Wall-Rock Alteration of Host Rocks at the Contacts of Gold–Quartz Veins of the Irokinda Ore Field

V. I. Bazhenov, A. P. Gribanov, V. A. Rubanov, O. A. Dulzon

(Presented by the scientific seminar of the Department of Mineralogy, Petrography, Geology, and Prospecting of Mineral Deposits)

The Irokinda ore field is located on the western margin of the South Muya crystalline block, at its junction with the Kilyan tectonic zone of northwestern strike. The South Muya block is composed of rocks of a complexly dislocated metamorphic complex, distinguished as the Kindikan Formation of Lower Proterozoic (Archean according to L. I. Salop [3]) age, represented by ancient primary sedimentary and igneous rocks. Within the Kilyan tectonic zone, metamorphosed effusive-sedimentary formations of the Kilyan sequence of Middle Proterozoic age (A. A. Gamyan, N. P. Andreev) and of the Mukhtun Formation of Upper Proterozoic, possibly Lower Cambrian (G. A. Kibanov, A. S. Matros, and others), age are developed.

As established by our work in 1964–1965, the principal folded structure of the ore field is a large anticline of northeastern strike, complicated by folds of higher orders, including up to isoclinal folding, and by ruptural disturbances. The latter form three systems of major fractures that possess a number of features allowing them to be assigned to the shear type: 1) disturbances of northwestern strike dipping to the southwest; 2) disturbances of northeastern strike dipping to the southeast and northwest; 3) disturbances of sublatitudinal strike with steep dip to the north and south. In the fractures of the first two systems, which are often characterized by considerable (up to 1.5 km and more) strike length and fairly persistent elements of occurrence, all known gold–quartz veins are localized.

Of all the ore bodies of the Irokinda gold-ore field, the Yurasovskaya-II vein is at present the most studied and interesting. It is localized in a gently dipping (35–40°) northwest-dipping shear fracture of northeastern strike, conjugate with fractures of northwestern strike, dipping gently

(30–40°) to the southwest. Its form is sheet-like. The contacts with the enclosing rocks are distinct. The thickness of the vein decreases down-dip from 3.05 to 0.6 m. In its material composition the Yurasovskaya-II vein belongs to the type of low-sulfide, essentially quartz, gold-bearing formations. The amount of ore minerals in it is no more than 1%. The ore minerals are represented by pale ores, chalcopyrite, pyrite, sphalerite, galena, gold, and arsenopyrite. In the upper levels of the vein, among the ore minerals, pale ores, chalcopyrite, and pyrite predominate; in the lower levels—sphalerite and galena. Gold is distributed extremely unevenly. Its main mass is concentrated in shoots.

At the contacts of the Yurasovskaya-II vein, as at the contacts of other gold-quartz veins of the Irokinda gold-ore field, the enclosing rocks underwent hydrothermal alteration. The width, in plan, of the zone of altered rocks at the hanging wall of the Yurasovskaya-II gold-quartz vein varies within 0.2–15.0 m, usually amounting to 6–10 m; at the footwall, from 10 to 25 m—usually 12–15 m.

Below is a brief petrographic description of the enclosing rocks, both those lying outside the halo of hydrothermal alteration surrounding the quartz vein and those within it.

A. Enclosing rocks not affected by processes of wall-rock alteration

1. Paragneisses. Among this group of enclosing rocks, varieties of garnet-pyroxene, amphibole-pyroxene, and amphibole composition are distinguished. They form thin interbeds, often alternating with one another and producing a linear-banded texture in the rock.

Under the microscope the rock structure is usually nematogranoblastic or lepidogranoblastic, caused by the predominance of isometric mineral grains represented by garnet, feldspars, pyroxene, quartz, and by the presence of a subordinate amount of elongated prismatic or platy grains of amphibole, chlorite, biotite, and more rarely quartz.

The mineralogical composition of the paragneisses is represented by plagioclase (40–60%), microcline (1–10%), quartz (10–30%), amphibole (5–50%), pyroxene (10–20%), garnet (3–17%), biotite, chlorite, carbonate, and sericite. Among accessory admixtures, sphene, zircon, apatite, and epidote have been found.

In many cases the paragneisses display a complex of later metasomatic processes superimposed on the metamorphic rocks; these processes are expressed in the replacement of plagioclase and pyroxene by an aggregate of epidote and clinozoisite, and in the microclinization of plagioclase, which is clearly manifested in a number of areas of the ore field.

