Genesis of Ferberite Ore Deposits in the Upper Racha Region (Transcaucasia)

Prof. F. N. Shakhov.

1. Introduction

In the summer of 1937, at the invitation of the Transcaucasian Division of “Soyuzredmetrazvedka,” I was able, in the course of consulting on prospecting work, to become acquainted with an original tungsten mineralization in the Upper Racha region, Western Georgia. Some of these deposits had been known earlier, but most of them were discovered by parties of Zakredmetrazvedka in the summer of 1937 (12). Inspection of the prospecting and exploration work enabled me to make a number of observations, the material of which I used in preparing the present article. In addition, for the same purpose I processed small collections of rocks and ores from the tungsten deposits of Upper Racha, kindly sent to me by the chief engineer of Zakredmetrazvedka, G. M. Gogoberidze, and by the geologist A. E. Bendeliani, and I partly examined stone material from the molybdenite deposit of Karobi, sent by G. I. Kharashvili. Goniometric studies of ferberite crystals were carried out by Docent of Tomsk State University V. K. Monich; chemical analyses of ferberite were performed by Docent of the Tomsk Industrial Institute I. P. Onufrienok; and drawings of the textures of the ferberite ores were made by laboratory assistant of the same institute T. F. Cherepanova. I take this opportunity to express my deep gratitude to these persons.

2. General Geological Data

The area of the prospecting and exploration work lies in Upper Racha (Western Georgia) and represents a band of northwestern, nearly latitudinal strike, bounded on the north by a zone of major dislocation, interpreted by geologists as an overthrust of pre-Paleozoic granites onto Jurassic deposits (2; 14; 10, 27; 29, 27). The granite rocks are exposed on the highest parts of the Main Caucasus Range, on the southern slope of which Jurassic (Liassic) shales emerge from beneath them. This band of shales, approximately (2, 14) from 3 to 4 km wide (counting southward from the overthrust boundary), was subjected to systematic examination over almost its entire extent (approximately 40 km), from the Mamison Pass to the upper reaches of the Zopkhito River, a left tributary of the Rioni River.

The principal formation composing the examined area consists of Jurassic Liassic clay shales. In addition to the ancient granites noted above, dikes of diabases are widespread, especially near the overthrust seam; small bodies of acidic hypabyssal rocks—“neointrusions”—are encountered occasionally; and near the Mamison Pass there are outcrops of andesites. Let us briefly consider the main characteristic features of the formations distinguished.

The granites are always represented by tectonites. A characteristic feature of the disinteg-

of rocks is the absence of schistosity phenomena leading to the roughly parallel textures of dislocation gneisses. Macroscopically the rocks have massive and, more often, blurred (when slightly abraded) breccia-like textures. Their fine-grained varieties, megascopically and microscopically, may be taken for felsitic porphyries, especially if the crushing has been healed by subsequent mineralization (usually silicification), which is often observed. The width—thickness—of the crushed granites, unfortunately, was not determined, but it may be assumed that it (taking into account the gentle dip of the disturbance) is small and in any case cannot be measured in several kilometers, as is characteristic of the Altai zones of crumpling, within which eruptive rocks are transformed into dislocation gneisses (30).

The Jurassic shales consist chiefly of perfectly schistose, sometimes phyllite-like, clayey rocks and, more rarely, sandstones. In the southern part of the shale belt, marly rocks are developed; geologists have distinguished them as a special series, lying stratigraphically above the clay shales adjoining the overthrust (on the north) and called carbonate. Contrary to earlier assumptions about a tectonic boundary between these series, local geologists consider the occurrence of the rocks normal and the formation genetically homogeneous (2, 15; 28, 74). It must be noted, however, that disjunctive displacements in this formation of schistose rocks are established with great difficulty, and since no special observations for establishing tectonic structures have been carried out in the district, the question of the relation of the “limestone unit” to the Lias shales can hardly be considered resolved.

The principal structural element of the clay shales is a northwestern-trending schistosity, represented mainly by two systems of fractures (not counting transverse jointing), which obscure the primary bedding. According to personal observations and data from the literature, the Jurassic shales are additionally folded into small folds (2, 14), which is in full agreement with A. D. Ershov’s observation in Svaneti, where in the ore field of the Tsenk mine, “fine secondary folding, developed, in his opinion, in connection with the thrusting of the ancient crystalline basement onto the sedimentary rocks of the Jurassic” (10, 271), has been recorded in the Jurassic shales. Examination of mineralized Jurassic shales, often charged with pyrite dissemination and stringers, shows that the schistosity planes, intersecting at an acute angle (about 30°), differ in morphological features. The surfaces of the most densely developed schistosity—a (Fig. 1)—are smooth and almost unmineralized. Along them only a weak spreading of pyrite dissemination—v—is noticeable, clearly extending from fractures—b, healed by pyrite stringers. The fractures—b—are less frequent, roughly uneven, and healed by pyrite with a small amount of quartz. A curvature of the surfaces—a—is noticeable at the place of intersection with fractures b, but always at one end. It may thus be thought that by the time the overthrust developed, the Jurassic shales had already been dislocated and probably schistosed¹. The overthrust created a secondary

Fig. 1.

additional platy structure; after it arose coarse-cleavage fractures—б—and only later were the rocks mineralized¹.

In composition, the clayey shales of the Lias are not uniform over the entire length and width of the belt studied. Deviation from the normal composition is due to two causes: contact metamorphism and thermal mineralization. Noticeable contact metamorphism up to biotite hornfels is observed only in the ore field of the Karobi molybdenum deposit, where it is undoubtedly connected with a young intrusion of acid rocks. Traces of thermal mineralization are observed more often; pyritization is especially strongly developed, and for the most part it is undoubtedly epigenetic and connected with thermal processes. Optical study of such pyritized shale reveals the following features. The relict clastic structure is quite clearly preserved in the shale, and it can always be recognized that the greater part of the fragments consisted of quartz and feldspar, and that the clayey mass cementing them was apparently represented by the same material, only more finely comminuted. The metasomatic minerals are: pyrite, quartz, and calcite. The latter two minerals, together with sericite (?) grow upon the faces of pyrite. It should be noted that the rims sometimes surround the entire section of a pyrite crystal in a thin section as a continuous halo. In such cases the most widespread mineral is sericite (?). Rarely, chlorite develops instead of sericite. Observations of the growth patterns show that the order of mineralization is characterized by the following sequence: pyrite, sericite (chlorite), quartz (calcite). The part of the rock most strongly affected by mineralization is the clayey mass, which is sometimes almost entirely transformed into a fine-lamellar micaceous aggregate, exhibiting pearly interference colors in crossed nicols, thereby resembling minerals of the kaolin or sericite group. In places, on the rims of the pyrite, needles of an ore mineral were observed among the sericite flakes; its nature remained undetermined.

Diabase dikes occurring in the Jurassic shales have been noted by many geologists (2, 15; 7; 8, 90–91; 9, 923; 10, 270) and, for our district, have been described in greatest detail by G. I. Kharashvili (28, 72–75), according to whose data these rocks in fresh varieties belong to “augite diabases”; more often the rocks are altered and are represented by uralite diabases and diabase porphyrites². It is most probable that these rocks in our district are for the most part effusives of dike facies, after M. A. Usov (24, 41–44; 25, 799–801), if one takes into account their occurrence within the thrust boundaries, where their bodies can be interpreted only as dikes, and the constant uralitization of autometasomatic origin. This question, however, requires additional observations.

The age of the diabases in the Lias shales of the Caucasus is usually regarded as Jurassic (3, 23; 8, 90–91; 9, 923; 10, 270), and sometimes even as Lower Jurassic (7, 342). In our district the diabases occur both in the thrust granites and in the rocks of the Lias. I did not succeed in observing sills or dikes of diabase among the rocks of the limestone series, while A. E. Bendeliani asserts that in the area of the Notsara ferberite deposit and the Talakhian cinnabar deposit all rocks, “with the exception of the carbonate sequence, are cut by diabase and diabase-porphyrite dikes” (3, 23). The diabases are undoubtedly older than the ore mineralization, since they are often fractured and even silicified. The confinement of these rocks to the thrust belt, which is noted not only for Upper Racha (3, 23), but also for Svaneti³ (10, 271), compels

to regard the question of the age of the diabases as an alternative one, since the crushing of these rocks cannot prove that they were emplaced before the formation of the main thrust, but may have been caused by subsequent movements, which occurred repeatedly in this area.

