ON THE MINERALOGY OF GOLD ORE DEPOSITS OF THE SOUTH MUYA RANGE

L. F. MITROFANOV, A. D. MIKOV, B. A. RUBANOV, A. P. GRIBANOV

(Presented by Professor A. M. Kuzmin)

This article presents certain materials collected by the authors in 1964–1966 on the mineralogical characteristics of gold ore deposits that we group into the Irokinda–Kindikan ore field.

The Irokinda–Kindikan ore field is composed of deeply metamorphosed rocks of Archean age, represented by crystalline schists, gneisses of varying composition, skarnified and marbled limestones, and also migmatites. In 1962 A. A. Gamchyan grouped these rocks into the Kindikan Formation. The magmatic rocks of the ore field are represented by small dike bodies of leucocratic microcline–plagioclase and pegmatitic granites, by small veins of pegmatites of Archean age [3], and by dikes of diorites and porphyritic granites, assigned by N. P. Andreev and A. A. Gamchyan to the Mesozoic intrusive complex.

The formations of the Kindikan Formation are gathered into tight folds, complicated by folds of higher orders, often with the formation of foliation and corrugation. Flexures of various sizes are characteristic. In addition, the Archean crystalline rocks are cut by numerous fault disturbances, which for the most part are subsidiary fractures of the Kilyan regional fault, separating the South Muya block of Archean rocks from the Proterozoic deposits that frame it.

The gold ore deposits of the ore field described tend toward the zone of the Kilyan fault, being in immediate proximity to it, and are controlled by systems of submeridional tectonic disturbances with a dip angle of 40–50° in the western quadrants. Along with gold- and sulfide-bearing quartz veins confined to these disturbances, completely barren quartz veins are also observed in them, represented by milky-white quartz with rare disseminations of early pyrite. The quartz in these veins is usually massive and saccharoidal.

Gold-bearing quartz veins are persistent along strike and dip—

[[continuation from previous page]]. Pinching-out of the ore bodies is simple. It is characterized by the usual gradual decrease in thickness. More rarely, more complex forms of pinching-out are observed. In all cases, along the continuation of the veins, lenses and veinlets of quartz are noted. The average thickness of the veins varies within 0.2–1.0 m, reaching 3.0–3.5 m in swells. Swells in the ore bodies are distributed rather uniformly, approximately every 20–30 m. Pinches are common in the quartz veins, up to complete pinching-out. Pinches and veinless intervals in the zones enclosing the quartz veins have different extents—from the first meters to the first tens of meters. The form of the veins is usually simple, plate-like, but it may also be distinctly lenticular.

The mineral composition of the ore bodies is fairly diverse. Among the minerals composing the ore bodies, three groups may be distinguished: vein, ore-hypogene, and hypergene. The vein minerals are represented by quartz, carbonates (calcite, ankerite), sericite, and chlorite (delessite). The hypogene ore mineralization is represented mainly by sulfides: pyrite, arsenopyrite, sphalerite, chalcopyrite, chalcocite, and galena. In addition, there are fahlerz, native gold, and silver. The hypergene mineralization at the deposits described is also fairly diverse: covellite, chalcocite, bornite, goethite, limonite, malachite, azurite, cerussite and anglesite, native copper and sulfur.

Below is a brief description of the vein and hypogene ore minerals in the order of their deposition from ore-bearing solutions. A characterization of the hypergene mineralization is not given because of the limited scope of this article.

Vein minerals

Quartz is the principal vein mineral, and four of its generations are clearly distinguished at the deposits. The first generation is represented by dark-gray (to black) barren quartz, with massive and breccia-like textures. Quartz of this generation is of limited distribution within the commercial ore bodies, forming lenses and veinlets, and in a number of cases independent small veins. Under the microscope it is observed as aggregates of irregular form with grain sizes from 1 to 4 mm. It can often be observed that fragments of early quartz are cemented or cut by veinlets of milky-white quartz of the second generation.

