ON THE COMPOSITION OF UPPER CRETACEOUS SEDIMENTARY IRON ORES OF TOMSK OBLAST

Yu. P. Kazanskii, P. G. Usov

(Presented by Corresponding Member of the Academy of Sciences of the USSR Yu. A. Kuznetsov)

Sedimentary coastal-marine iron ores have been established over a considerable area of the Central Ob region [4, 8], including large parts of the basins of the Bakchar, Parbig, Parabel, Chuzik, Tym, and other rivers. The ferruginous rocks make up several iron-ore horizons confined to Turonian, Senonian, and Maastrichtian–Paleogene (?) deposits. In the ore-bearing horizons, the richest in iron are oolitic hydrogoethite, chlorite-hydrogoethite, and hydrogoethite-siderite rocks. The present note is devoted to describing the composition of these three principal types of Upper Cretaceous ores of Tomsk Oblast.

Table 1 gives the chemical composition of individual samples of hydrogoethite-siderite, chlorite-hydrogoethite, and hydrogoethite ores. The analyses were made separately for the hydrochloric-acid extract and for the insoluble residue. Similar determinations were made for separate parts of the ores, among which we selected two fractions: 1) particles 0.5–0.25 mm in size, reflecting the composition of the oolites and pellets, and 2) particles less than 0.088 mm in diameter, which correspond to the composition of the cement.

A comparison of the results of studying the ores themselves shows that the different types hardly differ in chemical composition (Table 1). They contain approximately the same ratios of the principal oxides. At the same time, in each type substantial differences are visible in the composition of the cement and of the cemented particles, proving their different mineralogical content. In the hydrochloric-acid extracts for all types of ferruginous rocks, an increase in the amounts of SiO₂, Fe₂O₃, MgO, and P₂O₅ is noted in the 0.5–0.25 mm fractions. Particles less than 0.088 mm in diameter in the hydrochloric-acid extract are richer in Al₂O₃, FeO, CaO, and MgO, and in the insoluble residue in SiO₂ and Fe₂O₃. In addition, the cement has greater loss on ignition.

The remaining oxides, both in the hydrochloric-acid extracts and in the insoluble residues, are distributed differently in the corresponding fractions of each type of iron ore. Here, apparently, the mineralogical features of the rocks are manifested, as well as the element of chance in the selection of samples. An example of the noticeable influence of mineralogical composition on the ratio between individual oxides may be the distribution of ferrous and ferric iron in the 0.5–0.25 mm fractions and in the fractions finer than 0.088 mm. In rocks relat-

relatively poor in authigenic chlorite, the amount of FeO in the cement relative to the oolites increases by approximately a factor of 1.6, while in chlorite-hydrogoethite ore this ratio reaches 6.

The data of spectral analysis, obtained at the Geological Institute of the West Siberian Branch of the Academy of Sciences of the USSR, prove the presence in all types of ferruginous rocks of 0.001–0.01% Cu, Pb, Ni, Co, and others.

The mineralogical composition of the iron ores is very complex. Within each type the following are distinguished: 1) fragments of minerals and rocks, 2) oolites and beans, and 3) cementing material.

Fragments of minerals and rocks

The clastic material was studied in thin sections and immersion preparations. Its composition is dominated by quartz, feldspars, and rock fragments. The distribution of these components in the various fractions is different. Fragments of siliceous, clayey, and clayey-chloritic rocks, as well as ore fragments, are abundant in the gravel-pebble fraction. Here, rounded grains of phosphatic rock and fragments of wood are sometimes also encountered, which are often replaced by chlorite. The sand fraction, represented chiefly by fragments 0.1–0.3 mm in diameter, consists mainly of quartz, feldspars, and fragments of siliceous and clayey rocks. Many grains of quartz and feldspars in the hydrogoethite and hydrogoethite-chlorite rocks are replaced to one degree or another by authigenic minerals. Corrosion is usually manifested first of all along cleavage, cracks, and the edges of grains, and then it involves the entire fragment.

In the aleuritic fraction, an increase in the amount of quartz is observed in comparison with sandy fragments.

The composition of clay minerals in the ore horizons was determined by staining according to the method of N. E. Vedeneeva and M. F. Vikulova [2]. The results obtained confirmed Vikulova’s conclusion regarding the predominance, in the fraction smaller than 0.001 mm, of clay minerals of the hydromicaceous type.

