Journal of Conference Abstracts

Session H05:1B

H05 : 1B/25 : F5

Geodynamic Controls on Processes of Mineralisation

Derek J. Blundell (d.blundell@gl.rhbnc.ac.uk)

Department of Geology, Royal Holloway University of London, Egham, Surrey TW20 0EX, England

It is well established that the tectonic setting has a major influence on the style and extent of mineralisation. For example, conditions of arc-continent collision or the later stages of continent-continent collision in orogenesis can lead to extensive provinces of mineralisation, although not always. Essential elements for mineralisation are a heat source to generate a sufficient flux of magmatic fluids from partial melting in the upper mantle and/or the lower crust and a sufficient charge of metallic elements to source the volume of ore that is eventually concentrated within a mineral deposit. Numerical models of the thermo-mechanical evolution of the lithosphere/asthenosphere system undergoing arc-continent or continent-continent collision are now capable of predicting the timing and spatial extent of regions of partial melting and of producing P-T-t pathways for all points within the volume of the model, thus providing predictions that can be tested against observation. Whilst these can show the thermal events and magma sources, the metallic charge needs further consideration, in which the role of mantle plumes must be explored. Metallogenic provinces in Europe such as those associated with the Alpine-Carpathian chain, the Variscides and the Urals, for which much is now known about their tectonics, are key regions in which to examine geodynamic controls on the processes of mineralisation and these form a focus for integrated research in the European Science Foundation Scientific Programme GEODE (GEodynamics and Ore Deposit Evolution).

H05 : 1B/26 : F5

The Geodynamic Framework of Late Cenozoic Mineralization in the European Alpine Belt

Hugo de Boorder (hdboordr@geo.uu.nl),

Wim Spakman (wims@geo.uu.nl),

Stan White (swhite@geo.uu.nl) &

Rinus Wortel (wortel@geo.uu.nl)

Institute of Earth, Sciences, POB 80.021, 3508 TA Utrecht, The Netherland

Hydrothermal mineralization in orogenic belts involving precious and base metals often occurs in extensional settings late in the collisional history. In many areas within the European Alpine Belt this mineralization is hosted by calc-alkaline volcanics. However, sparse radiometric ages suggest that considerable time may pass between the subduction-related calc-alkaline magmatism and the formation of the mineral deposits, in the order of up to two million years. An association with generally younger, asthenosphere-derived alkaline magmas should therefore not be excluded.

In search of the underlying geodynamic processes we have analysed the spatial and temporal distribution of Late Cenozoic hydrothermal mineralization (De Boorder et al., in press). There is a close spatial association between Late Cenozoic mineralization and regions of orogenic collapse. Collapse is thought to result from detachment and sinking of either a thickened, gravitationally unstable root or of a relatively cold subducted slab. In both cases emplacement of hot asthenosphere at shallow levels increases the heat flux into the continental lithosphere. This creates a heat source which may rekindle the formation of magma, in the wake of the waning calc-alkaline magmatism, and intensify fluid migration at higher levels, which are conditions favourable for mineralization.

Consequently, a spatial correlation should exist between the mineralization and hot regions at lithosphere levels which are associated with processes in the mantle wedge and with lithosphere detachment. We demonstrate this correlation using recent results of seismic tomography. It is supported by modelling experiments of parameters controlling slab detachment in continental collision zones (Wong A Ton & Wortel, 1997). The modelling results show that breakoff of the slab as shallow as 50 km is realistic in terms of its thermal structure and rheology. Emplacement of asthenosphere into the void created by slab breakoff at this relatively shallow depth allows decompression melting and the generation of alkaline magmas behind the subducting slab (e.g. Mason et al., in press).

It is suggested that mineralizing processes in convergent belts may involve the interaction between (i) progressive lateral detachment of a subducting slab and consequent widening of the slab window, (ii) the evolution of steep tears in subducting slabs as potentially major channels of the plumbing system, and (iii) the generation of magmas within particular depth ranges, or magmatic windows.

De Boorder H et al, Earth Planet. Sci. Lett, in press, (1998).

Mason PRD et al, Tectonophysics, in press, (1998).

Wong AT on SYM & Wortel MJR, Geophys. Res. Lett, 24, 2095-2098, (1997).

H05 : 1B/27 : F5

Contrasting Ore Provinces in Collisional Orogens: The Alpine-Carpathian Belt

Franz Neubauer (franz.neubauer@sbg.ac.at)¹,

Gerhard Amann &

Werner H. Paar²

¹ Inst. of Geology, University of Salzburg, Hellbrunner Str. 34, A-5020 Salzburg, Austria

² Inst. of Mineralogy, University of Salzburg, Hellbrunner Str. 34, A-5020 Salzburg, Austria

Different types of mineralizations were formed within the Alpine-Carpathian belt during Late Oligocene to Neogene collisional processes. The Alps are thought to have formed by oblique convergence, due to indentation of the Adriatic microplate which resulted in subsequent eastward extrusion of fault-bounded blocks into the Carpathian arc. The Western and Eastern Alps are dominated by epigenetic mineralizations, which are linked to the formation of metamorphic core complexes, mineralzations of the Western and Eastern Carpathians and Apuseni mountains to subduction - and/or collisional volcanism. As an example, the mineralizations related to the Tauern metamorphic core complex have been studied in detail. These include: subvertical Au-quartz veins and replacement As-Ag-Cu ore bodies within the metamorphic core complex, remobilization of metals (Pb-W-Au) along low-angle ductile normal faults along upper margins of the metamorphic core complex, mineralized (Sb-Au) strike-slip faults and subvertical Au-Ag-Sb-bearing tension veins outside of the metamorphic core complex. Together, these structures indicate that mineralization occurred within a wrench setting by tensional failure within approximately orogen-parallel fault oversteps. Deep-reaching circulation of fluids likely linked a deep-seated plumbing system with shallow structural levels. In contrast, nearly all Late Oligocene to Neogene ore deposits within the Carpathians are related to volcanic activity contemporaneous with the invasion of fault-bounded blocks into the Carpathian arc. Beside hydrothermal activity within shallow volcanic edifices, common principal structures are steep tension veins parallel to the motion direction of crustal blocks, and along Riedel shears in wrench corridors separating blocks. Main metals are Au, Sb, Cu, Pb. Mineralizations postdate volcanism and are intimately linked to formation of structures as invasion progressively advanced towards to the southeastern Carpathians.

We suggest that the formation of ore deposits was induced by slab break-off after subduction of oceanic and marginal continental lithosphere. The slab-break-off may have provided additional heat that was channeled due to the uprise of magmas. Consequently, the distribution of mineralizations may reflect the control on locations by (1) the slab windows and (2) the areas with tensional failure within middle to upper levels of the crust. The latter may also allow to establish some rules to predict the locations of ore deposits as e.g.: (1) accomodation zones along terminations of strike-slip faults; (2) zones of bending of metamorphic core complexes around a vertical axis, and (3) zones of extension perpendicular to the motion direction of extruding blocks.

