On the "siderite" of the Bazyr deposits
Abstract Generated abstract
This paper reexamines brown carbonate veinlets in the southern Bazyr deposits, previously identified as siderite, within dolomitized limestones. Using microscopic observations, chemical analysis, and immersion-method refractive-index measurements, the author compares the material with siderite, ankerite, dolomite, and calcite. The results indicate that the carbonate is predominantly calcite with minor Mg, Fe, and Mn impurities, while its brown color is attributed to secondary limonite impregnation, likely derived from oxidation of sulfides in the enclosing limestones. The paper also notes associated quartz as an earlier vein-forming phase and interprets the calcite as a late product of mineralizing thermal solutions.
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On the “Siderite” of the Bazyr Deposits
V. K. Cherepnin
In the area of the Bazyr deposits, especially in its southern part, among dolomitized limestones, veinlets of a comparatively coarse-grained brown carbonate are widely developed. This carbonate was at one time taken by V. S. Domarev [1] and by some other investigators to be siderite.
Microscopic study of this material, carried out by the author, showed that it is composed of an extremely uneven-grained aggregate with a predominating rhombohedral form of the individual grains. In addition, in thin section it is clearly seen that the coloration of the carbonate veinlets is not as uniform as it appears macroscopically. Usually the densest coloring is observed in the parts of the veinlets adjacent to the selvages, whereas in the middle grains completely devoid of pigmenting substance are not infrequently encountered. In places it can be observed that the coloring material penetrates the carbonate grains along cleavage cracks. The grains therefore often appear, as it were, zonal. All the foregoing makes it possible to consider that the brown coloration of the carbonate described is a secondary formation.
Chemical analysis of our carbonate material revealed the following contents: CaCO₃—55.46%; MgCO₃—1.15%; FeCO₃—1.63%; MnCO₃—0.68%; Fe₂O₃—12.30%; H₂O⁺—11.39%; H₂O⁻—0.92%. If all the carbonates are taken as 100% and recalculated, the following is obtained: CaCO₃—94.12%; FeCO₃—2.76%; MgCO₃—1.95%; MnCO₃—1.15%.
The high content of CaCO₃, with an insignificant amount of MgCO₃ and FeCO₃, does not permit this mineral to be regarded not only as siderite, but even as ankerite or dolomite. The analytical data correspond most closely to calcite. As for the very slight admixture of Mg, Fe, and Mn, the latter is encountered rather often in calcites. Thus, for example, Hintze [3] gives chemical analyses of calcites in which a small amount of the above-mentioned impurities is often found (the highest content being MgO—1.44%; FeO—3.01%; and MnO—8.48%). The presence in the material studied of a rather large amount of ferric oxides is apparently entirely due to the brown coloring material, which is nothing other than limonite.
For a more complete characterization of the mineral, its refractive index was determined by the immersion method. The results of repeated observations showed that the largest and constant refractive index—Nm—is equal to 1.654. As for the second—the smallest—refractive index, Np, as is known, if the measurements are not made on a Fedorov universal stage, its values will always deviate from the normal ones.
In this case, for an optically negative carbonate this deviation will be in the direction of increase. Taking into account only what has been stated, the value of the smallest refractive index is not used by us in the calculation, although it is nevertheless very close to Np of calcite.
Below is a comparative table of refractive-index values, \(Nm\), for carbonates of the trigonal series according to Larsen and Berman [2].
Table 1
| Minerals | Minerals | Minerals | Minerals | Minerals | |
|---|---|---|---|---|---|
| siderite | ankerite | dolomite | calcite | mineral studied | |
| Refractive-index value | from 1.830 to 1.875 | from 1.698 to 1.749 | 1.681 | 1.658 | 1.654 |
As can be seen from the table presented, the refractive index of the carbonate under study differs very sharply from all the other minerals similar to it in appearance and almost completely coincides with the refractive index of calcite.
Thus, taking into account the data of chemical analyses and the results of optical investigations, we may consider that the mineral we have studied is normal calcite impregnated with iron hydroxides. The latter, apparently, are products of the oxidation of sulfides, inclusions of which are widely developed in the enclosing limestones.
Among the veinlets of the carbonate described, quartz is often encountered in the form of separate grains or in accumulations of veinlike form. This quartz represents an earlier formation than calcite, since the latter often develops as thin veinlets running along the boundaries of the quartz grains.
Genetically, calcite is apparently the final product of the activity of thermal solutions that caused the mineralization of the district.
References Used
- Domarev V. S.—The principal copper deposits of the Khakass-Minusinsk district, Vestn. Zap.-Sib. geol. tresta, issue 5, 1932.
- Larsen E. and Berman G.—Determination of transparent minerals under the microscope. ONTI, 1937.
- Hintze C. Handbuch der Mineralogie, Band I Carbonate, 1931.