THERMAL AND X-RAY DIFFRACTOMETRIC STUDIES OF LIMONITES FROM GOLD ORE DEPOSITS OF THE KUZNETSK ALATAU

A. D. MIKOV (TPI)

In carrying out work at the gold ore deposits of the northern part of the Kuznetsk Alatau—Tsentralnoe, Komsomolskoe, Berikul’skoe, and Natal’evskoe—various types of limonitic formations were encountered. Their brief characterization is given in the present paper.

Limonite is widely developed in the oxidation zone of all the ore bodies studied. It forms chiefly as a result of replacement of pyrite and, more rarely, chalcopyrite, arsenopyrite, and certain other minerals. Macroscopically, under field conditions, four varieties of limonite were distinguished; the division was based on the form of their occurrence, structure, color, etc. The presence of most similar varieties was noted, for example, by M. F. Zenin [3] in the oxidation zone of copper-porphyry ore deposits in Kazakhstan.

The generally accepted division [1, 3, 6] of limonitic formations into residual and transported is also of fundamental importance and has been adopted by the author. In this respect, the varieties of limonites encountered in the oxidation zone of the Tsentralnoe, Komsomolskoe, Berikul’skoe, and Natal’evskoe deposits may be divided into residual—pseudomorphic and resinous—and transported—nodular (crustiform) and ocherous.

For the most complete characterization of the minerals under consideration, chemical, thermal, semiquantitative spectral, and X-ray structural analyses were carried out on most of the varieties distinguished.

The first variety—pseudomorphic limonite (limonite pseudomorphs after pyrite)—is widespread in all the above-mentioned deposits, where it replaces individual pyrite crystals or their aggregates, both directly in the vein mass and in the enclosing beresites. The surface of the crystals is usually smooth; the striation on their faces, characteristic of pyrite, is often preserved. The color is dark brown and black. This limonite is characterized by a dense structure and high hardness. Microscopically it is established that it is represented by goethite. It is usually composed of aggregates of grains of various sizes, from 0.05 to 2 mm across. Very often

among goethite, relicts of pyrite are observed. The goethite described is often associated with nodular limonite (the Tumanševskaya vein of the Central deposit).

Resinous limonite is distributed only in the oxidation zone of the Natal’evskoe deposit. In the opinion of M. F. Zenin, who studied the distribution of resinous limonite in the oxidation zone of Almalyk [3], it is characteristic of those areas where the processes of oxidation and leaching manifested themselves rather weakly. At the Natal’evskoe deposit it replaces chalcopyrite along an irregular network of the finest cracks or along the periphery of grains. If oxidation processes proceeded rather actively, chalcopyrite is finally replaced by resinous limonite. Usually, however, among the fine veinlets of limonite, segregations of chalcopyrite of various sizes are preserved. Resinous limonite composes veinlets up to 5 mm thick and disseminations with diameters from 0.5 to 10 mm in skarns. Its color is black. A dense structure is characteristic. An important feature is its resinous luster. In polished sections it is seen that resinous limonite forms pseudomorphs after chalcopyrite and is represented by goethite.

According to thermal-analysis data, it has been established that, in accordance with the classification of E. Ya. Rode, the mineral (sample No. 5072) may be assigned to the goethite–lepidocrocite class and identified as hydrogoethite. Comparison of the X-ray study data (Table 2) for resinous limonite with the standard data, according to V. I. Mikheev [5], confirms the identification made.

Chemical analyses of most of the limonite varieties described are given in Table 1. The composition of resinous limonite basically does not differ from the chemical composition of other limonites. It shows only an increased copper content (0.11%), whereas its content in other samples is 0.03–0.06%. The results of the chemical analysis of resinous limonite confirm the data of thermal and X-ray analyses and make it possible to identify it as hydrogoethite (Table 1). Spectral analyses established in it an increased content of copper and bismuth and the presence of zinc, nickel, cobalt, lead, silver, and a number of other elements (Fig. 4), which agrees with the chemical composition of the primary ores.

Nodular (crustiform) limonite is comparatively widely developed in the oxidation zone of the Central deposit and, to a lesser extent, of the Komsomol’skoe and Berikul’skoe deposits.

According to A. Lokka [11], it forms at the expense of chalcosine and pyrite and belongs to the group of residual limonites. M. F. Zenin [3] relates its occurrence to the presence of chalcopyrite and pyrite in the primary ores, considering it transported (more precisely, adjacent to the site of sulfide decomposition). Our observations confirm M. F. Zenin’s data.

Crustiform limonite composes typical brown ironstones at the outcrops of a number of veins, or forms veinlets and fills cavities in quartz. The thickness of the veinlets and crusts reaches 5 mm. The color is grayish brown. The structure is dense. The field identification of the mineral is goethite.

