ON THE QUESTION OF THE GENETIC RELATIONSHIP BETWEEN CONTACT IRON-ORE MINERALIZATION AND INTRUSIONS

S. S. IL’ENOK

(Presented by the scientific seminar of the Department of Petrography)

The practice of prospecting and exploration requires the direct application of theory to the solution of practical problems. One of the most important questions of this kind is the study of the genetic relationships of mineralization with intrusions. Numerous petrological studies of recent times, together with experimental work, have made it possible to approach more closely the solution of this important problem. The studies of S. S. Smirnov, A. E. Fersman, Yu. A. Bilibin, A. G. Betekhtin, D. S. Korzhinskii, K. I. Satpaev, Kh. M. Abdullaev, and others have revealed a number of regularities in the relationship between rock formation and ore formation, and in the physicochemical and geochemical connection of these processes. An enormous role here has been played by the large community of field geologists in our country, as a result of whose work a vast amount of factual material has been obtained on the geology, structure, and composition of magmatic and ore complexes. Of great interest is the work of Yu. A. Kuznetsov [12] on identifying regularities in the distribution and analysis of magmatic complexes or formations. This work may serve as a basis for solving many metallogenic questions.

In addition to features associated with the primary composition of magma, most researchers assign a major role in the metallogenic specialization of magmas to phenomena of assimilation. This is usually understood as the assimilation by magma of foreign solid material from the enclosing environment by melting (the granophyres of the Karroo in South Africa) or by chemical interaction with the participation of volatiles.

At the same time, questions concerning the origin of volatiles, the presence of accumulations of individual elements in the absence of these elements in the surrounding environment, and especially the connection of mineralization with the natural history of the magmatic complex remain difficult to explain.

The results of our study of a number of magmatic complexes in Western Siberia, as well as an analysis of objective data on the petrology of individual intrusive massifs, make it possible to conclude that phenomena of magmatic replacement played a significant role both in the formation of these complexes and in the ore occurrences associated with them. In this case, the process of assimilation by melting is characteristic of the early stages in the formation of basaltic magma and is connected with its high temperature. In addition, it manifests itself when acting upon easily fusible rocks whose composition is close to the granitic eutectic (for example, aleurolites). When acting upon carbonate rocks, however, dissociation is more likely to occur.

carbonates with the formation of CO₂ and CaO, or MgO, which are chemically active substances. For the later stages of the formation of magmatic rocks, phenomena of magmatic replacement are characteristic.

The connection between ore mineralization and parent intrusions is most clearly manifested in contact-type iron-ore deposits. An analysis of the compositional features of the intrusions with which contact iron-ore mineralization is associated led Yu. A. Kuznetsov [13] to the conclusion that they are related to derivatives of basaltic magma. These derivatives are associated with the trachytic trend of magma differentiation. He notes the rarity of large contact iron-ore deposits associated with granitoid intrusions, and their occurrence where the latter have massively assimilated basic rocks.

The present article presents facts indicating features of magmatic replacement in massifs with which both contact iron-ore mineralization and other types of ore occurrences are associated. These features make it possible to conclude that there is a connection between the formation of magmatic intrusive complexes and ore mineralization.

The study of intrusive massifs composed of the basaltoid group of rocks shows, first of all, a sharp difference in the manifestation of magma differentiation in gently dipping and horizontal plutons, on the one hand, and in steeply dipping and vertical plutons, on the other. In the first group of plutons, expressed in the form of intrusive sheets, laccoliths, and lopoliths, stratification is distinct and even sharply developed, due mainly to the features of magma differentiation in place within the body of the pluton; the “ferruginous” type of magma differentiation is well manifested, with accumulation of a residual liquid rich in iron and titanium and somewhat enriched in phosphorus. Connected with this is the constant presence in intrusive massifs of poor disseminated mineralized rocks and, more rarely, massive ores with a titanomagnetite or magnetite-ilmenite, sometimes ilmenite, type of mineralization; a complex of sulfide mineralization is also often encountered, likewise usually in the form of disseminations and very rarely as accumulations of massive ores. The alkaline trend of differentiation here is almost not expressed, or is weakly manifested, if intrusions of magmas that are obviously alkaline are excluded from consideration. Such a feature of differentiation is clearly visible in the traps of the Siberian Platform [14].