Clastic minerals of the heavy part of the 0.25–0.01 mm fraction, separated from the iron ores and enclosing rocks, are represented by ilmenite and magnetite, leucoxene, epidote, common hornblende, zircon; more rarely observed here are tourmaline, apatite, sphene, andalusite, garnet, and, in individual grains, staurolite, disthene, sillimanite, rutile, tremolite, glaucophane, diopside, clastic oxidized glauconite, and chlorite. By the composition of the clastic transparent minerals, two types of heavy fractions are distinguished: epidote and epidote-hornblende. The epidote association is characterized by a large relative amount of minerals of the epidote group (30–54%), zircons are common (up to 1%), sphene (1–3%), common hornblende (2–10%), and some others. The epidote-hornblende complex is distinguished by a significant amount of amphiboles (10–30%), epidote (20–50%), and zircon (up to 3%). Significantly

Fig. 1. Differential heating curves of ores.
I—hydrogoethite-siderite ore,
II—hydrogoethite ore,
III—chlorite-hydrogoethite ore.

Table 1

Chemical composition of iron ores

less frequently, a zircon association of heavy minerals is encountered. It is distinguished by an abundance of zircon (10–15%), tourmaline (2–7%), metamorphic minerals (up to 3%), and some others. The amount of epidote and amphiboles decreases sharply. The heavy-mineral complexes do not form persistent horizons that could be used for correlating sections. It is quite possible that such diversity of composition is a consequence of the different grain-size and facies composition of the rocks.

Oolites and boboviny

Oolites and boboviny are widespread in all types of iron ores, making up mainly the 0.2–0.3 mm fraction. Grains of 0.4–0.5 mm and larger are noted more rarely. They are composed of iron hydroxides, as well as relicts of glauconite and fragments of quartz, feldspars, epidote, amphiboles, and other minerals located in the central parts of the oolites. The iron hydroxides are represented mainly by hydrogoethite, which is established by optical, X-ray, chemical, and thermal analyses. In powder the color of hydrogoethite is brown, rusty-brown; in a polished thin section it is light gray. The mineral gives a brown internal reflection. The mean refractive index measured in melts is \(2.43 > N > 2.35\). The specific gravity of the oolites and boboviny is given in Table 2. The heating curves of oolites of the 0.5–0.25 mm fraction from different rock types (Fig. 2) give characteristic hydrogoethite arrests at 220–230° and 360–370°. For oolites of cemented hydrogoethite ore an exoeffect is noted at 560°, possibly indicating the presence of xanthosiderite in this rock.

Fig. 2. Differential heating curves of the oolitic part of ores (fractions from 0.50 to 0.25 mm).
I — hydrogoethite-siderite ore, II — hydrogoethite ore, III — chlorite-hydrogoethite ore.

Table 2

Specific gravities of fractions of iron ores

The chemical composition of oolites and boboviny can be judged from Table 1, which gives analyses of the 0.5–0.25 mm fractions, consisting in all types of iron ores predominantly of oolites and boboviny. For this fraction, the hydrochloric-acid extract is characterized by an increased content of \(\mathrm{Fe_2O_3}\) and a smaller amount of \(\mathrm{Al_2O_3}\) and \(\mathrm{FeO}\) in comparison

with cement. In the insoluble residue this fraction contains the smallest amount of silica.

Chemical analysis of the ooids separated by electromagnetic means from the friable hydrogoethite rock shows \(\mathrm{Fe_2O_3}=82.7\%\), \(\mathrm{H_2O}=17.3\), revealing an increased amount of water in comparison with the theoretical formula of goethite.

Table 3 gives the X-ray pattern of the electromagnetic part of the \(0.5\text{--}0.25\ \mathrm{mm}\) fraction (borehole 10, sample 4), also obtained from hydrogoethite sand. It is identical to the standard X-ray pattern of goethite (3,5).

Table 3

X-ray patterns of individual fractions of ferruginous rocks

By origin, two types of ooids and pisolites are distinguished: 1) ooids and pisolites arising by replacement; 2) accretion ooids, formed as a result of the contraction of matter around a center.

