FEATURES OF THE GEOLOGICAL STRUCTURE

AND PATTERNS OF DISTRIBUTION OF GOLD MINERALIZATION

IN THE BASIN OF THE MAMA RIVER

(NORTHWESTERN BAIKAL REGION)

V. A. BURYAK (Institute of the Earth’s Crust, Siberian Branch, USSR Academy of Sciences)

Small gold-bearing placers in the basin of the Mama River were discovered as early as the pre-revolutionary period. Their primary sources were considered to be very weakly gold-bearing quartz veins and veinlets developed in weakly metamorphosed sandy-shale strata of Proterozoic age. On the whole, the prospects of the district for gold, especially lode gold, until recently were assessed as rather limited.

The detailed prospecting and mapping work carried out in recent years by the Irkutsk Geological Administration makes it possible to substantially refine and alter these views. During 1963–1967 a large number of major primary gold occurrences were identified here for the first time, and in a number of cases increased gold content was established in alluvial deposits. The best-studied Mukodek ore occurrence was discovered in 1963 (Yu. A. Tumolsky et al.).

The district is composed of magmatic and sedimentary-metamorphic formations. The magmatic rocks are the most widespread and are represented by gabbroids and plagiogranites of the Lower Proterozoic Muya volcano-plutonic complex and by Paleozoic granitoids of the Konkudero-Mamakan intrusive complex. Outside the limits of development of the gold ore occurrences, in the zone of distribution of high-temperature metamorphic facies, there are granite-pegmatites and granite-gneisses of the Upper Proterozoic metamorphogenic autochthonous Mama-Oron complex [2]. All known gold ore occurrences characterized by elevated gold content are spatially confined to altered gabbroids. Quartz veins and zones of sulfide dissemination, developed no less frequently among sandy-shale strata, are distinguished, as a rule, by significantly lower gold contents and usually by a lower sulfide content.

The gabbroids are everywhere intensely schistose, up to orthoschists, and are crumpled into small folds. As a result, in them there are observed

...segregation-metamorphic banding and gneiss-like character are observed in the arrangement of dark- and light-colored minerals, as well as an intensive development of secondary minerals. The structure is gabbroic or gabbro-ophitic in unaltered and weakly altered varieties, and blastogabbroic, nematogranoblastic, and lepidogranoblastic in altered varieties.

The mineral composition of the gabbroids is, qualitatively, fairly uniform. In addition to plagioclase, which, as a rule, is entirely albitized and intensively replaced by secondary minerals (sossurite, sericite, epidote, carbonates), hornblende, biotite, and occasionally quartz are present. The content of the latter is usually insignificant and amounts to no more than 1–5%. The content of amphiboles and plagioclase varies within broad limits. There are areas both with a sharp predominance of amphibole, up to amphibolites, and with an insignificant amount of dark-colored minerals. Transitions between such leucocratic and melanocratic varieties are most often sharp, conformable with the bedding of the enclosing effusive-sedimentary sequence, and sometimes gradual. Monoclinic pyroxene (augite) occurs extremely rarely, and then only as relics. Accessory minerals are represented by magnetite, titanomagnetite, sphene, leucoxene, apatite, pyrite, and pyrrhotite. Magnetite and titanomagnetite are the most widespread. Their total content in the gabbroids, according to the results of mineralogical analyses, sometimes reaches weight concentrations. Pyrite and pyrrhotite are present as rare individual grains. Very rarely, melanocratic varieties of gabbroids were noted with increased dissemination of pyrrhotite, associated with an insignificant amount of chalcopyrite. The gabbroids occur as irregular lenticular and sheet-like bodies and participate in the formation of folded structures together with the enclosing Lower Proterozoic sedimentary-volcanogenic sequences [2]. A genetic connection of the gold mineralization with them has not been established.