Steeply dipping plutons have a different character of structure and of the complex of rocks composing them. They are associated with the intrusion of magma that forms vertical and steep columns extending to great depths. In this case more varied and complex processes take place. The basic gabbroid rocks formed in the upper part of the magmatic column as a result of magma crystallization undergo continuous changes under the influence of the influx of magmatic liquid whose composition is constantly changing. The manifestation of tectonic movements or the development of fractures during the formation of the intrusion may lead to a multiphase structure of the massif, with the development of intrusive contacts for rocks associated with individual magma injections, the presence of chilled contacts, etc.

As a result of tectonic movements, new portions of magma often produce independent forms of bodies, which may be spatially separated from the earlier products of magma crystallization. Assimilation phenomena, which in this case are developed rather widely, impart considerable diversity to the magmatic products.

A study of the composition of such massifs as applied to the northeastern part of the Altai-Sayan region shows the usual sequence—

a complete series of magmatic differentiates of the following type: gabbro, monzonites, syenites, quartz syenites and granosyenites, granites; more often syenites and granites of the alkaline type, more rarely normal ones.

Another series of intrusive differentiates is usually situated in the enclosing mass of greenstone effusives and is associated with phenomena of assimilation of the enclosing rocks. This series has the following sequence: gabbro, diorites, quartz diorites, plagiogranites.

Indicators of magmatic replacement

A number of researchers have noted the connection of metasomatism with the localization of mobile magma during granitization phenomena. According to the works of MacGregor, Wilson, Barth, and others [20, 21, 22], the process of granitization is divided into two stages: a) metasomatism under the influence of heat and the active action of migrating solutions; b) mechanical penetration of mobile magma and its mechanical mixing with the metasomatized rock.

All the above-mentioned researchers draw a sharp boundary between metasomatism and the mechanical penetration of magma. Insufficient account is taken of the fact that the penetration of mobile magmatic solutions into rocks causes not only a change in the composition of the constituent solid phases, but also a continuous increase in the pore liquid and a continuous change in its composition. The process leads to the formation of a local magmatic liquid that completely replaces the original solid rock.

The distinctive character of the process defined as magmatic replacement was first singled out and formulated by D. S. Korzhinskii [8, 9]. This process is associated with the infiltration into solid rocks of transmagmatic solutions containing water and alkali metals, with the manifestation of a stage of their dissolution or melting. In this, an intermediate colloidal stage is admitted. Noting the incorrectness of ideas about the metasomatic origin of granites, D. S. Korzhinskii points to the difficulty of explaining the polymineral composition of granites, as well as the leveling of their composition, if a metasomatic mode of formation is assumed.

By magmatic replacement we understand, in accordance with D. S. Korzhinskii’s definition, the penetration into solid rocks of a mobile solution of magmatic origin, usually of alkaline-acid composition. As a result of the interaction of the solution and solid crystals, not only does diffusion exchange and equalization of the composition of the solid and liquid phases occur, but also a continuous increase in the amount of liquid phase, promoted, apparently, by the unleashing of exothermic reactions during the alteration of high-temperature magmatic materials. Local formation of secondary magma is manifested; it either crystallizes in place or is slowly squeezed out into somewhat higher horizons. This is also aided by the upward displacement of the front of magmatic replacement.

However, magmatic replacement is not a mechanical process of replacement of the original material. Undoubtedly, this process is complex and is associated with a continuous change in the composition of both the liquid and the solid substrate. At the same time the composition of the volatiles changes continuously, the vapor pressure of the latter increases, and their migration into the enclosing rocks increases. The composition of the liquid changes toward the most easily fusible eutectic and depends to a considerable extent on the replenishment of the solution being consumed by new portions from depth.

As a result of the action of alkaline-acid solutions on the basic minerals of the rock, a transition into the mobile state and removal of ...

Mg, Fe, Al, Ca, together with the principal components of the solution in the form of silica and alkalis. Volatile OH, H, CO₂, halogens, and others play a major role; they not only accelerate and regulate the process of replacement, but also promote the mobilization of metals from the medium and determine the specificity of the hydrothermal solutions associated with them.

In many intrusive massifs we have diverse rocks connected by mutual transitions, which are explained by phenomena of replacement of earlier formations. The latter are preserved as small areas along the margins of the massifs, sometimes in the inner part; not infrequently they have the character of large xenoliths. In connection with incomplete processes of assimilation of early rocks by portions of later magma, hybrid formations or schlieren textures are encountered. Let us give several examples.