H05 : 1B/28 : F5

Thermo-mechanical Behaviour of Large Ash-Flow Calderas: Faulting Initiation, Thermal and Structural Traps for Ore Deposits

Laurent Guillou-Frottier

(l.guillou-frottier@brgm.fr) &

Evgenii Burov (e.burov@brgm.fr)

BRGM, LGM, 3, av. C. Guillemin, BP6009, 45060 Orleans Cedex 2, France

Growing interest to the exploration potential of the ash-flow calderas resulted in a number of modeling studies, mostly focused on fluid circulation through the fracture networks associated with the caldera faults. This flow can effectively lead to mineral deposition and release of geothermal heat. In particular, a number of low-sulfidation epithermal ore deposits within ash-flow caldera settings are located at the external sides of the border faults. In contrast, the initiation of faulting, the mechanisms for caldera collapse, and the associated changes in the thermal regime were less investigated. Until present, only simple (mostly elastic) mechanical models of caldera collapse were proposed, incapable of reproducing neither formation of the faults nor account for the brittle-ductile transitions and fault-associated thermal anomalies. There is even no clear understanding of the relationships between the surface features such as faults and the dimensions of the chamber. Since the neglected factors may control the overall behavior of the system, we developed an analytical and numerical thermo-mechanical model that accounts for both the elastic-plastic-ductile rheology, ignimbrite deposition, and contrasting physical properties of the caldera rocks. In this model, the overpressured magma is evacuated through a central vent, and transforms in thick ash-flow units which are deposited within the forming caldera. It is shown that the brittle-ductile cover can bend until faulting ("snapping") occurs at the borders. The model shows that in the absence of the regional stress field, border faulting and caldera collapse may start only if the aspect ratio (caldera diameter to depth of the magma chamber) exceeds 5 or 6. At larger ratios (> 10), internal "embedded" faults can be observed. Genesis of "nested" calderas could thus be explained by the mechanical behavior of a thin ash-flow cover overlying a large overpressured magma chamber. The conductive border faults limit the low conductive caldera infill, resulting in heat refraction effects that localize heat flow anomalies at depth, towards the external side of the faults. These anomalies coincide with the model-predicted location of the fractured zones. The coupling between mechanical and thermal processes confirms that the external sides of border faults are particularly suitable areas for secondary fracturation and important heat exchanges to take place. When regional extension is imposed, the border faulting may be attenuated or not appear at all, and faults localize above and around the central part of the magma chamber roof, involving efficient and deep seated heat exchanges (conditions for porphyry-type deposits to form). Our studies shows that the geometry of the brittle-ductile transition largelycontrols the locations of the faulted zones and defines the geometry of the brittle part of the chamber roof and embeddings.

H05 : 1B/29 : F5

The Geodynamic Setting of Massive Sulphide Deposits in the Southern Urals

Dennis Brown (dbrown@ija.csic.es)

Instituto de Ciencias de la Tierra Jaume Almera, c/Lluis Sole i Sabaris s/n, 08028 Barcelona, Spain

The southern Uralides contain an exceptionally well-preserved arc-continent collision that, in detail, can be readily compared to those active in the southwest Pacific. Significantly, the arc region contains a number of large (several over >100 Mt) VMS deposits generally considered to be "Ural type" deposits; that is, distinct from the general classification of VMS deposits in that they are Cu-Zn rich. Understanding the geodynamic setting of the arc-continent collision that hosts these deposits will therefore help to constrain a model for their development. The early convergent history in the Magnitogorsk forearc is marked by the generation of Emsian-age arc-tholeiites that locally contain boninitic lavas, dikes and sills. By the late Emsian and through the Eifelian volcanism was predominantly andesitic, switching to more dacitic during the Givetian. It is these dacites that host the largest of the VMS deposits. During the Givetian, HP metamorphism of the leading edge of the East European Craton occurred, proving the earliest indicator that continental crust had entered the subduction zone. Coincident with this, arc volcanism waned and stopped. The arrival of the full thickness of the continental crust at the subduction zone is marked by increased sedimentation in the forearc basin and deposition of arc-derived volcaniclastic turbidites across the subducting slab. These, together with an ophiolite fragment and offscraped continental material, and the exhumed HP rocks formed an accretionary wedge. A broad, ophiolite-bearing melange zone seprates the forearc from the accretionary wedge, and marks the damage zone that developed along the backstop region. Shallow water carbonates deposited unconformably on top of the mildy deformed arc record the end of the collision and collapse of the arc.

H05 : 1B/30 : F5

Palaeozoic Massive Sulphide Deposits of the Southern Urals

Richard Herrington (R.Herrington@nhm.ac.uk)¹ &

Victor Zaykov²

¹ Natural History Museum, London, SW7 5BD, UK

² Institute of Mineralogy, Miass, Russia

The Palaeozoic volcano-sedimentary complexes of the Uralides are host to around 100 volcanogenic or volcanic-hosted massive sulphide (VMS) deposits, the bulk of which are located in the southern part of the belt. At least 5 "camps" contain reserves of over 100 million tonnes of sulphide ore with thesuper-giant Gai deposits reputedly comprising over 400 million tonnes of sulphide ore at around 2% contained Cu. The VMS deposits of the Uralides fall into two distinct age groups, Ordo-Silurian andDevonian which are located respectively west and east of the main Urals fault. The older western group of deposits is related to apparent arc volcanics developed in the Silurian, spatially related to slivers of ophiolitic terrain that hosts the world-class Kempirsai chromite deposits.The originalsetting for these ophiolite and early arc assemblages is currently poorly understood. The VMS deposits in this Silurian arc are related to bimodal volcanics intercalated with graptolitic shales which confirm the ca. 412 ma deposit ages derived from dating of alteration minerals. Exceptional textural preservation and low degrees of metamorphic overprint is confirmed by the unique preserved vent-fauna assemblage at the deposit of Yaman Kasy. The younger Devonian deposits developed in the Magnitogorsk arc to the east, which seems to be marked by a progression from arc-tholeiites in the Emsian through more andesitic lavas with thick dacitic sequences by the Givetian. Parts of this sequence are host to the medium-sized (20 million tonnes) copper-zinc-gold deposits of the Baimak region whilst the thick dacite-dominated sequences seem to be the hosts to the giant copper-zinc deposits such as Sibay (115 million tonnes), Gai (>400 million tonnes) and Uchaly (225 million tonnes).Research to date shows that all deposits in the belt share basic features of a dominantly volcanic host stratigraphy, clear regional structural controls to deposit distribution, sulphide development marginal to footwall dacitic domes, extensive footwall alteration zones and clear evidence of deposits having formed on the seafloor. The enormous difference in size of deposits across the belt appears to relate to more fundamental geological factors. The giant deposits are all dominated by acopper-zinc chemistry with generally lower gold contents. Tin is significantly enrichmed with respectto the smaller orebodies. This distinctive geochemical signature needs to be assessed in terms of the inferred tectonic setting for the deposits within the evolving arc system as well as being related to the apparent changes in petrochemistry of the extrusive volcanism as subduction evolved.