Undoubtedly younger than the thrust are the small bodies observed in the region of the neointrusions. In the area of the Karobi molybdenum deposit, G. I. Kharashvili (28, 77) recorded and described three intrusive bodies of dacitic composition. The outcrop area of one of them reaches 0.2 km². All of them cut the thrust zone and occur chiefly in ancient shattered granites. The form of the bodies is lens-like—approaching stock-like. The rocks have been affected by thermal mineralization; in particular, at the Karobi mine the molybdenum ore is a sharply silicified dacite with inclusions of molybdenite. On the strike from Karobi, in the area of the Notsara ferberite deposit, A. E. Bendeliani (2, 15) discovered “outcrops of andesites in the form of small interstratal dikes.” A specimen of similar rocks in my collection, brought from the left slope of the Buba River, is very close in character to the dacites of Karobi. Megascopically, we are dealing with a light-colored, friable rock of liparitic appearance, often clayey and clearly affected by processes of thermal metamorphism. The microscope confirms the dacitic composition of the rock and sharply expressed traces of thermal action, consisting in the development of sericite and carbonates, which sometimes completely fill the groundmass. It is possible that, in comparison with Karobi, in the Notsara area we have an erosional section more distant from the main parent intrusion; and it must be supposed that the surface at the Mamison Pass is even farther removed from it, where “post-Miocene lavas—andesites” are known (2, 15) and where, possibly, their covers have been preserved from erosion¹). However, examination of an andesite specimen from the region of the Mamison Pass shows the identity of its mineralogical composition with the dacite of Notsara and only the following differences from it: the rock has a felsitic appearance and a fluidal structure of the groundmass. At the same time, products of thermal activity—sericite and carbonates—are also considerably developed in this rock. It may be considered with considerable certainty that in the Mamison Pass–Karobi area the facies of dacitic rocks, according to Usov (25), change from possibly cover-like at Mamison to clearly stock-like at Karobi.

In addition to the dacite-andesites, dikes of albitophyres have been established in the region of the Karobi mine, cutting the diabases, the rocks of the thrust, and its structures (27, 9). These formations are clearly older than the dacites, since they are cut by them. Finally, the Jurassic shales of the Karobi ore field have, over a considerable extent, been transformed into biotite hornfels²). The bands of these metamorphosed rocks, according to the observations of G. I. Kharashvili (28, 76), extend in accordance with the general direction of the thrust, i.e., perpendicular to the strike of the albitophyre dikes and of the dacite body. It seems to us that the metamorphism of the Jurassic shales is due to an intrusive body of considerable size extending along the tectonic seam, while the albitophyre and dacite bodies should be regarded as an additional injection of residual magmas rich in volatiles, which at times broke through to the surface and with whose activity all ore mineralization in the region is connected. From the small amount of material at our disposal it is evident that the additional injection occurred more than once and, consequently, from the time of the thrust to the end of ore mineralization, repeated movements took place in the thrust zone, accompanied by volcanic activity.

¹) G. I. Kharashvili notes “Quaternary andesite lavas on the Domba and Karobi ranges” (28, 80).

²) Metamorphism is also noticeable in the shattered ancient granites (28).

3. PRINCIPAL FEATURES OF ORE MINERALIZATION IN THE UPPER RACHA REGION

In the territory studied, deposits of molybdenum, tungsten, cinnabar, and antimony are known. The ore fields of all these deposits are confined to the lower Liassic sequence—argillaceous shales; but it cannot be supposed that the limestones are unfavorable for mineralization, since the known Lukhumi deposit of realgar–orpiment ores occurs in calcareous-argillaceous shales, which I. Kuznetsov assigned to the upper Jurassic or lower Cretaceous (9a, 170). Thus, the general confinement in our region of ore mineralization to a comparatively narrow belt of homogeneous rocks should be connected causally and spatially with the thrust zone, which, incidentally, has the same elements of occurrence. In the vicinity of this crumpling zone all the subsequent events that led to the formation of the ore deposits took place; in particular, volcanic phenomena are spatially and temporally connected with it, as a result of which the bodies of the young intrusions of the Caucasus arose, which are regarded as the cause, in general, of all ore mineralization (5, 11, 18, etc.).

Within the belt of argillaceous shales, the arrangement of the ore bodies may be regarded as en echelon only in general outline, since the deposits clearly do not follow any single line of rupture, but are located along a number of fracture zones that arose in different parts of this belt and not simultaneously. As a result, within one ore field (Mamisonsky Pass) one can observe two roughly parallel belts of mineralization, one of which is represented by cinnabar ores and the other by ferberite ores. Thus, it may be thought that within the belt of Liassic argillaceous shales there is a complex zone of disturbances of the imbricate overthrust type. Naturally, the disturbance is most sharply expressed at the boundary between the granites and the shales, and is only weakly visible within the shale sequence, where only brecciated and mineralized areas reveal it to the observer’s eye. Movements in this weak and unstable zone occurred repeatedly and, judging from the phenomena of coarse brecciation of the shales (and granites), as well as from the small width of individual zones of crumpling (31), were resolved at a shallow depth, near the surface.

The principal features of the ore composition of the distinguished types may briefly be characterized as follows. In each individual deposit the predominant assemblage of minerals has a sharply expressed specific character, but as admixtures, sometimes rather significant ones, one may expect to encounter the entire range of minerals characterizing the ore mineralization of the region as a whole. Thus, in the Karobi deposit of quartz–molybdenite ores, pyrite, arsenopyrite, chalcopyrite, enargite, and antimonite are noted in small quantities. G. I. Kharashvili emphasizes that, with minor exceptions, these minerals were formed as a result of the latest mineralization and are represented by a special phase of mineralization (28, 83–87). Within the ore zone of the antimonite ores of the Zopkhito deposit, ferberite occurs. In the cinnabar ores of the Talakhian mercury deposit, A. E. Bendeliani (3,24) found ferberite, pyrite, chalcopyrite, and antimonite. In almost all ores of the ferberite deposits, antimonite and other sulfides are encountered. Within individual ore veins, zonality in the mineralization was not observed, and it may be thought that it is absent, since the same specific features are also characteristic of the arsenopyrite Tsensky deposit (Svanetia), where a considerable massif of a young intrusion is exposed and where, nevertheless, wolframite, various sulfides, and even orpiment and realgar have been found in the arsenopyrite veins (3a).

Certain regularities in the spatial distribution of differ-

deposits can be observed, on the basis of their spatial association with magmatic rocks of different facies. Thus, in the Karobi deposit, where additional injections of dike and stock facies are developed and where, apparently, the parent intrusion approaches close to the surface, the highest-temperature quartz–molybdenum mineralization is observed.¹⁾ The ore-bearing dacites are surrounded by deposits of sulfide ores, produced by later mineralization, as a result of which “arsenopyrite and predominantly polymetallic veins” were formed (28, 87). It should be noted that in Karobi we have a local, weakly expressed horizontal zonality. In the areas farther east (Mamison Pass) and west (Zopkhito, Sagebi), where volcanic rocks are represented by surface facies or are entirely absent, we have an association of bands of ferberite ores with parallel bands of cinnabar (Mamison Pass), or a spatial coincidence of ferberite and antimony mineralization, with a transition from ores of one composition into others (Sagebi, Zopkhito).

Thus, the spatial association of high- and low-temperature minerals and mineralized bands suggests that, on the whole, mineralization took place under conditions of rapid temperature change, which permitted the spatial superposition of different stages of mineralization. Such superposition was greatly favored also by the narrow belt along which intramineralization movements were released from time to time, separating the different stages of mineralization and sometimes (the case of the Mamison deposits) leading also to the spatial separation of different mineral associations. In the Karobi area, however, where the heating of the wall rocks was stronger, where the mass of the parent intrusion came closer to the surface and the bodies of additional injections were more substantial, we may observe, expressed to one degree or another, a horizontal zonality which in its principal features recalls the zonality of the Tyrnyauz ore field (19, 785–786), where, owing to the considerable size of the parent pluton, it is manifested more sharply and more extensively. In our region this horizontal zonality, judging from the Karobi ore field, is strongly obscured by phenomena of abrupt superposition. It must be assumed that in the Upper Racha region mineralization took place near the surface, which was reached by thermal waters and additional injections that often extended for a considerable distance from the parent intrusive body, in any case beyond the rocks of its contact aureole.²⁾ In this case the local zonality could have been causally connected and spatially confined only to the bodies of additional injections, which undoubtedly created equally heated areas; along them, up to the very surface, there also rose ore-bearing solutions heated to differing degrees, which retained the material and the capacity to deposit one or another mineral. Naturally, the paths of movement of the thermal waters could have been, first of all, the bodies of the additional injections themselves, as is characteristic of Karobi, where the solutions retained a high temperature and produced the corresponding mineralization.

