GARNETS OF GOLD-BEARING SKARNS OF THE SAYAN–ALTAI FOLDED REGION

L. F. Korobeynikov

(Presented by the scientific seminar of the Department of Geology and Exploration of Mineral Deposits)

In the Sayan–Altai gold-ore province, gold-bearing skarns are widely developed and often have industrial significance. The principal rock-forming minerals of calcareous skarns, along with pyroxenes, are garnets; the present communication is devoted to their detailed mineralogical-geochemical analysis.

The gold-bearing skarns of the region occur among Upper Proterozoic (Kommunar) and Cambrian (Natal’evskoe, Lebedskoe, Sinyukhinskoe, Tardanskoe) effusive-sedimentary sequences of the lower structural level. The gold-bearing skarns formed in connection with the emplacement of granitoid and gabbro-syenite intrusions of the Salairian and Caledonian cycles of tectogenesis, in areas where their domes and apophyses developed. The largest skarn zones arose in areas of development of carbonate rocks interbedded with terrigenous-volcanogenic material, and were controlled by lithologic composition, contacts of intrusions, faults, and zones of tectonic fracturing.

In the skarn-ore zones of the Kommunar, Lebedskoe, Sinyukhinskoe, and Tardanskoe gold-ore fields, garnets compose garnet and garnet-pyroxene endo- and exoskarns. They occur in place of hornfelsed effusives, diorites of the marginal parts of intrusions, and especially limestones. The shape of skarn bodies is most often lenticular, veinlike, and more rarely subbedded. In the distribution of garnets within skarn bodies, zoning is observed: at the contacts of intrusions, grossular is more often developed in endoskarns, whereas in exoskarns—among andradite. In all the skarn zones studied, three generations of garnet are distinguished.

Garnet I takes part in the composition of the main skarn bodies, which consist of garnet, pyroxenes, and scapolite with an admixture of wollastonite, vesuvianite, late amphiboles, and epidote. Within the Kommunar ore field these skarn bodies cut pyroxene and pyroxene-scapolite metasomatites. Massive aggregates of garnet I of fine- and medium-grained (up to 0.5 cm) structure predominate, while drusy intergrowths (crystal size 0.5–0.8 cm) occur sporadically. On the crystals the forms (110) and (211) and their combinations are clearly distinguishable. The color of the mineral is brownish yellow, sometimes greenish yellow or light brown. For the most part it is optically isotropic. In the largest crystals, indistinct zoning is recorded. In chemical composition and properties (Tables 1, 6-k), garnet I

among the endoskarns belongs to grossular with an andradite molecule content of up to 24% (No. 24 according to the classification [1, 7]). This garnet is associated with diopside II and, more rarely, with scapolite II.

Among the exoskarns carrying magnetite and gold mineralization, garnet I forms lenses, veins, and irregularly shaped nests of various sizes and outlines. In composition and properties (Table 1, class −1, \(N_o\) 1.848 and 1.876, sp. gr. 3.77 and 3.83) it belongs to andradite No. 80–94. Garnet is associated with salite and is replaced by magnetite, epidote, amphiboles, chlorite, and calcite.

In the Tardan skarn–gold-ore field, garnet I also forms garnet and garnet–pyroxene lenses in tectonic zones among marbles and marginal protrusions of the granitoid massif. It yields massive aggregates, nests, and inclusions of crystals (up to 0.5–1 cm), and more rarely drusy aggregates of medium-grained structure (0.3–0.5 cm), cutting diopside skarns and fassaite–forsterite calciphyrs. Inclusions of well-faceted crystals occur sporadically in the marginal parts of metasomatic zones and are represented by the forms (110) and, more rarely, (210). The mineral is mostly isotropic. Its color is brownish yellow to light brown. Zonal individuals of garnet I are occasionally noted: brown, light-brown in the core (\(a_0=11.943\) kX, \(N=1.770\)), gray-green (\(a_0=11.866\), \(N=1.752\)), and light-yellow zones (\(a_0=11.861\), \(N=1.740\)) toward the periphery of the grains. According to the properties cited, the content of the andradite molecule in garnet I is determined as from 10 to 30–60%.

