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The Mikheevskoe porphyry copper-gold deposit is located ~200 km south of Chelyabinsk in the Uralian Fold Belt (or Uralides) of the Urals Mountains in the Russian Federation.
The Uralides occupy a 2500 km long, north-south trending orogen that extends from the steppes of northern
Kazakhstan to the Arctic ocean, and were formed as a result of the collision of the Baltica (largely the East European craton)and the Siberia-Kazakh plates during the Late Carboniferous to Early Permian. Porphyry and porphyry related (skarn and epithermal) deposits are confined to three contiguous linear volcanic terranes, the Tagil-Magnitogorsk, East-Uralian volcanic and Trans-Uralian megaterrane. These terranes are bounded by large north-south trending structures. Most of the porphyry deposits and occurrences are concentrated in the Southern Urals and to a lesser degree in the Middle Urals where these subduction-related terranes are better exposed.
To the west of these volcanic terranes, there are tectonic mélanges associated with the continued collision of the two bounding plates in the Central and West Uralian megaterranes. The key divisions of the Uralides are:
• The Sakmara zone, which occupies the Central Uralian megaterrane, is bounded in the west by the West Uralian megaterrane, largely comprising the western margin of the Eastern European craton. It is separated from the Magnitogorsk zone to the east by the ~20 km wide, east dipping mélange of the Main Urals fault zone. The Sakmara zone is part of the foreland thrust and fold belt of the Uralides, and in the Southern Urals is largely occupied by a ~150 km wide, west-vergent thrust stack. It represents an obducted accretionary complex of Neoproterozoic and Lower Palaeozoic sediments and arcrocks, and ophiolites/mafic-ultramafic complexes, thrust over the eastern margin of the East European craton,
ahead of the approaching Magnitogorsk arc to the east (Herrington et al., 2005).
• The Magnitogorsk megaterrane is bounded in the east by the steeply dipping mélange of the East Magnitogorsk-Serov-Mauk fault, which separates the low- and high-grade metamorphic rocks respectively of the Magnitogorsk
zone and the East Uralian megaterrane. In the southern Urals, this zone comprises the Middle to Late Devonian Magnitogorsk arc, an oceanic arc sequence, composed of early tholeiites, overlain by younger, thick, calc-alkaline volcanic rocks, and a westward thickening volcaniclastic pile, including arc derived sedimentary rocks preserved to the west on the East European craton. These arc rocks of the main Magnitogorsk zone are overlain by Lower Carboniferous carbonates and intruded by Early Carboniferous granitoids.
• The East Uralian megaterrane, which forms the suture between the East European craton (which by the Early Carboniferous included the accreted Magnitogorsk arc) and the Kazakh plate to the east. It is composed of extensively strike-slip faulted, deformed and metamorphosed Proterozoic and Palaeozoic continental and island arc fragments, intruded by Late Devonian to Early Carboniferous tonalite to granodiorite masses, and by Late Carboniferous to Permian granitoid batholiths with subordinate diorite and gabbro. The megazone is bounded to the east by the Troitsk fault, a west dipping mélange zone of serpentinite containing relics of harzburgite (Herrington et al., 2005).
• The Trans-Uralian megaterrane bounded in the west by the Troitsk fault and the Kartaly Reflection Zone (below cover to the north) and in the east by the older rocks of the Kazakh collage. This zone is poorly exposed and comprises a Lower Palaeozoic basement overlain by the Andean-type Valerianovskoe arc composed of Devonian and Carboniferous calc-alkaline volcanoplutonic complexes, and overlying terrigenous red beds and evaporites (Herrington et al., 2005).
