LOCALIZATION CONDITIONS AND GENESIS ISSUES OF THE MNOGOVERSHINNOE GOLD DEPOSIT

(Coast of Sakhalin Bay)

M. S. MIKHAILOVA (TsNIGRI)

The large Mnogovershinnoe gold-ore deposit is located in the NE part of the Lower Amur synclinorium at its intersection with the structures of the northern termination of the superimposed Primorye volcanic belt. It is localized in a node of intersection of deep, extended faults of NE, NW, and sublatitudinal strike; the first of these correspond to the general strike of folding of the Mesozoic basement. Thick, extended ore bodies of auriferous quartz metasomatites occur in a Senonian andesite effusive-pyroclastic sequence, and are also traced in Lower Cretaceous siltstone-sandstones.

The ore-forming process at the deposit proceeded in connection with the formation of a Paleogene–Upper Cretaceous volcano-plutonic complex. Manifestations of intrusive and various forms of extrusive magmatism (hypabyssal granitoid massif, stocks, dikes, volcanic domes, vent structures, etc.) are, in their distribution, strictly controlled by the dominant NE and, more rarely, other trends of the region’s structures, and are closely spatially associated with volcanism of the subplatform stage of the region’s development. Individual members of the comagmatic series are close in time and reveal signs of genetic kinship. Characteristic features of the rocks of the complex are a high alkali content, predominance of potassium oxide over sodium, indications that the original magma was enriched in volatiles (boron, chlorine, fluorine, OH, carbon dioxide), a limited suite of accessory minerals, of which the leading ones are apatite, zircon, cassiterite, wolframite, and tourmaline; and a sharply expressed gold specialization with elements of rare-metal specialization. The elevated gold content in all facies of the comagmatic complex permits this element to be regarded as typomorphic for the complex. The extrusive rocks are derivatives of a magma of elevated alkalinity (potassic character), whose differentiation leads to the formation of a residual melt enriched in potassium oxide.

The increasing role of potassium in the late phases of manifestation of the magmatic process is confirmed by data on the chemistry of rocks both of the intrusive,

both in the intrusive and in the effusive line. A sign of the evolutionary development of a magmatic chamber toward enrichment of the latest melts and subsequent solutions in potassium is the presence, within the ore field of the deposit and in the area, of greisens of two-mica granites, as well as muscovite metasomatites and adularites. The elevated potassium content of the erupted rocks, which reveal a distinct gold geochemical specialization, should apparently be regarded as a magmatic evaluation criterion in areas prospective for the search for mineralization. In the literature this question has received positive treatment in the works of E. P. Izokh and I. V. Kunaev for the Lower Amur region, and of Galager for New Zealand and Fiji [3, 11]. As a rule, the greater part of gold-silver deposits characterized by a high silver content is associated with potassium-bearing rocks.

For the Mnogovershinnoe deposit, a paragenetic relationship is established between the gold-quartz mineralization and Upper Cretaceous effusive formations, of which it is a derivative of postmagmatic gas-hydrothermal activity. On the other hand, as recent work by the TsNIGRI Institute has shown, gold-ore mineralization and broad processes of hydrothermal metamorphism fall into the “forks” between different members of a single intrusive series (early and late phases in the formation of a hypabyssal granitoid massif), with which, as noted above, it reveals mineralogical and geochemical, though less pronounced, signs of comagmaticity. It is apparently legitimate to assume the possibility of two generating sources for the gold mineralization of the Mnogovershinnoe deposit: granitoid masses crystallizing at depth, of which the particular massif and dike-stock formations are derivatives, as well as centers of effusive-extrusive volcanism of the late stage of the region’s development. The gold mineralization is connected with both by the succession and commonality of a deep magmatic chamber that supplied comagmates to the upper structural levels along rupture disturbances. The geological position of the intrusive and a number of extrusive formations, and the features of their structures and textures, indicate formation of the rocks of the volcano-plutonic complex at the level of shallow (0.5–1 km) depths.

The entire complex of rocks enclosing the deposit underwent intensive reworking as a result of contact metamorphism, superimposed propylitization, and near-ore metasomatism accompanying the gold mineralization. In terms of the type of alterations preceding and accompanying the gold mineralization, and their temporal relations, the Mnogovershinnoe deposit is a typical example of a gold-silver deposit of the subvolcanic formation. At the same time it possesses a number of specific features not characteristic of deposits of the indicated formation.

