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The Bakal, iron mine is located in the Chelyabinsk district of Russia and is hosted by Upper Proterozoic sediments. It comprises a district in which 24 siderite and 'brown ironstone' (oxidised ore) deposits are known.

The stratigraphy of the district may be summarised as follows, from the base (after Smirnov, 1977):

Sataka Group - dolomites, marls, lime-clay shales, limestone and dolomitic limestone;
Bakal Group - subdivided into:
- a lower sub-group, consisting of chlorite-sericite-quartz, argillaceous and phyllitic slate and sandstones; and,
- the upper ore-bearing sub-group comprising alternating limestone dolomite and shaly rocks. This sub-group has been further divided into 10 members, in which carbonate members contain seams of siderite and brown ironstone;
Zigal'ga Group - commencing with 're-wash shales', conglomerates with cobbles and pebbles of quartzite, with phyllite and quartzite. Shallow water quartzites and sandstones are mapped in the upper part of the group.
Zigazin-Komarova Group - quartz-chlorite-sericite, carbonaceous-sericite-clay and phyllitic shales, with sandstones and marly dolomite. Thin seams of siderite, up to 0.2 m thick, occur in the shales.

Numerous dykes and sills of dolerite and gabbro-dolerite are found in the field.

The distribution of ore is determined by the inter-stratification of carbonate members with shale. The former host ore. More than 200 separate orebodies have been identified. Orebody boundaries are along faults, at thin non-ore intercalations, or along arbitrary grade boundaries. Individual orebodies occur as layers, lenses and nest like segregations, as well as ore veins. The largest layer like segregation covers an area of 1.5 x 2 km, with a thickness of 80 m. The contacts between dolomite and siderite are usually sharp, with the former containing 27 to 30% Fe, the dolomite 2 to 5% Fe. Limestones and siderite are always separated by a band of 'metasomatic' dolomite, varying in width from a few to hundreds of metres (Smirnov, 1977).

The ore is contained exclusively within the carbonates, while all other lithologies are un-affected by the Fe mineralisation. The ore is divided into:
i). siderites - the primary, un-oxidised ores, which are massive, stromatolitic and bedded; overprinted by post ore tectonic induced secondary veinlets, and banded, streaky and drusy textures. The principal ore minerals are sideroplesite (FeCO3.MgCO3) and pistomesite (a variety of magnesite with between 50 and 70% FeCO3) which form 80 to 90% of the ore, with the remainder being dolomite, ankerite and barite. Pyrite, chalcopyrite, hematite, galena and sphalerite occur in small quantities in the ore. The depth of oxidation varies from 3 to 110 m, with the base of oxidation being very irregular;
ii). semi-oxidised siderite - a zone between the siderite and oxidised ores;
iii). oxidised ores - consisting of hydrogoethite and hydrohematite, with sparse, sintery goethite (Smirnov, 1977).

Gangue minerals include quartz and clay minerals. Dolomite, magnesite, ankerite, barite, albite, aragonite, apatite, wad, pyrolusite, pyrite, chalcopyrite, azurite, hematite, galena, sphalerite, magnetite and vivianite are also present in minor amounts in various parts of the orebody (Smirnov, 1977).

The average composition of the two main ore types are:

Siderite: 28.0-37.5 % Fe; 7.5-13.8% MgO; 1-3.3% CaO; 1-2% MnO; 1.4-6.8% SiO
2; 0.8-3.5% Al2O3; 0.007-0.26% P; 0.06-0.82% S.
Oxidised: 38.0-57.7 % Fe; 0.16-1.8% MgO; 0.2-1.6% CaO; 0.7-2.3% MnO; 7.5-18% SiO
2; 2.3-6.6% Al2O3; 0.012-0.144%P; 0.006-0.9% S.

The contained ore in all categories quoted in 1977 was 560 Mt of siderite and 39 Mt of brown ironstone/oxidised ore (Smirnov, 1977).

The most recent source geological information used to prepare this summary was dated: 1977.    
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.

  References & Additional Information

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 takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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