In the oxidation zone of the veins of the Komsomol’skoe deposit, besides grayish-brown nodular limonite, analogous segregations of brownish-red limonite (sample 1048) are developed. At the deposit it is identified as hydrohematite. Under the microscope, it differs from the above-mentioned varieties by the presence of reddish internal reflections instead of the yellowish-brown and brown ones characteristic of goethite. Crustiform limonite is often present together with pseudomorphic limonite, which indicates an insignificant transport of limonitic material. Therefo-

...therefore, the assignment of nodular (crust) limonite to the group of transported limonites is to some extent conditional.

Thermal (Fig. 1, б, в) and X-ray-diffraction (Table 2) determinations of two samples (Nos. 0613, 1693) of crust limonite make it possible to diagnose them as goethite–hydrogoethite, which is confirmed by the data of chemical analysis (Table 1). The third sample (No. 1048), according to chemical-analysis data, is also determined as hydrogoethite. The results of the thermal determination of the latter are interpreted [8] as a mixture of 80% hematite and 20% goethite, which is also well confirmed by the X-ray-diffraction study. On the X-ray pattern, distinct peaks of hematite and weak peaks belonging to goethite are observed (Table 2).

Spectral analysis (Fig. 4) established in the first two limonites (Nos. 0613 and 1693) the presence of the usual complex of elements—copper, zinc, lead, nickel, cobalt, and others. In the third sample (sample No. 1048), however, a whole series of them is absent (copper, nickel, cobalt), which is consistent with the character of the primary mineralization.

Ochreous limonite is widely distributed in all the deposits studied. It usually fills cracks and cavities both in vein quartz and in the enclosing rocks—in the selvages of veins. It is formed both at the expense of the pyrite material of the veins and of the beresites. The color of ochreous limonite is variable and changes from yellowish-brownish to reddish and brownish-brown. In the distribution of ochreous limonite, a noticeable confinement to the lower part of the oxidation zone is observed. Often such limonitic material completely impregnates the mass of the enclosing rocks, and in this case it is impossible to separate it from them. Thermal analysis of such limonite (sample No. 1099) from the Central deposit gave a thermogram somewhat different from the ordinary limonite curves (Fig. 2, д). At a temperature of 450° an exothermic peak is observed on the curve, which, on the basis of the experiments of I. Kulga and A. Traitz [10], can probably be explained by the formation of the mineral in an alkaline environment.

The mineral may be identified as hydrogoethite. Chemical analysis of this mineral showed the presence of a significant admixture of SiO₂, Al₂O₃, CaO, K₂O, which is explained by the impossibility of separating it from the enclosing rocks. Recalculation of the chemical analysis of the indicated limonite (Table 1) confirms the data of the thermal study on the presence of hydrogoethite. Thermal analysis of ochreous limonites, samples 1099 and 1098, gave the usual pattern characteristic of hydrogoethite (Fig. 2, а, б), which is in good agreement with the data of the X-ray-diffraction determination (Table 2).

In the thermal study of ochreous limonites from the Natal′evskoe (sample 5094) and Central (sample 21) deposits, the presence of lepidocrocite was established, which is confirmed by the data of the X-ray determination (Table 2). Spectral analyses (Fig. 4) established in ochreous limonites, in a number of cases, elevated admixtures of lead, copper, zinc, arsenic, and in a number of samples silver, gold, bismuth, barium, and molybdenum.

Among the ochreous limonites, special mention should be made of modern neoformations of limonite encountered in the mine workings of the Central, Berikul′, and Komsomol′sk deposits. Such neoformations from the Central deposit were described by the author earlier [4]. Study of the neoformations showed that they are composed essentially of ochreous limonitic material ranging in color from yellowish-brown to reddish-brown. Thermograms obtained in the study of these neoformations from the Central deposit (samples 39 and 14a) differ significantly from the ideal heating curves of hydrogoethite.

Fig. 1. Heating curves of dense limonites:

a—resinous limonite (sample 5072) from the Natal'evskoe deposit, б—crustose limonite (sample 0613) from the Central vein of the deposit of the same name, в—nodular limonite (sample 1693) from the Tumashevskaya vein of the Central deposit, г—dense limonite (sample 1048) from the Komsomolskaya vein of the deposit of the same name.

Time

Fig. 2. Heating curves of ochreous limonites:
a—limonite (sample 1093) from the Komsomol’skoe deposit; b—limonite (sample 1098) from the vein of the Klyuchevskoe deposit, Komsomol’skoe; v—limonite (sample 21) from the vein of the Promezhutochnoe deposit, Tsentral’noe; g—limonite (sample 5094) from the Natal’evskoe deposit; d—limonite (sample 1099) from the vein of the Tsentral’noe deposit of the same name.

Fig. 3. Heating curves of recent limonite neoformations:
a — limonite (sample 39) from the Tsentralnaya vein of the deposit of the same name; b — limonite (sample 14a) from the Shkolnaya vein of the Tsentralnoe deposit.