H05 : 1B/33 : F5

Geological Environment and Mineralization Processes During the Formation of Lode-Gold Deposits in the Southern Urals

F. Michael Meyer (m.meyer@rwth-aachen.de),

Alexander F. M. Kisters,

Igor B. Seravkin,

Sergej E. Znamensky,

Alexander M. Kosarev &

Robert G. W. Ertl

Institut für Mineralogie und Lagerstättenlehre, RWTH Aachen, Wüllnerstr. 2, 52056 Aachen

The important gold districts of the southern Urals are confined to two tectonostratigraphic belts within the internal parts of the orogen, namely the Main Uralian Fault (MUF) that represents the fundamental suture zone of the Uralides, and the East Uralian Zone which is characterized by the emplacement of large, upper-Paleozoic granitoid massifs.

The MUF separates the foreland fold-and-thrust belt in the west from the internal parts of the orogen in the east and is represented by an up to 20 km wide mélange zone. The prolonged, polyphase deformation history of the MUF includes NW-vergent thrust imbrication, folding, reverse faulting and late-stage transcurrent shearing. Structurally, gold mineralization is controlled predominantly by second- and third-order brittle to brittle-ductile strike-slip faults that developed late in the kinematic history of the MUF. Strike-slip reactivation of earlier compressional structures occurred during the late-stage docking of the passive margin of the east European Platform with island arc complexes of the southern Urals. This event can tentatively be related to changes in plate motion during the final stages of terrane accretion during the upper Permian and lower Triassic. Fluid flow and associated gold precipitation was controlled by the permeability characteristics of the hydrothermal conduits, lithological competence contrasts, and fluid interaction with Fe-rich host rocks. Mineralization occurred at relatively shallow crustal levels (2-6 km) and post dates peak-metamorphism.

The East Uralian Zone is characterized by the emplacement of two main phases of granite and, thus, is referred to as the plutonic axis of the orogen. In the Kochkar gold deposit, mineralization occurs closely associated with mafic dyke swarms, the so-called 'tabashki', that intrude the upper-Paleozoic Plast massif. Genetically, the gold ore is related to the main phase of regional-scale compressional tectonics and granite plutonism during the upper Carboniferous to lower Permian. Controlling structures have a dominantly east-west strike and occur as hybrid shear-tensional vein systems in competent granitoids, subjected to E-W directed regional shortening. Deformation textures and alteration mineral assemblages point to mid-greenschist to lower-amphibolite facies conditions of mineralization, but these texture are invariably overprinted by mid-Permian regional metamorphism and later greenschist-facies retrogression.

The Uralides are perhaps one of the prime examples, where a number of independent geologic factors have coalesced in time and space to produce a large number of world-class ore deposits. The formation as well as preservation of mesothermal gold deposits in the internal parts of the Uralides reflects the unique geologic evolution of the orogen, and in particular, its specific post-collisional development, and demonstrates that mineral deposits are sensitive indicators of tectonic regimes.

KEYNOTE

H05 : 1B/34 : F5

Metallogeny of Fennoscandia

Krister Sundblad (Krister.Sundblad@geo.ntnu.no)

Geology and mineral resources, NTNU, N-7034 Trondheim, Norway

Fennoscandia is a Precambrian crustal segment that includes the Fennoscandian (Baltic) Shield and Precambrian under platform sediments east of the Baltic Sea (Gorbatschev & Bogdanova, 1993). The metallogeny of Fennoscandia is complex and follows closely the crustal evolution.

1. Mineralization tied to Archaean greenstone-tonalite systemsThe earliest crust of Fennoscandia is composed of 2.7-3.1 Ga tonalites and greeenstone belts. Most significant ores are located to the Karelian province: BIF-type Fe in greenstone belts (Kostamuksha) and Mo in tonalites (Mätäsvaara and Aittojärvi). Late (but still Archaean) Au is found in shear zones in greenstones (Ilomantsi).

2. Mineralization tied to Palaeoproterozoic failed rift systemsAfter cratonization of the Archaean, several failed rifts occured in the Paleoproterozoic, which resulted in a variety of ore-bearing environments in the Nordkalott province: Cr and PGE in 2.4 Ga layered intrusions (Kemi), 2.1 Ga Ni-Cu (Petsamo) and 2.1 Ga stratiform Cu in greenstones (Viscaria).

3. 1.9 Ga mineralization within the Svecofennian DomainWhen the Archaean continent eventually rifted and an ocean opened southwest of the present Archaean crust, a variety of Svecofennian terranes developed, which subsequently amalgamated to the Archaean continent during the Svecokarelian orogeny. a) The earliest environment is the 1.96 Ga Jormua ophiolite and the associated Outokumpu Cu-Zn-Co ore. b) Terrestrial 1.90 Ga volcanism, along the southwestern margin of the Archaean continent (comparable to present-day Chile), was associated with the Kiruna magnetite-apatite Fe ores. c) Continental margin, shallow-marine 1.90 Ga volcanism along the southwestern margin of the Archaean continent (from Tjåmotis to Varkaus), was associated with formation of VMS Cu-Zn deposits (Vihanti and Pyhäsalmi), which often erronously are confused with island arc metallogeny of the Skellefte district. d) Island arc 1.89 Ga magmatism resulted in Kuroko type Cu-Zn deposits in the Skellefte (Boliden and Kristineberg) and Tampere districts as well as comagmatic plutonic complexes of the Central Finnish Batholith with associated Cu-Mo-W (Kopsa, Ylöjärvi and Kåtaberget). e) The island arc environment (d) is spatially surrounded by discontiuous pods of ultramafics with significant Ni ores (Lappvattnet, Kotalahti and Vammala), revealing deep and regionally significant sutures, which probably represent terrane boundaries. f) The Bergslagen district in south-central Sweden is the historiacally most important ore district of Fennoscandia and forms a terrane that can be followed to southwestern Finland, Estonia and Latvia. It is characterized by 1.89 Ga felsic continental margin volcanics and continent-derived clastic sediments. Associated ores include magnetite-apatite Fe (Grängesberg), exhalative Fe (Dannemora and Norberg), volcanogenic contact-metasomatic Cu-Zn-Pb (Falun and Grängesberg) and volcanic-exhalative Zn-Pb (Zinkgruvan).

4. Stiching plutons within the Svecofennian (and Archaean) DomainsAfter metamorphism and amalgamation of the Svecofennian terranes onto the Archaean craton, the Svecofennian (and Archaean) Domains were penetrated by stiching granitoid plutons of several generations: a) 1.8 Ga late collisional uplift granites with intragranitic Mo (Bispberg, Allebuouda and Tepasto) and associated exoskarn-W (Yxsjöberg), b. 1.8 Ga subduction-related TIB batholiths with Pb-Zn (Ålatorp). c) 1.7 Ga TIB batholiths in extensional tectonics and 1.65-1.54 Ga anorogenic rapakivi granites with polymetallic (Cu-Zn-Pb-Sn) intragranitic greisen (Van and Eurajoki) and exoskarn ores (Pitkäranta).