2. Granites. In typical form, the granites occurring within the ore field as dike-like bodies with predominantly steep dips are medium- or fine-grained rocks, containing rare porphyritic segregations composed of microcline or acid plagioclase (albite). Fine-grained varieties of granites, which usually occur in the endocontact parts of large dikes and in dikes of small thickness, are characterized by enrichment in hornblende, distributed more or less uniformly in the rock.

The texture of the rocks as a whole is massive, sometimes bearing traces of weakייש

gneissification. Macroscopically, a pegmatitic structure is observed in some granite dikes.

Under the microscope the structure of the rock is, in most cases, hypidiomorphic-granular; a cataclastic structure is often manifested. The latter is due to large, somewhat rounded grains of plagioclase or microcline and to small areas, with a mosaic texture, of fine-grained quartz and feldspars, formed by blastesis and filling the angular interstices between the large grains. The mineralogical composition of the granite is fairly simple and stable. The principal rock-forming minerals are plagioclase (60–70%), quartz (20–30%), and microcline (0–20%). Amphibole, clinozoisite, and chlorite occur in very small amounts. Sphene, orthite, zircon, and apatite are present as isolated grains.

Among the secondary alterations of the granites, the following should be noted:

1. Microclinization of the rocks, expressed in the replacement of plagioclase by microcline and in the formation of quartz–microcline veinlets. The degree to which microclinization is manifested varies sharply because of the local character of this process.

2. Epidotization of plagioclase and chloritization of dark-colored minerals.

3. Garnet–mica gneisses. This group of host rocks is represented by gneisses with an indistinctly expressed lenticular-banded texture, which sometimes bears traces of intense schistosity and weak cataclasis. The lens-shaped accumulations are formed by biotite, with rare porphyroblastic segregations of garnet, and occur in a fine-grained groundmass of quartz–feldspar composition.

Under the microscope, the structure of the gneisses is microgranoblastic or lepidogranoblastic. Quite often these rocks display a blasto-cataclastic structure, with the secondary blastic recrystallization typical of it in material that had undergone comminution and flattening during dynamometamorphism.

The garnet–mica gneisses are characterized by constancy in both qualitative and quantitative mineralogical composition, represented by quartz (20–40%), plagioclase (40–50%), biotite (10–20%), and garnet (10–20%). In isolated cases, instead of biotite, amphibole and pyroxene are present in the same amount, the former intensively replacing the latter. In addition to these minerals, small amounts of kyanite and sillimanite (2–4%) are encountered fairly often. Accessory minerals are represented by isolated grains of zircon, rutile, and apatite. Alterations of secondary character, which somewhat modify the mineralogical composition of the rock and the appearance of the primary minerals, include some silicification of the rocks, replacement of garnet, kyanite, and sillimanite by biotite, and opacitization of biotite.

B. Rocks Altered by Wall-Rock Processes

The host rocks near the quartz veins bear traces of intense alteration, as a result of which the original paragneisses, garnet–mica gneisses, and granites were partly transformed into metasomatic neoformations.

Such neoformations, taking into account their quite definite geological position (development exclusively near quartz veins, gradual transitions to fresh rocks with increasing distance from the veins, etc.) and mineralogical features, are undoubtedly due to their

by origin to the action on the host rocks of solutions circulating along fractures and participating in the formation of the quartz veins.

Since the character of the wall-rock alterations and their intensity in the various types of host rocks have their own specific features, we consider it expedient to describe the altered rocks separately for each type.

Paragneisses. Wall-rock alterations in the paragneisses surrounding the vein are most clearly manifested within a halo of 10–12 m; beyond this, within 1–2 m outside its limits, they become so weakened that in external appearance the rock is almost indistinguishable from fresh varieties.

Macroscopically, the changes are expressed in the appearance in the paragneisses of a monotonous greenish-brown coloration, the disappearance of the linearly banded texture, and their transformation into dense rocks fractured only immediately at the contact with the vein.

