SECONDARY ZONATION OF GOLD ORE DEPOSITS IN YAKUTIA

N. V. Nesterov (YaTSU)

Across the vast territory of the Yakut ASSR, G. P. Volarovich and I. S. Rozhkov [2] distinguish four gold-bearing provinces: part of the North-Eastern and Aldan provinces of Cimmerian age, the Siberian Platform, and the Stanovoy Range of Archean and Lower Proterozoic age. Of practical importance are the gold mineralization of Cimmerian age and the numerous rich placers associated with it. The gold ore deposits of Yakutia are concentrated in the Upper Indigirka, Allakh-Yun, and Central Aldan gold-bearing districts of the western part of the Verkhoyansk-Kolyma folded region and of the Aldan Shield. Within these districts various Late Mesozoic magmatic formations and the gold mineralization associated with them are developed.

Over an 80-year period (since 1889), a considerable amount of gold has been mined in Yakutia, of which the share of ore gold is about 10%. All explored deposits have shown a sharp decrease in gold content with depth, 50–100 m below the surface; as a result, geological exploration did not undergo further development at depth. An exception is the Nezhdaninskoye deposit, represented by mineralized crushing zones of considerable size with a moderate gold content in the ore.

At six small vein deposits of the folded region (Yur, Bular, Talalakh, Pozdnee, Zhdannoye, Khangalass), mining was carried out within the upper enriched part of large vein bodies to depths of up to 50–100 m. Numerous steeply dipping veins and horizontal lodes of the Lebediny deposit have been worked to this same depth, and unique weathering-crust deposits in karst cavities of the kuranakh type in Central Aldan are being exploited. Questions of secondary gold enrichment have undeservedly received little attention. In the published works they were first set forth by L. V. Razin and I. S. Rozhkov [6] for deposits of the kuranakh type in Central Aldan.

Meanwhile, studies by M. N. Albov [1] in the Urals, E. D. Shlygina, K. M. Mukanov, and others [7] in Northern Kazakhstan, and A. D. Mikov [3] in the Mariinskaya taiga (Kuznetsk Alatau) have demonstrated the presence of hypergene concentration of gold in gold ore veins and lodes. The foundation for these studies was laid by Soviet scientists.

V. I. Vernadsky, S. S. Smirnov, A. E. Fersman, F. V. Chukhrov,
V. M. Kreiter, and M. N. Albov.

The Miocene–Lower Pleistocene stage in the development of the relief of the territory of Yakutia is characterized by the formation of a planation surface under conditions of a hot humid climate, now raised in the Verkhoyansk–Kolyma folded region to an elevation of 800–1200 m and within the Aldan Shield to 500–800 m.

This preglacial time was exceptionally favorable for the formation of a thick oxidation zone and for the development within it of secondary gold enrichment, preserved as relics in positive forms of the modern mid-mountain relief to depths of 50–100 m in quartz veins. In the negative forms of the folded region, however, which arose during the Late Lower Pleistocene stage of relief development, when permafrost formed and oxidation processes were arrested, the vein bodies are composed of primary ores. Under conditions of the low-mountain relief of the Aldan Shield, the primary ores have not been exposed by erosion. The author’s investigations at worked-out vein deposits of Yakutia established a sharply expressed subzone of secondary gold enrichment, extending down the dip for one or two levels of mine workings [4]. This subzone is clearly revealed when average contents are compared by 10–20 m intervals according to sampling data. Depending on the magnitude of the Late Lower Pleistocene erosional truncation, when oxidation processes were almost completely preserved under permafrost conditions, the subzone of secondary gold enrichment proved either to have been completely cut off, or brought directly to the ground surface, or separated from it by subzones of complete oxidation and leaching, extending to a depth of 50 m. Almost complete destruction of the subzone of secondary gold enrichment is characteristic of deposits of the folded region. Here, the preserved enriched parts of the oxidation zone, as a rule, begin immediately at the ground surface (Fig. 1). The deposits of Central Aldan are characterized by better preservation of the subzone of secondary gold enrichment, which begins either immediately at the ground

Fig. 1. Diagram of the longitudinal vertical projection of the vein of the Vtoroi deposit, Talalakh

1–7 — gold content in the ore from exceptionally high (1 — more than kg per tonne) to low values; 8 — sandy-clayey shales; 9 — parts of the vein with visible gold; 10 — gold sample 960; 11 — gold sample 880; 12 — underground workings; 13 — pillars of worked-out blocks.