Thick, extended ore bodies of gold-bearing quartz-muscovite metasomatites are localized within narrow zones of chlorite-carbonate low-temperature propylites and belong to a complex structural-morphological type. Belts of low-temperature propylitization form narrow local zones among fields of medium-temperature epidote-albite-chlorite propylites of areal distribution and mark tectonically weakened sectors. In the overwhelming majority of cases these zones are spatially associated with background-veinlet silicification and intense sulfidization carrying disseminated gold mineralization. The features of localization, petrochemistry, mineralogy, and gold content of the low-temperature propylites testify

testify to their syngenetic relation to gold mineralization. In our case, this is a “completed” series [8] of postmagmatic transformations which, in the understanding of L. N. Plyashkevich, are derivatives of a single process of gas–hydrothermal activity.

For the areal, medium-temperature propylitization of the epidote–albite–chlorite type, its postintrusive manifestation (after \(Pg^2_2\)) is unequivocally established, and thus its separation in time from the weakly manifested autometamorphic processes of effusive volcanism of the Upper Cretaceous is clearly fixed. The emplacement of postvolcanic intrusive formations caused metamorphism and weak dislocation of the subaerial effusives, and also led to the formation of rupture structures facilitating the ascent and mobilization of postmagmatic propylitizing solutions. From our point of view, identifying this series of rock alterations with regional transformations of the “green schist” type [2] is untenable. Near the intrusive hypabyssal massif and in areas of concentration of its late comagmates, among the medium-temperature epidote–chlorite–albite propylites, a facies with biotite, six-sided hornblende, and pyrrhotite is distinguished; it has a very broad distribution in comparison with the aureoles of contact metamorphism. The appearance of this specific facies is a consequence of the residual temperature field around the intrusions.

Gold-bearing quartz and quartz–muscovite metasomatites are most often located in belts of low-temperature propylites. They are accompanied by aureoles of silicification, muscovitization, intense pyritization, and disseminated adularization. The ore bodies are veins penetrated by the finest veinlets of gold-bearing chalcedony-like quartz with polymetallic sulfides, sulfosalts, argentite, etc. The formation of ore bodies took place in several stages: an early productive stage—quartz–pyrite with a small amount of gold; a rare-metal–tourmaline stage; a productive gold–polymetallic stage with pyrargyrite, freibergite, argentite, hessite, and others, accompanied by adularia, chlorite, carbonate, and podolite. The ore-deposition process ends with a stage of argillization. The ore bodies are characterized by persistence, but by a tendency toward bonanza concentrations of the metal. An increase in the concentration of minerals of the productive association, in particular gold, is observed near xenoliths of propylitized andesites, along belts and areas of accumulation of adularia and carbonate in early quartz. The nature of the host rocks has a substantial influence on the textural and structural features of the ores, and a lesser influence on the content of the valuable component.

Quartz of the early productive association is characterized by brecciated, cavernous, colloform-banded, and crustification textures, and by the presence of spheroidal and regular globular forms, emphasized by the synconcentric arrangement of adularia, chlorite, and other minerals. The presence of such forms is one of the indicators of the origin of quartz and other minerals from colloidal solutions that passed through a stage of gel formation [7, 10].

The concentrically zonal and rhythmically banded structure of the quartz aggregate—more rarely of quartz–adularia and quartz–adularia–chlorite with pyrite—may, in the opinion of a number of authors, be caused by diffusion phenomena in colloid-dispersed systems [1, 7].

At the Mnogovershinnoe deposit, the metacolloidal character of the mineral formations is also established for the main mass of vein filling, whose metasomatic character does not raise...

is beyond doubt. In addition to the features noted above, the metasomatic quartz of the early association also exhibits such indications of its formation from colloids as the concentrically zoned structure of spherulitic segregations and features of cracks caused by syngenetic phenomena. Apparently, along with true solutions, colloid-dispersed systems also participated in the formation of the metasomatites; as a rule, these are stable over a wide range of temperatures and pressures [5] at depths from the present surface down to 650 m. Features of a metacolloidal structure at the Mnogovershinnoe deposit are most often established also for chlorite globules and aggregates in quartz and quartz-adularia material participating in the rhythmically banded structure of veins. The emergence of gel systems was apparently associated with a sharp pressure drop, which is highly characteristic of near-surface conditions, as well as with the change from an alkaline to an acidic character of the medium at certain stages in the process of metasomatic replacement of rocks by solutions with a significant concentration of many components. The metacolloidal character of chalcedony-like quartz is manifested more sharply in the main productive association. Unlike the early productive association, where gold in dispersed droplet-like segregations is scattered throughout the entire mass of quartz, the gold of the main productive association forms closely related, nearly contemporaneous segregations with the entire complex of polymetallic sulfides, sulfosalts, argentite, etc.; in the form of the finest dispersed inclusions it is present within their grains, and also envelops and cements them. Carbonate, chlorite, and adularia often take part in this association. The latter shows no differences from the adularia participating in the structure of the main mass of quartz-adularia metasomatites of the early productive association and may have been redeposited. The productive gold–polymetallic association is manifested as veinlets and nest-like segregations in metasomatic quartz and constitutes an insignificant part of its volume. In the ores of the Mnogovershinnoe deposit, tellurium has been noted in amounts from 4–7 to 30 g/t. It is present as an isomorphic admixture in high-fineness (870 and higher) gold and in independent minerals—hessite, altaite, and petzite. The latter participate in the structure of the productive association together with other minerals and possess the same features of nearly contemporaneous deposition with gold and with sulfides. Some features indicate a slight lag in their deposition, which makes it possible to assume the existence of several stages of equilibrium within the productive association [7].