Quartz of the second generation is represented by a milky-white, unevenly granular variety and is the most widespread in the gold-ore veins of the ore field. It is characterized by a massive (less commonly drusy) texture. Cavities in the quartz have dimensions up to 5, less commonly 10 cm. Druses and brushes of quartz in these cavities are usually small. Well-formed semitransparent and transparent crystals are rare, but in all cases the pyramidal terminations of the crystals are well expressed. Breccia-like textures occur rather rarely in quartz of the second generation; they are caused by inclusions of sharp-angled fragments of hydrothermally altered wall rocks and dark quartz of the first generation. Breccia-like textures are most clearly manifested in the Yurassovsky-II and No. 30 veins. These textures are due to intramineralization movements. As a result of the deposition of ore minerals of subsequent stages of mineralization, disseminated textures arose. With quartz of the second generation, parage[[unclear: word continues on next page]]

...genetically associated with pyrite of the first generation. The remaining sulfides, as well as gold and carbonates, penetrate it along fractures and cut it, thus being later formations. The quartz being described cuts the quartz of the first generation and contains its fragments. With the quartz of the third generation it often has gradual transitions, caused by its dissolution during the deposition of the latter. In places a distinct structural intersection of them is observed.

The third generation of quartz is represented by a light-gray variety, sometimes with a bluish shade. The latter forms veinlets, lenses, and veins up to 1–1.5 m thick in the ore bodies. Deposition of the third-generation quartz along subparallel fractures in the quartz of the second generation accounts for the appearance of banded textures in the ores. A massive texture is also characteristic of this quartz. In some areas (Yurasovskaya-II vein) it has a coarse-platy (more rarely fine-platy) structure. Under the microscope, quartz of this generation is characterized by a medium-grained structure with grains up to 2, more rarely 5 mm in size. Their form is irregular, generally close to isometric, or elongated, resembling a poorly developed prismatic form. Many sulfides and gold are paragenetically associated with this quartz.

The fourth generation of quartz appeared in the concluding stage of formation of the ore bodies in the deposits of the ore field described. Its color is grayish- or watery-white, with a fine- to medium-grained structure and a massive texture. This quartz is present in small amounts among the mineral complexes of the preceding stages of ore formation. It forms isolated, thin lenses in the near-salband parts of the ore body and on its flanks in the ore-bearing zone of altered rocks. Its assignment to the fourth generation is made on the basis that, together with carbonates, it cuts the mineral complexes of the preceding stages of mineralization and the altered rocks in the ore-bearing zone, while itself being monolithic.

Carbonates are present in the ores in insignificant amounts. They are represented by calcite and ankerite, which form veinlets up to 1–2 cm thick and nests up to 5 cm in size in quartz of the first three generations.

Calcite is white, often light yellowish in color. Under the microscope, calcite is observed as irregularly shaped grains up to 8 mm in size, not infrequently with a clearly polysynthetic twin structure. Sometimes calcite veinlets develop in milky-white quartz. Spectral analysis of calcite from the Yurasovskaya-II and Nizhnyaya veins revealed lead—0.03%, copper—0.003–0.1%, zinc—0.3%, silver—0.005%, traces of nickel, antimony—up to 0.1%, strontium—0.1%, titanium—up to 0.1%, manganese, barium, and arsenic—up to 0.006%.

Ankerite occurs in the ores very rarely. It is characterized by a brownish-yellow coloration. It forms small—up to 1–2 cm—nest-like accumulations, consisting of aggregates of well-formed crystals or veinlets of varying thickness.

Since the carbonates are associated with quartz of the fourth generation and their veinlets cut all the previously deposited mineral associations, there is no doubt that they were deposited from the last portions of hydrothermal solutions.

Silicates occur in the veins very rarely and are represented mainly by sericite, more rarely by muscovite (Yurasovskaya, Petrovskaya veins) and chlorite-delessite (Verkhnyaya vein).

Ore Minerals

Overall, the ore mineralization in the veins amounts by volume to about 1–2%.

Pyrite is one of the most widespread minerals and is represented by two generations. First-generation pyrite has a light brass-yellow color. It occurs as nests up to 7 × 12 mm in size, sometimes up to 4 × 5 cm, and also as individual crystals of cubic habit. Electrolytic etching in an ammonia solution reveals a cataclastic structure of the mineral. As a result of the filling of fractures in its crushed grains and aggregates by third-generation quartz, galena, and gold, a cementation microstructure is quite often observed. Veinlet-like segregations of chalcopyrite occur in the pyrite. Paragenetically, the pyrite is associated with second-generation quartz, in which it is distributed rather uniformly. This association of minerals (pyrite and quartz) occupies the dominant position in all deposits of the ore field. Spectral analysis of sample No. 254 (Yurasovskaya-II vein) established in first-generation pyrite: lead—more than one percent, copper—0.1%, zinc—more than 1%, silver—0.01%, nickel—0.006%, arsenic and antimony—0.3%.