Plagiogranites of the Muya complex are younger formations in comparison with the gabbroids considered. In plan and especially in section, the granite bodies have the form of intrabedded deposits, lenticular bodies, and complexly constructed migmatites, individual injection components of which are often characterized by very insignificant thickness (from several meters to 1–2 cm and less) and, in places of pinching out, split into numerous thin apophyses resembling “horse-tail” structures. A zonal structure is established for such migmatization zones and individual migmatite bodies. The outermost marginal parts are composed of intensely schistose, cataclased gabbroids with small plagioclase veinlets and lenticular segregations. Toward the central part of the migmatite bodies, the content of dark-colored minerals gradually decreases, while that of plagioclase, on the contrary, increases, up to the formation of almost monomineralic plagioclase zones in migmatites and separate lenticular bodies and veinlets in the gabbroids. Such formations, conditionally termed by us plagioclasites, may be practically devoid of quartz or contain it in insignificant amounts. In the “normal” Muya plagioclase granites, however, the quartz content is elevated and may reach 45% of the rock volume [2]. No spatial or genetic connection of gold mineralization with these granites is observed.

Paleozoic granitoids of the Konkudero-Mamakan complex are typical intrusive formations. Their massifs occupy...

occupy a cross-cutting position with respect to the gold-bearing zones and, in all probability, are of a younger age.

Sedimentary rocks have a subordinate distribution and are represented chiefly by weakly metamorphosed rocks of Upper Proterozoic age. In the character of their occurrence they are very close to the sedimentary sequence of the Patom Series, in the central part of the Lena gold-bearing district. Magnesian-iron carbonates (ankerite, siderite) and rare disseminations of cubic pyrite crystals are noted in them almost everywhere.

It is characteristic that, as in the neighboring Lena gold-bearing province, gold mineralization in the district under consideration is developed only in rocks of the greenschist facies of the Upper Proterozoic–Lower Paleozoic (?) epoch of regional metamorphism and ultrametamorphism, which caused the formation of the Mama–Oron metamorphogenic magmatic complex of granite-gneisses and muscovite-bearing granite-pegmatites. In zones where higher-temperature facies of this epoch of metamorphism are developed, confined—as in the Lena district—only to the “central” (axial) part of the regional Abchad deep fault [2] and controlled by this fault, gold mineralization is absent, being replaced first by barren quartz veins and then by mica-bearing pegmatites. Consequently, gold mineralization is located only at a certain considerable distance (tens of kilometers) from the axial zone of the deep fault. This establishes a regional zonality in the distribution of metamorphic facies, gold and other types of mineralization relative to deep faults, a zonality characteristic of many ancient gold-bearing provinces (Lena, Western, Northwestern and Southwestern Cisbaikalia, Southeastern Transbaikalia, Biryusa, Yenisei Ridge) [1], and makes it possible to approach more correctly the determination of the genesis of the mineralization.

Within the greenschist facies of metamorphism, the controlling structures of a smaller order in the distribution of gold-ore mineralization are zones of intense schistosity of sublatitudinal and, to a lesser extent, northeastern strike, subparallel to the main Abchad deep fault. It is characteristic that the principal gold-ore occurrences—Mukodekskoye, Pravomamskoye, Andykskoye, and Gorbylyakskoye—tend toward the junctions of these zones of intense schistosity. Moreover, the main ore-hosting structures are zones of sublatitudinal, rather than northeastern, strike. In the overwhelming majority of cases, gold-bearing quartz veins, zones of gold-sulfide dissemination, and hydrothermal metamorphism accompanying the gold mineralization have a sublatitudinal strike. In this as well, a certain analogy is manifested with the central part of the neighboring Lena gold-bearing district.

The distribution of gold and sulfides in the schistosity zones is uneven. The most gold-bearing are most often schistosity zones developed among hydrothermally altered gabbroids (green orthoschists), while the least gold-bearing are zones among leucocratic plagioclase Muya granitoids. In sandy-shaly sequences, weight contents of gold and increased pyrite dissemination are sometimes noted. It is not excluded that this fact could, to a considerable extent, have been caused by a different primary (initial) Clarke of gold in the rocks under consideration, as shown by Yu. G. Shcherbakov [4]. It is also significant that the gabbroids are most enriched in dark-colored iron-bearing minerals, which probably determines the development within them of elevated concentrations of hydrothermal pyrite, contributing to the precipitation of gold from solutions [3].

