CONTENT and DESCRIPTIONS OF ORE DEPOSITS
Porter GeoConsultancy continued its International Study Tour series of professional development courses by visiting a representative selection of the most significant epithermal and porphyry gold and copper deposits, of the different styles and age/zones across the western Tethyan Belt of south-east Europe in Bulgaria, Serbia, Romania and Greece.
- overview with SE Europe Geoscience Foundation, Bulgaria,
Chelopech - high sulphidation epithermal Au-Cu, Bulgaria,
Assarel & Medet - porphyry Cu, Bulgaria,
Bor - porphyry & high sulphidation epithermal Cu-Au, Serbia,
Veliki Krivelj - porphyry Cu, Serbia,
Rosia Montana - Rosia Poieni - low sulphidation epithermal Au - porphyry Cu-Au, Romania,
Skouries - porphyry Au-Cu, & Olympias - replacement Au-Ag-Pb-Zn, Greece.
The tour commenced in Sofia, Bulgaria on the evening of Sunday 21 September and ended in Thessoaloniki, Greece on the evening of Thursday 2 October, 2008. Participants were able to take any 4 or more days, up to the full tour, as suited their interests or availability.
The main components of the itinerary were:
All mine/project visits and seminars were successfully undertaken and completed.
Sophia Seminar, with the SE Europe Geoscience Foundation, Bulgaria ...................... Tuesday 23 September, 2008.
A full day symposium (titled the 'Balkans Exploration Conference') was convened in association with the SE Europe Geoscience Foundation on the second day of the tour to provide a context to the tectonic, geological and metallogenic setting of the mineral deposits of the western Tethyan belt in the Balkans. The first day, Monday 22, a public holiday in Bulgaria, was devoted to visiting the important Chelopech mine, as described below.
The seminar was attended by tour participants and by local and regional delegates, allowing the participants the opportunity to network and interact with a wide cross-section of local experience. The first half of the seminar comprised an overview of the tectonics, geology and metallogeny of the region, presented by both local and invited external experts. The second half involved a selection of the tour participants providing presentations on similar copper and/or gold deposits, exploration projects or ore provinces elsewhere in the world.
The western Tethyan, or Alpine-Balkan-Carpathian-Dinaride (ABCD) Province, is part of the greater Tethyan (or Alpine-Himalayan) orogenic system that extends from western Europe to southern and southeast Asia and resulted from the convergence and collision of the Indian, Arabian and African plates with Eurasia. This on-going collision was for the greater part initiated during the Cretaceous. The complex geometry of the collision interface and the presence of several microplates within the orogenic collage resulted in a variety of collision products, notably some segments characterised by extensive regional metamorphism and others by calc-alkaline igneous activity.
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This segmentation has resulted in discontinuous distribution of mineral deposits within the province and a limited lateral extent of the various metallogenic belts along the trace of the orogen.
While magmatic activity is evident within the western Tethyan from the Cretaceous to the present, two overlapping, arcuate segments have been delineated representing the bulk of the magmatism, namely:
i). The more economically significant eastern zone which comprises 90 to 60 Ma, Late Cretaceous, subduction-related intrusive and volcanic rocks which forms an L-shape (concave to the east) extending from Romania through Serbia and into Bulgaria. This zone hosts deposits such as the Moldova Noua porphyry Cu in Romania, Majdenpek and Veliki Krivelj porphyry Cu-Au and Bor porphyry/epithermal Au-Cu in Serbia, and the Elatsite, Medet and Assarel porphyry Cu and Chelopech epithermal Au deposits in Bulgaria. Temporal variations in the ages of mineral deposits within this belt are attributed to variations in convergence direction relative to the geometry of the active margin. Specifically the deposits in south-western Romania, eastern Serbia and in central Bulgaria have been dated at 77, 85 and 91 to 80 Ma respectively.
ii). A parallel western arc, where Late Miocene activity predominates to the north-west hosting deposits such as the Rosia Montana epithermal Au and Rosia Poieni porphyry Cu-Au in Romania, and Early Miocene magmatism which is associated with the Buchim (Macedonia) and Skouries (Greece) porphyry Au-Cu ores to the south-west of the Cretaceous arc.
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Chelopech high sulphidation epithermal Au-Cu, Bulgaria ...................... Monday 22 September, 2008.
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 Au3Ag 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."
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Assarel porphyry Cu, Bulgaria ...................... Wednesday 24 September, 2008.
Three significant porphyry copper systems Elatsite, Medet and Assarel and a number of smaller occurrences including Vlaikov Vruh and Tsar Assen have been exploited within the Panagyurishte district of the Srednogorie zone in central Bulgaria, some 60 to 80 km east of Sofia.
