Distribution Patterns and Some Compositional Features of Gold-Bearing Placers in Mountain Shoria

V. A. Saraev (TPI)

The basis for prospecting for gold-ore deposits in Mountain Shoria is the presence of gold-bearing placers, the morphology and composition of placer gold, and also the minerals accompanying gold both among the coarse-clastic material and in heavy-mineral concentrates. Below are considered the distribution patterns of gold placers, the principal features of the morphology and composition of gold particles, the minerals accompanying gold and, proceeding from this, certain criteria for prospecting for primary sources.

An analysis of the gold-bearing placers, ore occurrences, and deposits of Mountain Shoria shows that, at the present erosional level, it cannot be said that where rich gold-bearing placers occur there are no primary deposits, or that weak gold content in ore deposits may give rise to rich placers. An example is the Lebed skarn gold deposit and the placer formed at its expense, where the ratio of explored reserves in the placer to the explored reserves of ore bodies is approximately \(2/3\). Moreover, exceptionally unfavorable conditions for the preservation of the integrity of the ore deposit are observed there.

Brief Geological Characterization

Mountain Shoria is a mid-mountain region with intensely dissected relief. It is located at the junction of three regional structures: the Kuznetsk Alatau, the Gorny Altai, and the Western Sayan. The central part (see map) of Mountain Shoria is occupied by the Shor median massif, composed of intensely dislocated, predominantly carbonate (dolomites, limestones) deposits of Upper Proterozoic and Cambrian age. From the west and northwest the Shor massif is bordered by the Tashelgino-Kondoma mobile zone, and from the south by the North Abakan mobile zone. These zones are composed chiefly of volcanogenic formations of basic and intermediate (partly acidic) composition of Lower Cambrian age. The Tashelgino-Kondoma mobile zone separates from the massif, on the northwest, the Biya horst, and on the southwest, the Uimen-Lebed synclinorium. The synclinorium is composed of Lower Paleozoic volcanogenic-terrigenous, partly carbonate formations; the horst, of Lower Paleozoic volcanogenic-terrigenous strata, among which Precambrian carbonate rocks occur as tectonic wedges.

Fig. 1. Schematic geological map of Mountain Shoria with gold-bearing placers. 1. Jurassic terrigenous deposits. 2. Devonian red-bed terrigenous deposits. 3. Clastic rocks of Cambrian–Ordovician age. 4. Volcanogenic formations of acidic composition. 5. Porphyry formation. 6. Volcanogenic formations of basic composition. 7. Volcanogenic formations of mixed composition. 8. Carbonate deposits of Upper Proterozoic and Lower Cambrian age. 9. Syenites. 10. Granites. 11. Granodiorites. 12. Gabbro-diorites. 13. Diorites. 14. Gabbro. 15. Gabbro-diorite–plagiogranite intrusions. 16. Foliation. 17. Gold placers. 18. Disjunctive faults. Gold-bearing districts are designated by Roman numerals.

Intrusive magmatism was widely manifested in two epochs of tectogenesis: the Salair and the Hercynian. Both volcanism and the subsequent intrusive magmatism (by epochs) were most broadly manifested in the peripheral parts of the Shoria massif, in tectonically mobile belts. Granitoid intrusions acquire the chief importance; gabbroid and alkaline intrusions play a lesser role. A broad differentiation of intrusive complexes is distinctly expressed, as are processes of magmatic replacement and assimilation of volcanogenic–terrigenous deposits (S. S. Il’enok, 1962).

In the most mobile districts of Mountain Shoria, metamorphism became widespread. Cataclastic metamorphism (cataclasites, mylonites, phyllonites, phyllites) assumes primary importance, especially within the Tashelgino-Kondoma mobile zone. Manifestations of contact metamorphism and autometamorphism are quite common. Regional metamorphism is widespread everywhere. Among metasomatic formations, skarns, secondary quartzites, propylitized rocks (propylites) are widely developed; among vein formations—quartz, quartz-calcite, quartz-barite, and barite veins, and lens-shaped bodies with ore mineralization. To one degree or another, skarns, propylitized rocks (propylites), and secondary quartzites in Mountain Shoria are related to one another both spatially and genetically. Gold-bearing placers and, consequently, gold mineralization occur in the fields of development of the above-noted ore-forming formations and vein hydrothermal formations.

