Ore Mineralization of the Lebed Skarn Zone

N. F. Stolbova

(Presented by the scientific seminar of the Department of Petrography)

The Lebed skarn zone is located in the northern apical part of the Lebed intrusion (southwestern part of Gornaya Shoriya), which extends far into the enclosing rocks as a narrow, elongated embayment and gradually wedges out. The rocks composing this part of the intrusion are dioritic, granodioritic, and syenitic. The enclosing rocks are represented by pyroxene porphyrites, tuffs, and sandstones with lenses of marbles, assigned to the Mrasskaya Suite $Cm_1$.

The skarn zone consists of a number of endo- and exoskarn bodies arranged en echelon along the tectonic contact between the intrusion and the enclosing rocks. The en echelon structure is closely connected with fractures abutting against the tectonic contact of the intrusion and the enclosing rocks. Under conditions of relative displacement of blocks along the tectonic zone, the most accessible paths for postmagmatic solutions were the opening abutting fractures. It is precisely with them that the processes of skarn formation, the emplacement of dikes, and the formation of hydrothermal veins are associated.

Without dwelling on a detailed characterization of the skarn bodies, it is necessary to note that their composition is complex, which is expressed in differences in the mineralogical composition of the endo- and exocontact zones and in their multistage character.

In the endocontact, the most highly developed are monomineralic garnet and magnetite skarns, characterizing the initial stages of skarnization, as well as recrystallized garnet, pyroxene, amphibole, magnetite skarns and their varieties, characterizing later stages. The exocontact skarns have a wollastonite and garnet-wollastonite composition.

Each of the skarn bodies is characterized by its own features, which reflect the intensity of the mineral-forming processes, their pulsational character, and the manifestation of subsequent hydrothermal activity.

The northern part of the skarn zone has been studied in the greatest detail. Here...

the First skarn body is known, a large body divided by the river into left- and right-bank parts. In the right-bank part is the worked gold-ore area “Lower Quarry.” To the north, arranged en echelon one after another, are the Second, Third, and Fourth skarn bodies. In the Second and Third skarn bodies, gold ore occurrences are known. In the Fourth body is the worked gold-ore area “Upper Quarry.”

The process of gold mineralization within the Lebed skarn zone proceeded in two epochs: hypogene and hypergene.

1. Hypogene epoch

Ore formation of the hypogene epoch is associated with the postmagmatic processes of the Lebed intrusion. This is proved by the very close spatial relationships of skarn, dike, and hydrothermal formations. The hydrothermal stage of ore formation has a multistage character. The following stages have been distinguished: gold-bismuth-telluride, quartz-epidote-hematite, sulfide-arsenide, and carbonate-sulfide.

The criteria used were: mineralogical composition and paragenetic associations, textural and structural patterns, spatial position, and geochemical features.

Gold-bismuth-telluride stage. Mineralization of this stage was manifested most intensively in the right-bank part of the First skarn body. It was precisely this stage that was the main supplier of gold in the worked “Lower Quarry.” In addition, gold-bismuth-telluride minerals occur in the skarns of the Second ore body.

Among the minerals of this stage the following have been found: native gold, tetradymite, bismuthinite, calaverite, weissite, and chalcopyrite. Under the microscope, structures of mutual boundaries and frequent inclusions of some minerals in others are observed. The most common are inclusions of gold in tetradymite—drop-shaped gold formations gravitate toward the periphery of tetradymite grains. Inclusions of bismuthinite and calaverite also occur here. Separate rounded inclusions of gold and tetradymite in chalcopyrite are known.

Semiquantitative spectral analysis of monomineral fractions of gold, tetradymite, and also of an aggregate of several minerals of the stage shows the presence of impurity elements: Pb and Ag more than 1%, Hg, Sb, Zn, Ba, Ba—0.03–0.3%; Ga, Ge, Ti, Be, Zr—0.001%. The diversity of impurity elements and their high contents are noteworthy.

The close structural interrelations of the minerals and the repeated occurrence of impurity elements in the minerals testify to a genetic kinship and close timing of crystallization of the minerals.

Minerals of the gold-bismuth-telluride stage are characterized by fine- and small-grained aggregates up to 0.1–0.5 mm. Only under especially favorable conditions, where significant accumulations of minerals form, can enlarged grains of gold or tetradymite up to 3.5 mm be encountered, with fairly distinct outlines of habit. In general, gold and its accompanying minerals are characterized by aggregate accumulations that take the form of the cavities they fill.