Replacement ooids and pisolites arise either during oxidation of glauconite or during the reworking of aggregates of iron hydroxides. Pisolites are often pseudomorphs, preserving the shape of the grains of the primary glauconite. Usually these are rounded and lobate formations of the same size as the glauconite grains. Pisolites are often replaced by ferruginous chlorite and metacolloidal forms of iron hydroxide. In individual cases, along the edges of grains a process of regeneration of glauconite is observed. Replacement ooids are usually found together with pisolites; they also arise in the process of oxidation of glauconite, the brown spots of which are often preserved in the cent-

ral parts of the oolites. In places the nuclei of the oolites are represented by oxidized varieties of vermiculite-like glauconite. There are frequent cases in which oolites of this type are replaced by chlorite. Measurements of refractive indices in such grains show that the composition of the chlorites in the different zones is not the same and varies in individual cases from 1.650 at the margins of the oolite to 1.658 in its central parts.

Accretion oolites are most often observed in cemented hydrogoethite ores, where they are often present in significant quantities (Table 4). Their distinctive feature is the presence in the central part of accumulations of grains of clastic minerals, which may be quartz, feldspars; epidote and common hornblende are encountered rather often, while fragments of oolites and grains of chloritic cement are considerably rarer. The ferruginous substance of accretion oolites usually replaces the clastic part to a considerable degree. Observations in thin sections show that in most cases accretion oolites formed in the process of oxidation of chloritic cement. In this case fragments of quartz and other minerals served as centers around which iron hydroxides were deposited in layers.

Table 4

In addition, fragments occur in small quantities in sandstones and chlorite-hydrogoethite rocks; these fragments yield grains, usually without noticeable traces of rounding. More numerous are fragmental, altered oolites in loose hydrogoethite rocks. The dimensions of the oolites here are the same as in other ferruginous rocks. However, they usually have a flattened form with an uneven, knobby surface. This, as well as the absence of cementing ferruginous material, makes it possible to regard loose hydrogoethite sands as products of the reworking of already formed hydrogoethite and chlorite-hydrogoethite ores.

Together with boboviny and oolites there occurs a mineral characteristic of ferruginous rocks, namely glauconite. This mineral, on oxidation, gives gradual transitions to boboviny. Unaltered glauconite forms green and dark-green grains with a microcrystalline structure. More rarely, vermiculite-like crystals up to 0.4–0.6 mm in length are observed. The sizes of glauconite grains are usually identical to the accumulations of iron hydroxides. The optical properties of glauconite, \(N_m = 1.604—1.612\), make it possible to assign it to the ferruginous variety. On

heating curves; characteristic endothermic effects at 175–185° and 600–625° are noted for it.

By origin, detrital and authigenic glauconites are distinguished. Detrital varieties yield isometric grains and often bear traces of oxidation. Authigenic glauconites have lobate and vermiculite forms and, together with chlorites, may cement the detrital material. Authigenic varieties are also oxidized in places.

The quantitative relationships among oolites, bobovinas, and glauconite grains are given in Table 4. The data of this table confirm the conclusion that hydrogoethite ores arise in the process of oxidation of deposits rich in chlorite. Oxidation of chlorites and glauconite is accompanied by the formation, in the ferruginous sediments, of replacement and accretion oolites.

Cement of the Rocks

Ferruginous rocks differ mainly in the composition of the cementing substance. Mineralogically, three types are distinguished: 1) chloritic, 2) consisting of hydrates of iron, aluminum, and silicon oxides, and 3) sideritic. Between these varieties there are all gradual transitions.

Chloritic material cements chlorite-hydrogoethite rocks and various “trash” rocks, aleurolites, and sandstones. Usually, in thin section, the chlorites are isotropic or weakly anisotropic. Judging from their refractive indices, their composition is not uniform. Table 5 gives the values of the refractive indices of chlorites in a section of one of the ore horizons. The highest refractive indices of chlorites are observed in rocks enriched in iron (up to 1.65–1.67%). In the enclosing rocks—sandstones and aleurolites—their values are considerably lower (down to 1.605–1.620). Apparently, in the ore horizon the chlorites belong to ferruginous varieties, while in the enclosing rocks ferruginous-magnesian chlorites are widespread. During oxidation of chlorites, their refractive indices may increase to 1.68.

Fig. 3. Differential heating curves of the cementing substance of ores (fractions finer than 0.088 mm):
I — hydrogoethite-siderite ore,
II — hydrogoethite ore,
III — chlorite-hydrogoethite ore.