The author studied in considerable detail the Bel’skii intrusive massif, situated on the Izekyul River, a right tributary of the Black Iyus River on the eastern slope of the Kuznetsk Alatau. Within the area of the massif, small bodies of gabbro are observed, with transitions to monzonites, quartz monzonites, and quartz syenites, which occupy the main part of the massif. In the southern part of the massif, quartz syenites and granites are developed. Quartz foyaite occurs as small patches among the noted rocks. The transitions between all types of rocks are gradual. A widely manifested albitization is confined to the final stage in the development of the magmatic rocks of the massif. It leads to the formation not only of foyaites, but also of albitic diorites, as well as peculiar ovoid albitites [5, 6]. In the contact zones associated with horizontally lying remnants of the roof, desmosites and adinoles are widely developed.

Belonging to this same intrusion is the nearby massif of Spasskii Golets, where rocks of the gabbro series predominate and there are transitions to quartz diorites and plagiogranites, forming very small patches.

In the Bel’skii area, clear signs of magmatic replacement are observed both in the gabbro and in the diabase porphyrites enclosing the intrusion. Signs of replacement of the gabbro are expressed in the presence of relicts of the gabbro itself among monzonites, as well as in the broad development of late xenomorphic potash feldspar, transitions of titanaugite into diopside, relicts of ophitic structure in monzonites and syenites, and relicts of titanomagnetite grains in the form of a network of ilmenite plates among titanomagnetite, replaced by chlorite, leucoxene, and sphene. In places no trace of magnetite remains, since all the iron has been removed. The general sequence of rock alteration is connected at first with a considerable accumulation of alkalis and silica, removal of magnesium, iron, calcium, and aluminum, with formation of monzonites and then syenites. The subsequent manifestation of replacement is expressed in a considerable increase in silica and sodium, with formation of quartz syenites and granites. Thus the process seems to split into two stages—the earlier stage of syenitization and the later one of granitization.

A very interesting fact is the correspondence of the mineral facies of the hornfelses to the magmatic rocks developed in the near-contact zone. Adjacent to the gabbroic rocks are plagioclase–pyroxene hornfelses; adjacent to the diorites, plagioclase–hornblende hornfelses, etc.

Iron-ore skarns are associated with the Bel’skii and Spasskii massifs, being confined to the contact area, as well as quartz–siderite–hematite and quartz veins with gold–copper–bismuth sulfide mineralization.

In the southern part of Mountain Shoria the Lebed intrusion contains, among granodiorites and weakly developed granites, bodies of syenites, diorites, quartz diorites, and gabbro. At the same time the gabbroic rocks have the character of relics with uneven boundaries among more alkaline and acidic rocks. Xenoliths of reworked gabbro are especially abundant among the diorites. It is interesting that iron-ore skarns are associated with the massif, occurring in the hanging wall (the Kourchak deposit). At the same time, not only a concentration of magnetite, but also sulfide mineralization and occurrences of gold mineralization of industrial significance are connected with the skarns.

The Kondoma group of iron-ore deposits is confined to small massifs of syenites, which are arranged in a chain in an NNW direction. These are the Kochurinsky, Tashtagolsky, Shalymsky, Kubessky massifs and a number of small exposures of this intrusion. In addition to syenites, monzonites and diorites have been described in them, and N. G. Sumin even distinguishes gabbroic rocks in the Tashtagol massif. According to V. G. Korel’ [11], in the monzonites there is development of xenomorphic calciphyte, corrosion of pyroxene and preservation of its remnants, and later development of amphibole. All this, together with the presence of uneven assimilation contacts between monzonites and syenites and the development of hybrid diorites, also indicates, in our opinion, the presence of phenomena of magmatic replacement of gabbroids with the formation of syenites. G. L. Pospelov [16] notes the close connection of metasomatic deposits with magmatic replacement, using as examples the Tashtagol district, where syenitization and porphyritization are developed, and also Salair, where a similar mode of rock formation is proposed for quartz keratophyres.

The Olginsko-Ampalykskaya intrusion (with an outcrop area of 130 km²) in the northern part of the Kuznetsk Alatau, according to V. G. Korel’, E. G. Bordyugov, and V. N. Kuznetsova, is a complex, multiphase, but genetically unified rock complex. The age of the intrusion is post-Ordovician; pebbles of rocks of this intrusion are found in the Telbes Formation of the Lower Devonian. The early phase of the intrusion is associated with the formation of gabbro, diorites, monzonites, and syenite-diorites, with the presence of gradual transitions between them. Subphases of more acidic rocks are distinguished—quartz diorites and monzonites, tonalites, and also granodiorites. In the quartz diorites, enrichment of the rocks with orthoclase begins from fine fractures, which indicates a later separation of an alkaline melt and the local formation of syenite-diorites at the expense of the previously formed solid rock.

