GOLD IN ORES OF THE KACHKANAR MASSIF IN THE MIDDLE URALS

V. G. FOMINYKH, E. D. USKOV, Yu. A. VOLCHENKO (UFAN USSR)

In the development of placer platinum deposits in the Urals, genetically connected with ultrabasic rocks of the platinum-bearing belt, attention has been and continues to be drawn to the constant presence in them of gold grains (Vysotskii, 1913; Latysh, 1959, and others).

N. K. Vysotskii regarded the occurrence of gold in platinum placers as due to the destruction of acidic rocks. But since acidic rocks are absent in a number of massifs of the platinum-bearing belt, the primary source of the gold, naturally, was for a long time not definitively established.

At the same time, analyses of native platinum from placers often note an admixture of gold of up to 0.3–0.4% (Vysotskii, 1925).

In connection with the exploitation of the Kachkanar titanomagnetite deposit in the Urals, the question arose of the comprehensive utilization of the low-titanium, vanadium-bearing ores of this deposit and of identifying the regularities in the distribution of the platinum-group metals. At the same time, we carried out studies on the distribution of gold in the ores of the deposit, preliminarily studied the composition of the gold, and provided an assessment of the gold content of the Gusevogorsk deposit.

The Gusevogorsk deposit of titanomagnetite ores is located in the pyroxenites of the Gusevogorsk massif, which forms part of the Kachkanar intrusive complex (Fig. 1).

The Kachkanar intrusive complex belongs to the platinum-bearing belt of the Urals and has, in plan view, an oval shape, somewhat elongated in the latitudinal direction. It includes the Kachkanar massif proper, forming the western, somewhat curved ridge, and the Gusevogorsk massif, situated in the eastern foothills of Kachkanar. The titanomagnetite mineralization in the pyroxenites of these massifs is localized, uniting the Kachkanar and Gusevogorsk deposits proper. The Gusevogorsk deposit is confined to the pyroxenite massif of the same name.

Its geological structure, petrography, and material composition have been described in the works of N. K. Vysotskii (1913), Z. V. Rupasova (1948), V. A. Reshitko (1959—1 and 2), A. F. Fadeichev (1961), V. G. Fominykh, P. I. Samoilov and others (1967), and others.

The massif is composed mainly of ore-bearing, olivine-free and olivine barren pyroxenites. Less commonly encountered are amphibolized and feldspathized pyroxenites and hornblendites, na-

Fig. 1. Structural-geological map of the area of the Kachkanar massif, after V. A. Reshitko (1959).

Sections along lines A–B and V–G.

1—pyroxene-plagioclase and plagioclase porphyrites of andesite-basalt composition; 2—amphibolized diabases with zones of schistosity; 3—amphibolized diabase porphyrites and lava breccias of diabase porphyrites; 4—amphibolized porphyrites, lava breccias, and tuffs; 5—dunites; 6—schlieren bodies of magnetite olivinites, peridotites, and dunites; 7—pyroxenites; 8—ore pyroxenites; 9—normal gabbro, commonly olivine-bearing, and forellensteins; 10—biotite gabbro, partly olivine-bearing, and gabbro-norites, to a significant extent uralitized and biotitized; 11—uralitized and biotitized pyroxenites, pyroxene-hornblende, hornblende, and epidote-hornblende rocks; 12—uralitized and biotitized gabbro; 13—plagioclase and epidote amphibolites; 14—orthogneisses; 15—chlorite-actinolite schists; 16—chlorite schists; 17—mica-quartz schists and, in part, quartzites; 18—banding with dip angles up to 45°, 45–90°, and vertical; 19—schistosity (gneissosity) with dip angles up to 90° and vertical; 20—bedding; 21—lines of tectonic disturbances; 22—old workings: a—Magnetnaya pit; b—Obodovskii adit; v—placers of boulder ores; g—iron mine; d—Vyiskii mine; 23—boundaries of the Kachkanar massif.

observed in zones of contact with gabbroids and in areas where plagioclasite veins, wehrlites, and olivinites occur. The latter are established in the form of elongated bodies, often irregular in shape, sometimes as angular blocks. These rocks are cut by ore pyroxenites. The mineralization in the pyroxenites is distributed unevenly, forming local areas with increased dissemination of titanomagnetite. One of these areas, the largest—the Main deposit—is currently being mined by the Kachkanar Mining and Processing Combine.

