Products of Hydrothermal Metamorphism of Volcanogenic Formations in the Eastern Part of the North Minusinsk Depression and Some Patterns of Their Distribution

V. G. Kryukov, V. K. Cherepnin

(Presented by Professor A. M. Kuzmin)

Recent years have been characterized by increased interest in the study of products of hydrothermal metamorphism in general and of near-ore alterations in particular. This is due to the fact that the process of ore formation, as a rule, is accompanied by the broad development of metasomatic aureoles. Determining the spatial position of newly formed minerals and the association of ore mineralization with a particular type of altered rock facilitates the search for concealed mineralization.

This article presents a characterization of hydrothermally altered rocks developed within the eastern part of the North Minusinsk Depression, and attempts to outline certain patterns in their spatial distribution.

The area in question lies in the zone of junction between the East Sayan Proterozoic mega-anticlinorium and the North Minusinsk Devonian megasynclinorium. This position determined the development of inherited northwestern fault tectonics and the volcanic and hydrothermal activity associated with it.

Devonian effusive and volcanogenic-sedimentary formations participate in the geological structure of the territory. Within them the following rocks are distinguished (from bottom to top): greenish-gray diabase porphyrites, dark cherry-gray amygdaloidal rocks, dark-gray basaltic, plagiobasaltic, and andesitic porphyrites, light-pinkish dacitic porphyries, brownish-red felsophyres and lilac-gray cryptocrystalline quartz porphyries, pinkish-gray trachyandesitic porphyries, reddish-brown orthophyres and trachytoid phonolites, reddish felsite-porphyries and dark-gray to black olivine-bearing basaltic porphyrites, alkaline rhyolitic porphyries and cherry quartz-bearing porphyries, brownish-cherry trachytic porphyries, as well as pyroclastic varieties of the listed effusives and green-colored tuffites, tuffaceous sandstones, and marls. The basic effusives are characterized by sheet-like covers or flows, the acidic and subalkaline ones by dome-shaped bodies, while beds and lenses are established for the tuffs and tuffites.

A fairly significant part of the area, about 15–20%, is occupied by subvolcanic and small intrusive bodies represented by two groups (from older to younger): 1) diabase and labradorite porphyrites, granophyric and alkaline rhyolitic porphyries, agglomerato-

tuffs, eruptive breccias, quartz porphyries, orthophyres, bostonites, and crystal porphyries; 2) syenite-porphyries, porphyritic syenites, granosyenites, granodiorites, dioritic and doleritic porphyrites. At the same time, the former are noted as belonging to subvolcanic facies, and the latter to intrusive ones.

Effusive and volcanogenic-sedimentary rocks form a trough-like, northwest-oriented plicative structure, complicated by an additional brachyform structure, which is emphasized by the presence of several smaller troughs with very gentle bedding of effusives on their limbs. The central axial part of the trough-like structure is, in addition, complicated by a flexure-like bend caused by the presence of a practically vertical fault of northwestern strike. This disturbance is a thick, 500–1000 m, zone of intense schistosity, crumpling, and, to a lesser extent, crushing of rocks. In addition, a sublatitudinal structure is recorded in the area, morphologically similar to the one indicated above, as well as a series of small tectonic sutures. These weakened zones control the spatial distribution of volcanic vents, dike and subvolcanic intrusive bodies, and also products of hydrothermal activity.

All altered rocks of the area described are caused by manifestations of the following processes: 1) solfataric-fumarolic and 2) hydrothermal. In studying the products of metasomatism, it is necessary to take into account the diagenetic alteration of the original rock, which arises, as follows from classical concepts [7, 8], as a result of the unstable state of the erupted magma, because in a number of cases diagenesis leads to the formation of new mineral assemblages very similar to the products of hydrothermal metamorphism. In the present case, the diagenetic phase of the rock’s state is determined by recrystallization of the mesostasis of effusives, clouding of feldspar grains, and decomposition, under new conditions of existence, of such minerals as olivine, pyroxene, amphibole, and biotite, with formation of epidote, albite, calcite, chlorite, serpentine, and magnetite. One of the criteria for distinguishing diagenetic alterations is the absence of feeder fractures, the monolithic appearance of the rock, and also the development of newly formed minerals practically only within the contours of the host mineral.

The most typical products of solfataric-fumarolic activity in the area are, to varying degrees, silicified rocks and secondary quartzites.

Silicified rocks, whose origin, as is known [5, 6, 7], is conditioned by exhalation processes, are everywhere confined to volcanic vents. A connection is also established with the northwestern fault, expressed in the fact that the areas adjacent to it are distinguished by a large quantity of newly formed or recrystallized quartz. These formations extend rather widely (1–4 km) in a northwestern direction for 30–40 km. Silicification mainly affects pyroclastic rocks and the near-contact parts of sheet effusives.