Among the syenites there appears a fine-grained quartz–orthoclase aggregate along fractures and boundaries of hornblende and feldspar. Titanomagnetite and ilmenite are always accompanied by products of their alteration in the form of sphene. Numerous data on the features of the distribution of the rocks, their composition, and structure clearly show that signs of magmatic replacement at this level are widely associated with rocks of the first intrusive phase. Skarns confined to syenites are developed.

The second phase of the intrusion forms an independent body composed essentially of adamellites, with transitions to trondhjemites and syenites. Replacement phenomena are weakly developed here. Supersaturation with silica is manifested. The main process of magmatic replacement apparently took place deeper. Higher up, a melt close to homogeneous penetrated.

The third phase of the intrusion is represented by alaskitic and alkaline granites in the form of small stock-like bodies and veins. Elements of monzonitic structure with replacement of plagioclase by calciphyte are widely represented. Supersaturation with alumina is manifested. Formation

rocks is connected with a high content of silica and alkalis. Characteristic is the abundance of postmagmatic processes with the development of carbonates, sulfides, etc. The above-mentioned investigators, incidentally, acknowledge the great role of transmagmatic solutions in the formation of syenites and granites.

In the area of the comparatively large Ampalyk iron-ore deposit, in the northern part of the intrusion, distinct traces are observed of the reworking of early gabbros by siliceous-alkaline solutions, with the formation of quartz diorites, monzonites, and syenites. Gradual transitions appear between the noted rocks. In the monzonites and syenites, relict minerals of gabbro are widely developed, with traces of their alteration and decomposition. Titaniferous augite in the gabbro is replaced by salite and diopside. All traces can be followed of the dissolution and removal of magnetite. At the same time, in place of grains of primary titanomagnetite there remains a network of plates of the more inert ilmenite, which is a product of the breakdown of the solid solution, while the magnetite disappears completely. Abundant leucoxene or well-crystallized sphene, replacing ilmenite, is encountered. The syenites, which are the extreme products of the reworking of gabbro, are very rich in sphene, whose content locally reaches 10–15%. In the contact zone with the syenites there are associated widely developed skarns with magnetite mineralization, and also later sulfides. According to E. A. Babina, magnetite is separated at the end of the process of skarn formation, after the principal skarn minerals in the form of diopside, scapolite, garnet, and epidote, following the amphiboles. In the skarns, too, a wide development of sphene is noted. In the area, later granites are widely developed, cutting the rocks of the early phase of the intrusion. Their younger age in relation to the magnetite skarns is noted.

A. N. Zavaritskii gave a detailed description of the magmatic complex of Mount Magnitnaya in the Urals, with which a skarn iron-ore deposit is associated [4]. He connects the diversity of rocks of the complex with the assimilation of limestones by granitic magma. However, this is contradicted by the presence of large remnants of granodiorites and diorites among the granites in the central part of the massif. In the marginal parts of the latter, diorites and gabbros occur. Gradual transitions from gabbro to diorites and granites, and also between granites and syenites, are observed. There are well-expressed signs of replacement of gabbro and diorites by syenitic magma and transitions of syenites into granites. Characteristic in the syenites is the predominance of albite and albite-oligoclase, incidentally dusted with hematite. Colored ferruginous pyroxenes—augites—are replaced by diopside (removal of iron). In the intermediate formations—diorites—there is much sphene and no magnetite. A considerable removal of iron and magnesium is present (in the granites, traces of magnesium) and an accumulation of aluminum.

One may cite a whole series of further examples of this kind, indicating phenomena of magmatic replacement of gabbroids. In this process there appears removal of iron, magnesium, calcium, aluminum, silica, and a number of other components by alkaline solutions, with the formation of skarns, of which the most interesting are magnetite, scheelite-bearing skarns, skarns with sulfides and gold, polymetallic and cobalt mineralization, and also skarns with boron mineralization.

It should be noted that magmatic replacement of enclosing rocks of sedimentary origin is not infrequently manifested. Thus, D. S. Korzhinskii established the formation of diorite porphyrites through replacement of carbonate rocks in the area of Turyi in the Middle Urals [8]. G. L. Pospelov, and later V. A. Vakhrushev [3, 16], note a similar mode of formation of rocks, called by the former hornblende porphyrites, and by the latter

… by dioritic porphyrites developed in the area of Tashtagol, Gornaya Shoria.