5. Gold in Proterozoic shear zones within the Svecofennian (and Archaean) DomainsLate gold in shear zones is found in several environments: a) Svecokarelian shear zones in the greenstone belts of the Nordkalott province (Bidjovagge, Saattopora and Pahtohavare). b) Svecokarelian shear zones along the margins of the island arc (Björkdal, Åkerberg, Laivakangas, Kutemajärvi and Osikonmäki), following the same general structural trends as the Ni-bearing ultramafics. c) Later (<1.8 Ga) shear zones in southeastern Sweden (Ädelfors and Solstad).

6. Mid to Late Proterozoic mineralization in southwestern Scandinavia.Southwestern Scandinavia records a very different geological and metallogenetic evolution compared with the Archaean-Svecofennian Domains. The following ore types are among the most important: a) Stratiform Zambia-type Cu deposits in 1.1 Ga continental sedimentary failed rift basins. b) Shear zone related Au associated with the 0.9-1.0 Ga Sveconorwegian orogeny (Eidsvoll and Harnäs). c) Stitching plutons with Mo (Knaben) and Ti (Tellnes).

Gorbatschev R & Bogdanova, S, Precambrian Research, 64, 3-21, (1993).

H05 : 1B/36 : F5

Precambrian Iron Banded Formations: Evolution Conception

Dmitri Yegorov (dyegorov@yahoo.com)

GI KSC 14 Fersman str., Apatity, Murmansk region, Russia

Precambrian Iron Banded Formations (BIF) can be subdivided into to major types: Algoma-type (lenticular deposits closely associated with effusive rocks) and Lake Superior-type (thicker deposits in schists). The Superior-type BIFs all over the world are confined mainly to the Lower Proterozoic rocks; in Europe these are huge deposits in Krivoy Rog and Kursk (2200-2000 Ma; Shcherbak et al., 1990). The Algome-type BIFs occur mostly near the Archean-Proterozoic boundary. In Europe, the largest deposits of this type are located in the Ukrainian Shield (Novopavlovsk complex, over 3400 Ma, Kosivtsevskaya formation of the Near-Azov region, over 3310 Ma, Konsko-Verkhovtsevskaya BIF, 3175-3150 Ma; Shcherbak et al., 1990), and on the Baltic Shield (Kola BIF, Olenegorsk, 2800-2750 Ma, original data; Kostomuksha, 2800-2760 Ma; Bibikova, 1989).

Most researchers interpret the Algoma-type BIFs to be sedimentary or volcanic-sedimentary formations, however (in contrast to the undoubtedly sedimentary BIFs of the Superior-type) this interpretation is more often than not based on insufficient factual evidence. According to our studies of the Kola BIF and the data on the Kostomuksha (Barabanov, 1985) and a number of Archean BIFs of the Ukrainian Shield (Shcherbak et al., 1990), these deposits were formed by metamorphic-metasomatic transformations of primary high-Fe volcanic rocks in Archean granite-greenstone areas.

Isotopic studies of sulphur (Huttori et al., 1983) and carbon (Melezhik & Fallik, 1996) showed that about 2300 Ma ago there was a sharp increase in the oxygen content in the atmosphere, from only a few percent to the modern level. In our opinion, this very event created the necessary prerequisites for huge (as much as 3 km thick) Superior-type BIFs to originate in the course of sedimentation.

And finally, BIFs ceased to originate about 1800 Ma ago. This is usually taken to be the consequence of oxidation of the deep-ocean waters which, as a result, became depleted in iron (Klein, 1997).

Barabanov V.F., Geochimiya Leningrad Nedra, 424, (1985).

Bibikova EV, Uranium-lead geochronology of early stages of the evolution of ancient shields. Moscow, Nauka, 180, (1989).

Hattori K, Krouse HR & Campbell FA, Science, 221, 549-551, (1983).

Klein C, Nature, 385, 25-26, (1997).

Melezhik VA & Fallick AE, Terra Nova, 8, 141-157, (1996).

Shcherbak NP, Artemenko GV, Bartnitsky EN, Tkachenko MV & Plotkina TA, Isotope dating of endogenic ore formations. Abstracts of the All-Union Conference. Kiev, 899-92, (1990).

H05 : 1B/37 : F5

Precise ¹⁸⁷Re-¹⁸⁷Os Ages for Au Deposits in the Ukrainian Shield

Holly Stein (hstein@cnr.colostate.edu)¹,

Richard Markey¹,

Krister Sundblad (Krister.Sundblad@geo.ntnu.no)²,

Albert Sivoronov (geomin@franko.lviv.ua)³,

Alexander Bobrov³ &

Igor Merkushin³

¹ Department of Earth Resources, Colorado State University, Fort Collins, CO 80523-1482, USA

² Department of Geology and Mineral Resources Engineering, The Norwegian University of Science and Technology, N-7034 Trondheim, Norway

³ Department of Geology, Lviv State University, 290000 Lviv, Ukraine

The Ukrainian Shield, comprised of at least six different terranes featuring contrasting Archean and Proterozoic geology, is situated in the southwestern part of the East European Platform. Gold mineralization of unknown extent is found in several terranes. Molybdenite (MoS₂), a common accessory to Au mineralization, is key in providing a venue for ¹⁸⁷Re-¹⁸⁷Os dating of Au deposits (Stein et al., 1998). We have dated molybdenite from drill core samples representing two major Au deposits, Maiske and Sergeevske. Absolute uncertainties for all Re-Os data are quoted at the 2-sigma level. The Archean Sergeevske deposit, in the Middle Dnieprian block consisting of 3.5-3.1 Ga tonalite-greenstone, is associated with an astonishingly well preserved volcanoplutonic complex with a Au-Cu-Mo core and a Au-Ag-Bi-Te peripheral zone of mineralization. The Re-Os age for Sergeevske is 3128 +/- 13 Ma [Re = 96.3(2) ppm, ¹⁸⁷Os = 3238(8) ppb] and represents the time of the magmatic-hydrothermal ore-forming process. This is supported by a Sm-Nd isochron, with large uncertainty, of 3117±204 Ma for associated felsic volcanic and plutonic rocks (Samsonov et al., 1993). The Proterozoic Maiske deposit, in contrast, is located in the complex Golovanevsk suture that stitches together two highly different geologic terranes. At Maiske, Au is associated with migmatites and pegmatites decorating intensely altered biotite-garnet plagiogneiss. There are two types of molybdenite in the Maiske region. The first type is regionally widespread as an accessory mineral in biotite plagiogneiss. The second type occurs in quartz veins and pegmatites at the Maiske mine and is believed to be associated with the formation of Au deposits during migmatization. We dated molybdenite from each of these two occurrences. The gniess-hosted molybdenite yielded an age of 2229±10 Ma [Re = 105.3(2) ppm, ¹⁸⁷Os = 2504(6) ppb], which may represent the time of biotite gneiss formation. A molybdenite hosted in a 5 mm quartz vein that clearly crosscuts the foliation in the hosting biotite gneiss yielded an age of 2060±9 Ma [Re = 182.3(4) ppm, ¹⁸⁷Os = 4002(10) ppb]. A repeat analysis of this same vein-hosted molybdenite provided an age of 2061±9 Ma [Re = 163.3(3) ppm, ¹⁸⁷Os = 3586(9) ppb], in superb agreement with our first results. This Re-Os age of 2060 Ma for vein-hosted molybdenite associated with Au mineralization is in excellent agreement with a U-Pb zircon age for Au-bearing pegmatite and U-Pb ages for potassic granites in the Bugsky region to the immediate west; it is also the suggested age for regional metamorphism (Stepanyuk et al., 1998).