Under the microscope, second-generation pyrite, in comparison with the first, is characterized by a lighter yellowish-white color. It forms fine-grained aggregates up to 0.2 × 1.0 mm in size in third-generation quartz, and is also observed in it as grains up to 1 mm in size. Together with third-generation quartz it cements crushed aggregates of first-generation pyrite. Paragenetically, second-generation pyrite is associated with third-generation quartz. But it itself, in turn, is corroded by sphalerite and galena.

Arsenopyrite was encountered only in three veins: Yurasovskaya-II, Verkhnyaya, and Petrovskaya, where its contents are very insignificant. It forms short-columnar prismatic crystals of very small size (tenths of a mm), which are sometimes gathered into aggregates. Twins on (010) are encountered. Arsenopyrite shows a paragenetic connection with third-generation quartz, in which it usually develops. It is an earlier formation in comparison with sphalerite, fahlores, galena, and chalcopyrite, since it has been observed that it is often corroded by the latter.

Sphalerite, in comparison with pyrite, occurs much more rarely and forms both nests and nest-like accumulations up to 1–2 cm in size, as well as fine disseminations in second-generation quartz, filling fractures in it. In the case of fine dissemination, sphalerite often has direct contact with pyrite, chalcopyrite, and galena. Sphalerite is in paragenetic association with third-generation quartz. Its grains usually contain fine emulsion inclusions of chalcopyrite confined to cleavage cracks. The chalcopyrite segregations in this case are products of decomposition of a solid solution. Sometimes sphalerite grains are intersected by chalcopyrite, which here is undoubtedly a younger formation. Arsenopyrite grains are often corroded by sphalerite. Rarely, fine disseminations of galena occur in it. Structural etching with a mixture of potassium permanganate and sulfuric acid establishes the confinement of galena grains to the intergranular spaces of sphalerite, which indicates a later separation of the former from solutions. Sphalerite is often corroded by fahlore, within whose formations only remnants of sphalerite aggregates are often preserved. In individual cases it is also corroded by gold.

Spectral analysis of sample No. 255 (Yurasovskaya-II vein) established the following accessory elements in sphalerite: lead—0.03%, copper—0.001%, silver—0.0005%, and cadmium—0.1–1.0%.

Chalcopyrite occurs in the ores in approximately the same quantity as sphalerite and develops as rare, irregularly shaped disseminations up to 3–4 mm in size. It is often associated with pyrite, galena, fahlores, and other minerals of the productive stage of mineralization. Chalcopyrite is paragenetically closely related to quartz of the third generation. In the latter it fills the interstices between its grains. Chalcopyrite segregations are observed as emulsion disseminations, small veinlet-like accumulations, and as rims in the marginal parts of sphalerite aggregates. In a number of cases, chalcopyrite grains are replaced by galena along cleavage cracks. But sometimes chalcopyrite veinlets cut galena segregations (adit No. 6, 120 m, Yurasovskaya-II vein). Completely irregular, corroded chalcopyrite grains 0.08–0.7 mm in size are contained in fahlore. Being a later formation relative to the milky-white quartz of the second generation, it fills cataclasis cracks in it.

Cubanite has been found only in the Verkhnyaya vein. It is located in chalcopyrite as small platy segregations and is observed only under the microscope. In individual cases the cubanite plates are cut by fahlore. As P. Ramdohr [2] indicates, at high temperature chalcopyrite dissolves a considerable amount of pyrite and occurs as “cubic high-temperature chalcopyrite,” which, with decreasing temperature, may break down into ordinary chalcopyrite + cubanite. Breakdown of the solid solution with exsolution of cubanite occurs at a temperature of 250–300°. Thus, the presence of cubanite in the deposits of the Irokinda-Kindikansky ore field indicates that the temperature of ore formation must have been, in any case, above 250°.

Chalcosine is a very rare hypogene mineral and is found only under the microscope. It is known in the Yurasovskaya-II and No. 30 veins. Its grains have an isometric form and are up to 0.4 mm in size. It occurs together with chalcopyrite and fahlore and, more rarely, in sphalerite. With chalcopyrite, chalcosine forms a structure of mutual boundaries, which testifies to their nearly simultaneous deposition from solutions. In addition, it is cut by fahlore, which served as the basis for assigning it to the group of hypogene minerals.