Distribution Patterns

The gold-bearing placers of Mountain Shoria are confined exclusively to the peripheral parts of the Shoria median massif (Fig. 1). The maximum number of placers is located within the Tashelgino-Kondoma mobile zone (Taenzinsky—VI, Suetinsky—III, Spassky—VIII, Dzhelsaisky—IX, Saizaksky—X, Malo-Kondomsky—XII, Chanyshsky—XIV, Talonsky—XV gold-bearing districts), then in the Uimen-Lebed synclinorium (Munzha-Kourinsky—IX, Atlinsky—XIII, Albassky—XV districts), in the North Abakan mobile zone (Talonsky—XVI, Kamzassky—XVII, Verkh-Mrassky—XVIII), and in the area of the Biya horst (Kazsky—I, Tazsky—II, Verkh-Mundybashsky—V). In the northeastern part of the Shoria massif two gold-bearing nodes are distinguished: Syor-Synzassky—VII and Petropavlovsky—IV. Certain regularities appear in the distribution of the gold-bearing placers of Mountain Shoria. The placers are observed to be confined to areas of:

1) apical and near-contact parts of diorite and granodiorite, complex gabbro-diorite–plagiogranite intrusions, with distinct traces of magmatic replacement and assimilation of terrigenous–volcanogenic formations (I, II, XV, XVIII);

2) zones of crumpling of long-lived (Lower Cambrian—Jurassic) faults both at the contacts of volcanogenic and carbonate sequences (III, VI, VII) and among volcanogenic formations (IX, XI);

3) strongly dislocated volcanogenic rocks of basic and intermediate composition with signs of propylitization (IX, XI);

4) fields of development of Devonian and Jurassic terrigenous sequences (IX, XIII, XV).

The richest gold-bearing districts are located in areas of conjugation of long-lived fault zones (XVI, XI).

The Shoria median massif is characterized by very weak gold content, with the exception of the Syor-Synzassky and Petropavlovsky zo-

gold-bearing nodes. Within the area of the massif, heavy-mineral sampling by the Shalym expedition of ZSGU and prospecting work by the Altai mine revealed minor placers and isolated gold grains in the alluvial deposits of the Mrassu River system. Heavy-mineral gold is confined to tectonic zones of crushing at the contacts of silicate and carbonate rocks. The Tashelga–Kondoma mobile zone is apparently characterized by a multiplicity and diversity of genetic types of primary sources of placer gold. The placers are confined chiefly to tectonic zones of crushing at the contacts of volcanogenic and carbonate rocks, to areas where intensely dislocated volcanogenic rocks of basic and intermediate composition are developed, to the near-contact parts of granitoid intrusions, and also to areas of junction of the noted tectono-magmatic structures.

In the Uymen–Lebed synclinorium, gold-bearing placers are confined to fault zones and to intensely dislocated volcanogenic formations, as well as to areas where Devonian red-colored terrigenous strata are developed.

The Biya horst and the North Abakan mobile zone are characterized by the development of gold-bearing placers within the apical and near-contact parts of granitoid intrusions.

Morphology and Composition of Placer Gold

The size, roundness, shape, and composition of gold grains are important factors in determining the location and, to some extent, the genetic type of the primary source. It is generally known that, with increasing distance from ore bodies, the coarseness of gold grains decreases as a result of their abrasion and electrochemical corrosion. However, these factors alone cannot explain the predominant prevalence of one or another size of gold grains in certain placers of Mountain Shoria.

The following features are observed. Fine gold (predominantly up to 2 mm) occurs in placers confined chiefly to fields of development of red-colored terrigenous deposits of Devonian age (rivers Chulta, Chuguna—XIII), to the apical parts of Salair granitoid intrusions (rivers Mrassu—XVII, Pushta, Sadra—XV).