Macroscopically, silicified rocks retain a brownish-cherry, light-brownish, or gray color; fragmental, dense, or fluidal textures. Under the microscope, recrystallization structures are established, obscuring the peperitic structure of the original effusive. The amount of quartz varies widely from 1–5% to 20–40%. Often, against the background of a fine-grained quartz aggregate, microveinlets with a drusy appearance of crystals are observed.

Within the field of development of silicified rocks, areas are noted, pre-

represented by secondary quartzites. The latter, as a rule, are confined to the northwestern and sublatitudinal faults, occurring within them at the sites of vent formations of ancient volcanic edifices. The linearly elongated bodies of secondary quartzites, stretched in a northwestern direction, as well as the isometric, oval bodies in plan, emphasize the fissure or central character of the volcanic structures. The dimensions of the bodies are about \(300–500 \times 2000–3000\) m for elongated forms and \(200–500 \times 500–800\) m for isometric ones. The transition from silicified rocks to quartzites is gradual.

Secondary quartzites are characterized by a light and light-gray color with pinkish, greenish, and yellowish shades, a very dense texture, often with platy parting. Relicts of slightly altered rocks are not infrequently established. Quartzites develop over practically all effusives; most often replacement affects subvolcanic alkaline rhyolite porphyries, quartz porphyries, orthophyres, and their effusive derivatives. Quite often alterations of this type include olivine-bearing basalt porphyrites, diabase porphyrites of the dike facies, and volcanolites of subalkaline rocks.

Secondary quartzites are characterized by mosaic or end-face structures, expressed in the development of fine (\(0.01–0.08\) mm) grains of quartz, dickite, and sericite. The newly formed minerals, constituting \(50–80\%\) of the rock volume, are represented by quartz, about \(40–70\%\); sericite, from \(5–15\%\) to \(30–50\%\); dickite, from \(1–5\%\) to \(20–30\%\); pyrite, \(1–10\%\); and by single grains or rare aggregates of rutile needles.

The products of hydrothermal metamorphism are very diverse. In accordance with views existing in the literature [2, 3, 4, 6], the entire complex of altered rocks may be subdivided into a number of formations, among which the following are established in the order of their formation: 1) alkaline metasomatites; 2) propylites; 3) quartz–sericite rocks.

Alkaline metasomatites occur mainly in areas that are remnants of ancient volcanic edifices, overprinting silicified rocks and secondary quartzites. The spatial distribution of the metasomatites is controlled here by sublatitudinal disturbances. In plan they form either narrow, linearly elongated bodies, or oval and irregularly shaped areas. Their dimensions are, respectively, \(50–250 \times 700–3000\) and from \(100 \times 150\) to \(1000 \times 1200\) m.

These formations develop in basic effusives, lithocrystal-clastic tuffs and tuff breccias, and to a lesser extent in quartz porphyries, alkaline rhyolite porphyries, and syenite-porphyries. In slightly altered varieties, colors and textures characteristic of the original rocks are observed; as the role of newly formed minerals increases, the color of the rock acquires a bluish tint, fluidal and clastic textures disappear, and a characteristic silky luster appears on fracture planes. In addition, the grain size increases to \(2–4\) mm, in contrast to the fine, \(0.1–0.5\) mm, crystals of the groundmass of the primary effusives.

Under the microscope, a blastose structure, metacrystals, and also structures close to hypidiomorphic-granular are observed. Very often, especially in slightly altered varieties, zonal overgrowths of alkaline dark-colored minerals are noted on the edges of phenocrysts or fragments. Of interest is the confinement of aggregates of newly formed minerals to fractures, the sheath-like or skeletal form of some of the largest grains. The amount of newly formed material ranges from \(1–5\%\) to

20–40%, often reaching 70–90% of the rock volume. The mineralogical composition of the alkaline metasomatites is as follows: arfvedsonite, aegirine-augite, syntagmatite; pargasite, diopside, biotite, and zircon are less developed.

Propylites are more widely developed than the preceding metasomatites, forming fairly broad and extensive fields measuring 0.7–1.2 km × 20–30 km. Within the area of their development there are elongated or rounded “islands” of alkaline metasomatites and secondary quartzites. The spatial position of the propylites is controlled by northwestern and sublatitudinal faults. Rocks affected by this type of alteration include both effusive varieties—alkaline rhyolitic porphyries, olivine-bearing basaltic porphyrites, quartz-bearing porphyries, trachytic porphyries, and their pyroclastic derivatives—and intrusive rocks, most commonly granitoids.

Macroscopically, the newly formed rocks retain the appearance of the original rock, expressed in the development of a dense, fluidal or fragmental texture, as well as a dark-gray, brownish, or reddish coloration. Nevertheless, in porous varieties of effusive rocks and in areas adjoining a tectonic seam, a weak “lightening” or “greening” of the rocks is recorded. The newly formed minerals of the propylites occur as spots or veinlets. Quite often, the rather irregular branching of the latter forms textures close to breccia-like ones.