Fig. 2. Diagram of the longitudinal vertical projection of the Osennyaya vein of the Lebedinoe deposit.

1—7 — gold content in the ore, from exceptionally high (1 — more than 1 kg per tonne) to low values, within the Lower Cambrian carbonate sequence; 8 — Archean granites.

surface, or from depths of 20–50 m (Fig. 2). The subzone of secondary gold enrichment is characterized by elevated gold contents in the ore, reaching hundreds of grams and the first kilograms per ton.

Gold in the primary and oxidized ores of deposits in Yakutia, in addition to a sharp difference in content, differs in the size of its particles, internal structure, fineness, and associations with other minerals. Gold in primary ores is, as a rule, represented by microscopic (less than 100 microns) and submicroscopic (less than 0.1 micron) particles occurring in close association with sulfides, forming droplet-like inclusions in grains or at the junctions of sulfide grains. Monomineral fractions of late sulfides at the Lebedinoe deposit of Central Aldan and in the folded region are distinguished by elevated gold contents (up to hundreds of grams per ton) [5]. In deposits of the Kuranakh type and with a quartz–pyrite composition of the primary ores of Central Aldan, which have a smaller amount of late sulfides in monomineral sulfide fractions, the gold content reaches several tens of grams per ton.

Primary native gold in the subzone of secondary gold enrichment preserves its internal structure, but very often hypergene gold of higher fineness grows upon it. Sometimes it forms intergranular inclusions. In weathering-crust deposits of the Kuranakh type, hypergene gold, noted by L. V. Ravin and I. S. Rozhkov [6], is predominantly developed. It forms brittle high-fineness segregations several millimeters in size, very rarely forming nests up to 2 cm in size. These segregations are characterized by a colloform structure with a growth center represented by limonite. In the general mass, secondary gold of the Kuranakh deposits is paragenetically closely associated with redeposited hydroxides of iron and manganese. Individual segregations have a drusy-spongy, branch-like form, where each of the branches is characterized by an elongated natrolite-like form. Along with ordinary crusts and inclusions of iron hydroxides, the gold segregations show an emulsion-like dissemination of gold in iron hydroxides and an extremely fine cobweb-like network of veinlets of high-fineness gold, penetrating along cracks into the iron hydroxides, or their mutual intergrowth in larger segregations with the formation of a looped subgraphic structure.

Visible gold in the deposits of Yakutia, as a rule, occurs in oxidized ores, forming segregations along cracks and contact planes of veins, 0.1–2 mm in size, more rarely 3–5 mm and larger. Large nuggets (over 100 grams) have been found only in placers.

Gold in the oxidized ores of the deposits considered is characterized by high fineness: the Lebedinoe deposit, 914–929; deposits of the Kuranakh type, 907–996; deposits of the folded region, 892–960. In the primary (slightly oxidized) ores of the Kuranakh type, the fineness of gold decreases to 946–880, and under the microscope, inclusions of electrum with a fineness of 590–600 occur in the general mass of high-fineness gold. In deposits of the folded region, many researchers note a widespread decrease in the fineness of gold with depth. Thus, at the Talalakh deposit, in the oxidized ores of the first mining level the fineness of gold is 960, while lower down, in the primary ores, it is 880.

To explain the mode of migration of gold, various scientific hypotheses may be used (mechanical, colloidal, haloidal, sulfate, electrochemical). The mine waters of Yakutia

are characterized by a high content of the sulfate ion. In the deposits of the folded region (Khangalass, Talalakh), its amount reaches 190,422 mg/l, and the highly mineralized, very acidic waters (pH decreases to 2.65) are, in terms of their chemical composition, defined as sulfate–iron waters. In the deposits of Central Aldan, upon emerging from the ore bodies into a carbonate environment, these waters are rapidly neutralized, and therefore, as a rule, waters of hydrocarbonate–calcium–magnesium composition with pH equal to 6.4–6.7 can be observed. Gold is found in the composition of the dry residues of mine waters.