Galena is the most widespread ore mineral and occurs in practically all ore bodies. In the Yurasovskaya-II, No. 30, Serebryakovskaya, Tuluinskaya, and other veins, it has predominant importance in comparison with other ore minerals. In other veins, in terms of abundance, it is subordinate to pyrite or fahlores (Petrovskaya vein). Occurring in the quartzes of the ore stages of mineralization, it forms in them small, rare, usually isometric disseminations from 0.4 to 2 mm in size, or rare nest-like, irregularly shaped accumulations from 0.5 to 3–4 cm, more rarely up to 10 cm or more (Yurasovskaya-II, Tuluinskaya veins). In individual areas the number of its accumulations and small nests in quartz increases sharply and, as a rule, these areas have increased gold content. Structural etching revealed traces of cataclasis in the mineral. In monomineral samples No. 256 and 260 of galena from the Yurasovskaya-II and Nizhnyaya veins, the following accessory elements were found: copper—0.001–0.03%.

zinc—0.06%, silver—0.1%, bismuth—0.003–0.01%, antimony—0.1–0.3%, and arsenic—0.01%.

Galena is paragenetically closely associated with quartz of the third generation, together with which it often cements crushed grains and aggregates of first-generation pyrite. It often almost completely corrodes grains of the indicated pyrite and arsenopyrite, and weakly corrodes second-generation pyrite, which occurs in galena as small inclusions. In a number of cases galena penetrates along cleavage cracks into chalcopyrite grains and corrodes it. But sometimes chalcopyrite penetrates into the marginal parts of galena grains or even cuts across them. It also occurs in the peripheral parts of grains of pale ore, but is corroded by the latter. It is often corroded by gold. Small segregations of galena are observed in sphalerite; sometimes it cements the grains of the latter.

Pale ore is comparatively widespread and occurs in almost all ore bodies. It is most extensively developed in the Khrebtovaya veins, at the absolute elevation of 1750 m, where, in comparison with the other ore minerals, it has a predominant development. Mineralogically, among the pale ores it is possible to distinguish tennantite and tetrahedrite. Paragenetically, pale ore is associated with quartz of the third generation, filling the spaces between its grains. Grains of pale ore sometimes contain, in their marginal parts, rounded inclusions of sphalerite and irregularly shaped grains of chalcopyrite. Grains of chalcosine are also encountered in it. In individual sphalerite grains it forms veinlets, corrodes arsenopyrite, galena, chalcosine, and gold. On the other hand, some microcracks in grains of pale ore are filled with gold. In quartz of the second generation it fills cataclastic cracks and corrodes it.

According to spectral-analysis data for sample No. 257, pale ore contains the following elements: lead, copper, zinc, silver, and antimony in amounts exceeding 1%, “traces” of nickel, 0.1–1.0% cadmium and arsenic, 0.001% titanium, and even up to 0.1% gold.

Gold in native form is present in almost all hydrothermal quartz veins. Gold segregations are very often visible to the naked eye. They occur in quartz both separately and in close association with sulfides and sulfosalts. The size of the gold particles varies over wide limits, from microscopic segregations to comparatively large aggregates of 1–2 mm, sometimes reaching even larger sizes. According to technological-test data, coarse gold makes up more than 50% of its initial content in the ore of the Petrovskaya vein. The color of the gold is everywhere pale yellow. It is due to its low fineness, which, according to TsNIGRI tests of a technological sample, lies within the limits 550–850 (according to Zh. Yurasovskaya-P). In isolated cases, when the gold particles develop in limonite, the gold has a reddish-yellow color, which indicates its higher fineness. Here, apparently, the fineness of the gold increased through the leaching of impurities from it in the oxidation zone.

The form of the gold segregations is irregular. Skeletal intergrowths and dendrites with an openwork porous-vesicular structure of separate intergrown small particles are observed. For fine gold (less than 0.1 mm), segregations in the form of plates and flakes are characteristic. Under the microscope, gold segregations have a rounded or irregularly elongated form. Gold veinlets are also encountered, located in intergranular spaces. The form of gold segregations is determined to a considerable degree by the outlines of the walls of cracks in quartz of the second generation and in sulfides, relative to which it is more

a late formation, and also by the shape of the intergranular spaces of third-generation quartz. In the latter case the gold segregations form chains. The outlines of the gold grains are usually sinuous and serrated; in some cases they are surrounded by a rim of limonite. The latter is characteristic of near-surface samples.