On the heating curves of ferruginous chlorites (Fig. 3), endothermic effects at 150 and 340°, belonging to iron hydroxides, are distinguished, as well as an endoeffect at 570°, attributable to chlorite. In some samples an additional weak exothermic stop at 760° was observed. The presence of effects at 570 and 760° makes the heating curve similar to the thermogram of chamosite, cited by D. P. Serdyuchenko [6].

It is not possible to isolate chlorites in pure form, as M. F. Vikulova pointed out, because they contain a significant amount of the finest aggregates of metacolloidal $\mathrm{Fe_2O_3}$, $\mathrm{Al_2O_3}$, and fine-grained hydrogoethite.

Cement consisting of hydroxides of iron, aluminum, and silicon occurs in oolitic, hydrogoethite, and hydrogoethite-siderite ores. This type of cementing substance forms as a result

oxidation of chlorites. In hydrogoethite ore, “islands” of unaltered chlorite are often observed; they are usually surrounded by a brown border of iron hydroxides no more than 0.004–0.008 mm wide. The cement of the rock outside the border is represented by a brown metacolloidal substance, cut by dehydration cracks. The composition

Table 5

Values of refractive indices of chlorites in a section of the second ore-bearing horizon (borehole 8, Kolpashevo)

of the light-brown cement is dominated by metacolloidal compounds of iron and aluminum hydroxides and, apparently, silica. Measurements of the refractive indices of this type of cement in melts at sites of concentration of iron hydroxides give the following values: \(2.31 > N > 2.28\). In powder the substance is brownish-red; its reflectivity is somewhat lower than that of hydrogoethite. Electron-microscopic observations carried out in the electron-microscopy laboratory of IGEM, Academy of Sciences of the USSR, showed that the cement particles correspond in shape to standard images of iron hydroxides. The presence of colloidal forms of iron and aluminum oxides is readily established by X-ray and other methods. In X-ray analysis of the fraction finer than 0.088 mm from hydrogoethite ore, a continuous strong veil forms on the film, practically completely covering the diffraction rings of crystalline phases. Satisfactory X-ray patterns of this ore fraction, with a slight veil, could be obtained only after preliminary treatment with a buffer mixture of oxalic acid and ammonium oxalate, with pH equal to 3.3 [5]. In this case the colloidal forms of iron and aluminum more readily enter into reaction with ox-

Fig. 4. Curves of weight loss during heating of ores: I—hydrogoethite-siderite ore, II—hydrogoethite ore, III—chlorite-hydrogoethite ore.

salation, forming oxalic-acid complexes of iron and aluminum, and are extracted from the ores during stepwise treatment. The characteristics of the X-ray patterns of ores treated with the buffer mixture are given in Table 3, and the amounts and chemical compositions of the extracts obtained with the buffer mixture are given in Table 6.

Table 6

Chemical composition of extracts obtained by treating iron-ore cement with the buffer mixture

By differential thermal analysis of the ores, two endothermic effects are clearly established on the heating curves, at temperatures of 160—180° and 360—380° (Fig. 1). The first low-temperature effect is due to the release of water by colloids [9]; the second, typical hydrogoethite effect, is due to the splitting off of crystallization water. By careful crushing of the ores with a wooden pestle and screening the material through a fine sieve with openings of 0.088 mm, it is possible to separate the cementing substance of the ore from the beanlike grains. When they are analyzed separately by the differential thermal method, the two recorded thermal effects are obtained separately. The low-temperature effect, characteristic of colloidal forms of Fe₂O₃ and Al₂O₃, is expressed considerably more strongly and appears only in the cementing substance, whereas the high-temperature hydrogoethite effect appears only in the oolitic, roe-like part (Figs. 2 and 3). These data are also confirmed by the weight-loss curves on heating (Figs. 4, 5, and 6).

Recrystallization of metacolloidal ferric oxide hydrate leads to the formation of hydrogoethite. This is demonstrated on thermograms, where the endothermic arrest at 340—375° belongs to hydrogoethite. In addition, this mineral is identified on X-ray patterns of the residue obtained after treatment of the fraction smaller than 0.088 mm with the buffer mixture (Table 2).

In places, recrystallization of colloidal SiO₂ is observed. In this case chalcedony is formed, which gives microgranular and radiating aggregates. Such new formations have been encountered in hydrogoethite and hydrogoethite-siderite ores, as well as in some sandstones with chlorite-ferruginous cement.