The geological structure of the deposit is in many respects analogous to that of other deposits of the ore field. Olivine pyroxenites, as a rule barren, border the ore bodies; sometimes, especially in the northern part of the deposit, they occur as blocks or small bodies within ore-bearing olivine-free pyroxenites, which accounts for the extremely “dissected” and “jagged” outlines of the boundaries of the ore contour. This picture is most characteristic of the se-

Fig. 2. Geological map of the Main deposit of the Gusevogorskoye deposit, according to data of the Kachkanar Mining and Processing Combine

1—olivinites; 2—wehrites; 3—olivine pyroxenites; 4—pyroxenites; 5—feldspathized pyroxenites; 6—amphibolized pyroxenites; 7—hornblendites; 8—hornblende gabbro; 9—areas showing serpentinization; 10—plagioclasite veins more than 1 m thick; 11—main tectonic zones; 12—banding; 13—outline of barren areas;

the northwestern and southwestern parts of the deposit (Fig. 2). Small bodies of olivine pyroxenites among the ore form scattered chains oriented in a northwestern direction. The rocks of the Main deposit, as well as of the deposit as a whole, are characterized by a banded-zonal structure and a northwestern strike of the rock varieties, with a steep eastern dip. A similar zonation, oriented in a northwestern direction, is observed in the distribution of various textural types of ores, in the distribution of iron, titanium, and vanadium. This feature of the geological structure of the Main deposit is in good agreement with the spatial position of the tectonic zones.

Fig. 3. Gold grains from the Gusevogorsk deposit. Magnified 15 times

The ore body of the Main deposit is intensely, but unevenly, dissected by vein rocks, among which monomineralic plagioclasites are predominant; quartz and hornblende plagioclasites occur more rarely. Still rarer are vein pyroxenites (gusevites).

The bulk of the plagioclasite veins is characterized by small thickness and has a northwestern strike (310–340°) and a dip of 65–80° to the northeast and southwest. Series of plagioclasite veins often have a crosscutting position with respect to the contacts of different rocks and ore types, as can be seen in Fig. 2, where several large plagioclasite veins are shown.

The data obtained by us during systematic sampling of the ustu-

…of the Main Deposit quarry of the Gusevogorsk deposit showed that gold occurs in all rocks (gabbro, pyroxenites of various types, hornblendites, etc.) in insignificant amounts \((1 \cdot 10^{-6}—1 \cdot 10^{-7}\%)\), and, consequently, areal destruction of these rocks could not have led to the formation of gold in placers. The source of the rich and now worked-out placers (Khishnicheskii Log, etc.) is apparently the zones of hydrothermally altered rocks identified by us, characterized by elevated concentrations of gold (from \(2 \cdot 10^{-5}\) to \(5{,}3 \cdot 10^{-4}\%\)). These zones have varying thickness and extent and are characterized by a complex internal structure. They represent a series of closely spaced lens-shaped and lenticular segments, combining into larger, conformable structures that often cut the structural elements of the massif (rock banding, contacts of different rock types, etc.). These zones are characterized by intensive development of amphibole after pyroxene, with the formation of hornblende rocks of the hornblendite type; serpentine after olivine in olivine rocks; feldspathization of pyroxenites, etc. Elevated contents of sulfides (pyrite and chalcopyrite), apatite, and others are established in them.

These zones are situated in areas of maximum development of plagioclasite veins. The largest zone of these altered rocks is located in the eastern part of the deposit and coincides with areas where plagioclasite veins develop (Fig. 2).

