On the Verkhne-Kondomsky Granodiorite Massif

B. F. Nifantov

(Presented by Prof. A. M. Kuzmin)

The Verkhne-Kondomsky granodiorite massif is located in Mountain Shoria, in the southeastern part of the basin of the Kondoma and Taimet rivers. This massif attracts attention because gold placers have long been worked within the area of its development; adjacent to it, in the surrounding rocks, there are deposits of copper, iron, and cinnabar, the genesis of which remains unclear to this day. The massif is also of interest for its geomorphological features. In the general relief, its territory is a lowered, gently hilly country with absolute elevations no higher than 580–650 m and relative rises of no more than 50–150 m. On the intrusive rocks there is developed a crust of physical weathering in the form of grus, the thickness of which, judging from individual old workings in the mouth part of the Steklyanny spring, reaches 25–30 m. The grus cover, creeping from elevated points toward their foothills, gives the relief soft outlines. To the east and south of the massif the relief becomes sharply mountainous, with elevations of 900–1300 m above sea level. To the west, the somewhat dissected Kondoma plateau rises above it as a high step. Thus, orographically, the entire area of the massif, elongated in a NW direction, is a broad depression with a comparatively leveled bottom. Its western and eastern margins almost coincide with the valleys of the Kondoma and Taimet rivers and, judging by the rectilinearity of their valleys and the sharply expressed relief of the surrounding uplands, it is bounded by zones of disturbances of NW strike. In the studied bedrock outcrops of the right sides of the Kondoma and Taimet valleys, small disjunctive disturbances, zones of crushing, mylonitization, and increased fracturing are widely developed. These phenomena also indicate the possibility of the existence of major faults at the bases of the valleys and, in addition, create favorable conditions for the intensive development of a crust of physical weathering.

In the published literature there is only brief information on the Verkhne-Kondomsky massif in the works of A. M. Zaitsev [1], V. S. Reutovsky [6], V. A. Obruchev [4, 5], A. M. Kuzmin [2, 3], and K. G. Tyumentsev [7].

Brief Characterization of the Intrusive Rocks

The petrographic composition of the Verkhne-Kondomsky massif is varied. Essentially it is composed of rocks of the diorite series: diorites

and closely associated with them gabbro-diorites and hornblendites; quartz diorites, granodiorites, and trondhjemites. In quantitative terms, rocks of intermediate composition occupy about 75% of the area studied, while all other varieties make up the remaining part. Light-colored granodiorites, trondhjemites, and the aplite-like granites closely associated with them, as well as some of the aplites, occur in the form of more or less large isometric bodies that cut the enclosing intrusive rocks of intermediate composition and form sharp contacts with them. The distribution and role of the diorite group of rocks are well controlled by positive values of the magnetic-field intensity, whereas small isometric bodies of pink aplites are outlined by the boundaries of negative fields.

The vein series is represented by pink aplites, which, in the form of thin veinlets or dikes of highly variable thickness, cut the massif in the most diverse directions. Among other representatives of the vein rocks, spessartites and kersantites occupy a significant place, as do white and dark plagioclasites.

Granodiorites are exposed in the central part of the massif, on the right side of the valley of the Kondoma River, between the mouths of its tributaries, the B. Bazas and the Taimet. They are pinkish-gray, coarse-crystalline porphyritic rocks with variable contents of quartz, potassium feldspars, plagioclases, and dark-colored minerals. The variable content of potassium feldspar in the rocks causes gradual transitions from granodiorites to trondhjemites. Such nonuniformity in the distribution of rock-forming minerals creates certain difficulties both in diagnosing the rocks and in mapping. The porphyritic character is expressed in the presence of large, up to 5 mm across, phenocrysts of quartz, plagioclases, or hexagonal plates of biotite against the general background of a medium-grained rock.

Microscopically, the granodiorites are composed of large and small grains of quartz (25–45%), oligoclases (40–45%), and biotite (3–5%). Potassium feldspars—orthoclase or microcline—are distributed extremely unevenly in this group of rocks. Their content ranges from 15–20% to traces, and therefore the composition of the rocks changes from granodiorites to trondhjemites. Hornblende and secondary minerals—chlorite, sericite, epidote, and calcite—are sometimes present. Accessory minerals are always present: sphene, apatite, and zircon.