Let us now consider the principal features of the forms of the ore bodies and the distribution of ore material within them. Within the limits adjoining the thrust zone of the Liassic shales, disseminated mineralization is very widespread. Especially frequent are pyritized and silicified shales and sandstones.³⁾ Mercury-ore deposits are represented, as in Dagestan (13 and 14), by—

¹⁾ A. E. Fersman (26, 290) considers molybdenite the first mineral in the paragenetic series of chalcophiles and places it before wolframite and even cassiterite.

²⁾ L. A. Vardanyants (6, 473) in the Tepli and Kazbek area measures the width of biotitized shales around young intrusions at 1–1.5 km.

³⁾ In Nochara there are ferberite disseminated ores (2, 16–17).

bands of disseminated mineralization in individual beds, or possibly zones of fracturing in the rocks of the Lias, silicification is very strongly developed. In the area of the Mochantsara and Buba rivers, cinnabar was observed in quartzes composing a series of ore bodies of indefinite, often vein-like, form. Similar features are also characteristic of the ferberite and antimonite deposits, where one cannot speak of a definite, and in particular vein-like, form of the ore bodies. Prospecting usually deals with a belt of fractured silicified rocks, mineralized to varying degrees in different sectors. Rich ore may quickly pass into poor, and even almost unmineralized, breccia. Thus, within sometimes extensive zones of mineralization, nests and columns of rich ores are observed. This is especially characteristic of the ferberite and antimonite deposits. The latter occurs in nests even within belts of ferberite mineralization. Despite the large-scale survey carried out, there are no data for correlating the structural elements of the enclosing rocks with the development of the general mineralization and columnar enrichment. One may only suppose that the silicified zones of fractured shales are connected with sectors of disturbance of the elements of bedding (schistosity) of the Jurassic rocks, and the richer ferberite ores with sectors of fracturing in already silicified rocks. That is, in the latter case, according to Hulin’s data, we may also expect that a change in the strike of the silicified zone during a subsequent pre-mineralization movement proved to be the place where fracturing occurred and an enriched sector formed (33, 38—39). The same mechanism, as we shall see below, may be expected in the process of formation of antimonite nests in zones of ferberite ores. In general, for the deposits of Upper Racha, sharply expressed and numerous stages of mineralization, separated in time by intra-mineralization movements, are extremely characteristic. The spatial superposition of these stages, under conditions in which local sectors of intensely fractured rocks arise, leads to the development of numerous and often very rich nests and columns of the bonanza type. The study of the ferberite ores fully confirms the propositions stated above.

4. FERBERITE ORES OF THE DEPOSITS OF UPPER RACHA

GENERAL CONSIDERATIONS

For the present chapter I have studied ferberite ores from the deposits of Notsara, Mamisonskoye, Sagebi, and Khvrelizto. The material obtained does not give an exhaustive picture of the possible mineralogical composition of these ores, since, on the one hand, I had at my disposal a limited number of samples (about twenty), and, on the other, all of them were taken from the oxidation zone, and the sulfides in them, as a rule, had undergone considerable alteration and destruction. This material made it possible to establish only the main fundamental features of the mineralogical composition of the ores, and also to determine the sequence of events and the setting of the process of ore formation, which was the task the author set himself.

TEXTURE OF THE ORES

Within the material studied we did not observe typical banded or massive textures. In rich ores, drusy and breccia-like textures are widespread. Drusy cavities are usually filled with beautiful ferberite crystals, on which quartz crystals often grow. In the dumps of the Notsara deposit, large scheelite crystals with well-expressed faces were found. In the drusy cavities of ores of the Mamisonskoye deposit, on quartz crystals, which often grow upon

ferberite, bright ferruginous ochres were observed, apparently formed by the oxidation of pyrite. In these same ores, drusy cavities filled with a silvery micaceous mineral—sericite—were observed.

Fig. 2. Ore of brecciated texture. 1—fragments of silicified shale; 2—ferberite; 3—quartz veins with fragments of ferberite. Magn. 2, Mamisonskoye deposit

It gives the impression that, in this case, sericite appears as a metasomatic mineral deposited by solutions (and not as a product of decomposition of feldspars, or of transformation of the clayey mass of the rock), and that this process takes place during the stage of sulfide development, especially after pyrite (see the description of pyritization in the Jurassic shales). Finally, the cavities are sometimes occupied by antimonite, in my specimens usually altered into antimony oxides. Observations on the nature of the filling of drusy cavities make it possible only roughly to determine the order of mineralization, expressed by the series: ferberite, quartz, antimonite, and other sulfides.

Greater details are revealed by analysis of brecciated textures. They are apparently more widespread than is seen in the field. Polishing of specimens of ferberite ores (Figs. 2, 3, 4) made it possible to identify brecciated textures in specimens from all the deposits. In Figs. 2 and 3 it is seen that the breccia is represented by fragments of varied composition and even age. In portions of massive ferberite ores from the Mamisonskoye deposit and Khvrelizeto, the fragments are represented by angular pieces of silicified shale and ferberite. It is also clearly seen that fragments of silicified rock were cemented by ferberite, and the latter was broken up before the introduction of a network, sometimes coarse (Fig. 3), of quartz veinlets with a frequently developed drusy texture. In one of the fragments (Fig. 3), quartz veinlets are noticeable, cut by ferberite cement. Three assumptions may be made: 1) the quartz veinlets arose earlier, before the silicified rock was transformed into breccia and the latter replaced by ferberite; 2) the quartz veinlets arose in one stage with the ferberite ore along cracks in the fragments; and 3) the quartz veinlets, during subsequent deformation of the ferberite ore, selectively affected

Fig. 3. Ore of brecciated texture. 1—fragments of silicified shale; 2—ferberite; natural size. Khvrelizeto.

the silicified rock was transformed into breccia and the latter replaced by ferberite; 2) the quartz veinlets arose in one stage with the ferberite ore along cracks in the fragments; and 3) the quartz veinlets, during subsequent deformation of the ferberite ore, selectively affected

remaining accessible for replacement by fissured fragments. It must be said that no conclusions can be drawn from the texture pattern for choosing among the assumptions stated. But, turning to observations of the texture of silicified rocks that are barren or only weakly affected by ferberite mineralization, we see that in most cases they are, as it were, shattered and penetrated by quartz veinlets. The latter have an irregular branching form, often end abruptly, or develop into a spot-lens. One gains the impression that these veinlets develop into a spot in places where the rock had been especially finely crushed and where circulation of solutions and replacement proceeded especially intensively. Thus the first assumption is the most probable: that quartz veinlets of indefinite form arose simultaneously with the general silicification of the shattered shale or sandstone in especially crushed areas accessible to circulation.

Undoubtedly the silicified and quartz-veined brecciated shales were shattered again before the appearance of the solutions that deposited ferberite. Mineralization also took place under conditions of formation, by leaching from the breccia material, of voids in which splendid crystals of ferberite grew. Ferberite druses are especially remarkably developed in the ores of Notsara, where, incidentally, silicification is expressed much more weakly than in the Mamisonskoye and Khvrelieto deposits. Apparently, leaching proceeds more successfully from breccias of weakly mineralized rocks than from rocks transformed into quartzites.

Thus, the appearance of ferberite was preceded by the crushing of already silicified rocks; but, examining the textures of breccia-like ores, we observed (Figs. 3 and 4) fragments of ferberite aggregates enclosed in vein quartz. These cross-cutting relations are developed unusually clearly. The quartz veins themselves, cutting the ferberite aggregates, have a drusy texture. In the drusy cavities nothing besides comb quartz was observed, possibly because of the limited amount of material. In samples from Sagebi and Khvrelieto it was possible clearly to establish an association of sulfides, and especially of antimonite, precisely with this stage of mineralization. Therefore there is every reason to consider that the quartz crystals, aggregates of antimonite, sulfides, and sericite that fill drusy cavities in the ores and usually grow upon ferberite crystals developed in a stage of mineralization distinct from the ferberite stage and are separated from the latter by a sharp deformational unconformity.

Fig. 4. Ores of breccia-like texture.
1—ferberite; 2—quartz; 3—inclusions of sulfides. Natural size. Sagebi.

This is confirmed by the fact that in antimonite ores from Notsara the following relations between quartz and antimonite were observed. Fragments of whitish, shattered, sericitized rock, bleached in the oxidation zone, are separated from the ore concentrations by a film of quartz approximately about 3 mm thick. The quartz is fine-grained, but nevertheless even with the unaided eye it can be seen that its fine, acicular crystals are arranged perpendicular to the surface of the fragment, which evidently was also the surf—

…by the rate of cooling. In some cases the impression was created that large accumulations of antimonite, encountered here and there in the Nonar ore zones, represent cavities of considerable size among coarsely crushed rocks, filled with antimonite, since a film of quartz throughout the whole hand specimen separates the antimonite from the enclosing rock.