For garnet I of the Lebed skarn–gold-ore zone [8], characteristic features are its association with pyroxene, massive structure, and weak crystallization of the faces (211) and (110). More often garnet I is isotropic; more rarely, finely zoned individuals are found. According to chemical analyses of garnet I, the content of the andradite molecule varies from 25 to 93%.

Garnet II is widespread in all the skarn–ore zones described. In the form of veins and small lenticles it cuts earlier-formed garnet and garnet–pyroxene aggregates, or forms thin (1–10 cm) lenticles and nests in the endocontacts of dikes of syenite-diorites and in the enclosing diabase porphyrites. The largest crystals of garnet II (1–1.5 cm) are represented by the forms (110) and (211), or by their combinations with predominance of the faces (110). The surfaces of the crystal faces have a microblock structure resembling fine-armored sculpture. The color of the mineral is yellow-brown, more rarely dark green to greenish brown. A zonal structure is very characteristic, caused by alternation of 2–4–6 zones, each 1–3 mm thick: the outer zone is greenish brown (\(a_0=11.891\) kX, i.e., Nos. 10–20), the second is dark green or red-brown (\(a_0=11.909\) and \(a_0=11.940\), i.e., Nos. 20–30 and Nos. 40–50), and the inner zone is yellow-brown or yellow (\(a_0=11.891\), i.e., Nos. 10–20). In transmitted light, garnet II is sharply anisotropic and shows a well-expressed zonal and sectorial structure. Usually hexagonal sections of crystals consist of 3–6 sectors of triangular and quadrangular form. Sectors and zones differ from one another in birefringence (from 0.002 to 0.009) and in the orientation of the optical axes. A zonal-sectorial structure of individual crystals is often observed: the marginal parts are zonal, and the inner parts sectorial. The zonality is caused by alternation of bands of isotropic and anisotropic structure. Sometimes anomalies of lattice and banded types with curved and rectilinear thin bands are recorded.

Individual anomalies resemble microperthites and are expressed in the alternation of narrow anisotropic bands up to 0.02 mm wide among

Table 1

Chemical composition and properties of garnets from gold-bearing skarns of the Kommunar and Tardan gold-ore fields

Tardan deposit: 7152 and 7154 — dark-brown garnets from exoskarn zone no. 1; 815 — garnet I from garnet-scapolite zone, site no. 8; 403-3 — garnet I of the bezrudnaya skarn zone near an outcrop of the quarry, X = 20 — black garnet II from the skarn–gold-ore zone no. 1 of the Kommunar deposit; 6-k — garnet I from endoskarns of the Malo-Berezovskaya zone; 2-k — garnet II from the tonalite of the Malo-Berezovskaya zone; k/1-1 — garnet from endoskarns of the Kailostrovsky.

of the isotropic garnet II. The extinction of these bands is diachronous. Anisotropic garnets are optically biaxial. Sometimes segregations of garnet II are observed in the form of skeletal and sheath-like forms up to 1–2 cm in size, owing to incomplete replacement of diopside-salite and scapolite I. In chemical composition and properties, garnet II belongs to grossular No. 31 (Table 1). Within the skarn zones of Kommunar, garnet II replaces and cuts diopside, scapolite I, cements fragments of garnet I, and itself is overgrown and replaced by epidote, albite, quartz, and calcite.

