Common Features of Gold Mineralization Zoning: Examples from Selected Districts of the Kuznetsk Alatau and Central Transbaikalia

B. D. Vasil'ev, G. V. Shubin (TPI)

In recent years, increased attention has been paid to questions of the zoning of ore deposits and districts in connection with the elucidation of regularities in the formation and distribution of mineralization. The regularities established within one district or another make it possible to conduct prospecting for new deposits on a more substantiated basis and to evaluate the prospects of already known ones. The conditions governing the formation of deposits in specific districts are distinctive; however, the geological materials now being accumulated already make it possible to raise the question of the existence of general regularities in the formation and distribution of mineralization in regions of different ages. This question is considered below through a comparison of the identified regularities in the distribution of gold mineralization in specific districts of the Kuznetsk Alatau and Central Transbaikalia.

In the structure of the northeastern part of the Kuznetsk Alatau, two structural levels are clearly distinguished. The first structural level—the geosynclinal-folded one—is represented by geosynclinal sedimentary and volcanogenic formations of the Late Precambrian and the Lower and Middle Cambrian, which are gathered into tight folds of submeridional strike and are cut by large intrusive massifs of a “variegated-composition” batholith formation.

The second structural level—the transitional one—is composed of terrigenous and volcanogenic formations of a transitional class and is subdivided into two sublevels separated by a major angular unconformity. The lower sublevel is represented by red-colored molasses of the Koblenzian, preserved mainly in grabens, where their thickness reaches 2 km. The upper sublevel is represented by a volcanogenic formation of complex composition of the Eifelian (?), lying both on the red-colored molasses of the Koblenzian and directly on various formations of the more ancient geosynclinal-folded level, and in different parts of the district having an irregular thickness on the order of 1–2 km. The structure of the second level is folded-block: along with horsts and grabens 10–20 km wide, the structure of the level contains broad open folds with dip angles of 30–40°. Near the boundaries of large blocks the folds become smaller and more compressed, even overturned.

The formation of the volcanogenic Eifelian sequence is associated in time with the establishment, in the northeastern part of the Kuznetsk Alatau, of the multiphase gabbro–alkali-syenite Kiisk complex [3], the intrusive bodies of which are integral structural elements of the second level. However, most of them at the present erosional

in the section it is situated within the limits of the more ancient geosynclinal-folded complex. The elongated bodies of gabbroids of phase I of the Kiya complex are emplaced along narrow zones of north-northwestern and sublatitudinal strike, marking deep faults, partly concealed. The later phases of the complex are represented by typical fracture bodies of alkaline syenites (northeastern strike), nepheline syenites (northwestern strike), microgranites, and aplite-pegmatites (sublatitudinal strike), which create a complex “network.”

Gold-ore deposits and ore occurrences in the northeastern part of the Kuznetsk Alatau belong to the gold-arsenic and gold-bismuth-copper formations. Genetically, they are associated with the post-batholithic Kiya complex of the Eifelian and are spatially controlled by the zones of emplacement of phase-I gabbroids of this complex, although the mineralization is clearly younger than the alkaline syenites and even the microgranites [1, 2].

Along the zones of localization of phase-I gabbroids of the Kiya complex, on a regional scale, horizontal zoning is distinctly manifested in the distribution of hydrothermal formations of different types, but not with respect to individual intrusive bodies; rather, it is relative to the axis of the Devonian syncline. This horizontal zoning is expressed in the successive replacement (with increasing distance from the axis of the syncline) not only of mineral associations but also of ore-localizing structures. Thus, crushing zones with polymetallic mineralization are replaced by irregularly shaped metasomatic bodies with gold-bismuth-copper mineralization and, further, by systems of quartz veins with gold-arsenic mineralization.

Taking into account the zonal arrangement of different types of mineralization in relation to the folded structure of the Devonian stage, and reconstructing the position of the land surface or at least the base of the Devonian volcanogenic sequence at the time of formation of the deposits, it became clear that the existing horizontal zoning is a reflection, in the present erosional section, of vertical facies (depth) zoning of deposition and depends on the depth of formation of the deposits [2]. Thus, deposits of the gold-arsenic formation are localized as persistent suites of quartz veins at depths of 1.5–2.5 km below the base of the volcanogenic Devonian, whereas deposits of the gold-bismuth-copper formation, in the form of metasomatic lenticular bodies of quartz-carbonate composition, occur respectively at depths of only 0.8–1.0 km below this base. Gold mineralization is replaced upward by polymetallic mineralization, localized as thick crushing zones cemented mainly by carbonates and cutting not only the underlying Devonian but also the Devonian formations proper.