In addition to the breccias of complex composition described above, simpler breccias are also observed. Thus, at Sagebi I was able to observe (Fig. 5) a breccia that consisted of fragments of almost unaltered Jurassic shale, cemented by quartz, among the aggregates of which antimonite was observed here and there. In the shale fragments a fine sprinkling of sulfides is noticeable, most often arsenopyrite. In this breccia only one stage of mineralization is clearly manifested—one rich in antimonite. It is quite obvious that after the deformations that separated one stage of mineralization from another, spatial superposition of mineralized zones did not always occur. We must take into account that, for our deposits, the superposition of different stages of mineralization in space is a frequent, but not a necessary, phenomenon. The frequency of the spatial coincidence of mineral associations differing in time clearly depends on the frequency of repeated movements along old fissures in the region of a belt of rocks weakened by the main rupture that led to the overthrusting of ancient granites onto Jurassic shales.

Fig. 5. Ore of breccia-like texture. 1—quartz with antimonite; 2—fragments of Jurassic shale with impregnation of arsenopyrite; natural size, Kherelishto.

Far from all movements were accompanied by mineralization. In a number of cases, slickenside surfaces and zones of crushing are noticeable in the ores, marked only by supergene mineralization. These movements are noted not only among ferberite ores, but also among ores of antimony and mercury. They should be regarded as post-ore: possibly, some of them are connected with landslide phenomena and, thus, genetically they may be heterogeneous.

If the examination of drusy textures made it possible to outline roughly the order of mineralization, then the analysis of breccia-like textures made it possible to determine stages of mineralization separated by deformational unconformity. The following stages of mineralization can be outlined quite clearly.

1. Formation of the main ruptures and silicification of the broken rocks; the earliest stage and one not everywhere manifested with equal intensity. Silicification occurred in patches—often rather large areas.
2. Quartz-ferberite stage of mineralization. The principal and abundant stage for the tungsten deposits of the region. It also manifests itself especially abundantly only in individual areas—bonanzas or columns—among extensive weakly mineralized zones.
3. Quartz-antimonite stage of mineralization. It is often superposed upon the preceding ones, but may also develop as spatially distinct, which is confirmed by the presence of ore fields (for example, Zopkhito) where this stage of mineralization is the main and almost the only one. It is interesting to note that the established stages are distinctly the same for all ferberite deposits of Upper Racha. Apparently, their origin is connected with phenomena common to the entire region. It must be assumed that they all arose…

simultaneous and causally connected with the development of a single volcanic process, since the uniform order of replacement of mineral associations throughout the entire district is difficult to connect with different, genetically unrelated volcanic phenomena. These latter arose or succeeded one another simultaneously throughout the whole district, and this replacement was everywhere preceded by renewed movements. Thus, tectonic phenomena, volcanism, and its stage—the ore mineralization—in the given district are very closely connected causally.

In conclusion it is necessary to note that both the number of mineralization stages outlined and the minerals participating in them are in reality considerably greater than we have been able to indicate. Thus, for example, the possibility is not excluded that such mineral associations as quartz-cinnabar or realgar-auripigment were superimposed on all the preceding stages. Unfortunately, however, we were not able to observe these minerals with complete clarity in the ores studied, and the judgment expressed remains for the time being theoretically possible. On the other hand, in optical study we did succeed in clarifying the composition and indicating a certain broadening of the mineral associations in a somewhat different direction. In the following section we shall attempt to set forth this material.

MAIN FEATURES OF THE COMPOSITION OF FERBERITE ORES

The principal minerals determining the general appearance and genetic features of the ferberite ores are quartz, ferberite, and antimonite. Rarely and in small quantities there were observed: pyrite, arsenopyrite, scheelite. Far more often one has to note products of decomposition of these minerals, especially limonite. This makes one think that in the primary ores, unaffected by oxidation processes, the sulfides are expressed more abundantly and are probably represented by a more complex assemblage. Among the decomposition products it is necessary to note a mica-like mineral—sericite (?), possibly in the upper zones partly passing into kaolin. As a relict mineral, graphite should be noted, often observed in Jurassic shales.

Quartz appears in a very diverse manner and in various generations. Three generations are clearly established: (a) silicification that preceded the appearance of ferberite; (b) quartz of the ferberite stage of mineralization; and (c) quartz of the antimonite stage. The separation of quartz of different generations into individual stages of mineralization is readily carried out if one compares the data obtained in analyzing ore textures with the character of the patterns of structural intergrowths observed in optical studies.

Fig. 6. 1—ferberite; 2—scheelite; 3—quartz. Polished section, 90×, Sagebi.

The first generation of quartz is sharply established in the analysis of breccia-like textures. Silicified fragments of clay shales often contain

and small quartz veinlets (Fig. 3). Examination of thin sections of such silicified rocks shows that the silicification was preceded by crushing, since angular fragments of argillaceous shale, almost entirely replaced by a fine mosaic of metasomatic quartz (the grain size varies from 0.01 mm to 0.1 and rarely to 0.23 mm), float among coarser-grained quartz that arose simultaneously in place of more finely crushed material. This quartz has a tendency toward idiomorphism and contains very many inclusions. Undoubtedly, it is formed in part on the walls of cavities produced by the leaching, by solutions, of finely crushed material. The quartz of this generation was clearly preceded, and in part accompanied, by abundant sericitization.

a—ferberite
b—quartz

Fig. 7.

The sericitization of fragments of the rock is observed to be greater the more intense the silicification. One might think that the appearance of quartz causes the transformation of the argillaceous and feldspathic material into sericite, but the decrease in the amount of sericite as the rock is transformed into quartzite compels one to think of the replacement of sericite by quartz. Apparently, at this stage of silicification the sericitization of the rock was an earlier phenomenon than its replacement by quartz. Chlorite develops here and there.

Together with ferberite we observe quartz of a different type. Around silicified fragments of argillaceous shale, rims of quartz and of ferberite growing upon it are visible. In sections it is seen how idiomorphic quartz crystals (Fig. 7) were encrusted by ferberite. In the drawing, the hatched area belongs to a single ferberite crystal. Thus, quartz, often of drusy texture, developed on fragments of silicified rock and earlier than ferberite, but frequently one may also observe quartz developing almost simultaneously with ferberite and somewhat later than it. Figure 8 shows a diagram of the spatial arrangement of fragments of silicified shale (c), ferberite (a), and quartz (b) of the second generation. Ferberite is selectively pressed against the walls of the fragments and is often encrusted by quartz. It may be thought that we are dealing with two varieties of quartz of the second generation; this mineral was deposited before and after ferberite. It is not impossible that the quartz at this stage was deposited continuously, i.e., that it crystallized also simultaneously with ferberite.

a—ferberite
b—quartz
c—fragments of silicified shale

Fig. 8.

The third generation of quartz is most distinctly expressed. This includes vein quartz enclosing sulfides and cutting ferberite ores, and scheelite veinlets (Fig. 6), as well as short-prismatic quartz growing on ferberite crystals in drusy cavities. Let us consider the structural relations of the quartz of this generation in polished sections.

The study of quartz from veinlets that clearly cut the ferberite ores shows that in this case also we are dealing with an uneven-grained mosaic of granular quartz, in the larger grains inclined toward idiomorphism and always cloudy from a large quantity of fine inclusions. In general, the inclusions are distributed uniformly, but often one can observe the disappearance of inclusions over considerable areas having

the form of vein-like bands cutting aggregates of cloudy quartz. In crossed nicols, within these veinlets—bands, the quartz is clearly sharply deformed, and sometimes entirely recrystallized. It is interesting to note that inclusions in the quartz disappear also near the sulfides. It may be thought that deposition of quartz began before the formation of the sulfides.

