Contact Aureole of the Chanysh Intrusion
Abstract Generated abstract
This paper characterizes the contact aureole of the Chanysh intrusion, a granodiorite, tonalite, and plagiogranite complex emplaced into Lower Cambrian effusive, volcaniclastic, and sedimentary rocks. On the basis of field relations, petrographic observations, optical mineral data, and mineral assemblages, it distinguishes three successive alteration stages: thermal metamorphism producing spotted schists and hornfelses of hornblende hornfels facies, contact metasomatic amphibolitization and skarnification, and later multistage hydrothermal mineralization. The study interprets amphibolitized rocks as products of postmagmatic magnesian metasomatism and regards the skarns as mainly bimetasomatic, with limited contact infiltration. Gold and telluride mineralization are associated with skarn development and later hydrothermal events, and the described alteration pattern is compared with processes near the Lebed skarn gold deposit.
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CONTACT AUREOLE OF THE CHANYSH INTRUSION
N. F. STOLBOVA
(Presented by the scientific seminar of the Department of Petrography)
The Chanysh intrusion, assigned by V. I. Yarygin (3) to the Lebed granodiorite–tonalite–plagiogranite intrusive complex (the junction of the Gorny Altai and Kuznetsk Alatau), in general has the form of a large dike elongated in a northwestern direction, in accordance with the strike of the enclosing rocks. The dip of the intrusion contacts coincides with the bedding of the effusive-sedimentary host rocks and has a northeastern direction. The rocks of the Chanysh intrusion comprise granites, plagiogranites, and granodiorites, which make up its inner parts, as well as quartz diorites and diorites, which replace them toward the periphery. The intrusion is accompanied by thin dikes of pegmatitic granites and aplites.
The rocks intruded by the intrusion belong to the Kandoma and Mrass suites of Lower Cambrian age.
The Kandoma suite—its rocks are intruded over a short distance by the northwestern part of the Chanysh massif. It is represented predominantly by basaltic and diabase porphyrites.
The Mrass suite is substantially of sedimentary origin. Its rocks, intruded by the southern part of the Chanysh intrusion, are represented by the following varieties: monomictic feldspathic sandstones and siliceous shales with thin interbeds of marbles, as well as separate thick (10–20 m) and fairly extended (300–500 m) lenses of marbles.
The rocks of the Mrass suite intruded by the northeastern part of the Chanysh intrusion are composed predominantly of volcaniclastic fragmental rocks, also containing thin interbeds of marbles, lavas, and tuffs of intermediate and moderately acidic composition.
The alteration of the host rocks caused by the emplacement of the intrusion covers a considerable aureole, the dimensions and intensity of manifestation of which are more substantial at the northern contact, dipping beneath the altered rocks. The southern near-contact zones are characterized by greater localization of the alterations and by their more distinct manifestation.
The study of the near-contact zones of the intrusion makes it possible to distinguish three principal stages of alteration of the host rocks, which succeeded one another in time:
- Thermal action of the intrusion.
- Contact-metasomatic alteration of the rocks.
- Hydrothermal mineralization.
The thermal effect of the intrusion was expressed in the formation of spotted schists, biotite and cordierite-biotite hornfelses developing at the expense of sandy-schistose rocks, and anthophyllite hornfelses developing chiefly after effusive and volcaniclastic rocks of the Mrassa Formation. The distinguished types of hornfels indicate the hornblende facies of thermal metamorphism.
Contact-metasomatic alteration of the rocks found expression in their amphibolitization and skarnification.
Amphibolitization is a widely distributed phenomenon and affects both effusive-sedimentary and intrusive rocks. It is characterized by the presence of three varieties of amphibole assigned, on the basis of optical constants, to the hornblende series according to Deer W. A., Howie R. A., et al. (1).
The spatial distribution of the various amphiboles is rather complex; only the following regularities can be outlined. Hornblende-I is predominantly distributed in the rocks of the diabase-porphyrite sequence. Amphibolitization with the formation of hornblende-II embraces more or less contact-distant areas of volcanogenic-sedimentary rocks. A spatial association is observed between it and the zones of skarnification, and it is very widely developed in comparison with the skarns. Hornblende-III participates in the amphibolitization of exocontact zones of the intrusion, immediately adjoining it or situated close to it, where it forms amphibole gneisses and amphibolites. This hornblende is characterized by an association with magnetite, by its frequent overgrowth of magnetite, and by the freshness of its appearance. Reaction relationships with the minerals of the hornfelses indicate its superimposed character.