Apparently, this can be explained by subsequent abrasion of the gold grains during the erosion of ancient placers in Devonian conglomerates and by the considerable vertical displacement of gold in fields of development of granitoid intrusions.

Another feature is that gold grains of larger sizes (more than 4 mm across) occur in placers confined to tectonic zones of crushing at the contacts of volcanogenic and carbonate rocks (rivers Aptaza—VI, Kaurchak—XVI, B. Sueta—III, tributaries of the Koura River—XI), to areas of development of intensely dislocated effusive-pyroclastic formations (rivers Aleksandrovka, Lyapinka—IX, Adiaksu, Sygzas—XVII), and also to zones of junction of intrusive and tectonic contacts (rivers M. Kondoma—XII, Soldatka—VIII). Such coarsening of the gold grains can be explained by the mobilization and redistribution of the metal in gold-ore bodies as a result of thermal and dynamothermal metamorphism, which is confirmed by the experimental data of V. G. Moiseenko (1965). Quite often, coarse gold (together with a large amount of fine gold) is observed in the near-contact parts of granitoid intrusions (rivers Talon—XVII, Kaz—I, Chanysh and Kamzas districts—XIV, XVII).

Gold nuggets (often intergrown with quartz) are fairly common in the placers of Mountain Shoria.

Almost all nuggets have been found in the upper parts of placers or in places where colluvial deposits have been eroded. The usual weight of nuggets ranges from several grams to 10–50 g; a considerable number have been found weighing up to 300 g, and rarely 3–5 kg. The largest nugget in Mountain Shoria was found in the bed of the Vorontsovskii creek (Chanysh district) and weighed 22.9 kg.

N. V. Petrovskaya (1947) notes a connection between platy and lump-like gold and quartz veins, between spongy and drusy forms and carbonate rocks, and between wire and droplet forms and polymetallic deposits. In the broadest terms, a similar dependence is also established in Mountain Shoria, but there is no reliable interrelation.

Platy (flat, rounded, thick) forms of gold particles are the most characteristic in the district. However, alongside these there occur irregular, hook-shaped, granular; porous (Bazas River), lentil-shaped and spherical (M. Kondoma River), and lump-like (Chanysh district and the headwaters of the Mrassa River) gold particles. Within the Lebedsk gold-skarn deposit, gold particles in the form of octahedra were found, and along Konyukhovskii creek (a tributary of the Chanysh River) in the form of wire up to 8 cm long. The placers of the Kondoma River are distinguished by the greatest variety of gold-particle forms.

In Mountain Shoria five principal morphological types of gold particles are distinguished: “rocky,” unrounded, semi-rounded or weakly rounded, rounded isometric forms, and rounded discoid and scaly forms. For purposes of prospecting for primary sources, the first three types of gold particles are the most important. Rounded gold particles, however, are characteristic of the lower and middle parts of gold-bearing valleys. “Rocky” gold is found within nearly all placers of Mountain Shoria. Three kinds of such gold are observed:

1. Fairly well-rounded pebbles of quartz with disseminated gold; 2. Unrounded fragments of quartz, barite, magnetite, garnet skarns, and brown ironstone with gold; 3. Unrounded and semi-rounded gold particles of irregular form with inclusions of grains of quartz, garnets, barite, magnetite, and limonite.

Intergrowths of gold with quartz occur in most placers of the district; intergrowths with barite are known in the headwaters of the Chanysh River; with magnetite and garnets—in the area of the Lebedsk deposit, Karagol creek (Kondoma River), and the Taza River; with brown ironstone—in the area of the Petropavlovka, Synzas, Saizak, and Kaurchak rivers. Unrounded intergrowths of gold with ore-bearing rocks and their minerals directly indicate the proximity of a primary source and are situated in the field of development of deluvial-eluvial and alluvial formations.