The main feature of the geology of the Panagyurishte district is the extensive late Cretaceous calc-alkaline magmatism, both intrusive and extrusive volcano-sedimentary rocks which are developed on and through a basement of Proterozoic (mainly granitic gneisses, schists and amphibolites) and late Neoproterozoic to Cambrian (greenschist phyllites, dolerites and mafic tuffs) metamorphic basement, and middle to late Palaeozoic intrusions (granodiorite to granite and smaller peridotite, gabbrodiorite, granodiorite, quartz-diorite to tonalite), overlain by Permian (red bed conglomerates and sandstones) and Triassic (limestone and sandstone) sediments.
The late Cretaceous sequences include:
i). Late Turonian (91-88 Ma) terrigenous sediments, chiefly coal and sandstone,
ii). Lower Senonian (88-86 Ma) volcanogenic complexes, largely extrusives with lesser volcano-sedimentary units and rare sediments, derived from a number of volcanoes, including lava sheets, agglomerates, block tuffs with distal lapilli tuffs, agglomerates, tuffite, sandstone, limestone, argillite and tephroid flysch. The volcanism is dominantly andesitic although dacitic rocks are more common to the south, while to the north there is a gradation to latite-andesite, latite and eventually trachyte. The volcanic and sub-volcanic to hypabyssal complexes includes numerous intrusive stocks to dyke like bodies, comagmatic with the extrusives, ranging from monzonite porphyry to granodiorite, quartz-monzodiorite and rarely quartz-gabbro. There is a possible younging sequence of intrusion from major centres at Chelopech-Elatsite in the north-west to Medet-Assarel to Elshita-Vlaikov Vruh and Pesovets in the south-east.
iii). Late Senonian to Maastrichtian flysch, followed by folding and faulting which produced a series of grabens that preserved the sediments and volcanics, and provided sites for subsequent overlapping Tertiary to Quaternary sedimentation.
The Assarel deposit is partly hosted by volcanics which are coeval with the main intrusive and is located in the central part of the main Assarel volcano in an area of intense radial and concentric faulting. The central part of the volcano is occupied by massive and brecciated andesitic and latite-andesitic lava sheets and pyroclastics, while the Assarel granodiorite porphyry is found in the centre of the stratovolcano as two apophyses which join at depth. Post ore faulting has displaced the eastern part of the structure upwards to juxtapose weakly altered basement along-side advanced argillic alteration. The ore is present as a cone inclined at 80 to 85° S, with a surface expression as an ellipsoid elongated in a N-S direction. Alteration is present within the sub-volcanic intrusives, volcanics and Palaeozoic granitoids as i). propylitic, ii). propylitic-argillic, iii). propylitic-sericitic, iv). sericitic, v). sericitic-advanced argillic, and vi). advanced argillic-acid chloride and acid sulphate types. Relicts of K silicate assemblages suggest an earlier K silicate and K silicate-propylitic phase. Epithermal sericitisation and advanced argillic alteration overprint these earlier K silicate assemblages in the lithocap. The ore and alteration assemblages at Assarel are more complex than at the other deposits, although the early quartz-magnetite-hematite seen at the other deposits is only present to a limited extent while the late quartz-molybdenite association is rare. The principal ore assemblage is quartz-chalcopyrite ±pyrite accompanied by sericite and transitional sericite-propylitic alteration, with quartz-pyrite with lesser chalcopyrite in both the centre and marginal parts of the system. Rare galena-sphalerite are found at the top of the system, but are in well defined veins with chalcopyrite at depth. High sulphidation assemblages including enargite are found in the upper sections of the system associated with sericitic and advanced argillic alteration. It is also the only deposit in the district with a well developed 60 to 70 m thick supergene enrichment blanket of chalcocite and covellite, below a remnant 10 to 15 m leached cap. Native gold is rare in the hypogene ore, although it shows elevated concentrations at the contact between the base of oxidation and the zone of secondary enrichment.
Reserves quoted in 2002 were 254 Mt @ 0.41% Cu, + historic production of 100 Mt @ 0.53% Cu, trace Au.
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Travelling Sofia, Bulgaria, to Bor, Serbia ...................... Thursday 25 September, 2008.
Bor porphyry & high sulphidation epithermal Au-Cu -and- Veliki Krivelj porphyry Cu, Serbia ................ pm Thursday 25, Friday 26 & Saturday 27 September, 2008.
The Bor and Veliki Krivelj deposits are both found within the Timok Magmatic Complex (TMC), which is part of the greater geological framework that comprises the Alpine-Balkan-Carpathian-Dinaride metallogenic-geodynamic province (Heinrich and Neubauer, 2002), also elsewhere referred to as the Carpatho-Balkan Magmatic Belt (see Herrington et al., 2003 and references therein).