This is supported by the well-expressed bleaching of gabbroids in zones of intense hydrothermal alteration, up to the formation of albite–chlorite–sericite and albite–carbonate–sericite orthoschists, as well as by the presence of a direct relationship between the gold content of ore bodies and the intensity of hydrothermal alteration of the enclosing rocks. Quartz veins occurring among weakly altered gabbroids have the simplest mineral composition and are practically devoid of sulfides and gold.

Gold-bearing zones among gabbroids most often develop at the contacts of their different petrographic varieties of substantially leucocratic and melanocratic composition, or in gabbroids migmatized to varying degrees. Sheet-like bodies of plagioclasites (Muiskii granites), like ordinary dike formations, play the role of structural localizers of mineralization. At the same time, hydrothermal metamorphism and mineralization are superimposed both on the plagioclasites, which during hydrothermal metamorphism pass into albitites, and on the gabbroids enclosing them. However, elevated concentrations of sulfides and gold tend toward the altered gabbroids, and not toward the albitites and albitized granites.

This confinement of gold-bearing zones to contacts in gabbroids granitized to varying degrees, or to their petrographic varieties—especially to the contacts of plagioclasites and intensely schistose gabbroids—finds, in many cases, rather clear confirmation in ground and airborne magnetic surveys of various scales: gold-bearing zones developed among gabbroids are usually confined to the boundaries of positive and negative magnetic anomalies. Incidentally, the first Mukodekskoe ore occurrence in the area was identified during verification of an aeromagnetic anomaly in gabbroids, where, at their contact with plagioclasites, gold–sulfide mineralization was found.

Compositionally homogeneous, equigranular massive gabbroids are not very favorable for the development of schistosity zones and superimposed hydrothermal mineralization. Obviously, detailed mapping and subdivision of gabbroid massifs, in combination with aerogeophysical or ground geophysical work, with the aim of distinguishing mechanically the most heterogeneous areas, is absolutely necessary when conducting prospecting work for the type of gold mineralization under consideration.

Morphologically, the mineralization is represented by two types: vein and veinlet-disseminated. The vein type is the principal one and is usually characterized by the highest gold contents. The mineral composition of the veins is not constant and is determined by the composition and intensity of hydrothermal–metamorphic reworking of the enclosing rocks. In albitites the veins are usually albite–quartz or substantially quartz, with a small amount of albite and magnesian–ferruginous carbonates. In intensely schistose gabbroids, transformed into carbonate–sericite–chlorite orthoschists, the veins are carbonate–quartz or substantially quartz, with a small amount of magnesian–ferruginous carbonates, albite, and chlorite. The content of ore minerals rarely exceeds 4–5%, usually fractions of a percent or 1–2%. Most often pyrite is present in the form of cubic, predominantly large (0.3–0.8 cm) crystals or their aggregate irregular accumulations, tending toward the selvages of veins or toward relics of the original gabbroids (orthoschists). Chalcopyrite is sometimes noted in insignificant amounts, and very rarely galena and titanomagnetite (magnetite), formed, probably, at the expense of titanomagnetite.

of the enclosing gabbroids. Gold is usually present in insignificant amounts, and elevated contents are rarely observed. It is predominantly fine (0.001–0.1 mm, rarely up to 1.5 mm and more); the form of the gold particles is irregular, elongated, sometimes rounded, dendritic, or platy. The color is golden yellow. The gold is located along small fractures in pyrite, more rarely in quartz; sometimes it forms small isometric segregations in pyrite. A characteristically elevated gold content is found in monomineralic pyrite—up to 50–140 and more grams per ton.