Quartz very often forms porphyritic segregations of 1–5 mm, which in cross section have rounded, rectangular, rhomboid, or hexagonal outlines. Large segregations often consist of numerous unoriented small grains, 0.1–0.3 mm in size, each of which shows features of idiomorphism. Quartz not infrequently displays a blocky structure and, as a consequence, spotty, cloudy, or wavy extinction. In those areas where the rocks were subjected to increased unilateral pressure, it underwent brittle deformation manifested as cataclasis and mylonitization. In contact with plagioclases, quartz sometimes acquires xenomorphic outlines and in this case seems to fill the spaces between grains of prismatic plagioclases, which are often penetrated by it along with plates of biotite and small prisms of sphene. Not infrequently, idiomorphic grains of quartz are corroded by later quartz, which forms irregular, bay-like inclusions in them. With respect to potassium feldspars, quartz exhibits both features of idiomorphism and features of xenomorphism.

Plagioclases, in the form of shortened prisms 0.8–1.2 mm long, are very often zonal and polysynthetically twinned. Sometimes the zones za-

the grains do not differ from one another in composition and are emphasized only by sericite, which grows through and is distributed along the contacts of the zones. The most widespread twinning is according to the albite and Carlsbad laws, along the face (010). The central parts of plagioclase grains most often correspond in composition to oligoclase No. 18–20, while toward the outer parts of the grain the composition of its zones corresponds to oligoclase–albite No. 12–15, and the outermost zone usually corresponds to albite No. 5–8. The cores of zonal plagioclases are often intensely replaced by sericite and saussurite, which creates difficulties in determining their composition. Judging from the replacement products, one may infer a somewhat more basic composition of these cores than that of the oligoclase zones enclosing them. Often, along the cleavage in plagioclases, there is penetration by plates of biotite and sphene or by apatite prisms, which in the process of growth—especially biotite—capture and replace the plagioclases. In contact with potassic feldspars, oligoclases form tongue-like inclusions and embayments, in which thin, radially arranged, worm-like curved quartz myrmekites usually develop.

Potassic feldspars are represented by orthoclase–microperthite, more rarely by microcline; developing between grains of plagioclase and quartz, they often push them apart and, as it were, cement them. In this way large pseudophenocrysts of irregular outline are formed, representing polycrystals, the individual blocks of which within these large grains have almost the same optical orientation, as is clearly observed from their faintly noticeable non-simultaneous extinction. These feldspars are usually impregnated with fine pelitic and hematitic dust, which is distributed as separate spots or streaks, for the most part along the boundaries of blocks or along the directions of dislocation disturbances. Such “dusting” gives the potassic feldspars a light pink color, owing to which they can readily be detected in hand specimens. The indicated alkali feldspars are distributed unevenly in the rocks: their content averages 10–15%, but locally their amount decreases to traces or increases to 30–50%. Therefore, in some thin sections K-feldspar is completely absent, while in others its content is sharply increased, although these thin sections were taken from a single outcrop and the distance between the sampling points, in some cases, does not exceed 10–20 cm. In this connection, we note that potassic feldspars often form continuous accumulations of irregular shape in granodiorites, diorites, and quartz diorites, or penetrate the rocks in the form of a network of parallel and mutually intersecting thin veinlets. The density of the veinlet network varies from 5 to 30 veinlets per 1 m², with their average thickness ranging from 1 to 3 cm. Fine-platy muscovite, secondary quartz, and a fine disseminated impregnation of pyrite and chalcopyrite often appear in such veinlets. Potassic feldspathization is especially clearly observed near zones of tectonic disturbances and in areas of increased fracturing. The facts just indicated undoubtedly speak of feldspathization of the massif, which occurred long after its formation.