The TMC itself is an approximately north-south, lozenge shaped belt of extrusive and intrusive units emplaced during the late Cretaceous. The complex is approximately 100 km in length north-south, up to 25 km at it's widest, and has a total area of around 1130 sq km (Cocic et al., 2002).
The country rocks of the TMC comprise a basement of Proterozoic metamorphics overlain by Palaeozoic metamorphic and sedimentary formations intruded by Hercynian granitoids (Cocic et al., 2002). Xenoliths of these units have been observed within the TMC volcanic succession. For the most part, the Mesozoic of the region is dominated by carbonate units of Triassic, Jurassic and lower Cretaceous ages. These units are folded with a broad regional NW-SE strike. Volcanic facies analysis of the TMC volcanic successions describe the presence of andesitic crypto-domes with hyaloclastic margins, autobrecciated andesitic flows, epiclastics and tuffs. The wide range of facies and their variation with time suggests periods of both sub-aerial explosive, and relatively quiescent sub-aqueous volcanism, which lasted from the Turonian to the Campanian.
The Bor Porphyry and High Sulphidation Cu-Au deposit is located in the Timok Mountains of eastern Serbia, approximately 150 km SE of the capital, Beograd (Belgrade).
Sub-cropping massive, high sulphidation mineralisation was discovered at Bor at the turn of the 20th century with mining commencing in 1903. Subsequent mining and exploration drilling has shown that the massive sulphide orebodies are spatially related to deeper porphyry style mineralisation, the Borska Reka deposit. The two styles are continuous, linked by a transitional stockwork zone. The length of the altered and mineralised structural zone exceeds 2000 m with a width of around 1000 m, while the mineralised zone, which dips at 50° to the west, has been drill tested to a depth of at least 1500 m below surface. Past production + current reserves of the high sulphidation Cu-Au zone is estimated to total 3 Mt Cu, 160 t Au and 600 t Ag from 200 Mt of ore with an average grade of 1.5% Cu and 0.8 g/t Au (Monthel et al., 2002).
The host sequence at Bor is dominated by porphyritic hornblende-biotite andesites, andesitic tuffs and minor dacites. These volcanic rocks are overlain by pelitic sediments, and are underlain by a series of Late Cretaceous conglomerates and sandstones, which are nowhere mineralised.
High sulphidation mineralisation is characterised by a series of massive, cigar-shaped, or pipe-like bodies, together with mineralisation in fracture zones and in volcanic breccias. The largest of the sulphide bodies is Tilva Rosh with other major bodies such as Choka Dulkan and Tilva Mika. Tilva Rosh, which had maximum plan dimensions of 650 x 300 m and extended vertically for as much as 800 m, comprised both massive and disseminated sulphide mineralisation. The massive sulphide orebody of Choka Dulkan has a strike of some 150 m, thickness of 60-70 m and a vertical extent of around 300 m.
The massive copper ore at Bor contains 3 to 6% Cu and comprises up to 70% (by volume) fine-grained pyrite, accompanied by up to 2.5 to 3.75 g/t Au (locally to 18 g/t Au) and 10 g/t Ag (Jankovic, 1982; Monthel et al., 2002). The principal copper minerals are chalcocite, covellite and enargite, with associated marcasite, chalcopyrite, tetrahedrite and sulvanite. Spectacularly bladed hypogene covellite is common in the massive sulphide ore. Traces of galena and sphalerite are present in the massive sulphides, although these minerals form a major component of the Choka Marin high-sulphidation body which lies to the north of Bor. Associated gangue minerals are significant amounts of silica, barite, ubiquitous anhydrite and native sulphur. Barite is more common in the upper sections of the system, while the predominant sulphate mineral in the lower levels is anhydrite. Native sulphur accompanies high-grade enargite and covellite veining in one of the orebodies. Very late gypsum veins are also common.
There is a suggestion of a sulphide mineralogy zonation within the massive ore to a pyrite-chalcopyrite-bornite (occasionally pyrrhotite) association in the lower levels, possibly indicating a change in sulphur activity. The massive ores grade laterally and at depth to disseminated mineralisation. A system of thin sub-parallel veins is developed beneath the large Tilva Rosh body although post-ore faulting has removed the lower sections of bodies such as Choka Dulkan. The disseminated zones also carry significant sulphide mineralisation (>0.6% Cu) and form part of the ore reserves.
Alteration is distinctive around the high-sulphidation deposits with a change at depth towards the porphyry style mineralisation. In the upper parts of the high-sulphidation system, silicification is the common alteration with vuggy silica developed close to the interpreted palaeosurface. Outward zoned advanced argillic alteration, characterised by in an inner envelope of kaolinisation and peripheral chlorite surrounds the orebodies, in places accompanied by pyrophyllite, diaspore and alunite, and locally with andalusite, zunyite, sericite and some corundum. Alunite is most abundant in the upper parts of the alteration. Kaolinite is commonly associated with the alunite (Karamata et al., 1983).