Under the microscope they display a microlepidogranoblastic structure, caused by disorderly arranged fine flakes of sericite, very small plates of muscovite, and microscopic grains of quartz. Relics of a granoblastic structure, typical of unaltered rocks, are often encountered in thin sections. The mineral composition of the altered rocks is fairly simple and uniform. They consist of quartz, carbonate, sericite, and an insignificant amount of leucoxene, which forms pseudomorphs of rhombic outlines after sphene grains, a feature highly characteristic of this mineral. Relict remnants of plagioclase and quartz grains are not infrequently encountered in the quartz–carbonate–sericite mass. Sometimes in the rock, especially near the salbands, there is a rare fine impregnation of pyrite; in addition, pyrite sometimes forms aggregates up to 1–1.5 cm in size. Carbonate in the form of calcite develops mainly by replacement of pyroxene, clinozoisite, and garnet. In addition, it is contained in veinlets cutting the rock. Ankerite often replaces plagioclase. A considerable part of the quartz is formed as a result of the decomposition of feldspars. It is represented by a fine-grained aggregate and is associated with sericite and ankerite. A comparatively small amount of quartz is evidently relict. It is contained in the rock as rare large grains, differing from secondary quartz by a sharply anomalous extinction. In addition, quartz is sometimes contained in veinlets cutting the rock, as well as in the marginal parts, and sometimes also inside grains of coarse-grained quartz; in the latter case it is apparently a regenerative new formation.

Sericite arises in the rock mainly at the expense of the decomposition of plagioclases, forming together with carbonate and fine-grained quartz. Taking into account that a certain amount of sericite is contained in the fresh rocks, its total content increases markedly during wall-rock alteration.

Thus, the wall-rock alterations of the mineral composition in paragneisses are expressed in carbonatization, silicification, and rather weak sericitization of the rocks. Sulfidization is manifested extremely weakly and has a narrowly local character. In the spatial distribution of the indicated mineral new formations, a certain zonality is observed. Near the vein, silicification of the rocks and carbonatization are manifested intensively; with distance from the vein, silicification weakens, then ferruginous carbonates disappear, and in the weakly altered rocks located at the periphery of the wall-rock halo—

vein alterations, only comparatively weak sericitization of plagioclase and rare calcite veinlets are noted.

Granites. The change in the mineralogical composition of the granites as a result of hydrothermal metamorphism is most clearly manifested directly at the contact with the vein. With increasing distance from it, these changes appear less and less distinctly, and at 6–7 m from the vein the external appearance of the rock is almost indistinguishable from fresh granite.

Macroscopically, the altered granites differ from unaltered varieties by the presence of a light-brown color, caused by the development of secondary minerals, and also by the porcelain-like appearance of the feldspars. Immediately near the vein the primary structure of the granite disappears, and it is transformed into a dense massive rock, in some cases bearing fine disseminations of enriched pyrite.

Under the microscope, newly formed quartz, sericite, muscovite, carbonate, and albite are established in the rock. Quartz, sericite, and muscovite are formed mainly at the expense of the decomposition of plagioclases. Calcite is also present in these fine-grained aggregates in small amounts. Albite develops along potassium feldspar, in the grains of which a chessboard structure appears. In addition, near the contact with the vein the rock is cut by numerous veinlets containing carbonate, quartz, sericite, and muscovite.

A characteristic distinction of the altered granites from the paragneisses is the comparatively low degree of decomposition of the primary minerals and structures, which are always determined under the microscope without any difficulty.

Thus, the most typical feature of granites altered by near-vein processes is the decomposition of plagioclases, manifested chiefly near the contact with the vein and gradually disappearing already at 5–6 m from the contact.

Carbonatization and muscovitization of the primary minerals are expressed rather weakly; however, the halo of their manifestation around the vein is wider and reaches 7–8 m.

Garnet–mica gneisses. Macroscopically, changes associated with near-vein processes can be traced up to 6–7 m from the vein. They are expressed in the appearance in the rocks of a gray-green color, caused by the broad development of secondary minerals and the presence of fine disseminated pyrite, which is almost absent in fresh rocks.

Under the microscope, the altered garnet–mica gneisses consist of quartz, sericite, ferruginous carbonate (ankerite), and muscovite. The structure of the rocks is lepidogranoblastic, conditioned by platy grains of mica minerals and generally isometric grains of quartz. A large part of the secondary minerals, including some quantity of quartz, arises primarily at the expense of destruction of plagioclase. In addition, sericite, muscovite, and carbonate are formed as a result of decomposition of garnet, biotite, disthene, and sillimanite.

As was shown above, macroscopically the rock alterations can be seen up to 6–7 m from the selvages; however, under the microscope they can be traced considerably farther from the vein and are expressed mainly in the chloritization of biotite, observed up to 20–22 m from the contact of the gneisses with the quartz vein. Chlorite develops along the cleavage of biotite, often replacing its grains entirely.