The distribution of gold-ore mineralization is closely associated with garnet, magnetite, garnet-magnetite, magnetite-pyroxene, and magnetite-amphibole recrystallized skarns, that is, with those varieties that complete the process of skarn formation. The listed varieties of skarns possess

distinct grain size, increased brittleness and, consequently, intense fracturing, and the presence of significant interstitial cavities. Gold and its accompanying minerals in such rocks fill interstices, penetrate along fine fractures into the inner parts of crystals, and are readily deposited along the cleavage of pyroxenes and amphiboles. Extended veinlets of considerable thickness are not encountered.

The location of gold mineralization in the “Lower Quarry” area corresponds to the spatial position of the enclosing skarn rocks of the late generations. The temporal position of the stage described is determined by the fact that it is spatially associated with subsequent products of skarn activity; its crystallization occurs somewhat later. Its products fill interstices, fractures, and cleavage in the skarns.

In pyroxene- and amphibole-bearing skarns enriched in gold, replacement by chlorite, calcite, and apatite is observed. In the inner parts of replaced pyroxenes and amphiboles, gold-ore minerals are deposited very frequently. This fact also indicates a later time of deposition of gold and its accompanying minerals relative to the skarns, and a change in the character of the solutions toward increased alkalinity.

In accordance with the view of S. S. Il’enko [1], the gold-bismuth-telluride stage of mineralization, following the formation of magnetite ores, is associated with the halide stage of manifestation of ore solutions. The subsequent quartz-epidote-hematite stage reveals degradation of the solutions of this stage and a significant increase in oxygen potential, which is not characteristic of solutions with substantial participation of halides.

Quartz-epidote-hematite. The mineralization of this stage is represented by quartz, epidote, and hematite. This stage was manifested most widely and intensively in the northeastern part of the left-bank skarns and skarn formations of the “Lower Quarry,” that is, in the central parts of the skarn bodies. Here hematite is deposited in quartz and quartz-epidote nests and lenses. The sizes of the nests and lenses are 20–30 cm and more. All minerals have a coarse-grained structure. Individual crystals reach 5–10 cm. Along the periphery of the skarn bodies, minerals of this stage occur as thin veinlets, persistent along strike. Their composition is predominantly monomineralic—quartz or epidote; in this case associations with hematite are rare. Thus, it is evident that the mineralization of this stage was manifested most fully and intensively in the central parts of the skarn bodies. Study of the structural and textural relationships of the minerals shows that hematite crystallizes last, cementing individual crystals and druses of quartz and epidote.

Spectral analysis of monomineralic fractions reveals insignificant amounts of impurities of Cu, Zn, Mn, V, Be.

The age position of the stage is determined by the fact that veinlets of minerals of this stage cut skarns and skarnized rocks and, in turn, are cut by sulfide veinlets. On this basis we may conclude that formation of the quartz-epidote-hematite stage took place in the post-skarn stage, but before the appearance of sulfides.

It should additionally be noted that in magnetite skarns in which manifestation of this stage of mineralization is observed, an intensive process of martitization takes place, which indicates its substantially oxidizing potential. The solutions from which deposition of the material occurred were, in all probability, high-temperature. This is indicated by the presence of gray quartz and epidote and the complete ab—

absence of sulfides. Judging from the trace elements, the solutions were very poor in metals. The superposition of the subsequent sulfide stage causes musketovitization, which indicates the manifestation of a reducing regime.

Sulfide-arsenide stage. The minerals of this stage are: pyrite, pyrrhotite, arsenopyrite, löllingite, magnetite, hematite. In many cases they are accompanied by quartz. This mineralization appeared almost everywhere within the zone, both in skarns, skarnified porphyrites, gneisses, altered diorites, and in diabase dikes. The textures are disseminated and fine-veinlet; in the area of the “Lower quarry,” nest-like textures are also noted together with them. In the central part of the quarry a body with a significant content of sulfides and arsenides has been exposed. Aggregates of ore minerals in it are cemented by a quartz-chlorite mass. Individual veinlets of sulfides near the body increase in thickness and merge with the general sulfide mass. The structures are fine- and medium-grained.