Paragenetically, gold is associated with third-generation quartz. It is located in the intervals between grains of quartz of this generation. Its segregations form veinlets in fractured grains of first-generation pyrite, and also penetrate grains of later sulfides—galena, sphalerite, and fahlores. It should be noted that gold grains are sometimes corroded by fahlore. These facts make it possible to suppose that gold is one of the latest hypogene ore minerals in the quartz veins of the ore field and was deposited, apparently, simultaneously with fahlore.

Silver is established only under the microscope, in the form of single grains in galena, for example in the Yurasovskaya vein, which is to some extent confirmed by assay analysis of a monomineralic sample that showed a silver content of about 0.3%. As P. Ramdor [2] indicates, in such a case silver must be present as a mechanical admixture, i.e., as native silver. Native silver was found as single small grains of irregular, hook-like form, 0.15–0.30 mm in size, in the gravity concentrate of the Petrovskaya vein. Its color is silvery-white. There are minor admixtures of copper and sulfur. The silver content in the gold is 37.5% in the Petrovskaya vein. Similar contents of it may apparently be assumed in the other gold-ore bodies as well.

All ore bodies of the Irokinda–Kindikansky ore field have a complex structure, which is due to the presence in their composition of products of several independent stages of mineral formation. In the deposits described, four main stages of mineralization are established, separated from one another by intramineralization breaks: 1) quartz, 2) quartz-pyrite, 3) quartz-polymetallic, 4) quartz-carbonate.

The first, quartz, stage, as already noted above, includes barren dark-gray to black quartz, having massive and breccia-like textures. This quartz occurs in insignificant quantities, and in some ore bodies it was not encountered at all. The latter evidently indicates the absence of a sufficient number of cavities of considerable size in the ore zone at the time the first portions of thermal solutions entered. The first stage of mineralization ended with weak tectonic movements, which led to the formation of open cavities, as well as numerous thin fractures of varying extent in the ore zone and in the first-generation quartz. This is indicated by the characteristic breccia-like structure of the first-generation quartz and by the intense cracking and crushing of the enclosing rocks.

In the second period there occurred the quiet deposition of milky-white second-generation quartz in open cavities among the previously formed quartz, which is demonstrated by the presence of drusy textures; with this quartz is also associated the formation of first-generation pyrite.

After a certain deformational break, circulation of solutions resumed. This stage is associated with the deposition of light-gray third-generation quartz with sulfides, sulfosalts, and gold. Deposition of the ore minerals took place almost simultaneously. Somewhat earlier among them, second-generation pyrite separated out. The deposition of gold and silver occurred after crystallization of the main mass

sulfide minerals. Sulfides and gold were deposited both in close intergrowth with quartz of the third generation and along cracks in the minerals of the preceding stages. This stage of mineral formation is rightfully the principal ore stage, since the release of the predominant amount of the useful component is associated with it.

In the final, fourth stage, carbonates and quartz were deposited. These formations are observed in the ore bodies in insignificant quantities. Their deposition was also preceded by weak tectonic movements, which are recorded by the presence of cracks in quartz of the third generation, as well as by deformation of galena crystals.

After the completion of the processes of ore formation and the exposure of the deposits at the surface, the sulfide minerals underwent secondary changes, which led to the formation of secondary sulfides, carbonates, oxides, and hydroxides.

The study of the mineralogy of certain gold-ore deposits of the South Muya Range, occurring among ancient Archean metamorphic rocks, makes it possible to assign them to the formation of low-sulfide ores of relatively great depths according to the classification of N. V. Petrovskaya [1] and to draw a number of conclusions regarding the prospects of these deposits.

REFERENCES

1. N. V. Petrovskaya. Mineral associations in gold-ore deposits of the Soviet Union. Transactions of TsNIGRI, issue 76, 1967.
2. P. Ramdohr. Ore minerals and their intergrowths. Izd. IL, 1962.
3. L. I. Salop. Geology of the Baikal mountainous region. Vol. II. Magmatism, tectonics, history of geological development. Izd. “Nedra,” 1967.