Early quartz of this stage of mineralization sharply corrodes ferberite. The phenomena of this corrosion are very clearly visible in polished sections and are not noticeable in thin sections, where these structures, delicate in their fineness, are apparently polished out. In the sections, skeletal grains of ferberite are observed (Fig. 9). In crossed nicols, over the whole area outlined by the dotted line (Fig. 9), the quartz appears black, whereas outside it, on a surface well illuminating it, it possesses a bright internal reflection. Small and large islands of ferberite among the quartz go extinct simultaneously. It is clear that we are dealing with a single crystal of ferberite, shallowly corroded by quartz. Examination of sections from drusy cavities also shows sharp corrosion of ferberite by quartz (Fig. 10) and deposition of antimonite on quartz. In addition, we observe quartz growing on arsenopyrite (Fig. 11) and pyrite (rims of quartz), and therefore we have every reason to think that quartz was deposited throughout the entire antimonite stage, but possibly with some interruptions, as is indicated to us by the presence of bands of deformation with the development of recrystallized quartz.

a — ferberite
b — quartz

Fig. 9.

a — ferberite
b — quartz
c — antimonite

Fig. 10.

Ferberite is the principal ore mineral. Its external morphological features are extraordinarily distinctive. In drusy cavities one can often observe beautifully formed crystals; especially well-developed crystals were supplied to me from the Notsara deposit. Goniometric studies of them, carried out by Associate Professor of Tomsk State University V. K. Monich, in brief gave the following results¹.

The ferberite crystals from the Notsara deposit are comparatively small, having an average size in the larger crystals of 6–8 mm in length and 3–6 mm in the two other dimensions. The habit of the crystals is thick-tabular or prismatic, approaching isometric, since the faces of the prismatic zone and the large face of the 3rd pinacoid enjoy exceptional development. “Pyramidal” faces, although they do occur, are only in the form of the very smallest faces, though, it is true, very well formed. The habit of the ferberite crystals from Notsara is unusual for minerals of the wolframite group, since short-prismatic—

...the ferberite crystals resemble crystals of the tetragonal or rhombic systems. In especially well-formed crystals, flattening along the “x” axis and blunting of the edges of (001) by prism faces of the 1st and 4th orders are clearly indicated. Such crystals somewhat resemble the ordinary tabular type of wolframite crystals.

In the four measured crystals and among several dozen examined under a magnifying glass, the following forms were found (in order of prevalence): a) large habit faces—c (001), b (010), m (110), h (310); b) ordinary habit faces—a (100), m (110), l (210) and (121); c) subordinate small faces—f (011), k (211), w (111) and (553) (?). In addition, on some crystals other weakly expressed and difficult-to-measure faces are also occasionally found: in the zone h (310)—a (100), possibly (810), in the zone k (211) and b (010), and in the zone a (100)—c (001). The most common combination of faces, occurring in large crystals, is as follows: c (001), b (010), a (100)—weakly developed, h (310), k (211), and weakly developed, sometimes absent faces—m (110), l (210). A similar crystal is shown in Fig. 12. Small, more perfectly formed crystals contain the whole complex of forms, and the face

![Fig. 11 diagram: a—ferberite; б—quartz; в—arsenopyrite.]

a — ferberite;
б — quartz;
в — arsenopyrite

Fig. 11.

a is displaced by well-developed prisms l and h; the beautifully formed face m (110) is clearly distinguished, and among the subordinate faces the faces f and k have great importance (Fig. 13). In morphological features, the ferberite crystals are quite unlike the known forms of this mineral described in the literature. In the development of the third pinacoid and the blunt prisms they somewhat resemble crystals of megabasite from Schlackenwald, now assigned to hübnerite.¹

A chemical analysis, carried out by Associate Professor of the Tomsk Industrial Institute I. P. Onufrienko, gave the following ratios of iron and manganese in ferberite.

The analytical data fully justify the name of the mineral and are very close to the results of analyses of ferberites from the Khulandoi and Kusapcha deposits of the Northern Caucasus (21, 349), explored for antimony (the first) and arsenic (the second). Storozhenko, noting the “cubic habitus” of ferberite crystals, speaks of the deposition of this mineral together with antimonite in a single stage, which was preceded by the formation of “barren quartz and quartz-carbonate veins” (21, 343). However, for

for the Kusparti deposit, data are given indicating a more complex process, since there a later polymetallic stage is distinguished (21, 344).

Ferberite polishes more poorly than quartz, but nevertheless more readily than magnetite. Differently oriented crystals take the polish differently. On well-polished surfaces triangular cavities or rectilinear cleavage cracks are visible. In some cases they are filled with quartz, and sometimes with a gray, weakly polarizing, transparent, unknown mineral of low hardness, possibly products of decomposition of ferberite. The reflectivity is higher than in normal wolframites, but the color is gray. In comparison with antimonite it appears gray with a brownish tint, while next to limonite ferberite appears brown and even pinkish-brown. Under observation with dry objectives dichroism is almost imperceptible, but in oil, when two grains are compared, a change of color in light-yellow and brownish tones is clearly observed; this distinguishes our mineral from ordinary wolframites (41,606). In crossed nicols the mineral changes color in gray tones with pinkish, bluish, and greenish

Fig. 12. magn. 10×              Fig. 13. magn. 40×

shades. The colors change more vividly and brightly than in ordinary wolframites. When aggregates of ferberite were examined at high magnification, the finest inclusions, resembling pyrite or pyrrhotite, were often recorded. Sometimes it is clearly visible that the mineral penetrates into ferberite along a crack. The relations of ferberite with quartz were discussed above; from Fig. 11 the relations with arsenopyrite and antimonite are visible. In the process of mineralization ferberite occupies a special and definite place; its appearance is separated by sharply manifested deformations from the early stage of silicification of the rocks and from the later development of antimonite. Deformational structures are often observed in the mineral itself. On weathering it sometimes passes into a mixture of tungsten ochers with limonite.

Antimonite occurs among fractured silicified rocks in the form of independent nests composed of a coarse-grained aggregate, or in druse cavities of ferberite ores. The latter case is observed more rarely. Brecciated clayey shales cemented by quartz containing antimonite and other sulfides can often be observed. Here and there, red coatings of kermesite were observed on weathered surfaces of antimonite ores. More often, however, weathering leads to the replacement of antimonite by light, almost colorless earthy aggregates of antimony oxides. In polished sections it is for the most part possible to establish a fibrous structure of the polarizing mineral, possibly valentinite (?), more rarely

isotropic earthy masses are observed, perhaps of cervantite or stibioconite (?). In one case it was possible to observe that earthy minerals form, as it were, a crust on a crystal that is a pseudomorph of valentinite (?) after antimonite. Probably the earthy varieties of antimony oxides appear later at the expense of valentinite (?).

Optical study of antimonite did not reveal in it any anomalous properties, except for the frequent development of deformation twins. Aggregates of antimonite are always crushed. The relations of this mineral with quartz and ferberite have been described above, and here we shall allow ourselves to dwell only on the structural relations of this mineral with later formations.

We did not have occasion to observe quartz developing later than antimonite. On the contrary, quartz veinlets, occurring in considerable quantity in the rocks enclosing the antimonite nests, were never observed in hand specimens and thin sections from antimonite ores. In quartz ores, together with antimonite, one often observes such sulfides as pyrite and arsenopyrite, after which quartz clearly continues to form. In addition, in one hand specimen from the Sagebi deposit it was possible to see a zonal distribution of colloids in a quartz vein. Arsenopyrite appeared near the selvage, and antimonite in the central part. Only on the basis of these considerations can we regard antimonite as the latest mineral in its stage of mineralization.

Nevertheless, it must be noted that the deposition of quartz, antimonite, and other sulfides is apparently not the end of mineralization. Study of antimonite ores from the Notsara and Motsantsara deposits shows that there was yet another stage of mineralization, the nature of which, owing to the paucity of the material studied, has remained unclear to us.

In a number of polished sections we were able to establish veinlets of an unknown transparent mineral cutting antimonite. The hardness of the mineral is close to that of antimonite, perhaps somewhat higher. Its reflectivity is much higher than that of quartz and close to that of sphalerite. Cleavage is noticeable, for the most part in the form of small triangles. The mineral is clearly anisotropic, with internal reflections weakly colored in yellow tones. It often composes crystals and aggregates of prismatic habit. Acids do not act on it. Together with this mineral there develop small round bodies—aggregates of melnikovite. This stage of mineralization somewhat recalls the mineral assemblage that appears after antimonite in the Lukhumi deposit (9a, 193), where, incidentally, Demchuk considers the process of mineralization continuous, with which it is hardly possible to agree.

Arsenopyrite and pyrite were observed rarely, since, on the one hand, they are generally present in the ores in small amounts, and, on the other hand, in the samples of our collection these minerals proved to be, to one degree or another, replaced by products of decomposition, most often limonite. Sometimes in thin sections one had to observe light-greenish scorodite, in turn replaced by limonite. Direct observations on the structural relations of these minerals could not be made. It may nevertheless be thought that arsenopyrite is an earlier mineral. Thus, for example, it usually develops in the selvages of veins or in fragments of argillaceous schists cemented by quartz containing pyrite. Apparently, the greater part of the quartz appeared later than the pyrite and arsenopyrite.