Samsonov AV, Zhuravlev DZ, & Bibikova, YV, International Geology Review, 35, 1166-1181, (1993).

Stein HJ, Sundblad K, Markey RJ, Morgan JW, & Motuza G, Mineralium Deposita, 33, 329-345, (1998).

Stepanyuk LM, Claesson S, Bibikova, YV, & Bogdanova, SV, Geophysical Journal, 4, 20, 118-120, (1998).

H05 : 1B/38 : F5

Numerical Modeling of Ag- and Pb- Antimony Sulfosalts Formation in Sulphide Ores of the Beregovo Epithermal Ores Field (Transcarpathian, Ukraine)

Alexander V. Emetz (geomin@geof.franko.lviv.ua) &

Leonid Z. Skakun (geomin@geof.franko.lviv.ua)

Lviv State University, Lviv, Ukraine

There is the following sequence of the mineral formation for sulphide assemblage in the Beregovo ores field: pyrite - sphalerite - galena + boulangerite - pyrargyrite - polybasite (Emetz, Skakun (1998)). At the beginning for formation of these minerals the temperature has equalled 160-230°C (by homogenisation of water inclusions in sphalerite) approximately.

The modeling of forming conditions for these minerals have been actualized by means of "CHILLER" computer program (Reed M.H., 1982), which account multicomponent heterogenic chemical equilibrium, with the utilization of "SOLTHERM" thermodynamic database (Reed & Spycher, 1987). The modeling was completed during quartz's, galena's, sphalerite's and pyrite's stability, and unstability of the undetected minerals (acantite, antymonite etc). Changing of pH and Ag concentration showed that stability fields of the studied minerals not shift greatly.

The temperature decrease displayed the decay of the hydrothermal system; and the decrease of Sb concentration (Fig. 1) reflected the progressive meteoric watering. In the ores boulangerite was precipitated from the hydrothermal solution directly together with galena. The formation of Ag-bearing sulfosalts began when in the solution Sb concentration was low for formation of Sb sulfosalts. The equilibrium was established between the thermal solution and sphalerite-galena aggregates. In these conditions the high-Sb sulfosalts formation occured in the way of destruction (replacement) of the boulangerite. At the condition of a fast dilution of the thermal solutions the replacement of boulangerite by polybasite and pyrargiryte occured directly, and the slowly dilution led to the formation of the intermediary mineral - bournonite. The temperature of polybasite formation we estimated from 115 to 140°C. Highest temperature level of pyrargirite and bournonite crystallisation equals from 145 to 160°C approximately.

The Received Stability Fields of the Following Minerals: Boulangerite (bl), Bournonite (br), Polybasite (pl), Pyrangyrite (pr), Acanthite (ak). In Modelling Solution Ag Concentration Equalled 1,8*10^-8m. The Arrows Show the Probable Evolution of Hydrothermal Solution in the Ores Field.

Emetz A V, Skakun L Z, Formation of silver ores in sulphide bodies of the Beregovo ores field (Transcarpathian), Ukraina J. Conf. Abs. CBGA XVI Congress, 154, (1998).

Reed M H, Calculation of multicomponent chemical equilibria and reaction processes in systems involving minerals, gases and an aqueous phase. Geochem. et Cosmochem. Acta., 46/4,, 513-528, (1982).

Reed M H, Spycher N F, SOLTHERM, data base. A computation of thermodynamic data from 255°C to 305°C for aqueous species, minerals and gases. Eugene, Univ. Oregon., 30p, (1987).

Session H05:1P

H05 : 1P/01 : PO

Geochemical Constraints from Tourmaline Hydrothermal Overgrowths on the Evolution of Mineralising Fluids

Ben J. Williamson (Ben.Williamson@bristol.ac.uk)¹,

Justine T. Adams (jusa@nhm.ac.uk)²,

Andy G. Tindle (a.g.tindle@open.ac.uk)³,

John Spratt (js@nhm.ac.uk)² &

Chris J. Stanley (cjs@nhm.ac.uk)²

¹ Department of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK

² Department of Mineralogy, The Natural History Museum, London SW7 5BD, UK

³ Department of Earth Sciences, The Open University, Milton Keynes MK7 6AA, UK

The evolution of mineralising fluids in southwest England has been studied through geochemical analysis of tourmaline from a <2 cm wide fault breccia within massive quartz-tourmaline rocks at Roche, Cornwall. Brecciated tourmaline clasts have <400 µm wide hydrothermal overgrowths (Fe/(Fe+Mg) = 0.31 to 0.99) within which four chemically distinct zones have been identified (1 to 4, towards the margins). Chemical variations through the overgrowths are interpreted as being due to episodic mixing between Mg-, Al-rich fluids, of magmatic origin, with an increasingly large component of more oxidising, Fe-rich connate waters, dominant in zones 2 and 4. More oxidising conditions are supported by relatively high concentrations of Sn in zone 2 (<0.35 wt.%) which is due to Sn⁴⁺ being more easily accommodated within the tourmaline structure than Sn²⁺, the usual form of Sn in low redox hydrothermal fluids.

The timing of tourmaline nucleation and growth is indicated from X-ray element maps of the breccia. During the formation of zones 1 and 3, tourmaline crystallisation as overgrowths was kinetically favoured over tourmaline nucleation. Increased nucleation and growth during the formation of zones 2 and 4 resulted from mixing with connate fluids which caused 'undercooling' and supersaturation with respect to tourmaline.

