Mineral inclusions of iron ores of the Bakchar deposit (Western Siberia)

Asochakova E.M.

Tomsk State University, Tomsk, Russia

asem@sibmail.com

The composition of the oolite iron ores from the Polynyanka site of the Bakchar deposit (Western Siberia) has been studied by the energy dispersion analysis combined with the scanning electron microscopy in the Analytical Centre of the natural systems geochemistry of Tomsk State University (analyst Ph.D. O.V. Bukharova).

The Bakchar deposit is confined to the Upper Cretaceous and Paleogene deposits overlapped by a rather thick Neogene-Quaternary rock body (160-200 m). Iron ores are related with several horizons: Narymskian, Kolpashevskian, Tymskian and Bakcharskian. The depth of productive strata varies from 2 to 40 m. The iron ore horizons are traced throughout the entire area of the deposit, as well as outside its borders and are separated by the barren and weakly ferruginous rocks that overlap each other often with washouts.

When studying the ore horizon of the Polynyanka site, we have distinguished the iron ore types that differ in their structural-textural characteristics, mineral compositions of the cementing mass, as well as in the specificity of their position within a section. Three essential ore types are distinguished in the section structure. Related to the first type are gothite-hydrogothitic (oolitic) ores representing cemented or loose sediments of brownish colour. The second ore type is glauconite-chloritic; these ores are distinguished for their dense, weakly cemented structure and the greenish-gray colour. The third transitional ore type bears the signs of both gothite-hydrogothitic and glauconite-chloritic formations.

Oolites as chief concentrators of iron hydroxides are the principal components of the Bakchar's iron ores. Oolites have always attracted the attention of scientists, because these are unusual spherical, ellipsoidal or similar mineral aggregates of conic-lamellar (shelly) structure 2 mm across. Formations similar to oolites, but of size over 2 mm, are called pisolites. Besides oolites and pisolites, ooids are recovered from the sedimentary iron ores. Those latter are mineral aggregates of a spherical or ellipsoidal form, sized from a fraction of mm to 2 mm, without signs of inner structurization, i.e. containing no nuclei.

Oolites of the Bakchar ores diversify in a form and colour. They are mainly spheroidal, oblate and angular by form; by colour they are black sniny, brown (fallow) shiny and dull. Sometimes, oolites are of irregular form, angular, flattened with a uneven surface, dull at times. In gothite-hydrogothitic ores, oolites are often oxidized and coloured brownish-rusty; in glauconite-chloritic ores, oolites are shiny, black, of regular shape. In contrast to greenish glauconite-chloritic ores and transitional varieties, in gothite-hydrogothitic ores there are often oolite fragments and clasts.

The REM-data indicate that oolites have often complicated, concentrically zonal texture with two pronounced zones: central and peripheral. The central part (100-200 µm in diameter) may be represented by waste minerals: quartz, magnetite, more rarely common potash feldspar; otherwise, they may have the nonuniform fabric resembling an ooid. The ooid often contains inclusions of the same waste minerals less than 50 µm in size. The peripheral oolite zone (20-100 µm thick) represents the successive concentric alternation of layers. The alternation of oolite concentres is so thin that their chemical composition is presented by mineral mixtures, the most part of which is composed of gothite and hydrogothite. The zonality of concentres is of the inversion character; the uniform alternation of the Σ Fe₂O₃ content in concentres is marked. Along with gothite and hydrogothite, if judged from the spectrum elements set, there are kaolinite, hydromicas, chlorites and phosphates in these ultrafine concentres.

The studies of mineral inclusions (~1μm) in oolite ores have demonstrated the presence of sulfids, free silver, zircon, ilmenite, rutil and rare-earth minerals. By the location and isolation character, all mineral inclusions can be classed as syngenetic (sulfids, free silver and rare-earth minerals) and epigenetic (zircon, ilmenite, rutil). The epigenetic minerals are commonly found in the terrigenous part of oolite ores together with quartz fragments. The syngenetic minerals mainly encountered in oolite aggregates and more rarely in the dominant bulk of glauconite-chloritic ores. Three associations are distinguished among these minerals: sulfide, phosphate and sulfide-phosphate.

The sulfide association comprises microinclusions in the contact of fragmented minerals and concentres of gothite-hydrogothite and leptochlorite composition. Pyrite is the essential sulfide mineral in oolite. Two generations of pyrite are distinguished: framboidal and euhedral. The pyrite framboids are composed of crystals > 1 μm in size. The euhedral pyrite is composed of aggregates in the form of several octahedral crystals 5-10 μm in size. Most likely, the growth of euhedral crystals is accounted for by the framboid enlarging. Trace elements As, Au and Pt are characteristic for pyrite.

Among other sulfides, sphalerite, covellite and antimonite have been revealed in the oolite aggregates. Covellite forms impregnations in oolites and the main ore mass. Sphalerite has been found in the nuclear part of oolites as colloform aggregates 20-50 μm in size. Antimonite has also been found within the oolite nuclei as microinclusions.

The phosphates are mainly represented by the association of REE minerals. In composition, they are solely Ce (Ce₂O₃ up to 27.63%), other rare earths identified La (La₂O₃ to 12.79%) and Nd (Nd₂O₃ up to 9.82%). In the rare-earth phosphate oolite occur very often in concentration and in the central parts, the number of micro-inclusions, the size of ~ 1 mm in one oolite may not exceed 10. Among the impurities observed Ca, Fe, Si, Al.

The sulfide-phosphate association presented by REE phosphates, sulfides, silver and free silver. The free silver is in the form of point or hair-like inclusions in the concentration oolites, localized along the concentric layers. In the central parts of the oolites silver forms a thin non-uniform impregnation, the number of inclusions in an oolite is rarely more than five points, less than 1 micron. In the ore horizon of the underlying mudstones are found the free silver dendrites (dimensions 250 × 300 mm), coated with silver sulfide (acanthite).