In the skarn-ore zones of the Tardan deposit, garnet II includes accumulations ranging from light green to dirty green, forming thin (5–10 cm) monomineral veinlets and lenses in a pyroxene-garnet aggregate of the early generation. This garnet often forms druses with well-faceted crystals represented by the forms (110) and (211) and their combinations. The surfaces of the faces also have a fine-block structure. Rarely, individual crystals show growth zoning with alternation of differently colored zones from 0.1 to 2–3 mm thick. The number of such zones does not exceed 3–4. For the most part, garnet II is isotropic, but in a number of cases zoning and sectoriality of individual crystals are manifested. The birefringence of individual zones varies from 0.002 to 0.007. According to the refractive indices and specific gravity (No. = 1.766 and specific gravity = 3.67), garnet II is assigned to grossular Nos. 20–25. It cuts and is replaced by magnetite, epidote, actinolite, chlorite, serpentine, quartz, and sulfides.

In the Lebed’ skarn-gold-ore field, the second generation of garnet was assigned by N. F. Stolbovoi [8] to varieties associated with microcline and albite-prehnite associations. In contrast to garnet I, it is characterized by distinct crystallographic forms (211) and (110) and by the presence of zoning. The zones are broad, sharply different in color and composition, and often change their habit. In composition, garnets II belong to andradite Nos. 70–97.

Garnets associated with vein quartz, prehnite, chlorite, and sulfides are assigned to garnet of the third generation.

Within the Kommunar ore field, garnet III has been found in exoskarn zones in close association with quartz. Here, together with grayish-white medium-grained quartz, it composes vein-like bodies up to 0.6 m thick. Garnet III occurs chiefly in the near-contact parts of veins and more often forms well-faceted crystals 0.2–1.3 cm in size, embedded in a quartz aggregate. Its crystals range from light brown to brownish-brown in color, with faces (110) and (211). Both isotropic and anisotropic varieties are identified. Zoning is recorded much more weakly than in garnet I, and is expressed in the alternation of thin bands up to 0.02 mm wide. Sectoriality, however, is manifested more distinctly. In addition to zoning and sectoriality, indistinctly expressed optical anomalies are recorded, which are distinguished by different birefringence of adjacent bands. It has been noted that the more intensely the garnet is deformed, the more anisotropic it is. In areas of contact between garnet III and quartz, it acquires an anomaly of a banded or spotted type. In chemical composition and properties, garnet III belongs to grossular No. 30 (Table 1, 3-k). A special variety of garnet III is represented by garnet found outside the zones of contact metasomatism, in association with prehnite, pyrite, and chalcopyrite. This garnet, together with prehnite, forms isolated thin veinlets and lenses (1–3 cm) among hornfelsed porphyrites of Podlunnyi Golets. The garnet is rather well crystallized with faces (110) and (211). The sizes

crystals 1–8 mm in size. The color is yellow-brown. According to X-ray structural data and specific gravity ($a_0 = 12.024\ kX$, sp. gr. = 3.73), it belongs to andradite No. 90. The andradite crystals are cemented by a fine-grained aggregate of prehnite, which makes up as much as 60% of the total vein filling. Pyrite and chalcopyrite are most often situated in the middle parts of the veinlets, penetrating into the prehnite and garnet along cracks or overgrowing them. The garnet itself is most often concentrated in the selvage parts of the veinlets.

At Tardan, accumulations and thin veinlets (1–2 cm) of black-brown garnet in association with chlorite, cutting pyroxene and pyroxene-garnet aggregates, have been assigned to the third generation of garnet. Well-developed crystals have not been encountered. It is cemented and replaced by hematite, chlorite, serpentine, calcite, sulfides, and gold. The mineral is anisotropic and microzonal. By chemical composition (Table 1, X-22) and other properties ($a^0 = 12.012 kX$, $N = 1.860$–$1.870$, sp. gr. = 3.83), it should be assigned to andradite No. 85–100.

In the Lebed skarn zone, garnets III associated with quartz [8] have also been found. They are characterized by crystals in the forms (211) and (110), zoning, and anisotropy.

Of all the described generations of garnets, in all skarn zones the dominant one is garnet I, which, together with pyroxenes, composes the principal metasomatic bodies.