The mechanical form of migration explains the formation of “pebbles” of limonites, the rolling of some gold particles in the subzone of secondary gold enrichment, with the formation of gaping cavities in the subzone of complete oxidation in the steeply dipping veins of the Lebediny deposit.

The principal condition for the migration of elements and the formation of the subzone of secondary gold enrichment is the presence of sulfides in the ore bodies. In the moderately sulfide ores of the Lebediny deposit they constitute 15–50% and often more; in deposits of the Kuranakh type, the primary ores contain up to 10% sulfides. Vein deposits of the folded region are assigned by all researchers to the low-sulfide gold–quartz formation and are characterized by a sulfide content of 1–3%. At the same time, the enclosing rocks, represented by sandy–clayey shales of the Verkhoyansk complex, are intensively sulfidized in the near-selvage part and contain up to 50% sulfides. The combination of widely developed banded and brecciated textures of vein bodies with post-ore crushing of the enclosing sulfidized rocks along the vein selvages was of great importance in the migration of metals in the oxidation zone and in the emergence of acidic mine waters of sulfate–iron composition (Khangalass deposit).

Thus, as a result of the investigations carried out and of the data from the operation of six vein deposits of the folded region, the Lebediny and Kuranakh deposits of Central Aldan in Yakutia, the presence in the oxidation zone of ore bodies of a subzone of secondary gold enrichment has been established. The latter penetrates to a depth of 50–100 m from the ground surface and is most distinctly developed along primary ore shoots.

The first indication of secondary gold enrichment of ore veins is the presence of iron hydroxides, which give the ores a bright coloration in brown and red tones. Redeployed limonites acquire special significance, convincingly proving migration under conditions of hypergenesis.

A decrease in the thickness of veins with depth, and consequently in the near-vein fracturing and permeability of mine waters, serves as a favorable condition for the formation of hypergene enrichment.

A vivid example is the presence of a secondary gold belt in the veins of the Lebediny deposit at the place where near-vein fracturing and vein thickness sharply decrease as they pass from Cambrian carbonate rocks into Archean granites; in karst cavities with clastic ore material above the underlying residual clays of Kuranakh-type deposits; above the lower boundary of thickness reduction in the veins of the folded region (Talalakh deposit, etc.). Hypergene gold enrichment also gravitates toward places where veins join downward along dip (Zhdannoe, etc.). And, finally, the zones of secondary enrichment are characterized by the presence of visible gold and comparatively large individual gold particles weighing up to 70 grams.

References

1. Al’bov M. N. Secondary zonation of gold-ore deposits of the Urals. Gosgeoltekhizdat, Moscow, 1960.

2. Volarovich G. P. and Rozhkov I. S. Gold-bearing provinces and regions of the USSR. In: Geology, patterns of distribution, and methods of studying gold deposits. Proceedings of TsNIGRI, issue 76, Moscow, 1967.

3. Myakov A. D. On the question of secondary zonation in gold-ore veins of the Mariinsk taiga (Kuznetsk Alatau). Proceedings of the Tomsk Order of the Red Banner of Labor Polytechnic Institute named after S. M. Kirov, vol. 138, Tomsk, 1965.

4. Nesterov N. V. Possible ways of increasing the extraction of ore gold in the Yakut ASSR. Abstracts of reports for the meeting of geologists of Yakutia on gold, Yakut Geological Administration, Yakutsk, 1967.

5. Petrovskaya N. V. Mineral associations in gold-ore deposits of the Soviet Union. In: Geology, patterns of distribution, and methods of studying gold deposits. Proceedings of TsNIGRI, issue 76, Moscow, 1967.

6. Razin L. V., Rozhkov I. S. Geochemistry of gold in the weathering crust and biosphere of gold-ore deposits of the Kuransk type. Publishing House “Nauka,” Moscow, 1966.

7. Shlygin E. D., Mukanov K. M., Grishin V. M., Magomedov S. G. “On the supergene concentration of gold at gold-ore deposits of Northern Kazakhstan.” Bulletin of the Academy of Sciences of the Kazakh SSR, No. 8, 1963.