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Varvarinskoye |
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Kazakhstan |
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Au Cu
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Super Porphyry Cu and Au
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The Varvarinskoye is a skarn gold-copper deposit is located 130 km south-west of the city of Kustanai in central northern Kazakhstan, (#Location: 52° 45'N, 62° 5'E).
The Varvarinskoye deposit is located within the southe-astern margin of the Urals Fold Belt, to the east of the lower to early-upper Carboniferous Valerianovsky volcano-plutonic Belt. It lies within both the Silurian to Devonian Denisovskaya structural belt and the superimposed, Carboniferous Pervomaisky trough.
The deposit appears to be associated with unconformities representing a Middle Devonian and Early Carboniferous break in deposition and is on the eastern, downthrown side of the major NE-SW striking early Palaeozoic basement Varvarinsky Fault Zone. NE-SW and north-south striking, east dipping faults in the basement are displaced by younger east-west trending structures. The geology in the deposit area dips generally at 40 to 50°E, steepening towards the West to form a monocline.
The succession within the district suggests a Back Arc basin environment, related to Island Arc - Arc collision.
The sequence in the deposit area comprises:
i). Middle Devonian basalts, andesites and tuffs with arenaceous sediment and limestone interbeds which are exposed in the western part of the deposit where they form a wide belt. They are also found as small fault-bounded inliers in the northern part of the deposit area. The basalts occur as 1 to 3 m thick interlayered, dark grey and greenish porphyritic lava flows in which pillow structures have been recorded. The rocks of this unit are chloritised and may contain albite and/or epidote alteration.
ii). Middle Devonian to Early Carboniferous clastics, comprising a 200 m thickness of grey to light grey meta-sandstones interbedded with dark grey to black, carbonaceous siltstones, all of which has been strongly altered and commonly exhibit schistose textures.
iii). Early Carboniferous basalts, volcaniclastics, quartzites and carbonates, with a basal conglomerate containing clasts of basalt, quartz, quartzite and siltstone marking the unconformable contact with the underlying sequence. Basalts comprise around 70% of the units, while marbles occur to the east of the deposit.
Palaeozoic weathering is evident, prior to the deposition of the Mesozoic cover sequence to form an irregularly preserved palaeosol characterised by kaolin and montmorillonite clays, unconsolidated breccias and highly oxidised zones of iron, quartz, feldspar and manganese wads and supergene enrichment of the cupriferous and auriferous mineralisation.
Superficial cover over the Palaeozoic sequence comprises 1 to 20 m of weathered Mesozoic, Cenozoic and Quaternary sands and clays which thicken to the southwest and west.
Intrusive rocks include small plutons of basic intermediate gabbroic and diorite-granodiorite rocks, and diorite and serpentinite dykes and sills. The serpentinites are commonly associated with fault zones and are frequently brecciated, with the oldest showing relict structures after dunites and peridotites and occasional veins of chrysotile within their core. The gabbroic rocks are mostly confined to the western portion of the area and are chemically and mineralogically heterogeneous, grading from gabbro to diorite within a single pluton.
The intrusions have been metamorphosed to produce an assemblage of pyroxene, amphibole and chlorite with secondary actinolite, epidote and chlorite, while plagioclase is totally converted to scapolite. Within the zone of weathering the diorites have been decomposed to kaolin. The Palaeozoic volcano-sedimentary sequence has been regionally metamorphosed to greenschist facies and typically display an alteration assemblage of chlorite, albite and epidote. Hornfels zones are developed in close proximity to intrusive margins. Calc-silicate skarn alteration grades from red to green garnet to pyroxene skarn with associated massive sulphide and magnetite mineralisation.
The mineralisation at Varvarinskoye is present as gold-copper skarns distributed over a strike length of 3 km, width of up to 20 m and to a depth of at least 215 m, with three main morphological associations as follows:
i). Stratabound massive and disseminated sulphide mineralisation accompanied by garnet-pyroxene skarn in calcareous volcanic rocks, marble and volcanic breccias. On the basis of the calcium-silicate skarn assemblage, the presence of magnetite and the copper/gold ratio, the centre of the Varvarinskoye skarn system lies between the Central and NE-1 pit areas. Outward from this core, copper decreases, the gold / copper ratio increases and mineralisation becomes more structurally, rather than stratigraphically controlled. The association of quartz-albite alteration in the western part of the Central and South pit areas with significant gold values has been taken to represent retrograde skarn alteration.
ii). Vein, stockwork and disseminated sulphides which are associated with the contact zones of porphyritic-diorite and serpentinite intrusives, and tectonic breccias in volcanic rocks, particularly in the western part of the Central pit, and the South, North Central, and River pit areas.
iii). Supergene oxide ores, gossans and massive to disseminated supergene sulphide ores, related with the palaeo-weathering profile. These styles are particularly prevalent in the southern part of the Central pit area.