The garnets of all skarn-ore zones are characterized by trace elements: Mn, Ti, Ni, Co, V, Cr, Zr, Zn, Cu, Pb, Au, Ag, Sn, Mo, As, Ba, Sr, Ga, Ge, Y, Yb in amounts from 0.0001 to 1%; moreover, such elements as Pb, As, Au, Ag, Ba, Sn, Cu, Mo are connected with the superposition of the hydrothermal-ore process on the skarn zones. This can explain the general tendency for the amount of trace elements to increase from early generations to late ones. In addition to the noted elements, Be, Ce, and La have been detected in the garnets of Kommunar, and W, Sc, and P in those of Tardan (Fig. 1). Consequently, all the examined skarn gold-ore zones are characterized by a similar composition of trace elements. Quantitative spectral analysis detected in the garnets of Kommunar and Tardan, respectively, Sn—0.00048 and 0.00089, Ga—0.00186 and 0.00154, Sr—0.0117 and 0.00178, Ba—0.00083 and 0.00123 (arithmetic mean of 5–10 analyses). The constructed histograms of the distribution of trace elements in the garnets of the Kommunar and Tardan gold-ore fields (Fig. 1) confirmed the necessity of distinguishing three types of garnets, probably corresponding to three generations. It is precisely in the histograms for Mn, Ti, Ni, V, and Cu that a three-peaked character is observed, which obviously indicates the heterogeneous nature of the distribution of these elements in different generations of garnets.

For a number of garnets from the Kommunar, Tardan, and Lebed ore fields, polarographic and neutron-activation determinations of gold were carried out; these confirmed the low gold content of garnets not subjected to hydrothermal metamorphism. The garnets of Kommunar showed a gold content from 0.1 to 22.1 mg/t, with an average value of 5.5 (from 31 samples); the garnets of Tardan—from 0.4 to 31 mg/t, with an average content of 6.6 (from 17 samples); the garnets of the Lebed deposit—from 7.4 to 1200 mg/t, with an average value of 134 mg/t (from 13 samples) [8]. Consequently, no substantial accumulation of gold occurs in skarn garnets, and only in the Lebed skarn zone was an increase of gold in garnets by a whole order of magnitude recorded. The latter circumstance is probably connected with the fact that the analyzed garnets had mostly been subjected to the process of hydrothermal metamorphism.

Studies have shown that garnet deposition begins after the release of the main mass of pyroxene II and ends before deposition of the main mass of magnetite and amphiboles. Garnets are distributed in the endo- and exocontacts, but chiefly in the latter. Metasomatic replacement of all rock types proceeded along tectonic zones by infiltration and, more rarely, by a bimetasomatic route. The appearance

Fig. 1. Histograms of the distribution of trace elements in garnets of the skarn zones of the Kommunar and Tardan gold-ore fields.

low-iron varieties of garnets in endoskarns is evidently due both to the specific character of the mineral-forming solutions and to the nature of the enclosing medium under conditions of uneven heating of the country rocks. The garnets of all the skarn zones studied were deposited under conditions of a variable tectonic regime and variable composition of the solutions, which was recorded in the deformation and zoning of the crystals formed. In contrast to endoskarns, the formation of garnets in exoskarn zones often took place at a considerable distance from the intrusive contacts, under conditions of less-heated carbonate rocks. These conditions, together with the active reactivity and lower degree of heating of the carbonate medium, probably also ensured the appearance of strongly ferruginous varieties of garnets. Deposition of garnet III occurred in the exo- and endoskarn zones during the final period of contact metasomatism.

Table 2

Chemical analyses of aqueous extracts of gas-liquid inclusions, pH and decrepitation of garnets from the gold-bearing skarns of Kommunar (Kuznetsk Alatau) and Tardan (Tuva)

Kommunar ore field: 6-k — grossular No. 23–25 from garnet-pyroxene endoskarns of the Malo-Berezovsky sector; 2 — garnet of the skarns of the Spassky adit.

