STRUCTURE OF THE KOMSOMOLSKOYE GOLD ORE DEPOSIT

Yu. P. Denisov (WSTU)

The Komsomolskoye deposit is located in the gabbro-diorite stock of the Ordovician–Silurian Martayginsky complex, which cuts through carbonate and volcanogenic Cambrian formations. The structure of the deposit is a combination of a number of heterogeneous structures. The structure of the massif, tectonics, the structure of the series of acidic dikes, the structure of the vein suite, and the structure of the series of basic dikes, which are elements of the structure of the deposit as a whole, are considered from the standpoint of their influence on the distribution of gold in the ore field.

The gabbro-diorite stock is elongated in a northeastern direction for 5 km, with a width of 3.5 km, and has an oval shape. According to preliminary data, the formation of the intrusion took place in two completed cycles. The rocks of the first cycle include porphyritic olivine microgabbro, microdiorites, and granites, known in the massif as xenoliths, hornfelsed and metasomatized. The second cycle of intrusion probably breaks down into three major phases.

The gabbro-diorites of the first phase of the second cycle are the principal rocks in the intrusion. They were probably emplaced with a successive change in composition from pyroxene–hornblende to biotite–hornblende. Gabbro-diorites of a definite structure and composition can be traced for 2–3 km along strike, with body thicknesses of only 100–200 m, and form more or less sharp contacts with other rocks, depending on the time of formation and the nature of the structure of the latter. Their common feature is an oriented texture, caused by crystallization under pressure and consistent with the general orientation of the structure and contacts of the massif; it is recorded both by the mafic components and by plagioclase.

Olivine gabbro-norites of the second phase compose a small lens in the southeastern part of the intrusion.

The rocks of the third phase—hypersthene gabbro, hypersthene quartz-bearing diorites, and quartz diorites—play a substantial role in the intrusion, mainly in its central and northern parts, and are the latest in the entire series of intrusive rocks. Quartz diorites, along with lenticular and irregular bodies, form dikes in the massif up to 1 meter thick.

Rocks of different composition and different age are subordinate to a single structural plan, forming more or less persistent bodies of northeastern orientation. Many xenoliths have been preserved in the massif.

of the enclosing rocks and of the rocks of the first intrusive cycle. The xenoliths are drawn out along the contacts in chains; some of them have been traced for a distance of 1.5–2 km and to a depth of as much as 600 m. The distribution of xenoliths over the area of the massif is very uneven, both quantitatively and in composition. The southeastern half of the massif contains separate chains along the southeastern contact with limestones. In the center and northwestern part of the massif the number of xenoliths increases sharply and in some areas reaches 40–80%.

The series of acidic dikes is divided into two groups: trondhjemite–plagiopegmatite and granite–aplite. Their thickness ranges from the first centimeters to a meter; along strike some have been traced for 400–500 meters. Usually the dikes are grouped into zones of sublatitudinal orientation, dipping north at an angle of 45–60°. All of them are pre-ore.

At the Komsomolskoye deposit about 150 veins are known, most of which are located in the southeastern half of the intrusion. In structural and spatial position all the veins are combined into five ore nodes: Bolshoi, Klyuchevskoi, Komsomolsky, Pionersky, and Andreevsky. In the first three nodes the structure is determined by large veins of northwestern strike, dipping east at an angle of 30–60°. The length of individual veins is up to 1.5 km, their thickness up to 5 m. Each node has certain distinctive features: in the Bolshoi there are two large veins, along the continuation of which powerful brecciation zones have been traced in the marbles of the enclosing sequence; the Klyuchevskaya vein is formed by a series of en échelon segments; the Komsomolskaya vein is accompanied by a mass of apophyses and gently dipping veins conjugate with it. The principal element of the Pionersky node is a vein zone parallel to the contact, with veins dipping to the northwest and developed tectonic disturbances of the same orientation, as well as comparatively persistent and thick sublatitudinal veins. A feature of the Andreevsky node is the combination of sublatitudinal veins with submeridional ones.

At the same time, the commonality of structure for all the nodes is beyond doubt. In all the nodes the sublatitudinal quartz veins dip northward, lie in the zones of acidic dikes, and often follow directly along them, with typical beresites formed at the contacts. All large veins are developed tectonic zones, the walls of which have undergone hydrothermal alteration, while the cavities in the displacement zone are filled with quartz. In all ore nodes the tectonic disturbances are numerous, have different orientations and parameters, but disturbances with dip angles steeper than 60° predominate. In individual areas silicification and sulfide mineralization are superimposed on them. Probably most of the vein fractures and disjunctive disturbances are syntectonic, and tectonic disturbances do not exert a decisive influence on the structure of the ore field.

The process of vein formation, according to the data of the author and I. V. Tsaplin, may be represented by the following scheme:

I stage — hydrothermal alteration of fracture zones.
II stage — filling of opened cavities with quartz.
III stage — sulfide stage: superposition of sulfides and gold on quartz veins and skarns; formation of metasomatic sulfide bodies in marbles. According to the principal parageneses, the sulfide stage is subdivided into a number of successive stages: pyrite–pyrrhotite, sphalerite, arsenopyrite, and galena. Gold and scheelite were deposited mainly during the arsenopyrite and galena stages.

The final act in the formation of the structure of the Komsomolskoye deposit was the intrusion of a series of basic dikes occupying up to 15% of the area of the massif. In composition and relative age

all the principal dikes can be divided into 5 groups: 1) lamprophyres, 2) alkaline dikes, 3) gabbro-diabases, 4) diabases, 5) labradorite (doleritic) porphyrites.

The relationship of the lamprophyres to mineralization is unknown; they may prove to be pre-ore. The dikes of the remaining four groups are undoubtedly post-ore and probably post–Lower Devonian.

With regard to the regularities in the distribution of gold, there are a number of more or less well-founded assumptions. The reason for the concentration of veins within the gabbro-diorite stock was probably the absence of other deep-seated formations in the regional ore-bearing zone of northeastern orientation, comparable in scale with the thickness of the zone. The sharp difference in the physical properties of the gabbro-diorites and the marbles of the southeastern contact caused the concentration of stresses precisely along this contact; and the absence here of a large quantity of xenoliths, and the associated relative monolithic character of the massif, promoted the formation of large fractures. The distribution of gold in the ore field was controlled by the structure of the intrusion, since all veins with commercial-grade contents are located in a near-contact zone about 1 km wide, and the ore shoot of vein No. 35—the southern one—has been traced down dip for 800 m along the chain of xenoliths. The distribution of gold in individual veins depends on the morphology of the vein and the vein tectonics. Ore shoots and streaks are quite distinctly situated in the areas of the greatest thickness gradients, which proved to be the most weakened during vein movements and served as pathways for the penetration of gold-bearing solutions.