Biotite is often present in the form of plates of hexagonal habit or flakes. Hornblende forms elongated crystals with well-developed faces {110} and narrow {010}; therefore its transverse sections look like rhombs truncated at the acute ends. Both minerals crystallize in the interstices between the rock-forming grains, penetrate along random fractures or along cleavage into plagioclases and quartz, capturing, enveloping, and replacing them in the process of growth. Biotite often replaces hornblende—especially often in the fields of feld-

...epidotization—and is itself, in turn, replaced by chlorite. In amphiboles and biotites the accessory minerals usually present are apatite, zircon, magnetite, and sphene, whose content in the mafic minerals is, as a rule, higher than in the other rock-forming minerals.

Apatite occurs as small crystals 0.06–0.15 mm in size. It is readily identified by its low birefringence, the hexagonal-prismatic habit of the crystals, and negative elongation. In biotites and chlorites apatite produces pleochroic halos around itself, i.e., it shows signs of radioactivity.

Zircon is encountered somewhat less often than apatite; it forms shortened prismatic tetragonal crystals with transverse dimensions usually from 0.01 to 0.05 mm. In thin section it is colorless, pinkish, or yellowish, and sometimes reveals zonal structure and traces of metamict breakdown, expressed in fine concentric zoning radiating from certain points in the central parts of the grain toward its periphery.

Spatially closely associated with the granodiorites are pink granite-aplites, which not infrequently envelop the granodiorites and trondhjemites and in this case, evidently, are their facies variety.

The granite-aplites and aplites are light pink, fine- and medium-grained rocks, almost devoid of femic minerals, with a characteristic texture expressed in the sinuous-banded distribution, within the general mass, of streaks with fine-, medium-, and coarse-crystalline composition. Separate, as a rule small, isometric bodies of pink aplites in their peripheral parts show a fine-grained structure with gradual transitions to coarse-crystalline structures toward the central parts of the bodies. Vein aplites are often cut by large, 3 to 10 cm, veinlet-like segregations of water-clear quartz, which forms pegmatitic intergrowths with the feldspar mass of the aplite.

Under the microscope the aplites are composed of quartz (15–25%), orthoclase (10–20%), and oligoclase (55–75%). Mafic minerals occur very rarely and only as large 0.5–0.7 mm porphyritic phenocrysts. The texture is aplitic.

Diorites and quartz diorites crop out along the right bank of the Taymet River; they are recorded from dredging in the basins of the B. and M. Bazas rivers, Klyuch Akpanak, and in a number of other places. They are medium- and coarse-grained gray rocks consisting of hornblende and plagioclases. Sometimes their composition includes large 3–5 mm segregations of quartz, the total content of which reaches 10–12% and more. Thus, the diorites gradually pass into their quartz varieties. A characteristic feature of the dioritic rocks is an increased content of accessory sphene, which is clearly visible macroscopically in the form of large, 0.5–1 mm, brown, crystallographically excellently faceted crystals with strongly developed faces {110}, and less developed {001}, {100}, and {111}. Such “sphenization” of the quartz diorites is observed very distinctly in the vicinity of the Tolpyshevo winter hut, where the rocks are distinguished by a coarse-crystalline structure and have undergone amphibolization.

Amphibolization of the diorites is a very characteristic phenomenon for the rocks of the area studied. This process is expressed in the fact that in the diorites the content of hornblende increases; it forms elongated, pole-shaped crystals oriented in one direction. In this process, as a rule, the rocks acquire a coarse-crystalline appearance, and the content of accessory magnetite and sphene in them increases. Thus, the diorites are transformed into peculiar gneiss-like gabbro-diorites. In subsequent stages of amphibolization, the hornblende crystals...

hornblendes grow and, gradually replacing feldspars and quartz, form coarse-crystalline and, not infrequently, giant-crystalline hornblendites, consisting of hornblende with relics of plagioclases.

Diorites, gabbro-diorites, and hornblendites usually occur in close association and form entirely gradual transitions among themselves: from diorites, through gabbro-diorites, to hornblendites. Like granodiorites, diorites are likewise subjected to processes of K-metasomatism; in the gabbro-diorites and hornblendites this process was not observed by us. Below a brief microscopic description of these rocks is given.

