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Chelopech

Bulgaria

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The Chelopech high sulphidation (acid sulphate) epithermal gold-copper deposit is located some 60 km to the east of Sofia in Bulgaria and falls within the Panagyurishte district in the central part of the Banat-Srednogorie zone. It is among the largest gold mines in Europe (in 2002) and lies within 7 to 8 km south-east of the large Elatsite porphyry copper deposit (#Location: 42° 41' 56"N, 24° 4' 30"E).

Bulgaria is located on the SE part of the Balkan Peninsula, and lies within the broader Alpine-Tethyan orogenic belt, which incorporates the arcuate Banat-Srednogorie zone. The Banat-Srednogorie zone trends SW from central Romania, curving to north-south through Serbia, to be oriented east-west through Bulgaria to the Black Sea coast. It constitutes a major metallogenic zone in Eastern Europe, linked to the subduction of the Tethyan oceanic crust.

The Panagyurishte mineral district is defined by a NNW alignment of porphyry copper (e.g., Elatsite, Assarel and Medet) and epithermal Cu-Au deposits (e.g. Chelopech, Elshitsa, and Radka). The alignment of these deposits is oblique to the east-west orientation of the Srednogorie belt in Bulgaria (Chambefort, 2005). Associated but small scale alluvial (Topolnitza and Luda Yana) and minor vein-hosted gold deposits (Svishti Plas) are also found in the belt.

The basement geology in the district is composed of Precambrian gneisses, amphibolites and metasediments of the Pre-Rhodopian Supergroup and lower Palaeozoic phyllites and dolerites (diabases), intruded by Palaeozoic granites and overlain by late Cretaceous conglomerates, sandstones and coal bearing shales. The basement rocks form a series of uplifted NE striking horsts and/or anticlinal structures between which a series sub-parallel grabens host Cretaceous sequences. To the north and towards Chelopech, the Srednogorie massif forms the basement.

These basement rocks are overlain by late Cretaceous volcanic rocks of the up to 2000 m thick Chelopech Formation, which has been subdivided into:
• The Lower Chelopech Unit - comprising of a basal sequence of siltstones and calcareous argillites with subordinate terrigenous sandstones and angular conglomerates. These sedimentary rocks pass upwards into intercalated volcanic sequences that eventually dominate, and include andesites, andesitic agglomerates, andesitic lapilli and psammitic tuffs.
• Upper Chelopech Unit - a Coniacian-Santonian age (Lower Senonian) complex of andesitic and dacitic lavas and tuffs with siliciclastic, volcaniclastic and argillaceous sedimentary rocks, intruded by sub-volcanic bodies of porphyritic andesites. The Upper Chelopech Formation passes from mixed terrigenous-volcanogenous gritty sandstones with volcanogenic exhalative iron-manganese oxide horizons, upwards and laterally into volcanogenic talus breccias and agglomeratic tuffs of andesitic affinity. These volcanic rocks are interpreted to represent the remnants of a stratovolcano, and is cut by later dykes and sub-volcanic bodies of andesite, dacite and porphyritic rocks emplaced between 88 and 75 Ma (late Cretaceous). The Chelopech volcanic centre is located on the northern side of a north-easterly trending jog in the regional, east-west trending, Balkan Fault. The northern and north-eastern part of the volcano has been eroded, whilst the southern part has been down thrust by a strand of the Balkan Fault complex.

The Chelopech Au-Cu deposit is developed within the Lower Chelopech Formation, hosted by altered andesitic and dacitic lavas, tuffs and agglomerates, in the centre of the Chelopech volcanic structure, with mineralisation being controlled by the radial and concentric faults of the associated caldera structure.