In one of the thin sections prepared from ores of the Khvrelieto deposit, small crystals of scheelite were observed. In a polished section from this ore scheelite was found in the form of sharp veinlets cutting aggregates of ferberite. The scheelite veinlets, in turn, are cut by quartz veinlets (fig. 6). It may be thought that scheelite appears in a special stage

mineralization that preceded, in time, the quartz–stibnite mineralization.

Graphite was observed in thin sections from mineralized black Jurassic shales; sometimes it can be seen in ores of brecciated texture among a felt of sericite. It is a typical relict mineral, having no direct genetic connection with the process of thermal mineralization.

PARAGENETIC RELATIONS OF MINERALS IN THE ORES

On the basis of the material set forth, the process of ore formation in the ferberite deposits of Upper Racha may be represented in the form of the following scheme (Fig. 14).

Fig. 14.

The following remarks must be made concerning this scheme. It may be thought that, with more detailed study, the number of minerals in the established stages should increase considerably. This applies especially to stages III, IV, and V. In addition, it may be supposed that stage IV is complex. The minerals placed here are represented in the ores poorly and are grouped together in time for lack of material on the basis of which they could have been separated. The discontinuity of mineralization is common to all deposits and, apparently, as we have already noted above, is due to general causes that led to the cessation and resumption of thermal activity in this region.

Let us now try, on the basis of analysis of this scheme, to form an idea of the physicochemical setting of the mineralization process.

The first stage is characterized by the formation of sericite and its replacement by quartz. A large part of the sericite is formed here from the mineral substance of the rock. The behavior of the thermal solutions during this stage is therefore clearly characterized by sharply expressed features of leaching, somewhat resembling the process of formation of high-temperature quartzites of the Climax type (40, 794) or of beresites (30). Schmedeman, who analyzed the chemistry of ore solutions (40), came to the conclusion that acidic high-temperature solutions, possessing a high leaching capacity, can absorb a considerable amount of material, and in particular during intense silicification; this is precisely how the properties and activity of the thermal waters in this area may be understood. The waters were acidic, highly heated, produced intense leaching, and deposited chiefly silica. At the end of the mineralization stage the acidity of the waters must have decreased; they may have become neutral or even alkaline. In this period of their activity one might have expected deposition of sulfides. For the mineral complex of stage I in our ores, sulfides are not characteristic. Therefore it may be thought that mineralization broke off earlier than, at this horizon, mineralization from acidic solutions could be overlapped by the action of alkaline ones. Schmedeman quite rightly believes that highly heated acidic solutions

act at a small distance from the parent intrusive body. In connection with this we must note that the distance at which the thermal waters preserve an acid reaction, other conditions being equal, must depend on the composition of the country rocks (their solubility) and the degree to which they have been heated. The less soluble, more strongly heated, and more permeable (for the thermal waters) the rocks are, the farther the thermal waters will move away from the parent intrusion while retaining their original composition. For our case we may assume, taking into account the permeability of the zones of crushing and the slight solubility of the shales, as well as the breakthrough of additional portions of magma to the surface, that thermal waters of acid composition could have moved some, though relatively not very great, distance away from the parent intrusion. In any case, they apparently only passed beyond the limits of the hornfels belt, the width of which may give us a rough idea of this distance. Nor is it excluded that in some cases the volatile constituents were transported by residual melts—additional injections—and, being released from them, the thermal waters could migrate only over a very limited distance.

The second stage of mineralization is also represented by such a complex of minerals as usually characterizes the activity of acid thermal waters (40, 26, 289). One might also assert a high temperature for these solutions, but the special specific features of wolframite, represented here by ferberite, require certain considerations confirming this proposition. For the ferberite deposits of Boulder County in Colorado, Lovering (16, 589) particularly emphasized that the temperature of ferberite formation is higher than that assumed for mineralization at shallow depth. A. E. Fersman assigns to ferberite a higher $eK$ than to hübnerite and, consequently, assumes for the former a higher temperature of formation (26, 298). For the ores of Transbaikalia, M. M. Tetyaev considered the replacement of ferberite by hübnerite with decreasing temperature to be characteristic (22), but later investigations by M. B. Besova (4, 60) and O. D. Levitsky (15, 40) did not confirm this rule. On the contrary, O. D. Levitsky notes cases where ferberite replaces hübnerite with decreasing temperature. However, the material presented by O. D. Levitsky shows that the author is comparing minerals of different stages (41, 136—137), noting, for the Ozernaya vein, the intersection of quartz with hübnerite by a veinlet of radiating chalcedony-like quartz with ferberite. This case characterizes not so much the temperature of formation as the rate of cooling of a highly heated solution. Similar mineral associations are also cited for hübnerite. Recently Yu. A. Speight (20, 79), describing the association of hübnerite with chalcedony-like quartz and rhodochrosite in the ores of Dzhida, nevertheless noted the high temperature at the beginning of mineralization. Meanwhile Gianella (32, 339) called hübnerite epithermal only because he encountered it in association with Miocene lavas and rhodochrosite. Finally, there are indications that wolframites close in composition to ferberite occur together with topaz and cassiterite in pegmatites (42), and in our typical Altai deposits with a high temperature of mineral formation in the principal stages, wolframite usually appears (17, 74). Associations of the latter with especially low-temperature minerals are scarcely noted in the literature.

Thus, the literature material on the temperature of ferberite formation is contradictory and does not provide a reliable solution to the question. But the contradictions become comprehensible if one takes into account certain data of a theoretical order. It is more probable to suppose that all minerals of the wolframite group form from highly heated acid solutions. Under conditions of slow cooling of the solutions and at comparatively higher temperatures, more complex formations arise—solid solutions (26, 251).

to which wolframites belong. Under conditions of constrained crystallization, near the surface, where the thermal waters may cool suddenly and rapidly, simple lattices of ferberite or huebnerite arise¹.

At the same time it may be assumed that, other conditions being equal, ferberite forms at a higher temperature. Thus, we may think that the second stage of mineralization was also caused by acidic and highly heated solutions in a setting of rapid cooling—near the surface.

The third stage has been poorly studied. It may be supposed that, besides scheelite, quartz also participated in it. Here, apparently, the solutions were still acidic, but already of lower temperature. It may be expected that, under the conditions of a later interruption in mineralization, alkaline solutions could readily have arisen here and sulfides could have appeared.

The fourth stage of mineralization is characterized at first by the appearance of such sulfides as pyrite and arsenopyrite, which are especially characteristic of alkaline solutions. Undoubtedly, during this stage the alkaline solutions must have become less alkaline and even acidic, so that antimony compounds, which are so readily preserved in normal alkaline solutions, could be precipitated from them (25, 289). In the fifth stage of hypogene mineralization we already observe marcasite, a mineral characteristic of acidic but low-temperature solutions.

Thus, the process of mineralization in the ore fields of the ferberite deposits proceeded in a very unsettled tectonic setting, where some stages of mineralization ceased and others arose. Each new stage of mineralization is generally characterized by lower temperatures of mineral formation, but in a number of cases the possibility is not excluded that the temperature at the beginning of a subsequent (for example, II) stage was higher than the temperature at the end of the preceding (for example, I) stage. Therefore there arises the possibility of supposing that, with more thorough investigations, the mineral series of the stages described may be supplemented.

Such supplementation may especially be expected at the expense of low-temperature minerals, since, on the basis of the small amount of material at our disposal, we were unable to distinguish generations among the ore minerals of the third, fourth, and fifth stages. The possibility is not excluded that such minerals as sulfides arose not only in the stages established by us, but also earlier. Since in the early stages they must have completed the mineralization and appeared in small quantity, more material than was at our disposal is necessary in order to establish their exact place in the scheme.

In the main, mineralization was due to acidic high- and low-temperature solutions. Alkaline solutions took a very weak part in ore formation. They apparently sometimes did not arise at all, which is possible only in the case where the leaching action of the acidic solutions was insufficient to change the acidic reaction to an alkaline one. Such a phenomenon is possible only on condition that the thermal waters, circulating over a limited distance, have time to cool rapidly, i.e., in the case where the parent intrusive lies not far from the surface and from the ore body.

