulphur, chlorine in sedimentations and magmatic rocks of ophiolite assotiation. When forming crust batholites of calc-alkaline geochemical type responsible for gold mineralization are the domelike parts, as a rule, of hybrid composition being formed against batholite development at high saturation with volatile components and heavy metals as a result of active interaction with host rocks containing water, fluid components and gold concentrations. In the presence of free fluid, gold due to the high coefficient of fluid-melt separation in favour of fluid was concentrated in the latter and didn't participate in the succeeding processes of magmatic (crystallization) differentiation. In this way gold in excess to solubility "was distilled off" against the batholite being formed, due to this the batholites proper and large intrusions do not provide geochemical information on possible specialization of the intrusion for gold. A similar mechanism allows for crust source of gold, for its transition into melt takes place during meta-magmatic replacement (granitization) of country rock. A great part as gold source should be played by carbonaceous terrigeneous and carbonate sediments characterized by high contents of gold, sulphur, barium, arsenic, chlorine as well as rock of ophiolite association also frequently having high concentrations of these elements.

Revised on August 11th, 2,000.

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