The porphyry style disseminated and stockwork mineralisation of Borska Reka is associated with dykes and minor intrusions of diorite which are restricted to its lower sections. The bulk of the mineralisation is hosted by altered andesite which has a distinct porphyritic texture. Alteration is characterised by silicification and illite-argillisation, accompanied by chlorite, alunite and carbonates, although alunite is widespread in parts of the deposit. The principal Cu sulphide is chalcopyrite with up to 6% (by volume) pyrite, and in the lower levels, molybdenite. These sulphides are accompanied by magnetite, minor pyrrhotite, enargite and bornite, and are overprinted in parts by chalcocite and covellite. This porphyry deposits contains around 600 Mt @ 0.6% Cu, 0.25 g/t Au.
NOTE: While it was not be practical to go underground at Bor, the group was given a detailed presentation on the deposit and district, and were able to study specimens of the ore and host rocks, traverse rocks at surface and on the margins of the deposit and have an overview of the mine. The deposit is very similar to the other high sulphidation ore seen elsewhere on the tour. The group was also able to inspect the ore in the mine at Veliki Krivelj.
The Veliki Krivelj porphyry copper deposit is approximately 3 km to the northeast of Bor deposit and is within the Timok Mountains of eastern Serbia, approximately 150 km SE of the capital, Beograd (Belgrade).
It has maximum sub-surface plan dimensions of 1500 x 700 m and a vertical extent of more than 800 m as indicated by diamond drilling. Historic production since operations commenced in 1982 has amounted to 0.51 Mt Cu, 120 Kt Mo, 60 t Ag and 10 t Au from ore with a recovered grade of 0.34% Cu, 0.4 g/t Ag and 0.07 g/t Au (Monthel et al., 2002). At a cut-off grade of 0.15% Cu, the deposit is quoted as originally containing a mineable reserve of approximately 2.5 Mt of Cu from ore averaging 0.44% Cu, within a larger geologic resource. The Mo content of the ore is generally only 0.003 to 0.005%, although it may locally reach 0.02 to 0.03%. The deposit has been previously described by Aleksic (1969, 1979); Jankovic et al., (1980); Jankovic (1990, 1990b).
The porphyry style mineralisation at Veliki Krivelj occurs in hydrothermally altered Upper Cretaceous sub-volcanic hornblende andesite and pyroclastic equivalent breccias, tuffs and agglomerates as well as the Upper Cretaceous volcano-sedimentary series of pelites and limestones. This mineralisation, prominent Late Cretaceous to Early Tertiary diorite and quartz diorite porphyry dykes as well as andesite dyke swarms, are all superimposed on earlier skarns formed in the Upper Cretaceous pelites and limestones.
Hydrothermal alteration is mainly developed in the pyroclastic facies of the host hornblende biotite andesite, predominantly as a potassic assemblage characterised by secondary biotite, accompanied by widespread silicification and by sericite on the margins. Pyrophyllite is found locally. On the periphery of the porphyry copper mineralisation, where silicification is weak, chlorite, epidote and calcite are developed. Late intense zeolite, gypsum and sporadically anhydrite are also apparent. Strong pyritisation is associated with the silicification.
Porphyry style mineralisation is dominated by chalcopyrite, pyrite, minor molybdenite, magnetite, pyrrhotite, hematite, traces of cubanite, enargite, bornite, covellite, chalcocite, galena and sphalerite. The occurrence of up to 50 m thick zones of pyrophyllite, alunite and kaolinisation in parts of the alteration system indicates the presence of advanced argillite alteration, although this has not been studied systematically.
The early skarn assemblage includes garnet, calcite, epidote, quartz, biotite, pyrite and sporadically wollastonite, with disseminated chalcopyrite.
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Travelling Bor, Serbia, via Sofia, Bulgaria and Bucharest, Romania to Rosia Montana ...................... Sunday 28 & am. Monday 29 September, 2008.
Rosia Montana low sulphidation epithermal Au -and- Rosia Poieni porphyry Cu-Au, Romania .................. pm. Mon. 29 & Tues. 30 September, 2008.
The Rosia Montana low- to intermediate-sulphidation, epithermal gold deposits lie within the Apuseni Mountains of the Transylvanian region of Romania, approximately 40 km north-west of the city of AlbaIulia and immediately to the north of the city of Deva and fall within the Carpatho-Balkan province of the Tethyan-Eurasian metallogenic belt.