Microscopically, the quartz diorites have a prismatic-granular or, in places, poikilitic texture: grains of plagioclase are enclosed in large grains of orthoclase or hornblende. The percentage ratios among the minerals are as follows: oligoclase-andesine (35–45%), orthoclase (10–15%), which in strongly feldspathized areas may amount to as much as 50%; quartz (15–35%), biotite and hornblende (20–30%). Secondary minerals: muscovite, sericite, secondary quartz, chlorite; accessory minerals: sphene, apatite, zircon; ore minerals: magnetite, pyrite, chalcopyrite.

Quartz forms irregular grains 1–2 mm in size, characterized by pronounced xenomorphism in relation to the plagioclases. It has a sharply expressed blocky structure and, in connection with this, blocky or cloudy extinction. Not infrequently several individual quartz grains create small clusters, forming porphyry-like inclusions.

Plagioclases are usually represented by oligoclase-andesines or andesines, which under the microscope appear as slightly elongated rectangles with ordinary dimensions from 0.3–0.7 to 1.4–1.8 mm. Zoning is often manifested, analogous to the zoning of plagioclases in granodiorites. The secondary products sericite and muscovite develop especially intensively after plagioclases in those areas where the rocks have undergone increased feldspathization. In this case all plagioclase grains are almost completely replaced by sericite, secondary quartz, and muscovite. The latter is distributed in plagioclase prisms as parallel chains along the seams of polysynthetic twins, along cleavage directions, or forms here disorderly scattered rosette-platy intergrowths. Thus, during feldspathization, diorites and quartz diorites undergo greisenization, in connection with which, as already noted above, disseminated sulfide mineralization appears.

Orthoclase occurs in the form of large, irregular segregations with sizes from 0.1–0.3 mm to 3–6 mm and more, which are unevenly distributed in the rock mass, engulfing, pushing apart, and dissolving other rock-forming minerals. In transmitted light it is cloudy, brown, owing to saturation with finely dispersed clayey products and iron oxides.

Common hornblende forms crystallographically regular short-prismatic grains ranging from 0.9 to 3 mm in size. It has sharp pleochroism from deep green along \(N_g\) to yellowish-green along \(N_p\); the extinction angle relative to \(N_g\) is 16–18°. In fields of feldspathization it is actively replaced by biotite, and then by chlorite. In turn, amphiboles, as is observed, in some cases develop after monoclinic pyroxenes.

Muscovite develops as parallel-scaly and rosette-like aggregates after plagioclases in fields of feldspathization. The intensity of muscovitization of the plagioclases varies and probably depends on

the intensity of the feldspathization process. With a small addition of potassium (orthoclase 10–15%), muscovite appears sporadically in individual plagioclase grains, whereas at a K-feldspar content above 50% all plagioclase grains undergo intense muscovitization; moreover, muscovite also develops after chlorite. In transmitted light the muscovite is transparent, with noticeable pseudoabsorption; extinction relative to the cleavage is almost straight.

Chlorite, together with calcite, magnetite, and chalcedony-like quartz, forms pseudomorphs after hornblende crystals. Here it is represented by a large number of small, curved flakes, uniformly intergrown throughout the entire grain. Chlorite (pennine) also develops along biotite plates, replacing the latter by individual packets or small lenses in the direction of cleavage. Evidently, in connection with incomplete replacement, chlorite often shows a dense violet anomalous interference color.

Sphene is present as large, well-bounded, envelope-shaped grains from 0.2 to 1.5 mm, usually developing in the interstices between the principal rock-forming minerals and occurring in close association with hornblende, biotite, apatite, and magnetite. It is weakly pleochroic in brownish tones; not infrequently, during growth, it captures individual small grains of plagioclase, which are observed in cross sections of sphene as inclusions. It is replaced by leucoxene.

Apatite forms small crystals of hexagonal-prismatic habit, 0.01–0.06 mm in size, enclosed in plagioclase, more rarely in quartz, and more often in hornblende. Sometimes peculiar intergrowths of several small apatite crystals with magnetite and hornblende are observed, as well as the formation of characteristic microsideronitic structures, enclosed as separate areas in the prismatic-granular aggregate of the rock.

