On the Nature of the Coloration of Limestone from the Yashkinskaya Member

P. N. Parshin

(Presented by the scientific seminar of the Research Institute of Hydrogeology and Engineering Geology)

Differences in the coloration and metamorphism of rocks are often used in geological practice (in the absence of more reliable data) for stratigraphic subdivision and correlation. Examples of this are also found in the practice of geological mapping in the Kolyvan–Tomsk folded zone, where, in particular, within the area of development of white marmorized limestones having the appearance of Middle Cambrian limestones of the Salair (the Ikovskaya Formation), Middle Devonian organic remains were subsequently encountered. Errors of this kind may occur in determining the age of the Yashkinskaya limestone member, whose age, on the basis of organic remains, is determined as Givetian–Frasnian. Yet for some white marmorized varieties a more ancient age is sometimes assumed.

The Yashkinskaya member is represented mainly by reef limestones and belongs to the lower part of the Pachinskaya Formation of the Frasnian and the upper part of the Givetian stages of the Devonian of the Kolyvan–Tomsk folded zone.

The limestones of the Yashkinskaya member are represented chiefly by two varieties: gray ones, usually with distinct bedding due to the interlayering of light and dark (sometimes clayey) varieties, containing fauna of crinoids, brachiopods, and corals in satisfactory preservation; and white, massive, marmorized ones, in which the outlines of organic remains are visible only occasionally on the surface of exposed parts during weathering, but are not noticeable in fresh fractures.

During geological mapping work in this area, some researchers, allowing for a difference in age between the gray and white varieties of limestone, suggested the possible presence of a break in sedimentation at the boundary between these varieties, which in turn would have made it possible to find bauxites of Paleozoic age under these conditions (mapping work of M. P. Nagorsky, 1932). We were unable to confirm the above assumptions with new factual material, but it became possible to clarify the nature of the coloration of some white marmorized varieties of limestones.

In the western part of the active quarry of the Yashkinsky cement plant, outcrops of organogenic gray, and sometimes black, bituminous limestones are observed, which, when struck, emit a sharp odor of hydrogen sulfide—

Sometimes, among the gray and whitish mass, black stems filled with bituminous calcite are observed. The impression is created that the dark and light varieties of limestone are the consequence of different intensities of removal of organic material during hydrothermal alteration of the rocks. Possibly the coral stems were zones of the most intensive movement of fluids transporting organic matter, and individual stems, filled in the center with bituminous material, may represent a case in which such movement was interrupted at the stage of intensive removal of organic matter. A subsequent stage was the silicification of the limestones, accompanied by sulfide mineralization (nests of pyrite with admixtures of chalcopyrite).

White, marblized varieties of limestones are developed along large faults that are significantly affected by sulfide mineralization. This is observed especially clearly south of the described area, in the middle course of the Vlas’kovo River, in the western part of the Vlas’kovo limestone quarry. Here, as in the Yashkinsky quarry, marblized white limestones west of a fault of northeastern strike are separated from clayey shales. Sulfide mineralization within this fault is indicated by dumps of brown ironstones, in which B. M. Levashov, from our collections, identified disseminations of native copper and elevated contents of elements of the polymetallic complex by spectral analysis. In birch leaves in this zone, an increased content of lead (0.005%), nickel (0.001%), cadmium (0.001%), and vanadium (0.003%) was noted. By the hydrogeochemical method it is difficult to establish the anomaly because of the presence of a geochemical barrier, which is represented by the limestones.

Possibly the bleaching and ocher coloration of the limestones is connected with a hydrothermal process that produced sulfide mineralization, exposed by drilling 4 km west of the Vlas’kovo quarry near the village of Baranovo. In that case the limestones of the Yashkinskaya member must be a geochemical barrier for ore-bearing fluids, and sulfide ores may be expected at the base of their bleached varieties.

Thus, differences in the coloration of the limestones and in the degree of their metamorphism cannot unconditionally be taken as indicators of their different stratigraphic position. In a number of cases they are the result of the local manifestation of secondary (in particular, hydrothermal) alteration, and in such cases they may be regarded as one of the prospecting indicators for sulfide ores.