The earliest recorded mining in the region that embraces Rosia Montana dates from the first century AD, prior to the siezure of production by the Roman Empire in 106 AD. The area, which extends over an area of around 500 sq km was known as the Golden Quadrilateral which falls within the Apuseni and Metaliferi Mountains, north of the regional centre of Deva. Production from the area, which has been virtually continuous over the last almost 2000 years, is thought to have been as much as 1300 tonnes (42 Moz) of gold. The most intense development and exploitation was under the Austro-Hungarian Empire between the late 17th century and 1918. Hard rock gold production from Rosia Montana is estimated to have totalled around 28 tonnes (0.9 Moz), augmented by colluvial and alluvial production.
The Apuseni Mountains region was subjected to clockwise rotation during early Tertiary time and was subsequently deformed by a major E-W to ESE-trending strike-slip fault systems. Major transtensional faults are interpreted to have generated pull-apart basins that acted as the structural loci for Tertiary epizonal intrusions and related hydrothermal systems. The Rosia Montana deposit lies within a generally NNW-trending structural corridor associated with strike slip faulting due to the easterly movement and rotation during the collision and northerly migration of the African continental plate into the European continent during the Miocene. A number of deposits and mineral occurrences occur within this 15 km-long corridor, including the Frasin and Rodu epithermal gold deposits and the Rosia Poieni and Bucium-Tarnit porphyry copper-gold deposits.
The Apuseni Mountains encompass a number of Tertiary calc-alkaline volcanic centres representing three main episodes of activity between about 14.8 and 1.6 Ma. Numerous epithermal and mesothermal Au-Ag, Cu-Au and Cu deposits are known within the district, associated with these mid-Miocene-Pliocene (Neogene) andesitic-dacitic volcanic and sub-volcanic bodies, which intrude a variety of lithologies. To the south of Rosia Montana, mafic bodies, which may represent mid-Jurassic oceanic crust basalts, are overlain by late-Jurassic to Cretaceous marine to deltaic sediments, including thick limestones. The country rocks at Rosia Montana comprise north vergent Cretaceous thrust sheets of shallow marine to terrigenous flysch-type sedimentary units. These sedimentary units are the host to igneous activity and mineralisation in the Rosia Montana area.
The Rosia Montana deposit is associated with, and hosted by, a Miocene age polymict "vent breccia" of a maar-diatreme complex, emplaced into the Cretaceous flysch-type sedimentary rocks and intruded by rhyodacite to dacite domes. These intrusive domes follow the lateral and vertical contact between the Cretaceous sediments and the vent breccias. Mineralisation in the breccia occurs as a series of relatively flat-lying zones which may be up to 200 m thick, while structurally controlled mineralisation is also found within Cretaceous sediments beyond the edges of the maar.
The deposit is interpreted to have been generated during episodic fracturing and brecciation events taking place over a period of several hundred thousand years, with mineralisation being punctuated by the eruptive events as indicated by crosscutting and overprinting relations of different breccia bodies in the maar-diatreme complex and age dating of gangue adularia and juvenile magmatic phases.
The bulk of the known reserves are hosted within two adjacent dacite bodies, namely Cetate to the west and and Cirnic to the east. Surrounding breccia units host the Igre Orlea and Cirnicel deposits.
At Cetate, the dacite body is surrounded by peripheral and internal phreato-magmatic breccias composed of dominant dacite clasts grading out into mixed volcanoclastic breccias. Mineralisation is present over an 800 m long by up to 500 m wide, north to NE trending zone as disseminations within the dacite and breccias as well as late carbonate-quartz-clay-sulphide veins.
The Cirnic dome complex is "mushroom shaped" in section which at depth forms a sub-vertical, approximately circular plug. Gold and silver mineralisation is found in two main zones, i). the larger within a NE-SW corridor on the western margin of the dacite plug; and ii). a roughly east-west trending zone on the southern margin of the same intrusive. The precious metal mineralisation occurs as disseminations associated with pervasive quartz-adularia-trace pyrite alteration overprinted by carbonate-quartz-clay-trace sulphide veining.
The epithermal low- to intermediate-sulphidation system is characterised by magnetite-destructive alteration with a paragenesis that comprises earlier stages of adularia, quartz (silicification), illitic clay and pyrite, generally disseminated in quartz-pyrite veinlets and silicified breccias. The late-stage mineralisation is associated with carbonate-quartz ± base-metal sulphide veins and disseminations and clays. Most of the gold deposition is related to the quartz-adularia-pyrite zones. The presence of silicification is the best field guide to gold mineralisation. Carbonate, with rhodochrosite and base metal sulphides, which accompany (often high grade) gold in narrow late-stage veins, was the focus of early mining, although this style of mineralisation does not constitute the bulk of the gold resource.