Tardan ore field: 403-3 — garnet No. 45 from the Barren skarn zone, 815 — garnet No. 85 from skarn-ore zone No. 8; 5 — garnet No. 10 from endoskarns; 6 — andradite from the Barren skarn zone.

Aqueous extracts were prepared from 100 g of sample material ground to 0.01 mm in 250 ml of bidistillate by L. B. Matsioshevskaya (TPI); to allow for the solubility of accessory minerals, repeated analyses were carried out successively for each sample; the pH of the bidistillate in the experiments was 6.5.

In order to study the character of the skarn-forming solutions, gas–liquid inclusions in garnets of the skarn zones of Kommunar and Tardan were investigated by preparing aqueous extracts, decrepitation, and homogenization (Table 2). The materials obtained showed that the composition of the solutions of inclusions in the garnets corresponds to sodium–potassium–calcium hydrocarbonate–chloride types of brines with Na\(^+\)/K\(^+\) ratios from 1:1 to 4.6:1 and Cl\(^-\):HCO\(_3^-\):SO\(_4^{2-}\) from 1:1.4:0 to 1:5:1. The temperatures of the mineral-forming solutions probably reached 560–400°C in the initial period of skarn formation and decreased to 360–260°C during the period of deposition of late generations of garnets. Garnets of different generations of gold-bearing skarns are characterized by the following features.

1. All the garnets studied belong to the grossular–andradite series with an insignificant content of the pyralspite molecule (up to 4.7–8.5%). In the early generations the grossular molecule more often predominates, and in the later ones the andradite molecule; the latter are usually richer in impurity elements. On the basis of the results of 136 chemical analyses of garnets from the skarn zones of the Kommunar, Tardan, Lebedskoye, and Sinyukhinskoye deposits, a three-component diagram of garnet composition was constructed (Fig. 2), as well as a histogram of the distribution

Fig. 2. Diagram of the composition of garnets of gold-bearing skarns of the Sayan–Altai folded region: 1 — garnets of the skarn zones of the Kommunar ore field; 2 — garnets of the Kommunar ore district according to [6, 10]; 3 — garnets of the Tardan ore field; 4 — garnets of the Lebed gold-skarn zone [8]; 5 — garnets of the Sinyukhinskoye gold-skarn deposit according to [5]; 6 — average compositions of garnets from various types of rocks according to [2]; ⊙ 1 — garnets of pegmatites; ● 2 — garnets of granites; ○ 3 — garnets of biotite crystalline schists; ▲ 4 — garnets of amphibole crystalline schists; × 5 — garnets of eclogites; ● 6 — garnets of kimberlites and peridotites; –·– 7 — garnets of basic rocks; ● 8 — garnets of contact-exomorphic rocks; 9 — boundary of the field of skarn garnets according to [7]. Gr — grossular. An — andradite. Pi — pyralspite (pyrope, almandine, spessartine) components of the garnet molecule.

of the andradite component (Fig. 3). From the three-component diagram of garnet compositions it is evident that they almost completely fill the entire series

Fig. 3. Histograms of the distribution of the andradite component in garnets from gold-bearing skarns of the Kommunar, Tardan, Lebedskoye, and Sinyukhinskoye deposits of the Sayan-Altai folded region

grossular-andradite, but nevertheless the greater part of the garnets studied lies in the andradite field. In contrast to the data presented in the work of N. V. Sobolev [7], the field of garnets of gold-bearing skarns on the three-component composition diagram occupies a smaller area of the pyralspite components. However, the compositions of garnets within the grossular-andradite series are distributed unevenly, which is most clearly expressed in the histograms of the distribution of the andradite component. These diagrams reveal three to four maxima, corresponding, probably, to three types (generations) of garnets. The first type includes garnets Nos. 10—40, the second—Nos. 40—80, and the third—Nos. 80—100 (Fig. 3).

1. The most developed crystal forms are (110) and (211), the former predominating both in aluminosilicate and in carbonate rocks. No regularity in the appearance of crystallographic forms of garnet as a function of the chemistry of the host medium has been established.