Unrounded gold (always with weakly rounded gold) occurs near primary sources. In general, it is found within tectonic crumpling zones at the contacts of volcanogenic and carbonate rocks (Bolshaya Rechka, Bolshaya Sueta, Saizak, Bazas, Synzas rivers), and also in the near-contact parts of granitoid intrusions (Chanysh, Taz, and Kamzas rivers, lower course of the Kaurchak River).

The gold of most placers of Mountain Shoria is of high fineness. Its fineness generally ranges within 800–920. The color is reddish-yellow, dark yellow, and sometimes light yellow. In some districts, especially in zones of post-Silurian thrust dislocations, the gold is low-fineness, brass-yellow or silvery in color. The lowest-fineness gold and electrum are present in the placers of the Saizak group of placers; less low-fineness gold occurs in the Syur-Synzas and Dzhelsai districts (fineness 550–850). In addition, silvery low-fineness gold is known in the placers of the Tikhaya River (a tributary of the Telbes River), Karagol creek (Kondoma River), and the left tributaries of the Bolshaya Sueta River.

According to Yu. G. Shcherbakov (1958), using as an example the study of the Sor-Synzass, Saizak, and Dzhelsai groups of placers, the principal difference in the composition of the gold of each placer is expressed in the silver content. Some of the impurities are undoubtedly lost by the gold during diffusion and self-refining. Yu. G. Shcherbakov showed a relatively higher content of impurities of copper, lead, zinc, antimony, nickel, cobalt, and arsenic in the gold of more ancient deposits, and their relative poverty in the younger ones. He also established that the ratio Au/Ag—the highest for the gold of the most ancient deposits—ranges from 9/1 to 7/3 and decreases to 3/7 in deposits of the Saizak group.

Associated Minerals of Gold in Placers

Below are considered those associated minerals of gold whose significance for elucidating the genesis of gold mineralization, as well as its location, has been definitely established. The associated minerals of heavy-concentrate gold among the tailings are: limonites, products of destruction of oxidation zones and karsts (sponges, variegated clays of the weathering crust, enriched with iron hydrates, ferruginized karst breccias), skarns of complex composition, sulfidized propylitized rocks, secondary quartzites with sulfide mineralization (ferruginized), magnetite, barite, quartz; heavy-concentrate minerals: tetradymite, bismuthine, bismuthinite, chalcopyrite, pyrite, antimonite, arsenopyrite, magnetite, cinnabar, copper minerals in the oxidation zone and native copper, scheelite, platinum and platinoids, barite, tourmaline, and skarn minerals with signs of recrystallization and deformation (garnets, epidote, pyroxenes, wollastonite).

Let us dwell on the characteristics of only some associated minerals of gold.

1. Fragments of vein quartz are encountered in almost all gold placers. It has been noted that there is no relationship between the quantity of quartz fragments in the alluvium and the gold content of the valleys. A large number of analyses have established a very weak gold content in quartz fragments; nevertheless, in many placers of Mountain Shoria, intergrowths of quartz with gold are encountered. Evidently, ore quartz, as the least physically stable, is rapidly destroyed; therefore its fragments are few in placers formed through the destruction of gold-ore quartz veins, whereas barren quartz, as the most stable of all ore-hosting formations, is preserved in the alluvium in the form of fragments and boulders. In the intergrowths there is white, yellowish, smoky, grayish, and sometimes transparent drusy quartz. These are mainly small fragments (up to 3–4 cm), rounded, semi-rounded, and angular.

Gold mineralization in Mountain Shoria is to a greater degree associated with the quartz-sulfide stage of mineralization; therefore quartz tailings are an important prospecting indicator.

1. In placers situated in the near-contact parts of granitoid intrusions, fragments of skarns and magnetite are encountered fairly often. Because skarns and magnetite, owing to their physical instability, disappear rather rapidly along the extent of the placer (the first tens of meters to 100–200 m from skarn bodies), finds of fragments of magnetite and skarns, especially of complex composition, taking into account other prospecting indicators, are of exceptional interest for the search for ore gold.

