Main Features of the Geological Structure of the "Udarny Pervenets" Deposit
Abstract Generated abstract
This paper describes the geological structure and ore localization of the Udarny Pervenets quartz vein deposit in the central Perevalninsk ore field. On the basis of petrographic observations, mineral assemblages, and structural relationships, it distinguishes progressive epidote-amphibolite facies metamorphism associated with formation of quartz and quartz-silicate vein bodies from later regressive greenschist facies alteration linked to northeastern rupture dislocations. The authors argue that vein zones are controlled by higher-order folding, lithological contrasts in the original sedimentary sequence, and subsequent differential movements that produced en echelon quartz bodies. The study relates the distribution of garnet-bearing, sericite-quartz, chlorite, and related schists to stages of mineralization including arsenopyrite, sulfides, and gold.
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Main Features of the Geological Structure of the “Udarny Pervenets” Deposit
A. M. Sazonov, S. S. Ilenok
(Presented by the seminar of the Department of Petrography)
General Information
The quartz-vein zones of “Udarny Pervenets” occupy the central part of the Perevalninsk ore field, the deposits and ore occurrences of which are localized on the northeastern limb of the Enamimin anticline and are controlled by large tectonic disturbances of northwestern strike.
The enclosing rocks of the area are represented by schists of the Gorbilok Formation of the Sukhopyt Series of Upper Proterozoic age. On a regional scale, the rocks of the Gorbilok Formation have undergone weak metamorphism, and the usual rocks of this formation are phyllites. Within the ore field we are dealing with crystalline schists; the phyllites, however, border the more highly metamorphosed rocks and are connected with them by gradual transitions.
Features of the Petrographic Structure of the Area
The petrographic composition of the rocks and the distribution of their varieties are closely connected with ore formation. A common staging is characteristic of the vein and enclosing rocks. In the formation of the rocks of the ore field, two stages are clearly distinguished: progressive and regressive.
The products of the first stage of metamorphism encompass the main mass of rocks. Their development is due to the formation of zones of folding of northwestern strike and to the dynamothermal metamorphism manifested in them. The main rock varieties of this stage are:
- Garnet-biotite-quartz and garnet-biotite-sericite-quartz schists.
- Garnet-biotite-albite-quartz and garnet-biotite-albite-sericite-quartz schists.
- Biotite-sericite-quartz schists.
- Biotite-quartz and biotite-albite-quartz schists.
- Sericite-quartz schists.
Analysis of the compositions of the principal rock-forming minerals of the rocks makes it possible to assign the products of this stage of metamorphism to the epidote-amphibolite facies. Synchronously with the metamorphic transformations of the phyllites, the vein zones of the deposit were formed. Quartz, quartz-biotite-garnet, quartz-andesine-biotite-gra-
garnet-bearing, andesine-biotite-garnet bodies. Arsenopyrite and gold mineralization terminate this stage.
In the distribution of the host rocks the following regularity is outlined. Garnet-bearing and sericite-quartz schists control the vein zones. Sericite schists appear in places of intense differential movements of the zones of folding. The increased circulation of metamorphic solutions of acidic composition in these places led to the formation of lenses of sericite schists up to 10–15 m thick. The temperatures of their formation lie within the range of 500°C, which corresponds to the temperature of the epidote-amphibolite facies.
Subsequently, as a result of a change in the direction of stresses, rupture dislocations of northeastern strike are formed; their formation is associated with regressive transformations of rocks from the epidote-amphibolite facies to the greenschist facies. The degree of formation of the rocks depends on the intensity of manifestation of stresses of the indicated direction. In this process the principal rock-forming minerals—garnet and biotite—are replaced by chlorite and stilpnomelane, and new formations of chloritoid appear. The main rock varieties of this stage are:
- Chlorite-quartz schists with chloritoid.
- Quartz albite-chlorite schists with chloritoid and stilpnomelane. The hydrothermal products of this stage should include veinlets of albite, orthoclase, and carbonates, and veinlet-disseminated formations of pyrrhotite, pyrite, galena, sphalerite, and gold.
Structural Features of the Localization of Vein Zones
As already noted, the vein zones of the deposit are located on the northeastern limb of the Enashiminskaya anticline. Study of the behavior of the crystallization schistosity shows that the ore zones are confined to plicative dislocations of high orders, clearly standing out against the background of monoclinal bedding.
The position of the quartz-vein zones, which owe their origin to the development of zones of folding in the section of the deposit, is controlled by the lithological heterogeneity of the primary sedimentary material. The alternation of substantially clayey and sandy interbeds subsequently created the most favorable areas for the formation of zones of folding, crushing, and zones of increased fracturing, which later became the localizers of quartz-vein bodies. The vein zones are represented by systems of convergence of quartz veins, lenses, nodules, and bodies of complex form. In the structure of the vein zones an en echelon arrangement is observed, which may be explained by differential movements of flat blocks cut out by the main faults. The succession of the en echelon segments proceeds with a retreat toward the southwest. This regularity is characteristic both of individual quartz bodies and of packets of veins and is due to pre-ore northeastern fractures.
The indicated regularities make it possible to link the features of the petrographic composition of the host rocks of the group of crystalline schists with the manifestation of quartz ore bodies and their mineralization.