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Zhayrem |
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Kazakhstan |
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Zn Pb
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Super Porphyry Cu and Au
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Zhayrem, which is located within an area of extensive superficial cover in central Kazakhstan, was discovered in 1951 by testing of a magnetic anomaly in an area of known Fe-Mn mineralisation. Drill testing of the magnetic anomaly encountered a barite body and associated Pb-Zn ore (Smirnov, 1977). It is a pyritic stratabound deposit hosted by upper Devonian carbonaceous and siliceous carbonates which overlie a Devonian volcano-sedimentary sequence. It is characterised by layered Fe, Mn and Zn ores with superimposed Ba-Zn-Pb rich vein mineralisation. Ore is present as stratabound sulphides and as veins and segregations and has low contaminants. The orebody is folded, has moderate to steep dips and is underlain by a trachytic porphyry.
Annual mine production in 1990 was 2.29 Mt @ 3.08% Zn, 1.51% Pb, 0.05% Cu. 40.75% BaSO4.
Zhayrem is some 50 km to the north-east of the Western Karazhal iron mine, hosted by a very similar sequence of the same age. The 'iron-ore' units in the hangingwall at Zhayrem may be related to the Western Karazhal iron mineralisation.
Mineralisation at Zhayrem has been shown to be in three independent deposits, termed the Western, Far Western, and Eastern ore sectors. The bulk of the mined resource is in the Western sector (Smirnov, 1977).
Zhayrem is a stratabound, sediment hosted, pyritic deposit, hosted by upper Devonian carbonaceous and siliceous carbonates, which overlie a middle Devonian volcano-sedimentary sequence. The middle to early-upper Devonian (ie. Eifelian or Givetian to Frasnian) is not exposed in the vicinity of the orebody, but regionally it comprises lavas and pyroclastics of acid, basic and alkaline composition, alternating with sediments. The upper Devonian is a marine sequence comprising carbonate, clay-carbonate and sandy argillite. In the vicinity of the ore deposits it comprises, from the lowest member encountered, upwards:
Upper Devonian
The lowest member commences with more than 150 m of argillaceous limestone containing bands of limestone and siliceous carbonate;
The Productive Member, which is made up of:
* Up to 90 m of thin bedded argillaceous-siliceous-carbonate rock;
* 70 m of unevenly bedded argillaceous-siliceous-carbonate rock;
* 30 to 50 m of pyritic rhythmites, consisting of argillaceous-siliceous-carbonate ribbon banded rock, saturated with seams of finely banded pyrite;
* 10 to 15 m of bedded carbonaceous-argillaceous-carbonate-siliceous rock, with bands of tuff and tuffite, and horizons of banded pyritic ore;
* The lower Flyschoid Horizon, comprising 50 to 890 m of flysch-like carbonaceous argillaceous-siliceous-carbonate rocks, with bands of finely dispersed pyrite and sphalerite in the upper parts of each cycle;
* A 10 to 14 m thick marker unit comprising a characteristic carbonaceous-quartzo-feldspar rock with carbonate nodules;
* The Middle Flyschoid Horizon, 10 to 15 m thick, which is very similar to the Lower Flyschoid;
* Bedded Iron Ore, which is 10 to 15 m thick and contains magnetite, hematite and ferruginous jasper, alternating with thin laminae of siliceous limestone.
The Intermediate Member, which is 60 to 80 m thick and comprises an alternation of rhythmically banded and nodular bedded limestone
Red Limestone Member, which is 80 to 100 m thick and composed of alternating layers of limestone, siliceous limestone, and pelites containing hematite-hydromica-sericite, and chlorite-sericite.
Lower Carboniferous, made up of:
* 30 to 50 m of nodular, bedded limestone with intercalations of argillaceous-sericite pelites;
* 50 to 80 m of carbonaceous limestone, which is made up of siliceous and argillaceous limestone, which is densely pigmented with carbonaceous matter;
* A 5 to 30 m thick horizon of psammitic vitro-clastic tuffs, pelitic tuffs and sedimentary breccias;
* A 40 to 50 m thick horizon of concretionary limestone, consisting of siliceous limestones and carbonate-siliceous rocks, containing bedded, compacted, carbonate nodules;
* A 350 m thick horizon of ashy grey limestones made up of siliceous limestones, and siliceous-carbonate rocks, with massive, coarsely bedded structures and seams of argillaceous-sericitic pelites and calcareous sandstone.
Magmatic rocks have a very limited distribution in the orebody area, being confined to the occurrence of sub-volcanic bodies of trachyte porphyry in the immediate footwall of the Productive Member in the core of the anticline over which the deposit is developed. Porphyry dykes are found locally cutting the Far Western sector mineralisation (Smirnov, 1977).
Mineralisation at Zhayrem is characterised by layered Fe and Mn oxides and Zn sulphides, with superimposed Ba-Zn-Pb rich vein mineralisation. Ore is present as stratabound sulphides and as veins and segregations and has low contaminants. The orebody is folded, has moderate to steep dips and is underlain by a trachytic porphyry (Smirnov, 1977).
In more detail, the following forms of mineralisation are present: i) 1 to 100 cm thick units composed of 0.1 to 0.15 mm bands of finely dispersed framboidal pyrite, interlaminated with quartz-calcite which contains fine sphalerite. Occasionally sphalerite is developed in place of pyrite; ii) conformable quartz-calcite and quartz-calcite-barite veins which are from 1.5 to 10 cm thick and contain nests and segregations of sphalerite and galena; iii) veinlets of quartz-calcite which cross-cut bedding, and contain nests of coarsely crystalline galena and sphalerite where they intersect laminated pyritic units; iv) lensoid developments of fine grained grey barite from 1 to 1.2 m thick, which replace the country rock and contain galena, sphalerite and less commonly, chalcopyrite; v) nests, veins and veinlets of white coarsely crystalline barite in masses of fine grained grey barite; vi) intercalated bands of hematite, magnetite, ferruginous jasper, and siliceous, sometimes hematised, limestone. The thickness of the bands vary from 0.5 to 300 m (Smirnov, 1977).
The bulk of the Zn ore is in the first type, forming orebodies from 5 to 25 m thick that are rigidly conformable and laterally extensive. The last type, the iron ore is also stratabound and laterally extensive. The other forms tend to be developed in fracture zones and fold hinges. The ore is low in contaminants although the stratabound ores contain lesser to trace amounts of Ag, Cd, In, Bi, Ni As, Ga, Tl and Ge; while the transgressive mineralisation has Hg, Sb, Co, but no As, Tl or Ge (Smirnov, 1977).
A lateral zonation across the Zhayrem Ore Field has been proposed, from: i) a core characterised by pyrite-chalcopyrite-barite in rocks that are altered to siliceous, quartz-pyrite and baritic products; to zones of ii) galena-barite; to iii) galena-sphalerite-barite; to iv) baritic rich lead-zinc mineralisation; to v) layered lead-zinc mineralisation with local baritisation; to vi) layered, essentially zinc mineralisation accompanied by lean lead. In the last two zones the stratabound mineralisation predominates, transgressive and stratabound mineralisation are found in the fourth, while replacement and veining predominate in the first three (Smirnov, 1977).
The most recent source geological information used to prepare this decription was dated: 1997.
This description is a summary from published sources, the chief of which are listed below.
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