Most of the gold mineralisation occurs as disseminations. The major gold mineral is electrum, associated with pyrite, base-metal sulphides, and a variety of Au-Ag sulphosalts with minor tellurides. The overall aspect of the gangue and alteration mineral assemblages, as well as the sulphide assemblage, is characteristic of intermediate sulphidation-state epithermal deposits.
A feasibility study completed in 2001 delineated a measured + indicated resource of 302 Mt @ 1.3 g/t Au and 6 g/t Ag for a total resource of 395 tonnes (12.77 Moz) of gold and 1900 tonnes (61 Moz) of silver.
The same study also defined total proven + probable reserves of 225.7 Mt @ 1.4 g/t Au and 7.5 g/t Ag for 325 tonnes (10.5 Moz) Au and 1700 tonnes (54.6 Moz) Ag.
Leary, et al., 2004 quote a total measured + indicated + inferred resource of 400 Mt @ 1.3 g/t Au, 6 g/t Ag.
The Rosia Poieni porphyry copper-gold and overprinting high sulphidation epithermal deposit lies within the Apuseni Mountains of the Transylvanian region of Romania, approximately 40 km north-west of the city of AlbaIulia and immediately to the north of the city of Deva. It falls within the Carpatho-Balkan province of the Tethyan-Eurasian metallogenic belt and is approximately 4 km ENE of the Rosia Montana intermediate sulphidation Au-Ag deposit.
The earliest recorded mining in the region that embraces Rosia Poieni and Rosia Montana dates from the first century AD, prior to the siezure of production by the Roman Empire in 106 AD. The area, which extends over an area of around 500 sq km was known as the Golden Quadrilateral and falls within the Apuseni and Metaliferi Mountains. Production from the area, which has been virtually continuous over the last almost 2000 years, is thought to have been as much as 1300 tonnes (42 Moz) of gold.
The Apuseni Mountains region was subjected to clockwise rotation during early Tertiary time and was subsequently deformed by a major E-W to ESE-trending strike-slip fault systems. Major transtensional faults are interpreted to have generated pull-apart basins that acted as the structural loci for Tertiary epizonal intrusions and related hydrothermal systems. The Rosia Poieni deposit lies within a generally NNW-trending structural corridor associated with strike slip faulting due to the easterly movement and rotation during the collision and northerly migration of the African continental plate into the European continent during the Miocene. A number of deposits and mineral occurrences occur within this 15 km-long corridor, including the Rosia Montana, Frasin and Rodu epithermal gold deposits and the Bucium-Tarnit porphyry copper-gold deposit.
The Apuseni Mountains encompass a number of Tertiary calc-alkaline volcanic centres representing three main episodes of activity between about 14.8 and 1.6 Ma. Numerous epithermal and mesothermal Au-Ag, Cu-Au and Cu deposits are known within the district, associated with these mid-Miocene-Pliocene (Neogene) andesitic-dacitic volcanic and sub-volcanic bodies, which intrude a variety of lithologies. To the south, mafic bodies, which may represent mid-Jurassic oceanic crust basalts, are overlain by late-Jurassic to Cretaceous marine to deltaic sediments, including thick limestones. The country rocks at Rosia Poieni comprise north vergent Cretaceous thrust sheets of shallow marine to terrigenous flysch-type sedimentary units.
Hydrothermal alteration and mineralisation at the Rosia Poieni porphyry Cu-Au deposit, which is in the northern part of the Apuseni Mountains, is apparently related to a single magmatic pulse of andesite porphyry intrusion, the Middle Miocene subvolcanic, the Fundoaia microdiorite. The deposit exhibits a transition from porphyry Cu-Au mineralisation with associated potassic alteration, minor dykes, and overprinting pyrite-sericite into high-sulphidation epithermal veins with advanced argillic alteration zones over vertical interval of 300 m. Several successive magmatic, breccia and hydrothermal events have been discriminated.
The transition from porphyry to epithermal style mineralisation is characterised by overprinting, fluid-assisted brecciation of the host rocks and the formation of breecia- and pebble-dykes which form the main channels that focused the epithermal fluids and host the high sulphidation epithermal quartz-pyrite-enargite veins. These veins, which are typically of vuggy silica with advanced argillic alteration haloes, are cut by late quartz-sphalerite-galena polymetallic veins associated with phyllic alteration.
