Distribution of Major Non-Ore Components in Deposits of the Iron-Bearing Horizons of the Bakchar Deposit

A. N. Kondakov

(Presented by Prof. A. M. Kuzmin)

The deposit is located in the southeastern part of the West Siberian iron-ore basin and is of Late Cretaceous—Early Paleogene age. The iron-ore sequence is characterized by a rhythmic structure. The upper iron-bearing parts of the rhythms are combined into iron-ore horizons (from bottom to top): Narym, Kolpashevo, Chigara, and Tym–Bakchar. The horizons are composed of loose oolitic ores and ores with siderite and leptochlorite cement, as well as chloritolites and oolitic sandstones. The article considers the distribution features of Si, P, Al, Mg, the sum of alkalis, Ca, and S.

Silicon. The bulk of silica is concentrated in authigenic formations and in clay minerals. A fairly high content of SiO₂ has been established in hydrogoethite oolites (10–14%) in the form of an admixture of terrigenous quartz, oxidized chlorite, and amorphous silica coagulates of the stilpnosiderite type. There is a very close negative correlation between the total iron content and silica, decreasing slightly in ores with siderite cement (Table 1). The correlation equations are sufficiently reliable and can be successfully used in calculation practice. The average silica contents in the deposits of the horizons are given in Table 2. The limits of average contents in sections of the horizons are 20.24–49.68% in the Narym, 19.48–51.97% in the Kolpashevo, 23.99–49.93% in the Chigara, and 17.53–38.18% in the Tym–Bakchar horizons. The average content in the ores is 18.84–28.95% (Table 2).

Phosphorus. The element occurs mainly in a dispersed state, in small amounts, accumulating in the form of “visible” calcium and iron phosphates. The highest phosphorus contents characterize hydrogoethite oolites and siderite cement. In other mineral components the phosphorus content is 5–20 times lower. The probable form of phosphorus accumulation is sorption in formations of the stilpnosiderite type, and in siderites—finely dispersed dissemination of collophane microsegregations. In the deposits of the horizons, there is a direct correlation between total iron content and phosphorus content, the closeness of which decreases from the upper horizons to the lower ones (Table 1). The value of the ratio

$$ \frac{P}{Fe} $$

reaches its highest value in deposits of the Narym horizon.

\((9.5 \times 10^{-3})\). In the Narym horizon, the weighted-average contents in sections vary from 0.03 to 0.65%; in the Kolpashevo, from 0.17 to 0.78%; in the Chigara, from 0.07 to 0.62%; and in the Tymsk–Bakchar, from 0.26 to 0.63%. The average content in the ores is 0.27–0.61%, whereas in Kerch ores it is 0.86–1.15% [1], and in Khoper Upper Cretaceous ores 0.38–1.47% to 5.52% [3], i.e., the ores of the Bakchar deposit are relatively low in phosphorus.

Aluminum. Aluminum is concentrated in the composition of leptochlorites and clay minerals. The lower aluminum content

Table 1

Results of correlation analysis of the relationship of certain components according to data from chemical analysis of group samples

Note. \(\bar{x}, \bar{y}\)—arithmetic mean content,
\(S_x, S_y\)—standard deviation,
\(r\)—correlation coefficient,
\(n\)—number of paired content values,
\(*\)—total iron content.

are characterized by hydrogoethite ooids. In oolitic ores with siderite-chlorite cement, the correlation between Al₂O₃ and Fe₂O₃ is absent, but in loose oolitic hydrogoethite ores there is a negative correlation. The latter circumstance is not accidental and is caused by the displacement of aluminum during the hydrogoethitization of the original hydrogoethite-chlorite oolitic material in the regressive stage of diagenesis, with accumulation of aluminum in deeper-water sediments. The limits of the weighted-average Al₂O₃ contents in individual sections of the horizons are 3,15—6,79% in the Narym horizon, 4,01—7,09% in the Kolpashevo horizon, 4,98—9,69% in the Chigara horizon, and 4,55—7,16% in the Tym-Bakchar horizon. The average Al₂O₃ content in the ores is 4,71—6,52% (Table 2).

