Metasomatic Formations Associated with the Alguy Intrusion of Quartz Diorites

A. I. Shevelev

(Presented by Professor A. M. Kuzmin)

The area described is located in the Kuznetsk Alatau, in the middle course of the Alguy River, a left tributary of the Amzas River, which flows into the Tom River from the right.

The area is composed predominantly of carbonate deposits (dolomites, limestones); quartzites and argillaceous shales are also present. Intrusive rocks within the area are represented by the Tygertysh intrusion of porphyritic granites and the Alguy intrusion of quartz diorites.

Metasomatic formations are developed in dolomites (assigned to the West Siberian Series of the Upper Proterozoic) cut by the intrusion of quartz diorites. The manifestation of metasomatic processes is confined to the Alguy tectonic zone, which is developed in the dolomites and extends in a northeasterly direction in accordance with their strike. The tectonic zone determined the form of the metasomatic formations and also, apparently, the form of the quartz diorite intrusion, which is slightly elongated in the direction of the zone’s extension.

The greatest distribution within the tectonic zone is shown by dolomites, occurring as thick deposits of north-northeasterly strike, dipping steeply (80–90°) to the east. In appearance the dolomite ranges from dark gray to light gray, is fine-grained, dense, and massive; under the microscope it has a granoblastic structure, with polygonal or rhomboid grains 0.02–0.04 mm in size. The dolomites are distinguished by a high purity of chemical composition, as can be seen from the analytical data for 5 samples taken uniformly along profile line XXVI across the strike of the sequence over a distance of 400–450 m (expressed in weight percent): silicon oxide 0.0–0.13, aluminum oxide 0.15–0.26; iron oxide 0.06–0.21; titanium oxide 0.01–0.03; calcium oxide 30.87–31.60; magnesium oxide 21.28–21.84; loss on ignition 46.78–47.19; insoluble residue 0.24–0.41; sodium oxide 0.04–0.08.

The intrusion of quartz diorites has, in plan, the form of an oval stock, slightly elongated in the NNE–SSW direction, measuring 1.5–2.0 km. The intrusion is composed of gray quartz diorites of medium-grained composition with a massive texture. The mineralogical composition of the diorites is: 60–65% plagioclase (oligoclase–andesine), up to 15% quartz, not more than 15% hornblende, not more than 10% biotite; second-

—chlorite, epidote, sericite; accessory minerals—sphene, apatite, zircon, and ore mineral.

Let us consider the metasomatic alteration of dolomites within a tectonic zone located northeast of the intrusion of quartz diorites.

In the immediate vicinity of the intrusion, the dolomites are altered and acquire the character of diopside rocks; with increasing distance from it, the dolomites undergo, to varying degrees, tremolitization, talcification, and replacement by quartzites.

The mode of occurrence of the diopside rocks is controlled by the position of the intrusive contact and by the tectonic structure of the replaced dolomites. The diopside rocks, in the form of a tongue up to 200 m thick, cut into the dolomites and extend within them along strike for 500–550 m, which is determined by the position of fractures in the tectonic zone.

Macroscopically, the pyroxene rocks (composed essentially of leuco-diopside) are white or grayish white, sometimes with a greenish tint; dense, massive; usually fine-grained (to 1.0 mm), more rarely medium-grained.

Near the intrusion, the size of pyroxene grains in the granular aggregate increases to 2.5 mm. Under the microscope the rock has a heteroblastic texture. Diopside occurs as short-prismatic, isometric, or columnar crystals 0.04 to 1 mm long. In the fine-grained pyroxene aggregate, larger individual grains or aggregates of large grains, up to 4 mm in size, are often disseminated.

In some places the white and grayish-white leuco-diopside rocks are cut by veinlets 1.5–2.0 cm wide, greenish in color, composed of columnar diopside crystals up to 4.5 mm long, which in the veinlets are arranged mainly perpendicular to their walls.

In addition to diopside, which predominantly composes the rock, other minerals are also present: tremolite, dolomite, serpentine, talc, and calcite.