A structural study undertaken for Dundee Precious Metals in 2007 and 2008 concluded that the architecture and kinematics of the Chelopech hydrothermal system are characterised by multiple fault and fluid flow events, with mineralising fluids injected as a series of repeated structurally controlled pulses. This pulsing created a complex high-sulphidation epithermal ore-bearing system with a series of ore bodies of differing geological character. Metal zonation, from Pb-Zn rich in the ENE to Cu-Au rich in the WSW, suggests that deeper parts of the hydrothermal system may be located to the SW. Late and post-mineralisation faulting modified the original shape and distribution of the epithermal mineralisation. Structural trends include i). steeply dipping, NW trending transfer structures with strike-slip displacement on the order of hundreds of metres; ii). north to NNW steep faults with normal throw offsets of 50 to 150 m, and iii). steeply dipping east-west structures which partition and offset the known ore blocks of copper mineralisation.

The host andesites and tuffs have been intensely altered from an outer propylitisation, through quartz-adularia, quartz-sericite and advanced argillic assemblages to an innermost intense silicification (50 to 75% SiO2), characterised by the presence of vuggy silica, massive silica and a chalcedony, which contains all of the economic mineralisation.

Three successive mineralisation stages have been recognised at Chelopech, including: i). an early Fe-S stage, mainly consisting of disseminated and massive pyrite, ii). a Cu-As-S stage which is the economic Cu and Au stage, and iii). a late Pb-Zn stage. Each displays different geometries and styles of mineralisation.

Mineralisation occurs as a complex system of massive sulphide bodies, stockworked masses of fine sulphide veinlets, discontinuous sulphide veins and disseminations in altered wall rocks. The sulphide-rich zones are characterised by replacement silicification surrounded by haloes of silica-sericite alteration. The massive sulphides are composed of steeply dipping, branched and overlapping lenses and pipes that plunge at 65 to 90° and extend to depths of 600 m below the surface.

The sulphide assemblage of the deposit comprises pyrite, marcasite, melnikovite, chalcopyrite, enargite-luzonite, tennantite and bornite, together with subordinate famatinite, sphalerite and galena and a variety of other sulphide, arsenide and telluride minerals The dominant gangue minerals are quartz, chalcedonic silica, barite and kaolinite, with subordinate chlorite, ankerite and gypsum. In gross terms, ~45% of the copper is in the form of arsenides and sulfosalts, 50% as chalcopyrite and 5% as oxides. Gold is paragenetically associated with arsenic and base metals minerals, and is found in a variety of forms, both as native metal with admixed silver in a stoichiometric form approximating to Au
3Ag and in auriferous tellurides. The free gold is fine grained (5 to 300 µm, with 5 to 20 µm the norm), accounting for ~10% of the endowment. Most is refractory, intergrown with pyrite, chalcopyrite and sphalerite (~45%), enargite, luzonite, tennantite, tetrahedrite and bornite (~25%) and finely intergrown with chalcedonic silica (~20%). Silver-bearing rock and native silver are usually spatially associated or finely intergrown with pyrite and galena (62%) with enargite, tennantite and tetrahedrite (15%) and as electrum (23%). Other major sulphides and arsenides have simple crystalline forms, and are intergrown with pyrite, occurring in intra-crystal spaces as replacements, as replacements of pyrite, as crosscutting veinlets and as overgrowths. Intergrowths of the cupriferous minerals are common, both as aggregates and as complex textures with several intergrown minerals.

The ore bodies are grouped into two mining areas. The central zone comprises eleven mineralised bodies whilst the western zone contains a further nine such bodies. Of these, seven are considered significant and are defined as mineral resources. The individual mineralised bodies, which are structurally controlled, vary from 150 to 300 m in length, are 30 to 120 m thick and can extend for at least 350 m down plunge.

Within the ore, there ore is a positive chemical correlation between Cu, Au, Fe, S, As and Ba. There is a negative correlation between Cu-Au and Pb-Zn, with the latter dominantly occurring peripheral to the core mineralised zones. The mineralisation has been dated at 78 to 74 Ma (which is younger than the nearby 92 Ma Elatsite porphyry deposit.)