Until recently, the presence of gold and silver tellurides in gold ores of the subvolcanic formation was considered a geochemical feature of the eastern branch of the Pacific belt, and their absence in deposits of the eastern USSR was emphasized.

In the vertical distribution of mineralization at the Mnogovershinnoe deposit, zonality is outlined, expressed in an increase in the total amount of sphalerite and chalcopyrite and a decrease in pyrargyrite and argentite down the section. This is consistent with a decrease in the role of quartz–muscovite rocks and an increase in substantially quartz facies in the same direction.

Another specific feature of the deposit is the presence in the productive association of Ca, Mn phosphates: podolite, triplodite, which differ in X-ray and other characteristics from the low-temperature apatite often described for subvolcanic deposits [9]. Podolite, triplodite, together with quartz and

with adularia form the closest intergrowths with ore components, in particular with gold, and sometimes include its droplet-like segregations.

The process of formation of the ore bodies at the deposit ends with the stage of argillization. The latter is manifested very weakly, which, apparently, is a consequence of the relatively shallow (0.5–1 km) level of formation of the deposit for this group of hydrothermal alterations. Argillization is expressed as dickite mineralization filling caverns and voids in vein metasomatites; more rarely, in the form of veinlets, it cuts formations of earlier stages and replaces adularia and chlorite in rhythmically banded aggregates. The presence of the argillizite facies of alteration, as the most near-surface one, makes it possible to assume a shallow erosional truncation of the deposit.

A specific feature of the deposit is also the presence within the ore bodies, along with gold mineralization, of rare-metal–tourmaline (cassiterite, wolframite, molybdenite, scheelite) mineralization. The latter occupies a position in the interval between the early productive and productive associations and reveals a close temporal, spatial, and genetic connection with the intrusion of crosscutting granite-porphyry dikes. For the Mnogovershinnoe deposit, quartz–tourmaline–muscovite greisen-like mineralization with wolframite, cassiterite, and molybdenite in a number of cases has an occurrence separate from the main ore bodies. According to the classification of N. V. Petrovskaya [6], it is “alien” to the deposit and reflects the metallogenic features of a territory situated at the junction of the trends of two structures: the Mongol–Okhotsk geosyncline and the Primorye volcanic belt.

In addition to quartz–tourmaline mineralization, the sequence of formation of the ore bodies and the general trend of the hydrothermal process were disrupted by the appearance of quartz–hematite and hematite mineralization. This association cements fragments of early quartz with pyrite and minor gold and is cut by veinlets of ore-stage productive quartz proper. A metamorphic transformation of hematite into magnetite is established near a granite-porphyry dike accompanying the rare-metal–tourmaline mineralization. The appearance of such mineralization is more characteristic of deposits of intermediate depths [4] and indicates a high oxidation potential at the time of its formation.

The entire complex of geological, geochemical, and mineralogical observations, taking into account analysis of the acid–base differentiation of postmagmatic solutions, makes it possible to divide the postmagmatic process into a number of stages and phases: pre-ore—the stage of areal propylitization; pre-ore—the stage of low-temperature propylitization; the stage of silica–potassium metasomatism; the ore-formation phase—the stage of the gold–pyrite association; the quartz–hematite stage; the stage of xenothermal rare-metal–tourmaline mineralization; the stage of the productive gold–polymetallic association; and the stage of argillization. The phased and stadial character of the formation of various types of metasomatic alterations and ores is a consequence of the continuously intermittent character of tectonic movements and the percolation of ore-forming solutions.

Analysis of the geological history of the deposit’s development, the facies and depths of the altered rocks, and direct and indirect data on the temperature of formation of the mineral associations shows that, in the course of the deposit’s formation, the conditions of pressure, the temperature regime, and the chemistry of the solutions changed repeatedly, being expressed in

in the successive formation of altered rocks and the formation of productive mineralization under conditions of moderate depths (0.5–1 km) and moderate temperatures (330–360°).

REFERENCES

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