2. A characteristic feature of the garnets described is their anisotropy. Usually the early generations are isotropic, while the later ones are sharply zonal and sectorial. A number of researchers have explained the occurrence of birefringence and zoning in garnets by the alternation of bands with different amounts of isomorphic impurities, which in the end could lead to the appearance of stresses within the zones. M. G. Sumina [9] put forward the supposition that anomalous garnet is a mixture of andradite with uvarovite. However, as has already been noted, nonzonal garnets are also anisotropic; they are characterized by optical anomalies of a banded type. In this case individual bands are often bent and displaced relative to one another. It is difficult to explain the occurrence of such anomalies from Brauns’s point of view. Most likely they arise as a result of mechanical deformations. Experimentally, the possibility of the appearance

optical anomalies caused by deformations, as shown by A. M. Kuzmin [4] on samples of rock salt. Other researchers, in explaining optical anomalies, take into account the influence of different conditions of mineral formation, which probably also occurred at the deposits described here. In this connection, the degree of progression of the host medium, the compositions of the country rocks, and the metamorphosing solutions undoubtedly played a role. However, this question is very complex and is not always resolved unambiguously. In general, our studies of garnets support the conclusions of a number of researchers that the formation of anisotropic varieties takes place at lower temperatures.

1. For the garnets studied, a dependence is established between the ferruginosity of the mineral and the composition of the host rocks: ferruginous varieties form in calcium-rich rocks, whereas low-iron varieties form in aluminosilicate rocks; i.e., the composition of the mineral and its position in the grossular–andradite isomorphous series is probably a function of the composition and properties of the solutions and the rocks being replaced.

2. The presence at a single deposit of garnets of different composition does not permit confident statements about relationships between the composition of garnets and the nature of the ore content of skarn zones, nor about predictions, based on garnets, of a definite type of mineralization—copper or tungsten—as some researchers have supposed [3]. However, one should agree with the opinion of many researchers that garnets with a substantial predominance of andradite are characteristic of iron-ore skarn deposits, with which superimposed gold mineralization is not infrequently associated.

References

1. Grudev A. P. Composition and nomenclature of garnets of skarn formations. Bulletin of the Moscow Society of Naturalists, Geological Section, 38, No. 3, 1963.

2. Deer W. A., Howie R. A., Zussman J. Rock-Forming Minerals. Vol. 1, Moscow, “Mir,” 1965.

3. Karpova E. D., Ivashchentsev A. G. Skarns. In: Altered Near-Ore Rocks and Their Prospecting Significance. Moscow, Gosgeoltekhizdat, 1954.

4. Kuzmin A. M. Planes of cleavage and slip in rock salt. Geology and Geophysics, 1960, No. 6.

5. Luzgin V. N. Geological structure and genesis of the Sinyukhinsk gold-ore skarn field in Gorny Altai. Author’s abstract of Candidate’s dissertation, Tomsk, 1970.

6. Mikheev V. G. The granitoid complex and conditions of gold content in the northern part of the Kommunar ore field (Kuznetsk Alatau). Author’s abstract of Candidate’s dissertation, Tomsk, 1971.

7. Sobolev N. V. Paragenetic Types of Garnets. Moscow, “Nauka,” 1964.

8. Stolbova N. F. Mineralogy and genesis of the Lebed’ gold-bearing skarn zone. Author’s abstract of Candidate’s dissertation, 1970.

9. Sumin N. G. On certain features of skarn minerals of iron-ore deposits. Proceedings of the Mineralogical Museum, issue 6, 1954.

10. Tyulyupo B. M. Garnets from the skarns of the Chebakovka group of iron-ore deposits (Kuznetsk Alatau). In: Materials on Mineralogy, Petrography, and Mineral Resources of Western Siberia and Krasnoyarsk Krai. Proceedings of the Tomsk Branch of the Moscow Society of Naturalists. Tomsk, 1965.