Fig. 4. Spectral analyses of limonites

Column headings:
Name of deposit; type of limonite; sample no.; Pb; Cu; Zn; Ag; Ni; Co; Bi; As; Sb; Sn; Ti; Cr; V; Mn; Te; Au; Ba; Mo.

Deposits, limonite types, and samples shown:

• Natal’evskoe — resinous limonite — sample 5072
• Tsentral’noe — nodular limonite — sample 0613
• Tsentral’noe — nodular limonite — sample 1693
• Tsentral’noe — nodular limonite — sample 1048
• Komsomol’skoe — ocherous limonite — sample 1099
• Tsentral’noe — ocherous limonite — sample 21
• Tsentral’noe — ocherous limonite — sample 21 a
• Tsentral’noe — ocherous limonite — sample 1109
• Komsomol’skoe — ocherous limonite — sample 1098
• Komsomol’skoe — ocherous limonite — sample 5094
• Natal’evskoe — ocherous limonite — sample 14 a
• Tsentral’noe — ocherous limonite — sample 39

Legend for contents:

• \<0.001%
• 0.001–0.01%
• 0.01–0.1%
• 0.1–1%
• 1–10%
• >10%

and lepidocrocite [8]. On them, endothermic effects at temperatures of 150–170° and 700–725° stand out clearly, as do less pronounced exothermic peaks at temperatures of 380–410° and 535–570°. In character these curves are similar to the curves of artificially obtained iron oxide hydrates [9, 10]. These minerals may be diagnosed as lepidocrocite. This is to some extent also confirmed by the X-ray-analysis data (Table 2), although the material was poorly crystallized and the Debye pattern obtained was indistinct. According to the cited data of chemical analysis (Table 1), the limonite described (sample 39) is determined as hydrogoethite. Most correctly, this limonite can probably be identified as hydrolepidocrocite. Spectral analyses (Fig. 4) established in the newly formed limonite the usual set of microimpurities, but their quantitative content is lower than in other types of limonites. Also characteristic is the absence of nickel, cobalt, and a number of other elements.

From the data presented, several conclusions may be drawn:

1. In the oxidation zone of the deposits studied, four varieties of limonite have been distinguished among the iron hydroxides: pseudomorphic, resinous, nodular, and ocherous. Chemical, thermal, and X-ray investigations have established that most of them are represented by hydrogoethite. The wide distribution of hydrogoethite in the oxidation zone of ore deposits has also been pointed out by other investigators [2, 7]. Goethite has been established only microscopically in the pseudomorphic variety.

Lepidocrocite is most widely distributed among the ocherous limonites (samples 14 a, 21, 39, 5094).

1. The application of thermal and X-ray analyses for the study of brown iron ores is quite accessible and makes it possible easily and accurately to diagnose mineral varieties of the limonite group.

2. According to spectral analyses, the content of the principal impurity elements—lead, copper, zinc, and arsenic—varies within wide limits, with the sharpest fluctuations observed for the last two elements. Manganese is present in approximately equal amounts. The presence of impurity elements is consistent with the composition of the primary ores. Gold was found in individual samples, but it was established that its content is higher in ocherous limonite, which reflects its association with transported limonites in the oxidation zone of the deposits studied.

Table 1

Reduced composition of limonites according to chemical-analysis data

References

1. Al'bov M. N. Secondary zoning of gold-ore deposits of the Urals. Moscow, 1960.

2. Vinnichenko N. T. Thermal study of brown iron ores of Turanguly-Sai. Transactions of the Uzbek Branch of the Mineralogical Society, issue 2, 1951.

3. Zenin M. F. Products of oxidation and leaching of sulfides as a criterion for evaluating copper-porphyry ore deposits from outcrops. Transactions of the Central Asian Industrial Institute, issue 1(9), 1938.

4. Mikov A. D. On modern mineral neogenic formations in quartz-sulfide veins of the Tsentral'noe deposit (Kuznetsk Alatau). Proceedings of TPI, vol. 165, 1968.

5. X-ray Determinative Tables for Minerals. Moscow, 1954.

6. Smirnov S. S. The oxidation zone of sulfide deposits. ONTI, 1936.

7. Tochilin M. S. Relict metasomatic microstructures of brown iron ores and their geochemical significance. Mineralogical Collection of the L'vov Geological Society, No. 4, 1950.

8. Chukrov F. V. Colloids in the Earth’s crust. Moscow, 1955.

9. Gheit M. A. Differential thermal analysis of certain iron oxides hydrates. Amer. Jorn. Sci., vol. 250, No. 9, 1952.

10. Kulp I. L., Trites A. F. Differential thermal analysis of the natural hydrous ferric oxides. Amer. Mineralogist, vol. 36, No. 1—2, 1951.

11. Locke A. Leaced outcrops as Guides to Copper Ore. London, 1926.

Results of X-ray determination of limonites

Table 2