Zonation of alteration within the porphyry copper/epithermal gold deposit extends outwards from the deep and central part of the porphyritic intrusion towards shallower and outer peripheries, with four alteration types having been recognised, namely:
i). Potassic - which mainly affects the Fundoaia subvolcanic body, with the andesitic country rocks only being altered in the immediate contact zone with the intrusion. Mg-biotite and K-feldspar are the dominant alteration minerals, accompanied by ubiquitous quartz; chlorite and anhydrite. Magnetite, pyrite, chalcopyrite and minor bornite are associated with this alteration. The early, extensive, potassic zone in the central part of the mineralised system originally passed laterally outwards into a propylitic zone.
ii). Propylitic - was developed peripheral to the early, extensive, potassic zone which represents the central part of the mineralised system, passing laterally outwards into the propylitic zone, which is charactyerised by an assemblage of chlorite, epidote, albite and carbonates.
iii). Phyllic - is peripheral to, and overprints the outer margin of the potassic zone and the inner parts of the propylitic zone. It replaces almost all early minerals with abundant quartz, phengite, illite, variable amounts of illite-smectite mixed-layer minerals, minor smectite and kaolinite. Abundant pyrite is the main sulphide in this alteration zone.
iv). Advanced argillic - is developed in the upper epithermal parts of the volcanic structure, with an assemblage of alunite, kaolinite, dickite, pyrophyllite, diaspore, aluminium-phosphate-sulphate minerals (woodhouseite-svanbergite series), zunyite, minamyite, pyrite and enargite (luzonite). Veins containing enargite (luzonite) and pyrite occur in a gangue of quartz, pyrophyllite and diaspore, and are found within and surrounding the subvolcanic intrusion, partially controlled by fractures. A zonal pattern of alteration is observed from the centre of fractures outwards with: a). vuggy quartz; b). quartz + alunite; c). quartz + kaolinite ±alunite and, in the deeper part of the argillic zone, quartz + pyrophyllite + diaspore; d). illite + illite-smectite mixed-layer minerals ±kaolinite ±alunite, and e). chlorite + albite + epidote.
Koneev, 2004 quotes a total measured+indicated+inferred resource of 350 Mt @ 0.36% Cu, 0.29 g/t Au.
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Travelling Rosia Montana, via Cluj & Bucharest, Romania to Thessaloniki, Greece ...................... Wednesday 1 October, 2008.
Skouries porphyry Au-Cu, and Olympias Au-Ag-Pb-Zn replacement deposits, Greece ...................... Thursday 2 October, 2008.
The Skouries porphyry copper-gold deposit is part of the Kasandra Mining District of the Chalkidiki Province and is located approximately 100 km east of Thessalonika in north-eastern Greece (#Location: 23° 50' N, 40° 36'E).
The tectonic framework comprises a series of north-west trending structural units, each representing successive subduction episodes resulting from the north-east movement of the African plate between the Jurassic and Eocene. These units are separated by major thrusts or transitional zones. Successive subduction events were accompanied by volcanism and arc-type plutonic igneous intrusions. The main zones, from NE to SW are:
i). The Rhodope Massif composed of pre Variscan Precambrian and Palaeozoic mica-schists, granites and marbles, the latter of which predominate to the west. These rocks have been metamorphosed to almandine-amphibolite facies with partial anatexis.
ii). The Serbo-Macedonian Massif composed of similar rock to those of the Rhodope Massif include Proterozoic and Palaeozoic schists that are intruded by Variscan granites. This massif is the host to the deposits of the Kasandra Mining District, including Skouries.
iii). The Varda Zone, the basement of which is composed of the Jurassic ophiolite (gabbro-peridotite) and metamorphosed rhyolitic and spilitic volcanic rocks and limestones, overlain by widespread Upper Cretaceous flysch. All of these rocks have been intruded by Eocene granitic rocks. This zone is separated from the Serbo-Macedonian Massif to the east and the Pelagonian Massif to the west by deep-seated regional structures which have been the locus of Neogene calc-alkaline volcano-intrusives complexes.
iv). The Pelagonian Massif comprises basal gneiss and mica-schist overlain by a mixed series of meta-sediments, represented by gneiss, schist, quartzite, marble and dolomite, locally intruded by granitoids and pegmatites.
The Skouries deposit lies towards the eastern margin of the Serbo-Macedonian Massif, which has in turn been subdivided into two north-west trending lithostratigraphic-tectonic units, namely the a). the Vertiskos Formation composed of amphibolite gneiss flanking biotite schists with intercalated amphibolites to the west; and b). the Kerdilla Formation, made up of granitised and migmatised mica gneisses, amphibolites and marble units.
These units have been intruded by Oligocene sub-alkaline porphyry stocks which host the Skouries deposit. The deposit occurs as a sub-vertical pipe like body, associated with an elliptical pipe of coarse grained porphyritic syenite, one of a north-west trending string of syenitic plugs and dykes intruding the Vertiskos and Kerdilla Formation country rock.