2. Secondary quartzites are no less widespread in Mountain Shoria than skarns. Gold mineralization of a whole series of districts is associated with secondary quartzites of the monoquartz, quartz-sericite, and quartz-kaolinite facies, with an abundance of iron and copper sulfides (the Synzas, Sor, Soldatka rivers, the area of Mount Uronnik near the Lebed deposit, etc.). In tectonic zones, secondary quartzites are ...

good collectors of auriferous solutions. Within the zone of development of secondary quartzites, there is an abundance of iron hats, sponges, sometimes with visible gold.

1. Propylitized rocks are known in Mountain Shoria among volcanogenic formations of basic and intermediate composition. Deposits associated with propylites have not yet been discovered, but, judging from the mineralogical composition of the concentrates and the manifestations of propylitization phenomena with gold mineralization in the area of Mt. Uronnik, one may confidently speak of the presence of gold deposits associated with propylites in the volcanic tectonically mobile zones of Mountain Shoria.

2. The usual companion of gold in the placers of the area is abundant iron hydrates, adsorbed by clays and loams. The latter, near disintegrating oxidation zones (the basins of the Koura, Saizak, Synzas, and Kaurchak rivers), acquire a characteristic rusty-brown or brownish-red color. Of great prospecting significance are the ferruginized loose masses of Meso-Cenozoic karsts developing in tectonic zones at the contacts of volcanogenic and carbonate rocks, and, apparently, these masses may serve as gold ore.

Conclusion

On the basis of the features of the distribution of auriferous placers, the morphology and composition of the gold and, chiefly, its accessory minerals, it appears to us that the principal ore-forming formations of gold in Mountain Shoria are skarns, propylites, secondary quartzites, and vein formations of quartz, quartz-calcite, and barite composition, which are interconnected to one degree or another both genetically and spatially.

With a sufficiently high degree of reliability, the location of the primary source of placer gold and its genetic type can be determined if, during prospecting, all geological factors, the conditions of distribution of heavy-mineral gold, and also the accessory minerals of gold both among coarse-clastic material and in heavy-mineral concentrates are taken into account.

The present work is based on the study by the author, in 1964–1965, of data from heavy-mineral sampling carried out mainly by parties of the Shalym Expedition of ZSGU, on the results of exploitation and exploration work of the Altai mine of the “Zapsibzoloto” trust, and also on data from geological prospecting work for ore gold in the upper reaches of the Lebed River with the author’s participation. Data from numerous researchers of Mountain Shoria were used [2, 3, 6, 7].

The author expresses deep gratitude to G. A. Safonov, chief geologist of the Altai mine, and to L. F. Perelitsyna, geologist of the Shalym Expedition, for assistance in selecting materials and discussing certain questions. The author expresses special appreciation to S. S. Ilyenok, head of the Department of Petrography of the Tomsk Polytechnic Institute, for consultations and critical review of the article.

References

1. Ilyenok S. S. On the question of the genetic connection of contact iron-ore mineralization with intrusions. Izvestiya Tomskogo Politekhn. In-ta, vol. 120, 1962.

2. Kazakevich Yu. P. Principal features of the gold content of the basin of the Lebed River in the NE Altai. OF APU, 1951.

3. Kuzmin A. M. Conditions of gold content in Mountain Shoria. OF ZSGU, 1940.

4. Moiseenko V. G. Metamorphism of Gold from Deposits of the Priamurye. Khabarovsk Book Publishing House, 1965.

5. Petrovskaya N. V. Morphological and structural features of native gold. In: Collected Materials on the Geology of Gold and Platinum, issue 3, 1947.

6. Potemkin K. V., Postnikov E. V. Report on the work of the Gorno-Shoria party in 1942–1944. OF APU.

7. Shcherbakov Yu. G. Conditions of gold content in the SE part of the Kuznetsk Alatau. Dissertation for the degree of Candidate of Geological-Mineralogical Sciences. Tomsk, 1958.