Some Features of the Host Rocks of the Eldorado Deposit (Yenisei Ridge)

L. M. Sazonov, S. S. Ilyenok

(Presented by the seminar of the Department of Petrography)

The Eldorado gold-ore deposit is located in the central part of the North Yenisei taiga (Yenisei Ridge), 40 km southeast of the settlement of Severo-Yeniseisky (Sovrudnik). Stratigraphically, it occurs on the watershed of the small watercourses Sevaglikon and Aktolik, tributaries of the Kalami and Vanagash rivers. The host rocks of the deposit are composed of sequences of primary terrigenous composition, assigned mainly to the Upper Proterozoic and having undergone a certain degree of metamorphism.

In the area of the Eldorado deposit, three formations are distinguished: the Kordinskaya, Gorbilokskaya, and Udereiskaya. The deposit lies within the Gorbilokskaya Formation, composed predominantly of crystalline schists of epidote–amphibolite facies. The underlying Kordinskaya Formation, made up of quartzites and fine-grained quartz–biotite–muscovite schists, occurs as a broad northwest-trending belt to the southeast of the deposit; the younger Udereiskaya Formation is composed of fairly diverse phyllites and occupies an extensive field adjoining the deposit area from the northeast.

The deposits of the above-mentioned formations are gathered into multistage folds of northwestern strike. The deposit is located on the northeastern limb of a large anticlinal fold. The strike of the fold axis is 310°, with a plunge to the northwest. In this direction, in the area of the Kalami and Enashimo rivers, a large granite massif is exposed.

The formation of the principal folded structures apparently proceeded in two main stages. In the first, pre-granite stage, the main structures of the region were formed. In the post-granite stage, powerful zones of crumpling of northwestern strike were formed, within one of which the Eldorado deposit is located. Along these zones, additional folding and metamorphism developed under the influence of large flows of hydrothermal solutions. The manifestation of the indicated folding and metamorphism was synchronous, as is proved by the coincidence of crystallization schistosity with the bends of the folds.

At the same time, metamorphism continued for some time after folding, with the development of large porphyroblasts of garnet and biotite. Subsequently, the stress pattern changed sharply with the formation of northeast-trending faults, with which the manifestation of retrogressive metamorphism of the green-schist facies is associated.

Structural and Textural Features of the Rocks

The host rocks of the deposit are characterized by varied textural and structural variability. The predominant rock varieties are distinguished by well-pronounced crystallization schistosity. Variations are observed in the fine and microscopic texture of the rocks. Along with gentle waviness, there occur small angular bends in folds with wing amplitudes of 3–5 cm, as well as still smaller ones.

The development of small additional folds is associated with differential movements along thin fractures in zones of crumpling. The scale of the indicated folding depends on the density of these fractures.

Very often in the folds one can find elongated lens-shaped accumulations of granoblastic quartz—“eyes.” They have an elongated form and are arranged with their long section along the plane of schistosity of the rock and emphasize it. Around the eyes, in the main fabric, a film of fine-scaly sericite develops, more rarely chlorite and biotite, thereby forming an eyed texture. The same microtexture is formed when fine-scaly films of the above-listed minerals develop around garnet porphyroblasts. Such a texture is usually associated with the schistosing of massive rock under the influence of pressure.

Bands enriched in platy and granular minerals are often encountered in the rocks, as a result of which features of two combined textures are observed simultaneously—schistose and banded. The alternation of bands of different composition reflects the primary bedding of the rock, which consisted of clayey and sandy or sandy-clayey interbeds.

In rocks containing a small amount of scaly minerals, schistosity is expressed less distinctly or is absent altogether, and the rocks are massive or nearly massive. Very rarely, plane-parallel textures are observed, due to the broad development, in certain planes, of prismatic tourmaline grains.

The structural features of the rocks are usually characterized by porphyroblastic inclusions of garnet, biotite, chloritoid, and more rarely muscovite; the latter is characteristic only of the schists of the Kordinskaya Formation. Porphyroblasts of the above-listed minerals, as a rule, are characterized by poikiloblastic inclusions of quartz, sericite, biotite, and more rarely other minerals.

In the rock-forming minerals, poikiloblasts of tourmaline, apatite, sphene, and zircon can very often be observed; the last forms pleochroic halos around itself, well expressed in biotite and chloritoid. Garnet porphyroblasts often contain quite regularly arranged quartz poikiloblasts, forming a snowball structure ($S$ and $R$—tectonites). This structure is also caused by differential movements along the schistosity of the rocks; the growing garnet porphyroblasts, as a result of these movements, begin to rotate, sometimes capturing adjacent portions of the rock as well, which is clearly emphasized by the arrangement of platy minerals (the pattern of a rotating fan).

The main fabric of the rock is usually characterized by a granolepidoblastic structure. A lepidoblastic structure is characteristic of zones of intensive schistosing or else is due to the primary pelitic composition of the original rock.

For the metasomatic rocks of the deposit, which have poor development within its limits, porphyroblastic, locally fascicular structures are characteristic, with a granoblastic main fabric.

Features of the Mineral Composition of the Rocks

The principal rock-forming minerals of the rocks are: quartz, biotite, sericite, muscovite, garnet, albite, chloritoid, zoisite; among the accessory minerals present are: tourmaline, apatite, rutile, sphene, zircon, ilmenite, magnetite, leucoxene.

Quartz plays a significant role in the composition of the rocks; its content varies greatly in the different rock types. Second in abundance is biotite. The content of muscovite is always subordinate, almost to complete disappearance. At the same time, the size of its grains is always smaller than that of biotite (except for the biotite-quartz schists of the Kordin Formation) and may be assigned to sericite.