In Central Transbaikalia there are also two structural stages, the lower of which—the geosynclinal-folded stage—is represented by metamorphic, dislocated formations, including Permian sandy-shaly sequences intruded by granitic batholiths of the Triassic (?), which have a large areal distribution. The upper structural stage reflects the transitional stage in the development of the region under continental conditions and is represented by terrigenous and volcanogenic Mesozoic formations that accumulated in depressions and graben-like structures associated with the development of the Daurian domal uplift. In the same period, in connection with the continuing development of deep faults, Upper Jurassic magmatism was manifested, represented by a series of dike rocks of acidic and intermediate composition, developed chiefly along zones of increased fracturing that mark deep faults.

Genetically associated with the post-batholithic Upper Jurassic dike complex of intrusive rocks of intermediate composition in Central Transbaikalia, within the Tura–Ilya and Upper Onon groups of gold-ore deposits, is mineralization of the gold–arsenic and gold–bismuth formations. The Ilyinskoye, Dybyksinskoye, and Lyubavinskoye deposits are located in near-latitudinal zones of regional faults, which also control the placement of ore-bearing intrusive formations. Within such zones a zonal distribution of types of mineralization and ore-localizing structures is observed. Thus, the stockwork-disseminated mineralization of the Ilyinskoye deposit proper, of the gold–arsenic formation, is replaced westward along the strike of the zone by the metasomatic type of gold–bismuth mineralization of the Dybyksinskoye deposit and then passes into the vein type of gold–bismuth mineralization (the Slavyanka ore occurrence).

At the Lyubavinskoye quartz-vein deposit, horizontal zoning within the zone is expressed in the westward replacement of gold–arsenopyrite ores (the Nikolayevsky area) by gold–polysulfide mineralization (the Bolshe-Fedorovsky area).

This horizontal zoning reflects the vertical zoning observed within the noted deposits from mine workings. Thus, on the upper levels of the Ilyinskoye deposit, quartz–sericite–tourmaline–pyrite ores are recorded; with depth, through quartz–arsenopyrite and quartz–carbonate–polysulfide ores, they are replaced on the lower levels by quartz–molybdenite–bismuth ores. At the Lyubavinskoye deposit this zoning is expressed in the replacement of arsenopyrite ores by polysulfide ores on the deeper levels of the gold-ore veins.

The brief characterization given above of the distribution of different types of gold mineralization and ore-localizing structures in districts with sharply different ages of mineralization makes it possible to note a number of common features.

1. There are two structural levels: the lower, geosynclinal-folded level with batholithic granitoids (the Martaiga complex of the Kuznetsk Alatau and the Kyra complex of Central Transbaikalia), and the upper structural level—the transitional one (the volcanogenic-sedimentary Devonian of the Kuznetsk Alatau and the volcanogenic-sedimentary Mesozoic of Central Transbaikalia).

2. In both districts, the formation of post-batholithic gold-bearing intrusive formations is connected with the development of the upper level (the Kiya gabbro–alkaline-syenite complex of the Kuznetsk Alatau and the intrusive formations of the volcano-plutonic formation of Central Transbaikalia).

3. In the districts considered of the Kuznetsk Alatau and Central Transbaikalia, gold-ore mineralization and ore-bearing intrusive formations are controlled by large tectonic zones of deep origin. Within these zones the following morphological types of zoning are observed: a) longitudinal horizontal zoning (after D. O. Ontoev [4]) and, corresponding to it, b) vertical zoning in individual deposits. This horizontal and vertical zoning should be regarded as a manifestation of deep facies zoning, determined by the depth of formation of individual types of mineralization relative to the land surface at the time the deposit was formed. The general zoning identified within ore fields, deposits, and individual bodies is complicated by other types of zoning, in particular pulsational zoning.

In the Kuznetsk Alatau, as a less rigid region, the folds of the second level are steeper, and therefore the horizontal zoning is expressed more distinctly—

... in the alternation of different formations and ore-localizing structures over a relatively short distance. In Central Transbaikalia the folds of the second level are more gentle, and the horizontal zoning is expressed less distinctly: the change of mineral associations here is to a considerable degree connected with the successive development of ore-localizing structures.

1. The influence of deep movements on the vertical distribution of mineralization types in the subsequent period is slight.

REFERENCES

1. Vasiliev B. D. Gold in the skarns of the Natalyevka deposit in the Mariinsk taiga. In: Materials on the Geology and Mineral Resources of Western Siberia, Tomsk State University Press, 1964.

2. Vasiliev B. D. Regularities in the distribution and forecasting of gold-ore deposits in the northern part of the Kuznetsk Alatau. Izv. TPI, vol. 151, 1966.

3. Vasiliev B. D., Kortusov M. P. On the question of the age of the Kiya intrusive complex of the Mariinsk taiga. Materials on Mineralogy, Petrography, and Mineral Resources of Western Siberia and Krasnoyarsk Krai, issue 2, Tomsk State University Press, 1964.

4. Onteev D. O. On the relationship of different types of hypogene zoning in certain rare-metal deposits of Eastern Transbaikalia. In: Conference. Problems of Postmagmatic Ore Formation, vol. 1, Publishing House of the Czechoslovak Academy of Sciences, Prague, 1963.