Table 1
| Optical properties | I 60% | II 73—70% | III 85% |
|---|---|---|---|
| Grain form | Elongate-prismatic, sheaflike aggregates | Broad-prismatic, lathlike aggregates | Prismatic-compact |
| Color by: $N_g$ | brownish | greenish | blue-green |
| $N_m$ | cream | greenish | yellow-green |
| $N_p$ | light yellow | light green-yellow | light yellow |
| Pleochroism | distinct | weak | intense |
| Pleochroism scheme | $N_g \gg N_m > N_p$ | $N_g = N_m > N_p$ | $N_g > N_m \gg N_p$ |
| Refractive indices $n_g$ | $1.677 \pm 0.003$ | $1.657 \pm 0.005$ | $1.659 \pm 0.002$ |
| Refractive indices $n_p$ | $1.656 \pm 0.003$ | $1.638 \pm 0.002$ | $1.628 \pm 0.002$ |
| $n_g - n_p$ | $0.021 \pm 0.003$ | $0.019 \pm 0.005$ | $0.030 \pm 0.002$ |
| $c : N_g$ | $21^\circ$ | $20^\circ$ | $18^\circ$ |
| $2V(-)$ | $82^\circ$ | $73^\circ,\ 73^\circ,\ 75^\circ$ | $72^\circ,\ 83^\circ,\ 71^\circ$ |
In general, taking into account the essentially magnesian composition of the amphiboles, their spatial association with the intrusion and the skarns, and their later time of formation than the manifestation of the hornfelsing stage, it may be assumed that the aureole of amphibolitized rocks is a product of postmagmatic magnesian metasomatism according to D. S. Korzhinsky (2).
Skarnification became widespread both in the northern and in the southern contact of the intrusion. Characteristic is bedded selective skarnification, preferentially affecting carbonate interbeds and their contacts with silicate rocks. In the northern contact of the intrusion, dipping beneath the altered rocks, skarnification appeared in the form of fairly broad bands and lenses. In the southern contact it is concentrated in a narrow band, and along the contacts of thick marble lenses it forms considerable skarn bodies. In addition to skarns controlled by the lithology of the enclosing rocks, one may note rare skarns associated with tectonic disturbances and small fractures.
The skarns include the following varieties: pyroxene, garnet, pyroxene-garnet, garnet-epidote, and epidosites. Monomineralic varieties predominate.
Pyroxene forms compact, short-prismatic grains of light-green color. In transmitted light it is colorless, \(c : Ng = 41—43^\circ\), \(n_g = 1.712 \pm 0.002\), \(n_p = 1.682 \pm 0.002\), \(n_g - n_p = 0.028 \pm 0.002\), \(2V(-) = 58^\circ—61^\circ\). By its optical properties it belongs to the diopside-hedenbergite series with 75% diopside molecule.
Garnet is recorded in two varieties—light brown and dark brown. The first variety, the most widespread, appears under the microscope colorless, isotropic, without zoning. Its chemical analysis reveals 37.27% andradite, 58.23% grossular, 3.07% almandine, 1.4% spessartite, and 0.03% pyrope molecules. The second variety, which replaces the first along fractures, has under the microscope a browner color, is often anisotropic, and displays zoning. Judging from its color, it has a more ferruginous composition than the first. Spectral analysis of the garnets reveals substantial amounts of silver, zinc, and titanium—hundredths of a percent—and also thousandths—of copper, lead, nickel, cobalt, tin, vanadium, and gallium.
Epidote forms enlarged crystals of pistachio-green color; under the microscope a distinct pleochroism is visible, from light yellow along \(N_g\) to very pale yellow along \(N_p\), \(n_p - 1.715 \pm 0.005\), \(n_g - n_p — 0.045\), \(2V(-) — 75^\circ\). By its optical properties the mineral belongs to pistacite with 30% ferric molecule.
The distribution of individual skarn varieties is fairly regular: in zones of finely interbedded skarnified rocks and on the flanks of enlarged skarn bodies they are represented by pyroxene varieties. The inner parts of the zones are composed of the remaining, more ferruginous varieties. Thus, metasomatic zoning is reflected in the skarn occurrences.
On the whole, from the features of the manifestation of skarnification, one may assume its bimetasomatic origin with a small participation of contact-infiltration processes.
Hydrothermal mineralization in the vicinity of the intrusion is developed everywhere and widely, and has a complex, multistage character. From the associations of individual minerals, from their relationships with one another, and from the intersections of veinlets, several stages of mineralization can be outlined: 1. Epidote-II, magnetite, actinolite, gold, and tellurides. 2. Albite, prehnite, zoisite, svanbergite. 3. Quartz, hematite, gold. 4. Quartz, sulfides, and arsenides. 5. Calcite, sulfides.
Thus, the contact aureole of the Chanysh intrusion is characterized by the manifestation of hornfelsing, assigned to the hornblende-hornfels facies of metamorphism; amphibolitization associated with magnesian metasomatism; and widespread, predominantly bimetasomatic, skar—
...ization, with which gold–telluride mineralization is found to be associated. In the field of development of the altered rocks, superimposed hydrothermal mineralization is observed, with which gold was also introduced.
In general terms, the processes described are repeated somewhat farther south in the area of the Lebed skarn–gold-ore deposit, which is located in contact with the Lebed intrusion, assigned to the same magmatic complex as the Chanysh intrusion.
REFERENCES
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W. A. Deer, R. A. Howie, J. Zussman. Rock-Forming Minerals, vol. 2, 1965.
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D. S. Korzhinskii. An outline of metasomatic processes. In: Fundamental Problems in the Study of Magmatogenic Ore Deposits. Publishing House of the USSR Academy of Sciences, Moscow, 1953.
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V. I. Yarygin. Petrology of the Lebed magmatic complex. Dissertation for the degree of Candidate of Geological-Mineralogical Sciences. Fundamental Library, TPI, Tomsk, 1966.