Summarizing the foregoing, it should be emphasized that, in the near-vein changes in the mineralogical composition of the garnet–mica gneisses, a definite zonality is observed in the spatial

in the distribution, with respect to the vein, of newly formed minerals. Up to 6–7 m within the halo of wall-rock alteration, silicification, carbonatization, sericitization, muscovitization, and sulfidization of the rocks predominate; farther on, up to the transition into fresh rocks and with the gradual dying out of these processes, chloritization of biotite—absent near the quartz vein—is manifested most clearly.

Conclusions concerning the chemistry of the process of hydrothermal metamorphism in paragneisses, garnet–mica gneisses, and granites are based on the results of analyses of fresh and altered rocks sampled from underground-drilling cores in rocks of the footwall of the Yurasovskaya-II vein.

For the quantitative determination of gain and loss of the principal rock-forming components, the “method of chemical analysis with allowance for porosity” was used.

General information on the features of migration during near-ore alteration of a number of trace elements (lead, zinc, copper, nickel, cobalt, molybdenum, silver, arsenic, tin, thorium, manganese, etc.) presented in the article was obtained as a result of semiquantitative spectral analysis.

A study of the chemistry of the process of near-ore metasomatism, which caused a high degree of hydrothermal alteration of the host rocks, made it possible to establish the high mobility of many components, as a result of which large masses of matter were set in motion. The mobility of the components reached a maximum near the ore body.

High mobility is established for silica, alumina, calcium, sodium, and potassium. Iron, titanium, and manganese proved to be relatively immobile in the process of hydrothermal metamorphism. A clearly expressed regularity in the distribution of zones of addition and removal along most metasomatic columns is, in most cases, not established. It should be noted, however, that the general character of migration of the principal rock-forming components differs substantially among the different types of host rocks. The same conclusions may also be drawn about the nature of migration of a number of trace elements on the basis of spectral-analysis data. Thus, in paragneisses and granites, trends of addition predominate in the migration of mobile components, which evidently indicates a favorable medium for hydrothermal solutions, creating the possibility of precipitation from them of a number of principal components of the host rocks (silica, alumina, iron, etc.) and of the overwhelming majority of trace elements (lead, zinc, copper, beryllium, manganese, chromium, vanadium, barium, nickel, zirconium, strontium, etc.), which quite clearly mark the interval of the most altered paragneisses by a higher and more persistent content in the samples, sharply different from samples located farther, 5–6 m from the quartz vein. This makes it possible to conclude that they were appreciably concentrated in the wall rocks near the quartz veins.

In garnet–mica gneisses, on the contrary, the medium proved less favorable for these solutions, as is indicated both by the wedging-out of quartz veins in these rocks, accompanied by a noticeable decrease in the content of gold and sulfides, and by the predominance of removal in the migration of a number of rock-forming components (silica, alumina, potassium, sodium), accompanied by a noticeable decrease in the content of a number of trace elements (lead, copper, cobalt, gallium, zirconium, chromium, zinc, strontium) in the most altered host rocks lying in contact with the quartz vein.

Without dwelling in detail on the question of the source of the material participating in hydrothermal metamorphism, it should be noted—

...that, as such, they may probably have been the host rocks themselves, from which, in the process of wall-rock alteration, a considerable amount of material was removed, with the exception of alkalis, carbon dioxide, and sulfur.

The hydrothermal metamorphism of the rocks of the Irokinda deposit took place at a temperature of 200–300° and was caused by solutions of alkaline or weakly alkaline composition. The latter is supported by the pH value of suspensions of certain minerals included in the hydrothermally altered rocks (chlorite, calcite, ferromagnesian carbonates). It is assumed that this value corresponds to the conditions under which the mineral was deposited.

References

G. I. Bushinskii. Geochemistry of the sedimentary process. In: Handbook of the Field Geologist–Petroleum Geologist. Gostoptekhizdat, 1954.

N. I. Nakovnik. Determination of the quantitative change of matter during hydrothermal metamorphism. Transactions of the All-Union Mineralogical Society, issue 4, 1958.

L. I. Salop. Geology of the Baikal Mountain Region. Vol. 1. Stratigraphy. VSEGEI, Gosgeoltekhizdat, Moscow, 1964.