H05 : 1P/02 : PO

Cretaceous Formation of the Trimouns Talc-Chlorite Deposit (Pyrénées, France) from Long-Lasting Hydrothermal Activity

Philippe de Parseval (parseval@insa-tlse.fr)¹,

Urs Schärer (scharer@ipgp.jussieu.fr)² &

Mireille Polvé (polve@lucid.ups-tlse.fr)¹

¹ Université Paul Sabatier - UMR 5563, 38, rue des 36 Ponts, 31000 Toulouse, France

² Université Paris 7, IPG Paris, 75251 Paris cedex 05, France

The Trimouns deposit is one of the largest talc-chlorite deposit in the world but no radiometric ages have been determined so far, leaving open the question on a Hercynian or younger origin. To temporally constrain deposit formation, 16 xenotime (YPO₄) fragments broken off from 4 different mm-size crystals were dated by the U-Pb chronometer, as well as a single monazite (CePO₄) grain. Together with other REE-minerals, xenotimes and monazite crystallized in centimeter size geodes in the hanging wall dolostones of the talc-chlorite ore body (de Parseval et al., 1997), being formed in direct relation to 320°C/0.25 GPa metasomatism along the host shear zone (Moine et al., 1989; de Parseval et al., 1993). Since xenotime and monazite are closed systems for U-Pb under such conditions, individual ages date crystal growth within the host geodes. All fragments measured yield concordant ages in the concordia diagram lying between 113 and 99 Ma. Dependent on the grain from which they were taken, the fragments plot differently within this range of ages: Six fragments have ages that are identical within analytical uncertainties defining a mean value of 110 Ma. Most other fragments yield younger ages, and a single monazite grain has an age of 99 Ma, which is within analytical uncertainty identical with the youngest xenotime fragments. The set of U-Pb dating results indicate that: (1) the talc-chlorite deposit was produced during the Pyrenean orogeny, (2) xenotime growth and related metasomatism occurred during a period of about 14 m.y., (3) monazite formed at the end of this period, and (4) associated ductile shear was active for at least the same period of time, most likely reflecting extension during late phases of Pyrenean orogeny. This scenario also suggests that Variscan high-temperature shear zones were re-activated at that time.

de Parseval Ph, Fontan F & Aigouy T, C. R. Acad. Sc. Paris, t324, 625-630, (1997).

Moine B, Fortuné JP, Moreau P & Viguier F, Econ. Geol, 84, 1398-1416, (1989).

de Parseval Ph, Moine B, Fortuné JP & Ferret J, Current research in geology applied to ore deposits, ISBN 84-338-1772-8, 205-208

H05 : 1P/03 : PO

Dynamic of Ores Formation in the Beregovo Epithermal Ores Field (Transcarpathian, Ukraine)

Alexander Emetz (geomin@geof.franko.lviv.ua)¹ &

Leonid Skakun (geomin@geof.franko.lviv.ua)²

¹ 37/3 Levitsky St., Lviv, Ukraine

² GF of LDU, 4 Grushevsky St., Ukraine

The ores field is situated in the Intercarpathian volcanic belt, in the Beregovo Sarmatian explosive caldera filled by the clayey sediments which are divided by two tuffs interbeds. There are Beregovo, Muzievo and Kvasovo gold-silver-base metal deposits in the ores field. The ores occur as veins, stockworks. Disseminated ores are widespread. For the deposits by the correlation of minerals in the ores, observations of homogenisation process of fluid inclusions in quartz, and metasomatic changes of the tuffs, a sequence of mineral paragenesis were established by Skakun (1994) here.

We have ascertained the relative continuity of these paragenesis formation. During decay of the hydrothermal system the control of mineral aggregates formation have been effected by the temperature gradient and migration of meteoric and thermal water mixing zone. During formation of the ores there were migrating zones for each mineral assemblage. The barrier between these zones was relative and was defined by permeability of rocks. There were the following forming ores zones in the deposit (the sequences of migrating replacement between basic minerals are specified in brackets):

- sulphide zone (siderite - pyrrhotine, pyrite, sphalerite, galena, native Ag, Sb-sulfosalts of Pb and Ag - tetrahedrite (higher horizons), chalcopyrite and bismuthinite (lower horizons), native gold, fine-grained quartz veins - recrystallization of quartz, barite).

- oxidation zone (anglesite - amethyst quartz, Fe hydroxide (higher horizons), hematite (lower horizons) - barite - Mn hydroxide - Ag sulphides, native Ag - cinnabar).

The homogenisation temperature of fluid inclusions both in recrystallizated and amethyst quartz are similar (200-300°C) and a little vary at a deepening. It corroborate the gradual decay of hydrothermal system. However, for one time this process was intensificated (the permeability of rocks has increased) because the cracks filled by amethyst quartz with chalcedone were observed in the ores. The intensification of hydrothermal process was accompanied by slight dissolution of earlier quartz, that have led to packing of the solution by SiO₂ and chalcedony formation. Fluid inclusions of amethyst quartz have a large salt content, that is the reflection of global dissolution of ores and a greatest saturation of the solution by salts in the oxidation zone.

Skakun LZ, Minerals-genetic model of the Muzievo gold-silver-base metal deposit. Ph.Dr. Thesis, Lviv St. University, 280, (1994)

H05 : 1P/04 : PO

Grade and Reserve Evaluation of the Tulovasi Borate Deposit (Balikesir, Turkey): A Geostatistical Case Study

Cem Sarac (cem.sarac@hun.edu.tr)

Hacettepe University, Geological Engineeering Department, Beytepe Campus 06532 Ankara, Turkey

This study describes a geostatistical grade and reserve evaluation of the Tulovasi borate deposit which is situated between Osmanca Village and the Simav River in Bigadic, Balikesir (Turkey). Commonly employed ore-reserve evaluation techniques use a weighted average of samples. The weighting coefficients are a function of the mining blocks in the deposit, but they shed no light on the variability of the orebody. Furthermore, these techniques do not allow a determination of the reliability of the estimates, Geometrical estimation methods, such as polygons of influence, triangular and cross-section, do not take into account spatial correlation and therefore result in biased estimates. However, geostatistical estimation techniques allow a calculation of the measure of the error associated with the estimates. In geostatistics, the spatial variability of a regionalized variable is characterized by the variogram function. It is possible to find weighting coefficients for a given mining block and also data configuration that minimizes this error.

The research begins with the composition, variable length of drillhole samples were composited to give equal length samples. 433 composite samples give the mean of 26.67 B₂O₃% with 142.65 variance and 0.45 coefficient variation. Experimental variograms representing three main directions were calculated, and a simple spherical-type model was fitted to the experimental variograms. This was followed by three-dimensional block kriging. The deposit was divided into 40 x 40 x 6 meter blocks and the kriged estimate, together with the associated kriging variances for each block, was determined. Finally, the actual grade-tonnage curves were calculated. These improved estimates and the grade-tonnage curves can be used as a basis for mine-planning purposes.