Hypogene sulphide ore predominantly comprises chalcopyrite, pyrite and pyrrhotite with accessory marcasite, arsenopyrite, silver minerals, sphalerite, magnetite, hematite, pentlandite, gersdorfite and niccolite. The sulphide assemblage at any point is related to the host rock lithology and chemistry. Gold is associated with the sulphides, but occasionally occurs as native metal within small calcite-pyrite veins. A further approximately 8% of the gold is held in solid solution as a trace element, or in sub-microscopic inclusions.
Most of the less abundant sulphides occur as inclusions within the dominant sulphide ore, associated with:
i). Fractures in the dominant sulfide matrix, associated with calcite, garnet, diopside, quartz and chlorite in skarns and;
ii). Muscovite, magnetite, rutile, feldspar, chlorite, calcite, quartz and clays in mafic volcanic rocks.
Supergene sulphide ores are composed of partially oxidised pyrite and chalcopyrite grains associated with chlorite, weathered feldspar, clays, muscovite, quartz, rutile, zircon, manganese oxides, goethite and hematite. The pyrite grains are usually coated with chalcocite. Coarse 120 x 50 µm gold grains have been observed in thin section. Gossans with well developed boxwork and ladder textures have also been observed in drill core from the South pit area suggesting a disseminated pyrite - arsenopyrite protore.
Mineralisation is interpreted to have been developed in four phases, as follows:
Phase 1: Metasomatism of mafic volcanics, sub-volcanic intrusives and limestones by fluid flow along stratigraphic discontinuities and tectonic conduits which produced skarn and serpentinite within selected host rocks. The hydrothermal fluids are believed to be related to a large underlying granodiorite intrusion. Sulphide deposition took place in favourable sites particularly as a massive replacement of skarn.
Phase 2: High grade gold and accompanying low grade copper mineralisation was deposited in cross-cutting calcite-quartz veins, some of which contain native gold and other base metal sulphides. These veins normally contain abundant fine-grained native gold but low (<1%) copper. Locally, brecciation of earlier sulphides accompanied by the precipitation of sulphide gold cement has further enriched gold grades.
Phase 3: Widespread, low grade copper-gold calcite and quartz veins responsible for a large tonnage of low grade (0.50 up to 6.0 g/t) gold-bearing material in the centre of the deposit with accompanying, generally low grade (averaging 0.06%) copper mineralisation.
Phase 4: Supergene enrichment of in-situ sulphides and gold to form gossan and underlying supergene sulphides during the weathering process.
Published measured + indicated mineral resources at a 0.01 g/t Au cutoff, are:
24.07 Mt @ 1.40 g/t Au, 0.69% Cu - High grade flotation copper;
5.89 Mt @ 1.26 g/t Au, 0.52% Cu - High grade supergene sulphide stockpiled leach ore;
82.55 Mt @ 0.89 g/t Au - Direct cyanide leach high Au, low Cu;
5.10 Mt @ 0.82 g/t Au - Direct cyanide leach high Au, low Cu supergene sulphides;
Total resource - 117.6 Mt @ 1.01 g/t Au, 0.66% Cu.
Published proven + probable reserves at a 0.01 g/t Au cutoff, are:
14.97 Mt @ 1.66 g/t Au, 0.813% Cu - High grade flotation copper;
42.13 Mt @ 1.05 g/t Au - Direct cyanide leach high Au, low Cu;
3.18 Mt @ 1.07 g/t Au - Direct cyanide leach high Au, low Cu supergene sulphides;
Total reserve - 60.3 Mt @ 1.21 g/t Au, 0.81% Cu.
The most recent source geological information used to prepare this decription was dated: 2005.
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.
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Selected References:
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Hammarstrom, J.M., Mihalasky, M.J., Ludington, S., Phillips, J.D., Berger, B.R., Denning, P.D., Dicken, C.L., Mars, J.C., Zientek, M.L., Herrington, R.J. and Seltmann, R., 2017 - Undiscovered porphyry copper resources in the Urals - A probabilistic mineral resource assessment: in Ore Geology Reviews v.85, pp. 181-203.
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Plotinskaya, O.Yu., Grabezhev, A.I., Tessalina, S., Seltmann, R., Groznova, E.O. and Abramov, S.S., 2017 - Porphyry deposits of the Urals: Geological framework and metallogeny: in Ore Geology Reviews v.85, pp. 153-173.
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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