The main Skouries syenite porphyry was represented at the surface by a 250 x 180 m exposure of an elliptical, composite pipe, and several E-W to NE trending dykes emplaced into the Vertiskos Formation. The Vertiskos Formation is represented by fine grained muscovite-biotite-chlorite-actinolite-feldspar-quartz schists. The Skouries porphyry comprises the following phases:
i). Early pink syenite porphyry which has undergone intense potassic alteration, accompanied by copper mineralisation associated with several veining stages, occurring as bornite, chalcopyrite and molybdenite with biotite and quartz;
ii). The Main stage porphyry which was accompanied by the high grade ore shell which contains abundant bornite and magnetite and was introduced in two pulses;
iii). Intra-mineral syenite porphyry which has only been subjected to propylitic alteration;
iv). Late oxidised porphyry accompanied by strong chlorite-pyrite-specularite alteration and remobilisation of Cu to higher levels where it was re-deposited into "S" veins of chalcopyrite-bornite ± quartz and carbonate.
The overall alteration pattern comprises a central potassic-siliceous core, surrounded by a propylitic halo. The potassic alteration comprises potassium felspar veinlets with lesser biotite veinlets in the intrusive and pervasive biotite replacement in the schists.
Mineralisation within the potassic zone is primarily chalcopyrite with lesser bornite inb 0.1 to 5 mm thick veins with disseminated chalcopyrite and bornite. Variable amounts of digenite, chalcocite, covellite, molybdenite and pyrite occur, together with rare galena and sphalerite. Magnetite is present as disseminations and in quartz veinlets. The propylitic zone contains <1% pyrite with only minor chalcopyrite. Gold occurs in its native form associated with gangue
minerals and is from a few to 160 µm in diameter. It is also found as blebs within sulphides and correlates with Cu. The ore also contains Pd.
An oxide zone is found from the surface to depths of 30 to 50 m, characterised by malachite, cuprite, chalcocite and minor azurite, covellite, digenite and native copper.
Total measured + indicated resources at Skouries as of October 2007 were: 191.2 Mt @ 0.55% Cu, 0.82 g/t Au (European Goldfields, 2007).
Tobey et al., (1998) quoted a resource of 500 Mt @ 0.47 g/t Au, 0.37% Cu,
The Olympias deposit is located approximately 15 km NNE of Skouries and is also part of the Kasandra Mining District. It is a massive stratabound polymetallic replacement deposit hosted in the marble-gneiss contact of the Kerdilla Formation. No skarn mineralisation has been identified to date, although it is predicted to occur at deeper levels, closer to the source of the mineralising fluid, believed to be a Tertiary age, sub-alkaline intrusive.
The deposit is characterised by an assemblage of pyrite, arsenopyrite, sphalerite, galena, tetrahedritetenantite, boulangerite and chalcopyrite, with gold being almost exclusively associated with arsenopyrite and pyrite. Minor amounts of bournonite, pyrrhotite, marcasite, mackinawite, enargite and geocronite are also found. Secondary cerussite, anglesite, chalcocite and covellite are found near the surface, while manganese oxides represent alteration of rhodochrosite. Gangue minerals include quartz, calcite, rhodochrosite, feldspar, kaolinite, chlorite, ankerite and graphite.
The ore is mainly massive in form, although disseminated sulphides are also present. Manganese oxides are found near surface and distally to the sulphides, and are regarded as representing a temperature zonation during deposition. From between 50 and 70 m below the surface, there is a gradual change to mixed sulphides. Kaolinisation is the main alteration type associated with mineralisation. Aplitic intrusions may be sericitised. Hydrothermal brecciation is common and chloritisation and silicification of host rocks immediately is frequently observed around the mineralised zone.
The deposit occurs within two orebodies, namely: i). the West orebody, which is approximately 250 m along strike and persists for 1200 m down plunge to the southwest. It has been intersected from surface to a depth of 500 m and is open down plunge, with a thickness that varies between 5 to 15 m, and dips averaging 30 to 35° east, although the attitude may vary locally from near horizontal to near-vertical. The horizontal width can reach 50 m in the flatter areas. The Upper West orebody comprises remnants left by previous operators who high-graded the lead and zinc mineralisation and left behind gold-rich pockets. ii). The East orebody, which lies some 150 m east of the West orebody. It represents an anticlinal structure, exhibiting axial thickening, with steeper dips toward the peripheries. It dips at 25-30° on average to the southeast, with an average length af thickness of 75 m and 7 m respectively. The mineralisation has been traced for 600 m down plunge.
The total measured + indicated resource at Olympias in 2004 (European Goldfields Ltd) was: 14.5 Mt @ 9.31 g/t Au, 128.6 g/t Ag, 4.2% Pb, 5.6% Zn.
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The summaries above were prepared by T M (Mike) Porter from a wide range of sources, both published and un-published. Most of these sources are listed on the "Tour Literature Collection" available from the Tethyan 2008 Tour options page.
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