Table 2

Average contents of elements and oxides in the sediments of ore horizons and in the ores of the Bakchar deposit according to chemical analyses

Note. Sediments of the ore-bearing horizons: 1 — Tym-Bakchar, 2 — Chigara, 3 — Kolpashevo, 4 — Narym. Ores: 5 — oolitic ores with siderite cement, 6 — loose oolitic ores, 7 — oolitic ores with siderite-chlorite cement; 8 — weight clarkes of the elements in the lithosphere.

Magnesium. The principal magnesium-bearing minerals are glauconite and hydromica; lepto-chlorites have a reduced content, and the lowest magnesium content has been established in hydrogoethite ooids and siderite. In the sediments there is a correlation between magnesium and aluminum. Like aluminum, magnesium is associated by a weak negative correlation with Fe₂O₃ in loose ores, but at the same time a reliable relationship between Fe₂O₃ and MgO has been established in ores with siderite-chlorite cement, caused by the accumulation of iron and magnesium in chlorite minerals (Table 1). In the Narym horizon, the weighted-average MgO contents in sections vary from 0,76 to 1,68%; in the Kolpashevo horizon, from 1,17 to 1,78%; in the Chigara horizon, from 1,33 to 1,88%; and in the Tym-Bakchar horizon, from 0,49 to 1,59%. In the ores the MgO content lies within the range 0,87—1,36%.

Potassium and sodium. The content of potassium and sodium was determined in total. Glauconite and hydromicas are characterized by the highest content of alkalis. According to the available determinations, the potassium content as a whole is considerably higher than the sodium content. In the sediments there is a correlation between the sum of alkalis and aluminum and magnesium. The weighted-average content in sections of the horizons is 0,70—1,96% in the Narym horizon, 0,62—2,34% in the Kolpashevo horizon, 0,81—2,66% in the Chigara horizon, and 0,44—2,18% in the Tym-Bakchar horizon. The average content in the ores is 0,81—1,36% (Table 2).

Calcium. In the deposits, calcium is dispersed in silicate minerals, in siderite, and, in small amounts, in phosphate concretions. The limits of the weighted-average contents in sections are: in the Narym horizon, 1.01–1.75%; in the Kolpashevo horizon, 0.37–2.5%; in the Chigara horizon, 0.55–1.43%; and in the Tym–Bakchar horizon, 0.47–1.47%. The ores with siderite–chlorite cement are characterized by the highest calcium content (Table 1). The ores are depleted in CaO in comparison with Kerch ores (1.48–1.89% [1]).

Sulfur. The bulk of the sulfur is in a dispersed state. A negligible amount of sulfur is concentrated in discrete mineral segregations—pyrite and melnikovite. The negative correlation, observed at other deposits, between sulfur and trivalent iron [2] is not found here, despite the fact that Fe₂O₃ and FeO are linked by a close negative relationship in cemented ores (Table 1). In sections of the horizons, the weighted-average sulfur contents lie within the ranges 0.01–0.41% in the Narym horizon, 0.01–0.3% in the Kolpashevo horizon, 0.01–0.74% in the Chigara horizon, and 0.01–0.16% in the Tym–Bakchar horizon. The sulfur content in the ores is 0.019–0.068%, which is substantially lower than its content in Kerch ores (0.13–0.17% [1]). The reduced sulfur content in the ores probably could not but affect the overall reducing balance in the sediments.

In general, the ores of the Bakchar deposit are characterized by comparatively low contents of phosphorus, calcium, and sulfur. The lowest contents of silica, alumina, magnesia, and alkalis have been established in ores with siderite cement, and the maximum contents in ores with siderite–chlorite cement. Phosphorus is distributed in the ores rather uniformly and correlates closely with the total iron content. The equations of the correlation relationship of silica and phosphorus with iron are reliable and may be used in calculations without involving chemical-analysis data.

REFERENCES

1. A. U. Litvinenko. On the geochemistry of the Kerch iron-ore basin. Lithology and Mineral Resources, No. 4, 1964.

2. V. F. Malakhovskii. Organic carbon and sulfur in the Kerch iron ores. Doklady of the Academy of Sciences of the USSR, vol. 128, No. 6, 1959.

3. N. Kh. Platonov. Geological Structure of the Khoper Region. Moscow, 1958.