Zircon is usually encountered in the form of small, 0.02–0.04 mm, tetragonal short-prismatic crystals, the faces of which most often correspond to {110}, {100}, {111}, and {211}. Colorless or slightly brownish, it often has brown pleochroic halos around it when enclosed in biotite or chlorite.

Magnetite forms acute-angled aggregates of irregular amoeboid shape, with transverse dimensions from 0.2 to 0.8 mm. Such aggregates, as they develop, penetrate into plagioclases, biotite, and amphiboles, often capturing them. The magnetite content in the rocks, as a rule, increases with an increase in the content of the dark-colored minerals hornblende and biotite.

Diorites differ from the quartz diorites described above by a lower quartz content (3–5%) and a somewhat increased content (25–35%) of green hornblende.

Gabbro-diorites are gray, medium- and coarse-crystalline rocks composed of strongly elongated, 1–3 cm grains of hornblende, more rarely of biotite and plagioclase, with a subparallel orientation, which gives the rocks a characteristic gneissoid appearance.

Under the microscope, the gabbro-diorites consist of plagioclase (30–35%) and hornblende (60–70%). In addition, they contain an increased amount of accessory apatite, sphene, and magnetite.

Plagioclases form strongly elongated crystals with uneven, step-like outlines of their contacts. Individual plagioclase grains seem to cut through and wedge into neighboring plagioclases, which greatly hampers the determination of the actual relationships between—

with them. The simplest determinations of plagioclases in a single thin section revealed sharp discrepancies in their composition, from No. 30 to No. 54, which requires further verification.

Hornblende also forms sharply elongated, step-like grains that grow in one direction, forming discontinuous chains. In large hornblende grains, poikilitic inclusions of plagioclases are often observed in the form of small rounded grains 0.1–0.2 mm in size. In addition, amphiboles often envelop plagioclases, and in this process the latter acquire rounded outlines. This probably indicates that, during crystallization, with the influx of the necessary material, the dark-colored minerals replace and displace the plagioclases. Hornblende exhibits distinct pleochroism from grass-green along \(N_g\) to straw-yellow along \(N_p\); its extinction angle relative to \(N_g\) is 16–18°, more rarely 20°.

Apatite, sphene, and magnetite are most often found in close intergrowth with amphiboles. In their mineralogical features they do not differ from the accessories described above.

Amphibolites extend as a narrow belt in the western endocontact of the massif; they have a fine-grained structure and a black color with a greenish tint. Under the microscope, the amphibolites consist of small acicular crystals of ordinary hornblende 0.1–0.5 mm long and similar small prisms of plagioclases. A gneissic character of the amphibolites is clearly observed, caused by a streaky, subparallel orientation of the plagioclase and amphibole crystals, which together compose the entire mass of the rock in approximately equal amounts.

Hornblendites usually occur among the gabbro-diorites as irregular isometric or elongated bodies. They are present both in the eastern endocontact of the massif and in its central part, and are dark-green rocks consisting of large short-columnar hornblende crystals 3–5 cm and more in size, and contain small, probably relict inclusions of plagioclases, more rarely K-feldspars.

Metasomatic Formations

From the factual material presented above it is evident that the intrusive rocks of the massif were widely affected by metasomatic processes of feldspathization, greisenization, and amphibolization. Especially noteworthy here is the formation of metasomatic secondary quartzites, developing along the intrusive rocks and mapped rather well within the intrusive field.

Feldspathization is usually observed on the sides of tectonic ruptures, in zones of increased fracturing, and is manifested in the form of veinlet or disseminated accumulations of metasomatic potassium feldspar. The veinlets, as a rule, have a thickness of 1–3 cm and are distributed in the rocks as a more or less dense network, forming an “orthoclase stockwork.” Sometimes thicker veins of 10–30 cm are encountered, in which orthoclase is usually in pegmatitic intergrowth with water-clear quartz. Fragments of such veins were found in the channel of the Steklyannyi spring. Dissemination of K-feldspars is manifested in the form of local accumulations ranging in size from 1 to 5 cm and more. Such disseminations not infrequently arise at the intersection of potassium-feldspar veinlets.

