PATTERNS IN THE DISTRIBUTION OF UPPER PALEOZOIC MAGMATIC AND ORE FORMATIONS OF THE KALBA GOLD-ORE BELT

P. I. POLTORYKHIN (VKGU)

The Kalba gold-ore belt is located in the central part of the Zaysan geosynclinal folded region. Its Upper Paleozoic stage of development is characterized by: a great thickness of deposits, a block-folded character of tectonics, a northwestern trend of major structures, intensive volcanoplutonism in the southeastern part of the belt and moderate intrusive magmatism in the northwestern part, and the hydrothermal nature of the principal gold-ore deposits—mineralized zones, stockworks, and quartz-vein deposits. These deposits reveal a close spatial and temporal connection with Upper Paleozoic magmatism.

Structural factors played a most important role in the distribution of the deposits. The role of faults is manifested in the linear distribution of the deposits associated with them. The most ore-bearing are the large Upper Paleozoic faults of northwestern and sublatitudinal strike. The northwestern faults, inherited and more ancient in their development, are ore-bearing in areas rejuvenated as a result of Upper Paleozoic block-folded movements of the basement. The most ore-bearing are the junction nodes of such disturbances with younger, post-folding Upper Paleozoic faults of sublatitudinal strike. Such junction nodes of disturbances are most saturated with Upper Paleozoic granitoid small intrusions, which are accompanied by hydrothermal metamorphism of the enclosing rocks and gold mineralization.

The sublatitudinal faults, expressed by zones of crushing (Kyzylovsky, Miyalinsky, Parallel, and others), cut the folded sandy-shale sequences at an angle, including the Upper Carboniferous, with a total thickness of up to 8–10 km. However, in the basement these faults correspond to ancient fractures.

Along the major crushing zones there are widespread stocks (up to 1–2 km across) of plagiogranites, granite-porphyries, and diorites, as well as dikes of varied composition. The latter are divided into three groups: 1) pre-ore plagiogranites; 2) subalkaline dikes (granosyenites, syenite-diorites, sodium syenites, granodiorites), cutting gold-bearing quartz veins and accompanied by sulfide-disseminated mineralization; 3) post-ore dikes of diabase porphyrites. For the Upper Paleozoic

...zoic stocks and dikes reveal considerable hybridism, increased alkalinity, an alkaline trend in the changes in rock compositions from early to late, as well as temporal proximity and petrochemical continuity among all varieties, which makes it possible to unite them into a single complex of small intrusions.

In the southeastern part of the gold-ore belt, where volcanic activity was fairly intense in the Upper Paleozoic, the complex of intrusions and dikes shows comagmatic relationships with Upper Paleozoic effusives.

In terms of the time of formation, the small intrusions are apparently synchronous with the Kalba granitoid complex or complete it. In all known gold-ore fields, a close structural and age relationship is noted between the small intrusions and the hydrothermal bodies (especially sulfide-disseminated ones), which is explained by their close paragenetic relations.

The host rocks of the Kalba gold-ore belt, according to their physicochemical properties, may be divided into three strata (from bottom to top, in the age sequence): 1) effusive-sedimentary, rigid Devonian strata, from which the ascending hydrothermal solutions borrowed iron, magnesium, and other elements; 2) substantially carbonate strata of the Middle and Upper Visean, which acted upon the intruding magmas and promoted the differentiation of the small intrusions;

3) thick sandy-shaly strata of the Lower and Middle Carboniferous, in which all known gold-ore deposits are localized. At the same time, a confinement of sulfide-disseminated deposits to substantially shaly (especially carbonaceous) members is observed both on the regional and on the local scale. The siltstone members are intensely crumpled and therefore served as screens for the hydrothermal solutions. Carbonaceous siltstones were good precipitants and sorbents of metals from solutions.

The ore bodies of sulfide-disseminated deposits are located in zones of concordant folding and have a complex form, which only in a general approximation may be called lenticular.

The principal ore columns, often closely associated with lenticular dikes, have been traced down dip to comparatively great depths (600–800 m) and apparently continue to depths of 1.5–3 km. Quartz-vein bodies have a lesser extent (300–700 m).

The ore minerals are pyrite, arsenopyrite, more rarely antimonite, sphalerite, chalcopyrite, galena, fahlores, and others. Fine-grained disseminated sulfides together usually make up no more than 3–5% of the total mass of the ore. The gold is submicroscopic or dispersed and, for the most part (75–80%), occurs as an admixture in sulfides (mainly in pyrite and arsenopyrite). In the quartz veins, which have only secondary importance in the overall balance, the gold is free and is distributed unevenly in the veins, in bush-like aggregates.

The sulfide-disseminated and quartz-vein deposits within the Kalba gold-ore belt are united by us into a single arsenic-gold-antimony ore formation.