In 2002 the remaining ore reserves+resources were:
    50 Mt @ 1.4% Cu, 3.3 g/t Au (using a 4 g/t Au equivalent cut-off)
    Total production+reserve is estimated to be 195 tonnes of Au
Lowering of the cut-off to 3 g/t Au equivalent doubles the reserve.

Moritz et al., 2005 estimate the production + resource at 42.5 Mt @ 1.1% Cu, 3.12 g/t Au.

Mineral resources in March 2008 (Dundee precious Metals Inc) at a cut-off grade of 3.20 g/t Au
equiv. were:
    Measured + Indicated: 33.1 Mt @ 3.8 g/t Au, 8.9 g/t Ag, 1.3% Cu,
    Inferred: 9.8 Mt @ 2.7 g/t Au, 11.4 g/t Ag, 0.9% Cu

Mineral resources in December 2013 (CSA Consultants for Dundee precious Metals Inc) at a cut-off grade of 3 g/t Au
equiv. were:
    Measured + Indicated: 28.7 Mt @ 4.03 g/t Au, 9.25 g/t Ag, 1.25% Cu,
    Inferred: 8.2 Mt @ 2.71 g/t Au, 11.23 g/t Ag, 0.92% Cu

This summary has been largely drawn from: "Titley, M., Bennett, J. and Meik, S., 2014 - Mineral Resource and Mineral Reserve Update, Chelopech Project, Chelopech, Bulgaria; An NI 43-101 Technical report, prepared for Dundee Precious Metals Inc., by CSA Global."

The most recent source geological information used to prepare this summary was dated: 2014.     Record last updated: 4/12/2015
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.


Chelopech

  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Bonev I K, Kerestedjian T, Atanassova R, Andrew C J  2002 - Morphogenesis and composition of native Gold in the Chelopech Volcanic-hosted Au-Cu epithermal deposit, Srednogorie zone, Bulgaria: in    Mineralium Deposita   v37 pp 614-629
Chambefort I and Moritz R,  2006 - Late Cretaceous structural control and Alpine overprint of the high-sulfidation Cu-Au epithermal Chelopech deposit, Srednogorie belt, Bulgaria: in    Mineralium Deposita   v41 pp 259-280
Chambefort I, Moritz R and von Quadt A,  2007 - Petrology, geochemistry and U-Pb geochronology of magmatic rocks from the high-sulfidation epithermal Au-Cu Chelopech deposit, Srednogorie zone, Bulgaria : in    Mineralium Deposita   v42 pp 665-690
Ciobanu C L, Cook N J and Stein H  2002 - Regional setting and geochronology of the Late Cretaceous Banatitic Magmatic and Metallogenetic Belt: in    Mineralium Deposita   v37 pp 541-567
Heinrich C A, Neubauer F  2002 - Cu - Au - Pb - Zn - Ag metallogeny of the Alpine - Balkan - Carpathian - Dinaride geodynamic province: in    Mineralium Deposita   v37 pp 533-540
Lips, A., Herrington, R., Stein, G., Kozelj, D., Popov, K. and Wijbrans, J  2004 - Refined tuning of porphyry copper formation in the Serbian and Bulgarian portions of the Cretaceous Carpatho-Balkan Belt: in    Econ. Geol.   v99 pp. 601-609
Moritz R, Chambefort I, Georgieva S, Jacquata S and Petrunov R  2005 - The Chelopech high-sulphidation epithermal Cu-Au deposit: in    Ore Geology Reviews   v27 pp 130-131
Moritz R, Kouzmanov K and Petrunov R,  2004 - Late Cretaceous Cu-Au epithermal deposits of the Panagyurishte district, Srednogorie zone, Bulgaria: in    Schweizerische Mineralogische und Petrographische Mitteilungen   v84 pp 79-99
Zimmerman A , Stein H J, Hannah J L, Kozelj D, Bogdanov K and Berza T,  2008 - Tectonic configuration of the Apuseni-Banat-Timok-Srednogorie belt, Balkans-South Carpathians, constrained by high precision Re ľOs molybdenite ages: in    Mineralium Deposita   v43 pp 1-21


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