In the part of the gabbro massif of Mount Patyn studied by us, there are indications of magmatic replacement of the enclosing limestones. Here, in the southwestern part of the contact of the intrusion, the limestones are transformed into rocks consisting of calcite, wollastonite, locally forsterite or titanaugite. Among these rocks there are very characteristic nodule-like and spotted segregations of melilite-nepheline or melilite-nepheline-garnet rock. These nodules have a morphological similarity to the nodular and conglomerate-like segregations of the above-mentioned porphyrites in the Tashtagol area. Large areas are formed by melanocratic rocks of melteigite and ijolite composition, having a directive texture. The formation of a local alkaline-lime undersaturated magma is indicated by the presence in this area of small dikes of nepheline-melilite, nepheline-sodalite-albite, or monticellite compositions, which are developed not only in the exocontact zone but also occur within the gabbro massif.

Time of Separation of Alkalis and Silica

It is interesting that the separation of alkalis and silica, leading to the formation of more alkaline rocks, apparently begins already in the magmatic stage, immediately after the emplacement of the gabbroic magma. This can be seen in examples of horizontal plutons that have a weak connection with the feeder channel. Among the intrusive traps of the Siberian Platform, the segregation of alkaline and acid rocks in the upper horizons of individual massifs was noted by A. P. Lebedev [14], V. L. Masaitis [15], M. G. Ravich and L. A. Chaika [17].

In the South African Bushveld lopolith, beneath the granites of the late intrusive phase, there occur upper layers containing granophyric norites and quartz syenites.

In the Valaam and West Onega sills, composed of gabbro-diabases, according to the data of K. O. Kratts, syenite-diorites, and locally granite-aplites, are observed in the upper parts of the massifs.

We have noted alkaline rims in small funnel-shaped massifs of Lower Devonian gabbroids of Gornaya Shoria (Patyn, Kul-Taiga). Thus, in the southeastern near-contact zone of the Mount Patyn massif, normal gabbros have narrow rims of monchiquite, which, in the direction of the contact with the enclosing sequence, passes into syenite. The thickness of such a zone is 5–30 m, i.e., it is insignificant, since the total area of the massif is about 50 km². Small dikes of selsbergites and grorudites are also confined to these areas.

Early separation of alkalis from the magma was noted by Yu. A. Bilibin [1], who proposed explaining this process by thermal dissociation of molecules.

Naturally, with a flow of alkalis and silica from below upward, the mass manifestation of such through-magmatic solutions should be expected in steeply dipping plutons. The segregation and comparatively rapid migration of alkalis and silica upward may cause the accumulation of an alkaline solution not only in the magmatic stage, but more often at the moment when the upper horizons of the pluton have already passed through the principal stage of crystallization. In this case, the paths of penetration of alkaline-siliceous solutions remain interstices or angular spaces between crystals. With a weak manifestation of such a process, orthoclase and granophyric gabbros and norites are formed, with a characteristic structure of alkaline-acid mesostasis.

The mass accumulation of such solutions may lead, as we often see in complex massifs, to replacement of the earlier gabbroid rock with the formation above all of monzonites and syenites. Syenitization at this level apparently characterizes gradual transitions between gabbroids and syenites, with the presence of monzonites or hybrid rocks. Thus, A. N. Zavaritskii, in describing small intrusive masses and veins of diorite porphyries in the area of Mount Magnitnaya, noted the replacement of andesine phenocrysts by orthoclase from the margins inward, up to the formation of pseudomorphs. Characteristic here is the appearance of late magnetite along the finest cracks in crystals of feldspar. In the hanging wall of the diorite porphyries there occur peculiar fine-grained rocks—atachites—formed as a result of the accumulation of potassium, aluminum, and silicon with replacement of earlier labradorite segregations by orthoclase.

Relation to ore formation

As is evident from the examples given above, the phenomena of magmatic replacement are closely connected with the processes of early metasomatism, the formation of postmagmatic solutions, and ore formation. With gabbro intrusions that do not carry later differentiates there are associated lime-magnesian metasomatic formations (skarns, hornfelses), generally weakly developed. Thus, for example, magnesian, almost iron-free skarns of the Karo and Bis-Tag areas on the eastern slope of the Kuznetsk Alatau are confined to gabbro.