H05 : 1P/05 : PO

The Evolution of the Tagil Volcanic- Arc Complex in the Middle Urals and Related Ore Deposits

Magnus Friberg (mf@geofys.uu.se)¹,

Gueorg A. Petrov (uricc@dialup.mplik.ru)²,

Monica Beckholmen (mb@geofys.uu.se)¹,

Alexander Larionov (al_larionov@hotmail.com)³,

Alexandet N. Glushkov (uricc@dialup.mplik.ru)² &

Ivan I. Zenkov (uricc@dialup.mplik.ru)²

¹ Dept. of Earth Sciences, Uppsala University, Villavägen 16, S-752 36 Uppsala, Sweden

² Urals Geological Survey Expedition, UGSE, 55 Veinera, Ekaterineburg, Sverdlovsk District 620014, Russia

³ Swedish Museum of Natural History, Box 50007, S-104 05 Stockholm, Sweden

The Urals mountains is one of the most important regions for Russian ore exploration. Major deposits of gold, silver, copper and iron have been mined for over 200 years, other, platinum, chrome and titanium etc. more recently. Many of these deposits are concentrated to the Tagil Volcanic-arc Complex (TVC). This Middle Palaeozoic island-arc extends for 1000 km in N-S direction throughout Middle and Northern Urals and was accreted to the eastern margin of Baltica in the Late Palaeozoic.TIC consists of ophiolitic, island-arc and back-arc basin igneous and sedimentary rocks. The ophiolitic units, exposed in the western margin of the complex, include gabbro massifs (e.g. Arbat), sheeted dike complexes and basalts (Kaban Suite) of Late Ordovician age. The Silurian is dominated by extensive, mainly andesitic island-arc volcanism, (Pavda and Imennaya suites) in the west and shoshonitic, (Tura Suite) island-arc volcanism and back-arc basins (Krasnouralsk suite) to the east. The Imennaya suite associates with ultramafic bodies of the Platinum bearing belt dated to 428±7 (Bosch et al 1997) and the Tura with syenite intrusions (e.g. Kusvha massif), zircons from the latter yields Mid Silurian ages (426±5 Ma). The magmatic zonation has previously been explained as an evolution of the arc with time; however, our new data imply that the arc was laterally zoned and one way of interpreting the data is by an E-dipping subduction zone, yielding a more evolved magmatism in the east (Tura suite) than in the west (Imennaya suite). There are strong indications (fauna in limestone lenses in volcaniclastic material) of Late Silurian and Early Devonian magmatic events also producing shoshonitic lavas. This event is not easy to separate from the Silurian Tura shoshonites. After a short break in volcanic activity (Siegenian-Emsian) the magmatism continued further east with andesites (Krasnoturisk Suite) in the Eifelian. Characteristic mineralizations are associated with each of the magmatic episodes. The Kaban ophiolite suite have small Cyprus-type of copper and iron sulphides. The extrusive Pavda and Imennaya suites contain Kuroko sulphide mineralizations and the Krasnouralsk suite gave rise to large deposits of copper, zinc and lead sulphides of Uralian type. The ultramafic rocks e.g. pyroxenite and dunite, associated with the Silurian volcanism have high concentrations of mainly titanium, iron, vanadium, platinum and chrome. Gabbros of the same age (e.g. Volkovo massif) contain titanium, iron, gold, platinum, and vanadium. Intrusive massifs associated with the Tura suite and the Kushva massif produced iron, copper and gold skarn mineralizations, the gold being often in quartz veins in hydrothermaly altered rocks. Similar mineralizations are found in the Late Silurian and Devonian rocks. The sedimentary rocks have strata bound manganese and aluminum deposits.

Bosch D, Krasnobayev AA, Efimov AA, Savalieva GG & Boudier F, EUG 9 Abstracts, 122, (1997).

H05 : 1P/06 : PO

Ore Facies of Urals Massive Sulphide Deposits

Svetlana Tessalina (S.Tessalina@brgm.fr)¹,

Victor Zaykov (zaykov@ilmeny.ac.ru)¹,

Valery Maslennikov (mas@ilmeny.ac.ru)¹ &

Jean-Jacques Orgeval (Orgeval@exchange.brgm.fr)²

¹ Institute of Mineralogy of Urals Branch of Russian Academy of Sciences, Miass, 456301, Russia

² BRGM, 3, avenue C. Guillemin - BP 6009 Orléans Cedex 2, France

Many types of massive sulphide deposits can be found worldwide. But every ore deposit is composed from some ore facies. A detailed study of distribution, mineralogy and geochemistry of the ore facies is necessary for reconstruction of processes of ore formation.

According to the definition of Lisitzin, Zhabin, Hekinian, Fouket by ore facies we understand: a set of ores with the same type of genetic features, through which the processes and conditions of ore-formation can be reconstructed. The ore facies are considered as a part of a geological body, belonging to a same it from ore-bearing system in their morphostructure, texture-structural, mineralogical or geochemical features. After the example of Ural's massive sulphide deposits following basic ore facies are allocated: hydrothermal-sedimentary, hydrothermal-metasomatic, clastic and hypergene. Every ore facies include any ore-subfacies. For example - hydrothermal-sedimentary ore facies include the deposits of the socle of sulphide hills, the relics of sulphide chimneys and biomorfic ore etc.

The hydrotermal-metasomatic impregnation and streaky ore form stockworks and bed-likes bodies in the footwol of the deposits. Hydrotermal-sedimentary massive and bedded ore compose central part of the sulphides hills and bodies of compound combination (double-mounds, multi-mounds). Clastic and hypergene ores situated at the top and the flanks of ore bodies in the form of bed- and lens-shaped layers.

Mineralogical zonality of hydrotermal-metasomatic and hydrotermal-sedimentary ore facies was described many times. The zonality of clastic ore body (for example Alexandrinka deposit) is inversed with respect to zonality of sulphide hills (upwards): (sphalerite-chalcopyrite-pyrite) zone - (pyrite-chalcopyrite) zone - (pyrite-sphalerite-barite) zone - hematite zone. The highest contents of gold are concentrated at the top of ore cyclites in the submarine leaching zone and silver - in the footwall in submarine sulphide enrichment zone, often with bornite.

The contents of the trace elements in the sulphide minerals differ from various ore facies - for example, contents of Fe and Cd in sphalerite and Se in galena.

Isotopic composition of sulphur in the sandstone and bedded ore is more "light" (to -31‰ in the sandstone from the top of Gay deposit) in comparison with the ore of hydrothermal-sedimentary facies (average for some deposits - +2,2 - +2,4‰).

Typomorphic properties of sulphide minerals from various ore facies, depending on the conditions of ore formation, are different: microhardness and thermoelectric properties of pyrite, two colours of luminescence of sphalerite in the clastic ore etc.

Support from Russian Scientific Foundation (RFFI) N 98-05-64818 and INTAS N 96-1699 is gratefully acknowledged.

H05 : 1P/07 : PO

Isotope Characteristics of the Rare-Metal Granitoids (Far East, Russia) as a Possible Tool for Estimation of Sn-W Ore-Deposit Scale

Robert Krymsky (robert@RK2258.spb.edu) &

Boris Belyatsky (boris@BB1401.spb.edu)

Institute of Precambrian Geology and Geochronology (IPGG), RAS, Makarova emb., 2, St.Petersburg 199034, Russia

To search of isotope-geochemical criteria for discriminate giant ore-deposits and ore-deposits of ordinary scale there were studied Sm-Nd and Rb-Sr systems of granitoids from two giant (Vostok-2, Pravourmi) Sn-W ore-deposits and two smaller ones (Zabytoe, Tigrinoe) from the Far East, Russia. Formation of these granitoids and ore-deposits took place in Late Cretaceous during rather short interval time at 98-80 Ma.

