GEOLOGY AND METAL CONTENT OF THE LEBED SKARN ZONE

(GORNAYA SHORIA)

V. A. Saraev (TPI)

The present article briefly sets forth the geology and metal content of the Lebed skarn zone, in the northern part of which the Lebed gold-ore deposit has been discovered. In connection with the growing interest of geologists [1, 2, 3] in skarns as one of the most probable gold-hosting rocks in gold-bearing districts, the author gives a characterization of the geology and gold content of one of the skarn fields of Gornaya Shoria.

The Lebed chain of magnetic anomalies was identified in 1948 (P. S. Uskov, V. I. Mineev), and in 1953 the first magnetite bodies were exposed within the Lebed skarn zone (R. A. Pipar); their exploration was carried out by the Shalym expedition of the ZSGU in 1954–1956 (E. S. Shinkarev, A. M. Shinkareva). In 1958 L. K. Batalov and N. E. Bevzenko discovered gold-bearing skarns within the Kaurchak magnetite deposit (the first ore body). In 1959–1965 the Shalym expedition carried out prospecting and mapping work in the area of the Lebed skarn zone (V. A. Saraev), as a result of which the II, III, and IV gold-ore bodies were identified; operational-exploration work was conducted by the Altai mine office (G. A. Safonov). From 1962 to the present, a group of geologists from the Tomsk Polytechnic Institute (S. S. Ilenok, V. I. Yarygin, N. F. Stolbova, D. G. Berlimble) has been carrying out research on magmatic complexes and the conditions of localization of gold mineralization in the area of the Lebed deposit.

The Lebed skarn zone is situated in the southwestern part of Gornaya Shoria, in the basin of the upper course of the Lebed River. The district is composed of sedimentary-volcanogenic formations of the Lower Cambrian, which are intruded and metamorphosed by the Lebed gabbro-plagiogranite massif—$Cm_{2-3}$ (V. I. Yarygin, 1966). On the eroded surface of the intrusion and of the sedimentary-volcanogenic rocks lie the red-colored deposits of the Andoba Formation of the Middle Devonian (sandstones, siltstones, conglomerates). The sedimentary-volcanogenic sequence (at the level of the Mrasskaya Formation—$Cm^2_1\, mr_1$) is represented by plagioclase and plagioclase-pyroxene porphyrites and their tuffs, among which there occur discontinuous, low-thickness lens-shaped bodies of marbles and sandstones. These rocks are folded into rather narrow folds of northwestern strike, but in the general plan the sequence is traceable in a northeastern direction, where beyond the limits of the area (approximately 2–3 km to the NE of the northern end of the Lebed zone) the effusive-tuffogenic rocks are replaced by formations-

... of the near-ore zone of the Lower Cambrian volcano of Mount Uronnik of the central type.

The Lebed massif within the zone is represented by its northeastern part, up to 2 km wide, composed of granodiorites, diorites, quartz monzonites, granosyenites, gabbro-diorites, and gabbro. The marginal facies are represented by melanocratic varieties. The contacts with the enclosing rocks are complex and reflect the folded structure of the sedimentary-volcanogenic sequence. The dike complex is very poor and is represented by diabases, diorite porphyrites, and syenite-porphyries of the post-skarn period of formation.

The complexity of the tectonic structure of the area is due to the confinement of the latter to the Tashelgino-Kondoma mobile zone, which passes along the junction of two large regional structures: the Shor median massif and the Uimen-Lebed synclinorium.

The Lebed skarn zone has been traced from the valley of the Lebed River to the right bank of the Kaurchak River (Fig. 1). Skarn bodies are localized both in the endo- and exocontact parts of the intrusion.

Plagioclase-pyroxene porphyrites, their tuffs, marbles, and rocks of the marginal facies of the intrusion are subject to skarnification. The total length of the zone is more than 5 km, with a width from 100 to 1300 m. According to the features of geological structure, geomorphology, and material composition of the skarns, the Lebed zone is subdivided into three parts: Southern—I, Watershed—II, and Northern—III. The zone is controlled by the roof of the Lebed gabbro-plagiogranite massif. By erosional truncation it is exposed vertically from 170 to 240 m. The general trend of the zone is northeastern (azimuth of strike 35–40°). Individual skarn bodies have a NW–NE orientation (300–360–50°). In the Southern part of the zone, some bodies have a sublatitudinal strike, thus being transverse to the general direction of the intrusion contact.