Siderite occurs in small amounts in almost all types of iron ores in the form of small crystals. Hydrogoethite-siderite ore contains fairly large grains of second-generation siderite, which, when the rock is crushed, do not enter the fraction smaller than 0.088 mm. On the heating curve (Fig. 1), siderite has an endothermic reaction at 550°, passing into a positive effect with a maximum at 630°. Thermograms obtained for siderite concretions are characterized by dissociation of the carbonate at 520° and by an exoeffect ending at 680—700°.

Together with siderite, calcium and ferruginous phosphates are encountered. Calcium phosphates have been found in hydrogoethite-siderite ores in the form of irregular concretions with diameters from several mm

up to 2–3 cm, or in the form of large concretions up to 5–7 cm across. In thin section the phosphate gives fine-grained aggregates with \(Nm = 1.620 \pm 0.002\) and with low interference colors. The phosphate cements the clastic material, oolites, and beans, replacing glauconite and chlorites. The siderite crystals encountered in these nodules are later formations. By origin, calcium phosphate is a diagenetic formation. The content of \(P_2O_5\) in the ferruginous rocks is comparatively small, not more than 0.5–0.6%. The concentration of phosphate in individual parts of the rocks is due to its migration in the sediment.

Iron phosphate was found in a single sample from the bed of green sandstones exposed in a borehole near the yurt of Korlik, on the Vakh River. Phosphate of the alphakerchenite type is the cement of a vari-grained sandstone. The mineral is demonstrated by chemical, optical, and thermal methods. Recalculation of the chemical analysis gives the following formula for the mineral: \(3.2 Fe_2O_3 \cdot 2.6 FeO \cdot 3P_2O_5 \cdot 21H_2O\). The refractive indices are

Fig. 5. Curves of weight loss of the cementing substance of ores (fraction finer than 0.088 mm).
I—hydrogoethite-siderite ore, II—hydrogoethite ore, III—chlorite-hydrogoethite ore.

Fig. 6. Curves of weight loss upon heating of the roe-like part of ores (fraction from 0.50 to 0.25 mm).
I—hydrogoethite-siderite ore, II—hydrogoethite ore, III—chlorite-hydrogoethite ore.

\(Ng = 1.702 \pm 0.002;\ Nm = 1.672 \pm 0.002\) and \(Np = 1.632 \pm 0.001\). Kerchenite is coarse-crystalline, with pleochroism from olive-green along \(Ng\) to blue along \(Np\). The heating curves are characterized by an endoeffect at \(180^\circ\) and an exoreaction at \(740^\circ\), and differ from the thermograms of Crimean kerchenites [7] by a small negative arrest at \(585^\circ\), which in the Vakh sample is due to an admixture of glauconite.

In comparatively small amounts, the ferruginous rocks contain iron sulfides. Most often they are small, spherical nodules of pyrite 0.05–0.1 mm in diameter. In polished thin sections of oolitic hydrogoethite rocks, the finest crystals of pyrite are observed.

Some Conclusions

The study of the composition of the ores shows that they belong to the widely distributed oolitic hydrogoethite–chlorite–siderite type. The ores differ mainly in the composition of the cement. Among them, hydrogoethite–chlorite, hydrogoethite with colloidal ferruginous cement, and hydrogoethite–siderite varieties have been distinguished. The scheme we propose for subdividing oolitic iron ores differs substantially from the classification of A. A. Babin, D. G. Gerasimova, and A. M. Kassir [1], who distinguish hydrogoethite ores with siderite–leptochlorite cement, hydrogoethite–glauconite ores with leptochlorite–glauconite–siderite or phosphate cement, and loose oolitic ores. Apparently, these authors did not have sufficiently complete petrographic materials. In their scheme, a widespread type of iron ore is omitted: oolitic hydrogoethite ore with reddish-brown ferruginous cement, consisting predominantly of hydrogoethite, colloidal compounds of iron, silica, and aluminum.

The oolites and pisolites of the iron ores are composed predominantly of hydrogoethite. In addition, in the central parts, detrital minerals and relics of oxidized glauconite are observed. The cement of the ores differs in the different types. The original cementing substance was represented by ferruginous chlorite. During oxidation of the latter, a complex cement arose, consisting of hydrogoethite and colloidal forms of iron, silica, and aluminum hydroxides. In places, during diagenesis, regeneration of chloritic material is observed. Siderite is the latest secondary formation, and it may cement loose hydrogoethite sands formed as a result of the reworking of other ferruginous rocks.

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