5. POSITION OF THE FERBERITE ORES OF UPPER RACHA IN THE SYSTEMATICS OF ORE DEPOSITS

From the material considered above there follow the following characteristic features of the genesis of the ferberite deposits of Upper Racha. The deposits are [[unclear: text continues on next page]]

situated at great elevations, where erosion has only slightly affected the surface; in places volcanic rocks of the cover facies are preserved. The mineralization clearly took place under conditions of very shallow depths and is controlled by tectonic zones likewise of the surface facies. The mineralization was preceded by the injection of intrusive rocks young in age (Alpine)—dacites and andesites—bearing clearly expressed features of the stock facies, according to Usov (5). The breccia-like and drusy textures of the ores, the multiplicity of stages of mineralization, and the sharp features of corrosion structures, so characteristic according to Niggli (36, 30) of deposits of the “volcanic” facies,¹ are also specific features of our deposits. The facies character of the general mineralization of the region is emphasized also by the presence in it of auripigment and realgar deposits, whose formation Niggli regards as typical of the volcanic type of deposits (36, 21).

For the deposits under consideration, the composition of the ores is extremely characteristic and original: minerals so different in temperature of formation as ferberite and antimonite occur as the principal minerals. This latter circumstance creates a particular difficulty in determining the place of these deposits in Lindgren’s system (34). But our deposits, as well as the inconsistencies in Lindgren’s system, cannot be regarded as an isolated phenomenon. Deposits with ores in which high- and low-temperature stages of mineralization are vividly expressed are cited in the literature more and more often. Similar properties are possessed by the tin-ore deposits of Bolivia (38 and 39), the quartz-tourmaline ores with gold and silver in Nevada (37), and the well-known ferberite deposits of Boulder County (35), assigned by Lindgren (34, 596–597) to the group of mesothermal deposits. Later L. Beddington noted the contradiction between Lindgren’s principles for his systematics and the position in it of the group of ferberite ores of Boulder County, and proposed that similar formations, not fitting into the framework of Lindgren’s scheme and arising near the surface but at high temperature, be separated into a special group of xenothermal deposits. But Beddington’s proposal also cannot escape objections. The point is not that mineralization occurs at high temperatures near the surface, but that mineralization near the surface proceeds at both high and low temperature, and therefore the ores of such deposits are unusually variegated in composition and original in the character of their mineral associations. Possibilities for associations of this kind arise only under conditions of surface volcanism, as P. Niggli and M. A. Usov (23) noted almost simultaneously and independently of one another. Subsequently M. A. Usov, pointing out that abyssal-facies intrusive rocks are almost never encountered in nature (24, 33), drew particular attention to the subdivision of rocks of the surface (effusive) facies (25), but he did not succeed in extending these ideas to the field of systematics of ore deposits of useful minerals, although he had prepared a considerable theoretical basis for this purpose. On the basis of the systematics that M. A. Usov proposed (25) for rocks of the effusive facies, we should have had to assign the deposits of Upper Racha to the stock facies, taking into account the character of the parent rocks of dacite-andesite composition.

It should be noted that in the systematics of ore deposits M. A. Usov distinguished the effusive facies only for the group of vein deposits. It is perfectly clear that, in the setting of surface volcanism in the now broader sense that he proposed, there may arise any—

¹ Niggli strongly emphasizes this feature: “It contains unmistakable signs of manifolds superimposed processes—e. g., corrosion or metasomatic alterations of early formed minerals.” In the Russian translation this phrase is rendered very confusedly and obscurely. P. Niggli, Genetic Classification of Magmatic Ore Deposits—Moscow, 1933, p. 36.

structural types of deposits, and also any temperature associations. Thus, for example, in the setting of the stock facies even contact deposits of the Tetiuhe or Campiglia Marittima type may arise. In particular, the deposits under discussion cannot, by their form and mode of formation, be assigned to vein deposits, although the element of filling of drusy cavities is very sharply expressed here. The ore bodies do not have the form of veins either, and the processes of leaching and replacement proceeded here rather sharply, in part probably because the thermal waters had a predominantly acid reaction. In the ore fields there occur bands of metasomatic replacements and impregnated ores, to which A. E. Bendeliani (2, 13) first drew special attention.

A question that is always very important and serious for understanding the genesis of a deposit is that of the source of the thermal waters. The great merit of M. A. Usov consists in the following: in distinguishing the genetic group of deposits of the effusive facies, he emphasized that the lavas of sheets cannot be the source of thermal waters, and assumed that this source is “the roots of such sheets of effusive bodies” (23, 66). In our case, quite obviously, one must go considerably further. The small bodies of the stock facies of the andesite-dacites, being possible roots of former effusions, at the same time are, as a rule, not sources of thermal waters, but only emphasize and in part create the setting—the facies—in which mineralization took place. The thermal waters come from below; their source is a hidden pluton; it is the common source for the rocks of the andesite-dacite magma, and also for the thermal waters that caused “autometamorphism” in these rocks and created extensive fields of ore mineralization. We should like to emphasize that, in the formation of the Upper Racha deposits, the hidden pluton played a certain role, but this should not be an obstacle to fixing the effusive facies as the setting of the development of the mineralization process, which conditioned the special features of deposits distinct from those formed at depth, under conditions of the intrusive facies. Only by a certain misunderstanding of this proposition can one explain the position of V. G. Grushevoi, who, for South Ossetia (9, 927), only because he did not consider it possible to relate the mineralization to processes of the “effusive magmatic cycle” (after Niggli and Schneiderhöhn), wrote that the author “is inclined to see the active role of intrusions not everywhere, but perhaps only where they have been exposed by erosion.” An overestimation of the role of rocks of the stock facies also leads to incorrect ideas. Thus, regarding the bodies of “neointrusions” as sources of emanations, K. Chichinadze (29, 27) notes that in the process of ore formation “the pegmatite stage is almost entirely absent, which is connected with the poverty of the neointrusive magma in volatile components.” We should like to note that there is no basis for considering the parent magma poor in volatiles, while the absence of a clearly expressed mineralization of the “pneumatolysis” type is determined by the setting, i.e., by the factor that leads to the formation from the residual magma not of pegmatites, but of aplites or even porphyries.

In their characterization of deposits of the effusive facies, Niggli and Usov consider a variegated complex of ores to be a specific feature of them. Assuming the source of the thermal waters to be located almost near the surface, M. A. Usov did not consider it possible to subdivide these deposits by formations, and regarded the composition of the ores of deposits of this group as conditioned by the composition of the emanations. In light of the new systematics of rocks of the effusive facies developed by M. A. Usov, and also in connection with the view set forth above regarding the possible source of the thermal waters, this proposition requires some revision—unfortunately, for the time being only in principle.

The parent intrusion as a whole lies relatively not far from the surface, which its additional injections and emanations reach. Nevertheless, taking into account the considerable unevenness of the roof of the pluto—

conditions, especially acidic ones, we must assume that the degree of separation of the Earth’s surface from the surface of the pluton may vary very considerably at different points. On the other hand, as the pluton crystallizes, the distance of the surface from the liquid magma will change considerably with time. We therefore believe that the change in environment over time, owing to the sharply discontinuous character of the ore-forming process, on the one hand promotes the formation of variegated complex ores, but on the other does not exclude the possibility of the formation of ores of a specifically differentiated composition, since each stage of mineralization, even under conditions of an effusive facies, is characterized by a special differentiated complex of mineral components. Let us examine, from this standpoint, the specific features of the ferberite ore deposits of Upper Racha.

In considering the scheme of ore formation presented in the preceding chapter, it is necessary to note in it one very distinctive feature. In all the deposits of Upper Racha, the stages of mineralization created by emanation solutions of acid reaction are expressed most abundantly. The activity of alkaline solutions, manifested in the development of a normal complex of sulfides, is very weakly expressed. Thermal solutions, replacing one another under the conditions of the given environment, persistently retained an acid reaction and, as a result, among the numerous stages of mineralization that show a gradual decrease in the initial temperature of the therms arriving at these horizons, we find almost no mineral complexes caused by alkaline solutions. It should above all be noted that differentiation of the ore material takes place in our deposits, and it is very sharply expressed. On the other hand, in our opinion, proceeding from the concept of the process of development of thermal solutions, we must regard such differentiation as a natural phenomenon for an environment in which cooling of thermal solutions occurs rapidly and, consequently, their solvent capacity also decreases rapidly. In a number of cases the therms were unable to absorb from the wall rocks such an amount of material as could quickly have caused a change from an acid to an alkaline reaction. Therefore, at high temperatures the stages of activity of acid solutions are so prolonged and sharply expressed. On the other hand, the change from an acid to an alkaline reaction occurred in the solutions already near the surface and, naturally, for a short time, since the influence of the oxygenated medium of the surface environment quickly caused a change of the alkaline medium to an acid one in the solutions that were already cooling. Therefore, in the ores we have a sharply expressed low-temperature stage of deposition of antimony ores. The question arises: is it possible, and where should one expect the appearance of sulfide pyrite ores of high- or low-temperature stages? In the Caucasus as a whole, and in Transcaucasia in particular, deposits of nonferrous metals genetically connected with Alpine volcanism have long been known, and therefore the first part of the question is answered in the affirmative. The second half of the question may be answered as follows. If our ideas about the process of development of thermal solutions are correct, then sulfide deposits may especially often arise in those cases when the parent intrusive lies at a considerable distance from the surface to which additional injections reached, or if the enclosing rocks are readily soluble and can quickly change the acid reaction of high-temperature solutions to an alkaline one¹. It is entirely natural that in these cases high-tempera-

…sulphides of the arsenopyrite-ore type will be found in areas adjoining the parent intrusive bodies, whereas lead-zinc ones will be discovered in connection with effusives of the dike or even cover facies. Thus, in this case we are compelled to allow for a certain vertical zoning as well.