Plagioclase occurs among the groundmass of the rocks, where within various zones it undergoes intense sericitization. It is probably a relict mineral of the primary rocks. In addition, later albite is manifested, forming lens-shaped segregations along the schistosity of the rocks.

Chlorite, chloritoid, and stilpnomelane are characteristic of rocks subjected to the regressive stage of metamorphism. These minerals usually form metamorphs after biotite, garnet, and amphibole-pargasite, and sometimes, in rocks intensively subjected to regressive metamorphism, form new growths.

On the basis of quantitative mineralogical composition, very many varieties are distinguished. We shall dwell only on the characterization of the rock-forming minerals identified for the stages of metamorphism.

The rock-forming coexisting minerals of the first stage of metamorphism characterize the facies features of metamorphism.

Biotite \(Ng \simeq Nm = 1.630 + 1.633\), \(Np = 1.585—1.588\), \(2V = -8^\circ\).

The crystallochemical formula, according to chemical analyses of the mineral, is

\[ (\mathrm{K}_{0.9}\mathrm{Na}_{0.1})_{1.00} (\mathrm{Mg}_{1.15}\mathrm{Fe}^{2+}_{0.95}\mathrm{Fe}^{3+}_{0.26}\mathrm{Ca}_{0.03}\mathrm{Ti}_{0.18}\mathrm{Al}_{0.43})_{3.00} \]

\[ (\mathrm{Si}_{2.73}\mathrm{Al}_{1.27})_{4.00}\mathrm{O}_{10} (\mathrm{OH}_{1.57}\mathrm{F}_{0.03}\mathrm{O}_{0.4})_{2.00}. \]

The indicated properties correspond to a mineral transitional from biotite to meroxene with a moderate content of the phlogopite molecule.

Garnet \(N = 1.83 \pm 0.04\), specific gravity \(4.15—4.20\).

The results of chemical analyses confirm the data of physical properties.

\[ (\mathrm{Ca}_{0.08}\mathrm{Fe}^{2+}_{2.31}\mathrm{Mg}_{0.39}\mathrm{Mn}_{0.22})_{3.00} (\mathrm{Al}_{1.9}\mathrm{Fe}^{3+}_{0.1})_{2.00} (\mathrm{Si}_{2.95}\mathrm{Ti}_{0.05})_{3.00}\mathrm{O}_{12.00}. \]

The content of isomorphous end-members in the mineral, in %: almandine — 76, grossular — 3, spessartine — 6, andradite — 5, pyrope — 10.

For metasomatic quartz-albite-garnet-amphibole rocks we shall consider a pair of coexisting minerals: garnet–amphibole (pargasite).

Pargasite \(Ng = 1.639\), \(Nm = 1.624\), \(Np = 1.620 \pm 0.002\), \(Ng—Np = 0.019—0.020\), \(C:Ng = 26^\circ\), \(2V = 64^\circ\).

The crystallochemical formula of the mineral, according to chemical analyses, is

\[ (\mathrm{K}_{0.2}\mathrm{Na}_{0.8}\mathrm{Ca}_{1.00})_{2.00} (\mathrm{Mg}_{3.68}\mathrm{Fe}^{2+}_{0.32})_{4.00} (\mathrm{Al}_{0.85}\mathrm{Fe}^{3+}_{0.15})_{1.00} \]

\[ [(\mathrm{Si}_{5.94}\mathrm{Ti}_{0.06})_{6.00} (\mathrm{Al}_{2.01}\mathrm{O}_{22})](\mathrm{OH},\mathrm{F})_{2.00}. \]

The crystallochemical formula of garnet, according to chemical analyses:

\[ (\mathrm{Ca}_{0.71}\mathrm{Fe}^{2+}_{1.90}\mathrm{Mg}_{0.19}\mathrm{Mn}_{0.21})_{3.00} (\mathrm{Al}_{1.95}\mathrm{Fe}^{3+}_{0.04}\mathrm{Cr}_{0.01})_{2.00} \mathrm{Si}_{3}\mathrm{O}_{12}. \]

The content of isomorphic end-members in the mineral, in %: almandine — 63, spessartine — 7, pyrope — 6.5, grossular — 2.5, andradite — 21.

The composition of the coexisting minerals of rocks of the first stage of metamorphism indicates the epidote–amphibolite facies of metamorphism.

Formation of the mineral composition took place in the temperature interval 500–650°C, mostly 500–550°C, at pressures from 7 to 10 kbar. The lower temperature boundary is marked by the replacement of almandine and biotite by chlorite and stilpnomelane.

The rock-forming coexisting minerals of the second (regressive) stage of metamorphism are as follows:

Chlorite, replacing biotite, has the following optical properties: \(N_m = 1.600\) to \(1.610\), more often \(1.607\), which corresponds to a total iron content of \(36\)–\(39\) (Kepezhinskas, 1965).

Stilpnomelane rarely forms independent segregations in the form of worm-like and radial-radiating aggregates. It more readily develops on garnet. The construction of the mineral requires additional alumina, which is assimilated from garnet. Optical properties: \(N_g \simeq N_m = 1.699 - 0.004\), \(N_p = 1.610 \pm 0.002\), which corresponds to 68% of the total.

Chloritoid is usually formed as independent segregations. The color of the mineral under the microscope ranges from green according to \(N_g\) to light yellow-green according to \(N_p\). A high relief, low interference color, and polysynthetic twins are characteristic. Optical properties of the mineral: \(N_g = 1.724 \div 1.725\); \(N_m = 1.719\)—\(1.721\); \(N_p = 1.716\)—\(1.719\); \(2V = 56^\circ\).

The properties of the coexisting minerals of rocks of the second stage of metamorphism characterize the greenschist facies. It developed within the range from 350–400°C to 500–550°C at a pressure of 5–7 kbar. The upper boundary of this facies is determined by the appearance of almandine, plagioclase, and amphibole.