Tremolite develops after diopside along the boundaries of its grains or along cleavage cracks, and forms platy or fibrous grains and aggregates. With intense tremolitization, diopside occurs as relics among tremolite formations. The size of tremolite grains varies from extremely small to 0.4 mm, and their size depends on the size of the diopside grain replaced by them.

Within the field of white diopside, irregularly shaped areas ranging in size from several centimeters to one meter stand out; they are greenish or lettuce-green in color and are composed of diopside, tremolite, dolomite, serpentine, and calcite. In these areas, intense tremolitization and dolomitization of diopside are manifested. With intense dolomitization, diopside occurs as irregular relics within the dolomite mass.

Serpentine develops after diopside and tremolite, replacing them up to the formation of pseudomorphs.

Talcification associated with cleavage cracks or with mineral-grain boundaries is observed after diopside and tremolite. Talc develops either as single flakes (<0.01 mm) or as aggregates.

Within the field of diopside rocks there are remnants of unreplaced dolomite in the form of rounded or irregular relics ranging in size from several centimeters to one meter.

Tremolitization of dolomites is developed quite widely within the tectonic zone and is manifested with varying degrees of intensity: from the development of isolated tremolite grains and the formation of their ag-

aggregates to the appearance of tremolite-calcite rocks, which occur as lens-shaped bodies up to 400 m long and up to 50–70 m thick, oriented in accordance with the bedding of the dolomite sequence.

Weakly tremolitized dolomites are a dark-gray or gray rock in which individual needles of tremolite or their aggregates in association with calcite are distinguished; with complete tremolitization the rock acquires a light color and is composed of tremolite and calcite. The tremolite-calcite rock sometimes has a greenish tint and is often considerably schistose, the elements of bedding and schistosity coinciding with the elements of bedding of the rocks. Tremolite-calcite rocks are exposed as narrow vertical cliffs, sometimes of considerable height (up to 10 m) and extent (50–60 m).

Microscopic studies have shown that the tremolite-calcite rock consists predominantly of these two minerals, with tremolite predominating (up to 70–80%). In addition, dolomite and talc are present in the rock. Tremolite occurs as acicular, fibrous, or platy crystals arranged randomly or gathered into sheaf-like and rosette-like aggregates. The length of the tremolite crystals is 0.4–0.6 mm, sometimes 4–5 mm. The spaces between tremolite grains are filled with fine-grained (0.06–0.08 mm) calcite.

The tremolite crystals are, to varying degrees, subject to talc alteration, which develops along grain boundaries or along cleavage cracks in the form of individual flakes or fine-scaled aggregates.

In the white tremolite-calcite rock there are relicts of unreplaced gray and dark-gray dolomite of irregular or rounded form, ranging in size from 2–3 mm to 0.7–0.8 m. Small rounded dolomite relicts give the rock a spotted texture. Dolomite relicts are usually bordered by a calcite rim. Dolomite remnants also occur as parallel, elongated strips-lenses among tremolitized dolomites.

Dolomites sufficiently removed from the intrusion undergo talc alteration and, under intense processes, are transformed into talcites, which are a gray rock composed of fine-scaled (to 0.01 mm) talc, the flakes of which are oriented lengthwise in the direction of the strike of the original rock. Talcite occurs in the form of lens-shaped bodies ranging in thickness from several centimeters to 8–10 m. Interlayers and remnants of unreplaced dolomite among the tremolite-calcite rocks are also subject to intense talc alteration.

Within the tectonic zone, bodies of metasomatic quartzites are distinguished. They occur as lens-shaped bodies of varying thickness (from several meters to 50 m) and extent (up to 200 m). Quartzites have been noted both in the zone of diopside rocks and in the zone of tremolitization of dolomites. In outward appearance, quartzite is a white or light-gray rock, massive, fine-grained. In some areas the quartzite acquires a spotted texture owing to remnants of unreplaced rock of various form and size.