In concluding the present chapter, we must note the following. In modern classifications there is no exact place for the mineralization type of Upper Racha. It seems rational and correct to us to assign it to the group of mineralizations of the effusive stockwork facies, according to M. A. Usov, indicating that mineralizations of this type are substantially represented by bands of metasomatic, often stockwork, ores and disseminations. A more detailed subdivision of the mineralizations of the effusive facies is absolutely necessary. If this subdivision is constructed on the same principles as those proposed by M. A. Usov for mineralizations of the intrusive facies, then we must assign the mineralizations of Upper Racha to the group of metasomatic mineralizations and disseminations of the wolframite formation.

LIST OF REFERENCES

1. L. Beddington.—Associations of high-temperature minerals at shallow and intermediate depths.—Transactions of the Gold Prospecting Trust and the “Nigrizoloto” Institute, 1936, issue 3.

2. A. E. Bendeliani.—Occurrence of rare-metal mineralization in the mountains of Upper Racha.—Exploration of Mineral Resources, 4–5, 1938.

3. The same.—The Talakhian mercury mineralization in Upper Racha.—Exploration of Mineral Resources, 3, 1939.

3a. The same.—The Tsena (Tsurungal) arsenic deposit in Upper Svaneti.—Exploration of Mineral Resources, 2, 1937.

1. M. B. Besova.—Geology and mineralogy of the Dzhida tungsten deposit.—Deposits of Rare and Minor Metals of the USSR, vol. I, 1939.

2. L. A. Vardanyants.—On deposits of non-ferrous metals in the Dagestan ASSR.—“Natural Resources of the Dagestan ASSR,” publ. Acad. Sci. USSR, 1935.

3. The same.—Materials on the neointrusion of Mountain Ossetia. The Telli, Kazbek and Adaikhokh massifs.—Problems of Soviet Geology, 5–6, 1937.

4. The same.—Intrusives of North Ossetia.—Problems of Soviet Geology, 4, 1938.

5. The same.—Neointrusions, their mineralization and relation to tectonics in the Main Caucasus.—Proceedings of the Academy of Sciences, geological series, 2, 1939.

6. V. G. Grushevoy.—A brief outline of the metallogeny of Transcaucasia.—Problems of Soviet Geology, 10, 1935.

9a. A. Demchuk.—Geological sketch of the Lukhumi realgar-auripigment deposit in Upper Racha in Western Georgia.—Transactions of the All-Union Mineralogical Society, part 1, 1935.

1. A. D. Ershov.—On the structure of the Tsurungal (Tsena) ore field.—Problems of Soviet Geology, 4, 1938.

2. The same.—On the occurrence of tin in the Tsena arsenic mineralization (Upper Svaneti).—Soviet Geology, 12, 1938.

3. N. S. Zaitsev.—On geological exploration works for rare metals in Transcaucasia.—Non-ferrous Metals, 8, 1938.

4. S. Konstantov. The Nikolaevskoe cinnabar mineralization in Dagestan.—Surface and Subsurface Resources, 2–3, 1917.

5. A. Sh. Kurbanov.—Cinnabar in the Dagestan ASSR.—“Natural Resources of the Dagestan ASSR,” publ. Acad. Sci. USSR, 1935.

6. O. D. Levitsky.—Tungsten deposits of Transbaikalia.—Deposits of Rare and Minor Metals of the USSR, vol. II, 1939.

7. T. Lovering.—Tungsten deposits.—“Geology of Ore Deposits of the Western States of the USA,” 1937.

8. G. V. Pinus.—Mineral associations of ores and wall-rock alterations of ore-bearing quartz veins of the Beloretsk tungsten deposit in the Altai.—Bulletin of the West Siberian Geological Administration, 5, 1938.

9. P. S. Saakyan.—Paths of development of the ore resources of the Central Caucasus.—Soviet Geology, 5, 1938.

10. S. P. Solov’ev.—Zonal distribution of metals in the Tyrny-auz district.—Problems of Soviet Geology, 9, 1937.

11. Yu. A. Speit.—Geological structure of the Dzhida tungsten mineralization.—Bulletin of the West Siberian Geological Trust, 4, 1937.

12. I. D. Storozhenko.—Geological-mineralogical characteristics of tungsten mineralizations in the North Caucasus.—Transactions of the All-Russian Mineralogical Society, part 68, issue 2, series 2, 1939.

13. M. M. Tetyaev.—Tungsten and tin deposits of the Onon–Berzinsk district, Transbaikal Region.—Materials on General and Applied Geology, issue 32, 1918.

1. M. A. Usov.—A Short Course on Ore Deposits.—Tomsk, 1928 and 1933.

2. Idem.—Geology of ore deposits of the West Siberian Territory.—Tomsk, publ. Zap. Sib. GGGT, 1935.

3. Idem.—Facies and phases of rocks of effusive appearance.—Probl. Sov. Geol., 9, 1935.

4. A. E. Fersman.—Geochemistry, vol. III.—Leningrad, 1937.

5. G. I. Kharashvili.—The Karob deposit in Georgia.—Razv. Nedr., 11, 1936.

6. Idem.—A geological-petrographic sketch of the Karob deposit and the conditions of its formation.—Izv. Akad. Nauk SSSR, 5, 1939.

7. K. Chichinadze.—Preliminary data on the metallogeny of the southern slope of the Caucasus.—Tsvetn. metally, 10, 1938.

8. F. N. Shakhov.—Genesis of the belorechites of Altai.—Proceedings of the scientific conference on the study and development of the productive forces of Siberia, Tomsk, 1940.

9. Idem.—The main features of the metallogeny of Altai.—Ibid., 1940.

10. V. Gianella.—Epitermal Hübnerite from the Monitor District. Alpine County, California—Econ. Geol., 3, 1938.

11. C. D. Hulin.—Structural control of Ore Deposition.—Econ. Geol., 24, 1929.

12. W. Lindgren.—Mineral Deposits.—New-York, 1933.

13. Fr. Loomis.—The Ore Wolfram of Baulder County, Colorado.—Econ. Geol., 1937.

14. P. Niggli.—Ore Deposits of Magmatic Origin their Genesis and Natural Classification London, 1929.

15. R. Pilz und M. Donath.—Betrachtungen über die Entstehung der Eruptivgesteine und Erzlagerstätten des Andinen Bolivien-Zeitschr. f. Pr. Geol., 1929.

16. V. Samoyloff.—The Llallagua Unica Tin Deposits.—Econ. Geol. 5, 1934.

17. D. F. Campbell.—Geology of the Bonanza King Mine Humbold Range, Pershing Counti, Nevada.—Econ. Geol., 1, 1939,

18. O. C. Schmedeman.—Notes on the Chemistry of Ore Solutions.—Econ. Geol., 8, 1939.

19. F. Wernicke und E. O. Teuscher. Die neue vogtländische Wolframit lagerstätte von Pechtelsgrun bei Legenfeld—Zs. deutsch. geol. Gesel 1936.

20. H. Schneiderhöhn und P. Ramdohr. Lehrbuch der Erzmikroskopie, Bd. II.—Berlin, 1931.

1. Finally, sulfide deposits may arise in the setting of an intrusive facies, i.e., under conditions when an outpouring onto the surface did not take place. It goes without saying that consideration of such deposits lies outside the scope of the question we have raised. ↩↩↩↩↩↩

2. Since Sb is preserved for a long time in alkaline solutions, the precipitation of antimonite is more easily connected with the loss of the solution’s alkaline reaction. ↩

3. A. D. Ershov (10, 271) regards the association of diabase dikes with the thrust zone as a consequence of “a certain constancy of zones of rupture.” ↩