To study the composition of the gold, grains of native gold were separated by us at the ore-testing station of the Kachkanar Mining and Processing Combine named after Ya. M. Sverdlov.

The gold grains have either an irregular, porous shape or a somewhat rounded form. The size of the gold particles ranges from 0.01 to 3–6 mm in cross section (Fig. 3).

Gold in the rocks of the deposit is heterogeneous. This heterogeneity is due to the presence of two principal varieties of gold.

The first variety is represented by somewhat rounded grains of high-fineness gold, often found intergrown with grains of titanomagnetite and pyroxene. The gold is yellow with a pinkish tint. It occurs in ore pyroxenites, verlites, and olivinites.

The other variety of gold particles has the form of porous grains. The gold is greenish-yellow in color and of low fineness. It has been found together with grains of sulfides (pyrite and chalcopyrite) in zones of intensive development of secondary processes of rock alteration.

When polished sections of gold particles mounted in Bakelite are studied, their homogeneity is established, as well as the absence of rims and inclusions of platinoid grains. True, in one case we observed an intergrowth of a gold grain with a platinum grain.

The chemical composition of the gold particles selected under a binocular microscope from these two large groups of gold is presented in Table 1.

Table 1

Chemical composition of gold from the Gusevogorsk deposit

Note: 1—irregular, spongy grains of yellow gold from ore pyroxenites; 2—irregular, somewhat rounded grains of greenish-yellow color; 3—the same. The last gold samples were separated from the contact zone of gabbro with pyroxenites. The gold samples were separated from platinum-concentrate tailings obtained during washing at the ore-testing station of the Kachkanar Mining and Processing Combine named after Ya. M. Sverdlov. The analyses were performed in the assay laboratory of the Uralmekhanobr Institute.

These data showed that the gold particles selected from the contact zone of pyroxenites with gabbro are characterized by an elevated silver content, which reaches 17.63%. At the same time, significant contents of platinum-group metals, reaching 1.5%, are recorded in all gold samples.

The formation of gold-bearing zones is associated with zones of increased permeability in the pyroxenites, along which hydrothermal solutions circulated; these were apparently connected with the emplacement and crystallization of the gabbroids that form part of the Kachkanar intrusive complex. These solutions caused hydrothermal alteration and deposited gold, which became the principal source of this metal during the formation of placers.

LITERATURE

1. Vysotskii N. K. Platinum deposits of the Isovsk and Nizhne-Tagil districts in the Urals. Tr. Geolkom, new ser., issue 62, 1913.

2. Vysotskii N. K. Platinum and the Regions of Its Extraction. Leningrad, KEPS Publishing House, part IV, 1925.

3. Latysh I. K. Forms of occurrence of platinum in ores of the Kachkanar type in the Urals. In: Problems of the Geology of the Urals, Tr. Gorno-geol. inst. UFAN SSSR, issue 42, 1959.

4. Reshetko V. A. Some features of the geological structure of the Isovsk district of the Middle Urals. In: Materials of the First Scientific Conference of Postgraduate Students. Rostov University Publishing House, 1959—1.

5. Reshetko V. A. Geological structure of the Kachkanar gabbro-peridotite massif in the Middle Urals. Ibid., 1959—2.

6. Rupasova Z. V. The Kachkanar deposit of titanomagnetite ores. Gorn. zhurn., 1948, No. 5.

7. Fadeichev A. F. The Kachkanar gabbro-pyroxenite massif and the titanomagnetite mineralization associated with it. Guidebook of the Tagil-Kuvshin excursion. First Ural Petrographic Conference, Sverdlovsk, 1961.

8. Fominykh V. G., Samoilov P. I., Maksimov G. S., Makatov V. A. Pyroxenites of Kachkanar. Tr. Institute of Geology and Geochemistry, UFAN SSSR, Sverdlovsk, 1967.