Under the microscope, quartzite has a granoblastic or heterogranoblastic structure and is composed of polygonal quartz grains ranging in size from 0.1–0.2 to 0.8–1.0 mm. Quartzites metasomatically replace dolomites, diopsides, and tremolite-calcite rock, as is evidenced by remnants of the latter in the quartzite. Relicts of diopside and tremolite-calcite rock are usually surrounded by small (0.01–0.05 mm) grains of quartz.

Insignificant silicification of dolomites, manifested in the form

irregular quartz segregations, is observed on both sides of the tectonic zone.

The intrusion of porphyritic granites, exposed as a small stock in the northeastern part of the area, caused the marmorization of the dolomites along its eastern contact.

Within the dolomite sequence of the area there are numerous dike formations, represented by diabases, diabase porphyrites, and diorite porphyrites. The dikes have a northwestern strike and dip steeply to the northeast; their thickness ranges from 0.5 m to 40–50 m. Exocontact alterations near the dikes, noted in isolated cases, were expressed in serpentinization of the enclosing rocks.

As is evident from the foregoing, within the tectonic zone there is exocontact zonality of metasomatic formations with increasing distance from the intrusion: in direct contact with the intrusion there is a zone of diopside rocks, followed by a zone of tremolitization and talcification.

The formation of metasomatic rocks took place mainly in the process of infiltration metasomatism, promoted by the intense fracturing of rocks within the tectonic zone, which favored the movement of solutions causing metasomatic alteration of the dolomites.

Bimetasomatism is of subordinate importance and evidently develops in the immediate contact between the intrusion and the dolomites.

Diopside rocks are the highest-temperature formations among the metasomatic rocks described. As the postmagmatic solutions cooled in the course of their movement along fractures and away from the magmatic source, the chemical potential of water [5] increased, and silicates containing the hydroxyl group (tremolite, talc) formed. The lowest-temperature formations include serpentinization, dolomitization, and talcification.

According to the classification of postmagmatic processes [3, 4], the formation of the metasomatic rocks of the area may be assigned to the following stages. In the early alkaline stage, diopside and tremolite-calcite rocks formed, the former at the beginning of the alkaline stage and the latter at its end. The acid stage was manifested in the formation of secondary quartzites under acid-medium conditions, with a general input of calcium oxide and magnesium oxide. In the late alkaline stage, talcification of dolomites, tremolites, and diopsides took place, as well as replacement of diopside by dolomite and serpentinization of diopside, tremolite, and dolomite. The formation of talc and serpentine evidently belongs to one temperature interval, but the former, as the more highly siliceous mineral (silica—63.5%) compared with serpentine (silica—44.0%), forms under conditions of excess silica; this is confirmed by microscopic study of the rocks. Thus, in particular, talcification of diopside occurs in areas where quartz segregations are present, whereas in areas of serpentinization both quartz and talc are absent. Similar phenomena were noted by A. P. Ganeev [2] in his examination of metamorphic conditions in the Lesser Karatau.

Thus, the metasomatic processes that led to the formation of the rocks described developed under conditions of input of silica by postmagmatic solutions, with the remaining rock-forming elements (calcium, magnesium) present in the original rocks being replaced; the mineral composition of the newly formed rocks is wholly dependent on the composition of the original dolomites and on the physicochemical conditions of the metasomatic processes.

References

1. N. A. Eliseev. Metamorphism. “Nedra,” 1963.

2. A. P. Gareev. A new talc-bearing province of the Maly Karatau Range. “Nauka,” 1965.

3. D. S. Korzhinskii. An essay on metasomatic processes. In: Fundamental Problems in the Study of Magmatogenic Ore Deposits. Academy of Sciences of the USSR, 1955.

4. D. S. Korzhinskii. General regularities of postmagmatic processes. In: Problems of Postmagmatic Ore Formation, vol. I. Publishing House of the Czechoslovak Academy of Sciences, 1963.

5. V. I. Kichul. Regularities in the association of minerals of metamorphic carbonate rocks. In: Physicochemical Conditions of Magmatism and Metasomatism (Proceedings of the 2nd All-Union Petrographic Conference). “Nauka,” 1964.