Wall-rock alterations are usually well expressed and are manifested in albitization of the plagioclase of the enclosing gabbroids, the formation of magnesian-iron carbonates (ankerite, siderite), calcite, chlorite, sericite, epidote, and increased pyritization. The following metasomatic zoning is indicated. The central parts of the veins are almost entirely composed of ordinary grayish-white massive quartz; closer to the selvages albite is added (if the enclosing rocks contained plagioclases), which is gradually replaced by magnesian-iron carbonates (if, in the enclosing rocks, they are highly developed). At the same time, increased dissemination of pyrite and relics of the enclosing orthoschists appear. The type of the latter is determined by the original composition of the country rocks. In plagioclasites and in intensely granitized gabbroids these are sericite-albite, sericite-carbonate-albite, and chlorite-sericite-carbonate-albite orthoschists, gradually passing into albite and chlorite-albite orthoschists. In ordinary gabbroids, in contact with the vein, there are pyritized epidote-carbonate-chlorite, chlorite, sericite-chlorite-carbonate, and albite-sericite-carbonate orthoschists, passing, with increasing distance from the vein, into sericite-chlorite and then into weakly pyritized chlorite, epidote-chlorite, and, finally, amphibole-chlorite orthoschists.

The thickness of the wall-rock-altered rocks reaches several tens of meters, sometimes increasing to 100–200 meters and more. Such intensely altered rocks, with increased disseminated pyrite and closely spaced quartz, quartz-carbonate, and quartz-albite veinlets and irregularly shaped metasomatic formations, represent the second—veinlet-disseminated—type of mineralization. The gold content in such rocks is usually lower than in the veins; sometimes it reaches several tens of grams per ton. As in the quartz veins, elevated gold content is characteristic of monomineralic pyrite fractions.

The contacts of veins with the enclosing rocks in orthoschists, especially in altered gabbroids, are often indistinct and extremely uneven; in phyllites and siltstones they are usually clear and sharp. The form of the veins is irregular, lens-shaped, with frequent lens-shaped and irregular swellings and pinches. The thickness of individual veins varies widely, from several centimeters to 3–5 m and more; their strike length reaches several hundred meters, but is usually several tens of meters. The thickness of gold-bearing mineralized vein zones is usually several tens of meters, with a strike length of up to 1–1.5 kilometers and more.

On the basis of the foregoing, we note the following:

1. The controlling structures in the distribution of gold mineralization in the area under consideration are sublatitudinal, and to a lesser extent northeastern, zones of intense schistosity developed in rocks of the greenschist facies of the Upper Proterozoic—Lower Paleo-

of the Zoisk stage of regional metamorphism. The development of processes of regional metamorphism, ultrametamorphism and, probably, gold mineralization, as in the Lena gold-bearing province, is controlled by the Abchad zone of an intrageosynclinal deep fault of northeastern strike. This fault should evidently be regarded as a regional ore-controlling structure.

1. Other conditions being equal, the most favorable sites for the localization of mineralization are zones of sublatitudinal schistosity developed in intensely hydrothermally altered gabbroids. The position of these zones is controlled by the mechanical anisotropy of the gabbroid massifs.

2. The mineral composition of the vein bodies, the nature of their gold content, and the wall-rock alterations are closely dependent on the initial composition and on preceding hydrothermal processes of alteration of the host rocks.

3. The vein bodies are characterized by relict-banded textures, an absence of infilling textures, and a sufficiently clearly expressed stage-by-stage character in the development of mineral formation.

4. The gold is predominantly fine; in elevated amounts it is concentrated in pyrite.

5. The considerable dimensions of the mineralized zones and individual ore bodies, and the presence in them of elevated gold concentrations, indicate the possibility of discovering, in the area under consideration, gold ore deposits profitable for industrial exploitation. It is also advisable to carry out detailed prospecting in order to identify gold-bearing placers, especially of the ancient buried type.

REFERENCES

1. Buryak, V. A. Endogenous zoning of mineralization in ancient gold-bearing provinces of Siberia. DAN SSSR, vol. 173, No. 5, 1967.

2. Salop, L. I. Geology of the Baikal Mountain Region. Vol. II, Nedra Publishing House, 1967.

3. Sakharova, M. S. The role of electrochemical factors in the formation of hydrothermal gold deposits. In: Endogenous Ore Deposits, Nauka Publishing House, Moscow, 1968.

4. Shcherbakov, Yu. G. Distribution and Conditions of Gold Concentration in Ore Provinces. Nauka Publishing House, 1967.