In connection with gabbro–syenite massifs that occur within the field of the Cambrian spilite–keratophyre formation in the Kuznetsk Alatau, during the process of syenitization a regional early metasomatism is observed, with mass removal of iron and magnesium. As a result, over broad areas of the enclosing rocks, phenomena of amphibolization and dioritization of basic effusives are developed.

Together with the formation of local syenitic magma, skarn solutions begin to originate, containing Mg, Fe, Al, Si, Ca, B, Cl, H, OH. During crystallization of the syenite they are transmagmatic and separate into the contact zone. With a more prolonged process of development of magmatic replacement, passing into granitization, the composition of the skarn solutions changes with an increase in the content of volatiles, sulfur, and silica. In connection with the more prolonged development of these solutions and the peculiarities of their composition, they become enriched in rare and nonferrous metals—W, Mo, Cu, Pb, Zn, Au—which precipitate as sulfides, while tungsten usually occurs as scheelite. In this case the sulfides appear in the skarns in the final stages of the process of their formation, or else they feed the hydrothermal solutions. Since the separation of skarn solutions follows the crystallization of syenitic or granitic magma, being a consequence of the latter process, the skarns are formed in the postmagmatic period. The high content of alkalis, especially sodium, is characteristic of these conditions; at the same time, being mobile components, they are fixed in the composition of minerals at the final moments of mineral formation. In areas of skarn development, albitization is widely developed, locally scapolitization. The study of intrusive bodies also shows a broad manifestation of sodium metasomatism in the form of albitization, more intense near skarn fields, with the development of perthites, antiperthites, and free albite, up to the formation of albitic rocks [5, 19].

The high sodium content causes increased mobility of iron and its accumulation in hydrothermal solutions. To this type of formation belongs the hydrosilicate type of iron-ore depos—

deposits distinguished by V. V. Bogatskii [2]. They are more sharply controlled by fracture structures and are accompanied by broad fields of albitized rocks (the Anzas and Abakan deposits). Such formations may be superimposed on previously formed ore skarns and may produce a high concentration of mineralization owing to an additional introduction of iron from deeper horizons. Because of the cooling of the intrusive body and the movement of the front of magma crystallization from depth, the formation of albitized rocks and the ores of hydrothermal type associated with them should be regarded as a regular process, provided the feeding magmatic channel remained stable over time.

Skarns spatially associated with syenites usually contain weakly expressed sulfide mineralization. For example, the latter occurs in all, without exception, iron-bearing skarns of Gornaya Shoriya, as well as on the eastern slope of the Kuznetsk Alatau. Sometimes the skarns in the outer zone are succeeded by relatively weak disseminated polymetallic mineralization (Ampalyk) and then by quartz gold-ore veins (the Belskii massif).

Where there are signs of syenitization of gabbroids, regional iron–magnesian metasomatism usually develops in their place, and the skarns are weakly manifested. The latter are more often products of bimetasomatism. This condition is not obligatory, however, since it depends on the development of fracturing in the medium and on tectonics. Where, within the massif, there are intrusive contacts between different derivatives of magmatic rocks, a predominance of syenites—indicating the manifestation of the process of magmatic replacement somewhat deeper—and the formation of a mobile syenitic melt, more abundant skarns appear, with infiltration metasomatism predominating. In this case a larger scale of iron mineralization is usually observed. Favorable tectonic conditions, leading to the localization of separating ore solutions along large fractures, may result in the formation of deposits with persistent mineralization to a considerable depth. An example of this, apparently, is the Tashtagol deposit of the Kondoma group, or the hydrosilicate deposits—the Anzas and Tei deposits.

Thus, the maximum removal of iron and the formation of iron-rich skarns are manifested at the stage of syenitization. By the stage of granitization the removal of iron weakens, but the role of sulfur-bearing solutions increases, leading to the formation of larger concentrations of sulfides of heavy metals. From what has been said above regarding the origin of skarn-forming solutions, it follows that their formation begins during the formation of gabbro and reaches maximum development during the formation of syenites and granites; at the same time the character of the skarns formed changes noticeably. There also clearly appear, in a number of districts, close paragenetic links between hydrothermal quartz veins with gold, copper, bismuth, tungsten, and other types of mineralization and skarn formations. With the noted mode of formation of magmatic complexes accompanied by iron mineralization, the multiphase nature and variegated composition of the rocks, the sources of emanations and their evolution, as well as the connection with the peculiarities of skarn manifestation, are naturally associated.

LITERATURE

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