Under the microscope, a rather significant quantity of newly formed minerals is established. At the same time, an increase in their proportion from 5–7% to 15–30% is noted in porous rocks or near tectonic fractures. The composition of the propylites includes: epidote, actinolite, chlorite, calcite, and pyrite; the epidote-actinolite facies has limited distribution and is established only along the periphery of the mullion-like structure.

Copper-zinc or lead-zinc mineralization is confined to areas of strongly propylitized rocks, where chlorite and albite show maximum development. Disseminations of chalcopyrite, galena, sphalerite, pyrite, and arsenopyrite are observed. Ore minerals form stable associations mainly with chlorite, calcite, and albite.

Quartz-sericite rocks are recorded only within fields of development of secondary quartzites and propylites, as narrow (50–250 m), extended (1000–5000 m) zones of sublatitudinal or northwestern orientation. They develop mainly after alkaline rhyolitic porphyries and their pyroclasts, as well as after felsophyres, quartz-bearing porphyries, and ash tuffs.

For quartz-sericite formations, breccia-like, platy-cellular, or veinlet textures are very characteristic. The fragments are represented by quartzites or propylites. Cases are not uncommon in which quartzites with superimposed propylitization occur among the fragments. In addition to such characteristic textural features, quartz-sericite rocks are distinguished by a light-gray to white color with brownish or pinkish shades. The color and structural features of the original rocks are almost completely obscured.

New formations usually constitute from 20–30% to 60–90% of the rock volume. The lepidoblastic and granoblastic structure of the aggregate of metasomatic minerals is complicated by the manifestation of banded and “streamy” microtextures. The mineralogical composition is very diverse: the principal minerals are quartz and sericite; in addition, albite, adularia, pyrite, and somewhat later hematite, dolomite, and fluorite are observed. Relict-

The ore minerals are represented by chlorite and dickite. It should be noted that fluorite, forming nests, veinlets, and veins up to several meters thick, is closely associated with quartz, albite, and orthoclase.

In addition to the characterized products of hydrothermal metamorphism, formations produced by the influence of intruding intrusions are observed quite often in the contact zones of granitoid bodies. These new formations are expressed in weak recrystallization of the enclosing rocks, which is very typical [1] of dikes, as well as in the development of tourmaline, biotite, potassic feldspar, and zircon. The amount of the latter does not exceed 1–3%, in contrast to alkaline metasomatites.

Thus, for the southeastern part of the Solgon Range, a significant metasomatic alteration of rocks is established, with the formation of various types of newly formed products, among which silicified rocks, secondary quartzites, alkaline metasomatites, propylites, and quartz-sericite formations are distinguished.

At the same time, it is noted that silicified rocks and secondary quartzites arose in the pre-intrusive stage, whereas all the others arose in the post-intrusive stage.

The spatial position of the various types of altered rocks is controlled by a single northwestern or sublatitudinal fault. The distribution of products of solfataric-fumarolic activity is governed by the position of ancient volcanic edifices. The emplacement of products of hydrothermal metamorphism, however, is associated with the manifestation of sublatitudinal and northwestern fracture tectonics.

The confinement of alkaline metasomatites, propylites, and quartz-sericite rocks to the same structures confirms the practically continuous evolution of the metamorphic process.

A definite type of mineralization is also associated with certain formations of hydrothermally altered rocks: with propylites—copper-zinc and lead-zinc; with quartz-sericite formations—fluorite and molybdenum-sulfide.

REFERENCES

1. A. I. Bazhenov. Features of contact phenomena associated with dike rocks of basic alkaline, intermediate, and acidic compositions (on the example of the Elekmonar massif in Gorny Altai). Izv. TPI, vol. 127, issue 1, 1964.

2. V. A. Zharikov. Some regularities of metasomatic processes. In: Metasomatic Alteration of Wall Rocks and Its Role in Ore Formation. Nedra Publishing House, 1966.

3. D. S. Korzhinsky. An outline of metasomatic processes. In: Basic Problems in the Study of Magmatogenic Ore Deposits. 2nd ed., Academy of Sciences of the USSR, 1955.

4. N. I. Nakovnik. Propylitized rocks, their mineral facies, genesis, and practical significance. Zapiski Vsesoyuznogo Mineralogicheskogo Obshchestva, series 2, part 83, issue 2, 1954.

5. N. I. Nakovnik. Secondary Quartzites of the USSR. Nedra Publishing House, 1964.

6. S. I. Naboko. Hydrothermal Metamorphism of Rocks in Volcanic Regions. Publishing House of the Academy of Sciences of the USSR, 1963.

7. A. Rittmann. Volcanoes and Their Activity. Mir Publishing House, 1964.

8. M. A. Usov. Phases of effusives. Collection: The Basic Ideas of M. A. Usov in Geology. Publishing House of the Academy of Sciences of the Kazakh SSR, 1960.