Primary Nd and Sr isotope characteristics of studied granitoids form linear trend in coordinates <epsilon> (Nd)-I(Sr) with positive slope. Direct correlation of these characteristics with the age of the corresponding massif is observed - from the older to the younger granitoids there is a shift towards positive values of <epsilon> (Nd) and negative values of <epsilon> (Sr). If this trend evidences a realization of certain genetic relations during formation of these granitoids, then:- Formation of granitoid sources could be the result of interaction and mixing of material of two reservoirs: depleted mantle (DM) and continental crust of Proterozoic age (Bureinsko-Khankaisky microcontinent ~1.0 Ga). The rocks of the last had higher Rb/Sr ratios and as a consequence ⁸⁷Rb/⁸⁶Sr=0.5-2.6 and ⁸⁷Sr/⁸⁶Sr=0.715-0.736. In the crust component Sr/Nd and/or ⁸⁷Sr/⁸⁶Sr ratios were increased during the time, that could be caused by progressive involvement of different rocks (with diverse composition and content of Sr) into the process of magma generation. Then the share of such crust material in the sources of these granitoids was not higher than 50%.- Granitoids could be formed in the result of mixing of Proterozoic continental crust material with the substance of metasomatized mantle with high Rb/Sr ratio due to higher phlogopite content. The observed shift of initial Sr composition from 0.7067 to 0.7116 during 20 Ma is provided at the primary Rb/Sr ratio in metasomatized mantle not less then 5.3, which is rather typical for the products of ultra-potassium magmatism (lamproites, kimberlites) - the derivatives of metasomatized mantle.

Granitoids of large ore-deposits (Vostok-2, Pravourmi) lie in the beginning of this trend (the lowest values of e(Nd)) and the small ore-deposits (Zabytoe, Tigrinoe) - in the opposite end, i.e. the ratio of mantle and crustal component (k=(Sr/Nd)mantle/(Sr/Nd)crust) varies from 1 (Vostok-2, Pravourmi) to 0.1 (Tigrinoe). The process of Sr/Nd ratio decreasing within the mantle component can be connected with metasomatic reworking of mantle substance in the source under action of fluids rich in CO₂ which mainly extracted REE. During crust material melting by fluid-rich melts of such composition mixed daughter magmas should be characterized by intermediate, in comparison with DM and crust, parameters of <epsilon> (Nd), and crustal values of <epsilon> (Sr). Such characteristics correspond to those for Tigrinoe and Zabytoe granites. The granites of giant ore-deposits are characterized by mixing of DM material and continental crust with usual for these isotope reservoirs Sr/Nd ratios (k=1-2).

Investigation of Sm-Nd and Rb-Sr isotope systems for ore-minerals from these ore-deposits do not demonstrate the same trends, which is caused by the superposition of more factors - not only by the deep sources, but host granites, sedimentary and metasomatically altered rocks also.

H05 : 1P/08 : PO

Gold Deposits in the Lower Proterozoic Gneiss Complexes of the Ukrainian Shield

German Yatsenko (sveshn@geof.franko.lviv.ua)¹,

Alexander Babynin² &

Eugenia Slivko (sveshn@geof.franko.lviv.ua)¹

¹ Department of Geology, Lviv State University, Grushevskogo 4, 290005, Lviv, Ukraine

² Ukrpolymetal, Kyiv, Ukraine

During 80-90th some gold deposits and prospects of new type were discovered in the central part of the Ukrainian Shield (the Kirovograd geoblock), and the first one was the Klyntsi deposit. Mineralization is located within Kirovograd deep-seated sub-longitudinal fault zone which surrounds from the east the Novoukrainskiy trachytoid potassic granite massif. The host rocks are presented by Lower Proterozoic highly metamorphosed and metasomatically altered ones of flisch-like metagreywacke formation. This type of deposits is mono-mineral one and includes gold objects essential in respect to their resources.

The Kirovograd geoblock belongs to the protoplatform structure type of the Early Proterozoic stabilization epoch. Its basement is built up with highly metamorphosed and granitized Archaean units. Broken and retrograded basement had been subsequently overlapped by thick proto-cover which has been in turn multy-facially metamorphosed and sometimes granitized. Metagreywacke formation is presented by intercalation of prevailing various dark-coloured gneisses (metamorphosed sandstones-greywackes) an greenish diopside-plagioclase-quartz crystalline schists.

The Klyntsi ore field have been formed during activization stages; metasomatic and metasomatic-hydrothermal activity which have promoted ore formation took place. Two types of metasomatites are distinguished: Fe-Mg-Ca ones of prograde stage and alkaline-siliceous metasomatites and hydrothermalites of retrograde stage. The former type includes biotite and biotite-phlogopite micaceous rocks, amphibole(cummingtonite)-biotite rocks (klyntsovites), diopside-amphibole-biotite-quartz, biotite-tourmaline, biotite-cordierite metasomatites, eclogite-like rocks. Metasomatic transformation have been facilitated by tectonic movements, sometimes mylonites, pseudo-tachylites and distinct fluidisite bodies are presented. Accessory diamonds of metamorphogenic ("koktchetav") type have been found. During retrograde stage orthoclase-quartz vein bodies, low-temperature skarns, epidosites, beresites, propylites and argillizites have been formed. Young cutting dykes of lamprophyres are distinguished.

Ore bodies of the Klyntsi deposit are subjected to general linear framework (Yatsenko et al., 1998). Their dipping is around NW 268-276 at the angles 82-89 degree. Ore zones with gold grade from 0.1 g/t are traced as much as 3 km. Almost entire gold enrichment occurs, accompanied by anomalous As and Bi concentrations. Widespread sulphides are presented by arsenopyrite, loellingite, pyrrhotite, pyrite, chalcopyrite, native minerals - by gold, arsenic, bismuth, lead. Ore bodies with gold grade more tha 3 g/t have mineralized stockwork zones appearance which are characterized by sharp increasing of vein oligoclase-quartz matter and by deformations of ore bodies owing to shear renew and welding by new quartz generations. The thickness of ore bodies varies from 0.4 to 6.0 meters. Free gold is of high fineness and of slightly decreased one (930-881), it is irregularly distributed, its upgraded amounts are restricted to veins external contacts.

From the known gold objects, the Klyntsi gold deposit in many respects is very similar to Homestacke deposit, of the same group is also Kalana deposit in the Western Africa and Amismissa deposit in the Western Ahaggar.

Yatsenko GM, Babynin AK, Gurskiy DS et al, 1998, Gold Deposits in the Precambrian Gneiss Complexes of the Ukrainian shield. - Kyiv: Geoinform, 225 p, (1998).