The Mikheevskoe deposit is hosted by Late Devonian to Early Carboniferous volcanic rocks (Belgorodskii et al., 1991). The lower part of the sequence comprises interbedded sandstone, tuffs, basaltic andesite, tuffaceous breccia, and subordinate siltstone, carbonaceous cherty rocks and basalt. On the basis of the contained fossil fauna, this sequence was dated as Late-Devonian to Tournaisian (Belgorodskii et al., 1991). The upper part of the host sequence is composed of aphyric basaltic lava and pyroclastics with intercalated sandstone, quartzite and carbonaceous–cherty rocks, intruded by numerous serpentinite bodies. The intrusive rocks belong to the Ulyanovsk and Mikheevskoe igneous complexes which are often considered as 'sub-stages' of a single composite igneous complex (Belgorodskii et al., 1991). The Ulyanovsk Complex intrusions comprise stocks and dykes of porphyritic diorite and andesite, and less frequent dacite, rhyodacite and dolerite. These intrusive rocks are, on geological evidence, interpreted to be of Late Devonian to Early Carboniferous age.
Porphyry copper-molybdenum mineralisation is related to the rocks of the Mikheevskoe Complex (Belgorodskii et al., 1991), which are diorite, porphyritic diorite, plagiogranodiorite porphyry and their porphyry equivalents as well as post-ore adamellite porphyry (Belgorodskii et al., 1991). At depth, the dykes and stocks may merge into the parental pluton (Grabezhev and Belgorodskii, 1992). Zircon from the the Mikheevskoe diorite porphyry was dated as 356±6 Ma (U-Pb SHRIMP determination; Grabezhev and Ronkin, 2011). This is in a good agreement with the Re-Os dating of molybdenite that gave ages of 357.8±1.8 and 356.1±1.4 Ma; Tessalina et al., 2016, in press).
Mineralisation and Alteration
The principal hypogene ore minerals are pyrite and chalcopyrite, with less abundant bornite, molybdenite, magnetite and rutile, whilst cobaltite–gersdorffite, arsenopyrite, sphalerite, galena, tetrahedrite and native gold are minor components. The hypogene mineralisation is overlain by an ~15 m thick zone of oxidation, composed of clays with residual quartz and nests and concretions of malachite and azurite.
Endogenous porphyry-approximately north-south between two ~1 km diameter diorite stocks of the Mikheevskoe Complex. This configuration makes it difficult to clearly determine the pattern of ore zoning and hydrothermal alteration (Plotinskaya et al., 2015b).
A central core of sodic–calcic alteration within the deposit is composed of an actinolite±epidote assemblage accompanied by magnetite mineralisation overprinted by later pyrite, chalcopyrite and molybdenite.
Abundant potassic alteration occurs in the central part of the ore stockwork, particularly in the northern sector of the deposit. Altered rocks composed of biotite-muscovite±potassium feldspar accompany bornite–chalcopyrite mineralisation.
Rare micron-size inclusions of native gold and tellurides of Au, Ag, Pb, Bi, etc are found within bornite grains, although these apparently belong to a later stage of the paragenetic sequence.
Phyllic (quartz–sericite) alteration accompany molybdenite–chalcopyrite mineralisation. Propylitic chlorite-epidote-sericite assemblages are abundant on the margins of the deposit.Propylitic-related mineralisation is estimated to have taken place at temperatures of 150 to 350°C and pressures of 100 to 650 bar from saline (10 to 35 wt.% eq.NaCl) fluids of Ca,(Mg,Na) and Na chloride composition with CO2 and minor N2 in the vapour phase (Groznova et al., 2015).
Other base metal sulphides (e.g., arsenopyrite, tetrahedrite, galena, sphalerite) are in rare cases superimposed on the earlier minerals.
Estimated resources of the Mikheevskoe deposit at January 1st 2014 are (Volchkov et al., 2015):
~347 Mt @ 0.45% Cu, 0.1 g/t Au with 1.54 Mt of contained Cu and 47 t of contained Au.
This summary is based on "Plotinskaya, O.Yu., Grabezhev, A.I., Tessalina, S., Seltmann, R., Groznova, E.O. and Abramov, S.S., 2016 - Porphyry deposits of the Urals: geological framework and metallogeny, Ore Geology Reviews (in press)."
The most recent source geological information used to prepare this summary was dated: 2016.
This description is a summary from published sources, the chief of which are listed below.
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