The Southern part of the skarn zone is located on the right side of the valley of the Lebed River at hypsometric elevations of 460–700 m, in the apical part of the Lebed intrusion. The dimensions of the exposed skarnification zone are 1.1 × 1.3 km, depth ~320 m. The zone is elongated in a northwestern direction and consists of a series of skarn bodies. Individual skarn bodies have different strikes, which vary within 300–360°. The dip of the skarn bodies is mainly northeastern, partly southwestern, with dip angles from 40 to 75°.

At the southern end of the zone, a trough-like structure is outlined, measuring 250 × 160 m, composed mostly of wollastonite and garnet skarns and their transitional varieties. At the contacts of wollastonite and garnet skarns within the trough-like deposit, calcite medium- to coarse-crystalline rocks develop (up to 5–6 m thick) with inclusions of quartz, wollastonite, and garnet, with dense disseminated copper sulfides, sometimes with veinlets up to 2–5 cm thick of bornite-chalcocite-chalcopyrite composition.

The Southern part of the zone is composed (from the periphery to the center) of hornfelsed and skarnified porphyrites, skarns (garnet, garnet-pyroxene, wollastonite, amphibole-garnet, magnetite-garnet). In the peripheral parts of the zone there occur small bodies of marble, amphibole rocks, and chlorite-carbonate rocks; in the center, small apophyses of skarnified granitoid rocks. A number of quartzized zones, nests, and quartz veins of insignificant thickness and strike are observed.

The Watershed part is located at the head of the watershed of the Lebed and Kaurchak rivers. The hypsometric elevations of the skarn outcrops are 580–700 m. Here, at distances of 50–200 m from one another, six skar-

Fig. 1. Geological map of the Lebed skarn zone (schematized). Compiled by V. A. Saraev.
1 — Devonian variegated clastic rocks; 2 — intrusive rocks of the Lebed pluton; 3 — gabbro-diorites; 4 — plagioclase, pyroxene–plagioclase porphyrites and their tuffs; 5 — marbles; 6 — gneiss-like hybrid-metasomatic formations; 7 — skarnized rocks; 8 — skarns; 9 — contact-altered porphyrites and their tuffs; 10 — gold-bearing placer; 11 — disjunctive faults; 12 — geological boundaries.

new bodies elongated in the NE and submeridional directions. The dimensions of the skarn bodies are 200 × 70—60 × 15 m. The skarns are located among peculiar gneiss-like hybrid-metasomatic formations, diopside hornfelses, and skarnized and hornfelsed porphyrites. All the bodies are composed of garnet-pyroxene, garnet fine- to medium-grained skarns. In the northeastern skarn body, a small lens-shaped body of magnetites measuring 30 × 15 m has been exposed.

The northern part of the Lebed zone is situated almost symmetrically along both banks of the Kaurchak River, extending for a distance of 1.7 km in a NE direction with a width ranging from several tens of meters to 600 m. The hypsometric elevations of the skarn outcrops at the surface are within 460—640 m. This part of the zone consists of a main skarn body measuring 1000 × 250 m and a number of small bodies ranging in size from 200 × 40 to 60 × 5—10 m and smaller.

The main skarn body, according to borehole data, is traced down to the +380 m level. This body is entirely located in the endocontact of the intrusion and in general has the form of an elongated trough, gradually plunging and widening southward. The thickness of the skarns, depending on the erosional cut and the structural features of the body, varies from 10 to 120 m. In section along the long axis, the skarn deposit has a trough-like form complicated by diagonal-transverse narrow asymmetric folds. More rarely, monoclinal dips to the east or west at angles of 40—50° are observed.

Such a structure of the skarn deposit is caused by the intrusion of the peripheral parts of the intrusion in accordance with the structures of the enclosing rocks existing at that time and with prototectonics. In this part of the zone, rather large magnetite bodies have been exposed, which emphasize the folded structure of the skarns.

A gold-ore body is confined to one of the synclinal folds (right bank of the Kaurchak River). At the base of the section of the bowl-shaped gold-ore deposit lie altered intrusive rocks; above them are skarns of various composition (garnet, pyroxene, garnet-amphibole, and their transitional varieties), then a sheet-like body of magnetite up to 30 m thick, which is replaced by substantially garnet skarns. In the core of the fold occur garnet-pyroxene, garnet-magnetite skarns and small nests and lenses of magnetite.

The form of the gold mineralization, with an extremely complex outline, is in general determined by the bowl-shaped form of the magnetite deposit underlying the gold mineralization. The most enriched sectors are confined to the body of skarns overlying the magnetites, as well as to places where fractures of different orientation are linked, chiefly northwestern (strike azimuth 330°) and sublatitudinal directions.