Greisenization is, it must be supposed, simultaneous with feldspathization and is recorded by the processes of muscovitization of plagioclases and silicification. Along with this, iron, copper, and molybdenum sulfides appear in the greisenized areas. Greisenization is most often confined to K-feldspar veinlets.

Amphibolization is expressed in the formation of gneiss-like gabbro-diorites, giant-crystalline hornblendites, and fine-grained amphibolites. The gabbro-diorites form rather large fields in the eastern endocontact of the massif and, owing to their elevated magnetite content, are clearly recorded by magnetometry, usually in the form of isometric positive fields with increased values of magnetic-field intensity. The distribution of hornblende and magnetite in these fields is uneven; the hornblendites richest in amphiboles and magnetite occur as small isometric, stock-like, or lens-shaped bodies among the gabbro-diorites. Fine-grained amphibolites, in bands 150–200 m wide, extend along the western contact of the massif. The amphibolites are cut by numerous quartz–potassium-feldspar and aplite veinlets, which carry disseminated mineralization of copper and molybdenum sulfides.

Mylonitization, silicification, and secondary quartzites are very often observed in the intrusive rocks of the massif under study, in close connection with tectonic ruptures. Silicification is usually manifested in feathering fractures as thin veinlets composed of quartz of platy and radial-radiating structure. Such quartz grows perpendicular to the fracture walls, forming radiating, fan-shaped aggregates, at the narrowed base of which, along the fracture walls, fine-grained chalcedony-like quartz is usually present. Evidently, the development of radiating quartz follows the formation of chalcedony along the mylonite.

Secondary quartzites are large, linearly elongated bodies along zones of tectonic ruptures, occurring in intrusive rocks. Such quartzites crop out on the watershed of the B. and M. Bazas rivers—the “Quartzite” hill—in the vicinity of the settlement of M. Bazas (lower), on the right side of the Tashtyk-Gol spring, and in other places. The secondary quartzites have a variegated, spotted coloration (gray, yellow, white, and cherry-red colors) and a fine-grained structure.

Under the microscope, they not infrequently reveal a relict structure of intrusive rocks: plagioclases, entirely replaced by chalcedony, and intrusive quartz preserve their original outlines. These quartzites are cut by barite and barite–quartz–calcite veins of varying thickness and extent. As a rule, the thickness of these veins ranges from 10–20 cm to 2–3 m. The indicated veins and the quartzites enclosing them contain disseminated cinnabar mineralization.

On the basis of the factual material presented, the following brief conclusions may be drawn:

1. The Verkhne-Kondomsky granodiorite massif evidently has two phases of formation: a diorite phase and a granodiorite–trondhjemite phase.

2. The intrusive rocks of the massif have been widely subjected to metasomatic processes of feldspathization, amphibolization, and silicification, in close connection with which disseminated mineralization of iron, titanium, copper, molybdenum, and cinnabar is noted.

LITERATURE

1. A. M. Zaitsev. On the question of primary gold deposits in the system of the Kondoma River. Collection in memory of E. G. Salishchev, Tomsk, 1903.

2. A. M. Kuzmin. Mineral resources of Gornaya Shoriya. Vestnik Zap.-Sib. geol. razv. tresta, vol. 1, 1939.

3. A. M. Kuzmin. Geological survey of Gornaya Shoriya. Collection “Mineral-Raw-Material Base of KMK,” 1933.

4. V. A. Obruchev. Geological sketch of the gold-bearing regions of Siberia. Zoloto i platina, No. 9, 1910.

5. V. A. Obruchev. Geological sketch of the gold-bearing regions of Siberia. Zoloto i platina, 1911.

6. V. S. Reutovsky. Mineral resources of Siberia. Tomsk, 1905.

7. K. G. Tyumentsev. Geological sketch of the basin of the Kondoma River in the southwestern part of the Kuznetsk Alatau. Izv. Zap.-Sib. geol. upravleniya, issue 2, 1931.