The northern end of the skarn zone is located in a tectonic crumpling zone of almost meridional direction with a westward dip at angles of 75—85° and is traced in the zone of direct contact of the intrusion with the sedimentary-volcanogenic sequence. In this part of the zone, three gold-ore bodies have been exposed. The structure of ore body II is analogous to the structure of ore body I. Its distinguishing feature is that on the eastern side it is truncated by a disjunctive fault (crumpling zone) of post-skarn, probably post-Devonian, time of formation. Ore bodies III and IV are stockwork-like quartz-gold-pyrite-chalcopyrite mineralization in the exocontact of the Lebed intrusion in intensely cataclased, mylonitized garnet, garnet-pyroxene, garnet-epidote skarns. The gold mineralization is confined to a skarn body elongated in a submeridional direction and located in a tectonic crumpling zone. Along the flanks of the skarn body are traced—

are intensely metamorphosed, in places transformed into epidote–chlorite–actinolite rocks, and porphyrites.

Disjunctive tectonics within the skarn zone is complex and multiphase. The emergence of disjunctives of submeridional trend and the subordinate sublatitudinal and NW-trending fractures accompanying them is probably due to the prototectonics of the volcanogenic formations and of the Lebed massif; their renewal and the development of new additional fractures ended only in the post-Lebed time (Fig. 1). Tectonic disturbances of the post-skarn period of formation are mineralized with epidote, quartz, calcite, barite, pyrite, chalcopyrite, and others (in the form of veinlets, nests, inclusions, disseminations, and thin veins); a significant number of disjunctives without any mineralization occur (mylonites, cataclasites, tectonic clay). The disjunctives are of thrust-fault, normal-fault, and strike-slip character.

Of practical value in the skarn zone is gold-ore mineralization, localized mainly in the northern part of the skarn zone, where magnetite mineralization attained its greatest development.

The following mineralogical–morphological varieties of gold-ore mineralization may be distinguished.

a. Disseminated gold–pyrite–arsenopyrite–chalcopyrite mineralization in skarns of complex composition, skarned porphyrites of the volcanogenic sequence, porphyrite dikes, and altered diorites (ore bodies I and II).

b. Disseminated and veinlet-disseminated gold–tetradymite–chalcopyrite–arsenopyrite mineralization in recrystallized and cataclased garnet–magnetite, magnetite, garnet–amphibole, and garnet skarns. Along with dissemination, the ore minerals form small, randomly oriented veinlets from 1 mm to 3 cm thick and up to 20–30 cm long (ore body I).

c. Disseminated stockwork-like quartz–gold–sulfide mineralization is manifested in ore bodies III, IV, and partly in the southern part of the skarn zone. This type of mineralization, the latest in time of localization, is observed in tectonic zones of crumpling, among garnet, epidote–garnet, and garnet–wollastonite skarns.

d. Vein quartz–gold–sulfide mineralization is weakly manifested in the skarn zone.

The greatest practical value is represented by recrystallized and cataclased medium- and coarse-grained garnet–magnetite, magnetite, garnet–amphibole, and magnetite skarns with gold–tetradymite–chalcopyrite–arsenopyrite mineralization, as well as mylonitized, cataclased medium- and fine-grained (up to massive) garnet, garnet–epidote, and garnet–pyroxene skarns with gold–pyrite–chalcopyrite–quartz mineralization.

In conclusion, it should be noted that gold mineralization is probably genetically connected with the formation of the Lebed gabbro–plagiogranite massif. Fracture structures, together with the petrographic–mineralogical composition of the gold-bearing rocks and with the quantity and pathways of circulation of solutions, were among the decisive factors in the distribution and structure of the gold-ore bodies.

REFERENCES

1. Vasil’ev B. D. On the geology and metal content of the skarns of the Bol’shaya Natal’evka area. Izvestiya Tomskogo politekh. in-ta, vol. 120, 1962.

2. Shakhov F. N. Geology of Vein Deposits. Publishing House “Nauka,” 1964.

3. Shcherbakov Yu. G. Some regularities of gold mineralization at the Sinyukhin deposit. Geologiya i geofizika, No. 2, 1961.

4. Yarygin V. I. On the petrography of the Lebed gabbro–plagiogranite massif. Abstracts of reports of the Novosibirsk Conference of Young Scientists and Specialists. Novosibirsk, 1966.