Sulfides in quartz-amphibole nests, widely distributed in the “Lower quarry,” show sharp xenomorphism toward drusy formations of quartz and amphibole, tending toward the inner parts of the nests and veins. In addition, veinlets of sulfides cut hematite formations and themselves, in turn, are cut by veinlets of calcite. These facts determine the independence of the stage and its position in the paragenetic scheme.

Paragenetic scheme

Among the formations of this stage, several paragenetic associations may be distinguished: pyrrhotite, pyrite-magnetite, and pyrite-hematite.

Pyrrhotite association is represented by pyrrhotite with a significant isomorphic admixture of gold (more than 1%) and milky-white quartz. The textures are disseminated and veinlet, the structures fine-grained. The distribution of the distinguished association is most characteristic of the Third and Fourth skarn bodies.

The pyrite–magnetite association is most characteristic of the sulfides of the Second and Third skarn bodies, and is also encountered in the First. The textures are disseminated; the structures are fine- and medium-grained. Under the microscope, replacement of pyrite by magnetite is observed. In this case, magnetite is deposited on the corroded surface of the pyrite and in the cracks contained within it. An admixture of finely dispersed gold has been found in the pyrite, the content of which reaches 0.05–0.1%.

The pyrite–arsenopyrite–hematite association occurs in the “Lower Quarry.” The textures are disseminated and veinlet-like; nests are characteristic, and there is a rather massive continuous sulfide body.

In this paragenesis a clear sequence in the crystallization of minerals is observed: first pyrite is deposited, then it is overgrown by aggregates of arsenopyrite, and somewhat later hematite with quartz is deposited. The lellingite encountered in the workings probably belongs to this same association. In the arsenopyrite and pyrite here, gold admixtures of about 0.001% are detected.

The appearance of different paragenetic associations within a single stage indicates certain changes in the physicochemical conditions of formation, in particular, a change in the sulfur and oxygen regime. If, in the first association, all the iron enters the sulfides, then in the second part of it already passes into magnetite. This indicates a lowering of the sulfur potential and an increase in the oxygen potential. The process continues further and leads to the replacement of sulfides by arsenides, and of magnetite by hematite. In this same sequence the content of finely dispersed gold in sulfides of the different associations decreases.

The carbonate–sulfide stage. Mineralization of this stage is represented by calcite with ore minerals: chalcopyrite, pyrite, bornite, magnetite, and hematite. It is expressed especially clearly in the “Lower Quarry.” This is a thick, steeply dipping vein, traceable along strike for 350 m and down dip for 150 m. Its thickness on the upper levels is 10–20 cm, on the lower levels 4–5 m. In the selvages of the vein are concentrated numerous fragments of the enclosing rocks, cemented by calcite. Cockade textures are also encountered. Wall-rock alterations are characteristic, expressed in intense carbonatization of the rocks.

Throughout the calcite vein, nests of ore minerals are unevenly distributed. The structural features of this mineral association indicate that pyrite is replaced by chalcopyrite and magnetite. At the same time, exsolution structures of bornite are observed in the chalcopyrite. These structural and mineralogical features indicate the prolonged existence of solutions and a change in the oxidation–reduction potentials of sulfur and oxygen toward an increase in oxygen concentration.

Hematite stands somewhat apart from the association described. It crystallizes not in the form of nests, but along individual thin veinlets, intergranular spaces, and cleavage in calcite. Evidently, hematite formation took place under conditions of a further increase in the oxygen potential, and its deposition occurred when the main mass of calcite had already recrystallized. Parallel with the calcite vein, siderite veinlets with chalcopyrite, bornite, and magnetite are formed. Spectral analysis of monomineralic fractions shows contents of Co, Zn, Cu up to 0.03%, Ag, As, Pb up to 0.005%; Ba, Ni, Sb, Ti, Cr occur sporadically.

II. Hypergene Epoch

Ores whose formation took place during the hypergene epoch are of practical interest. This applies, in particular, to the ores of the “Upper Quarry.” Gold mineralization is associated with the oxidation zone of intensely fractured skarns and skarnized porphyrites enriched in gold-bearing sulfides. The thickness of the oxidation zone is small—from 2 to 4 m. Within it, against a background of brown and reddish-brown formations, areas of dark cherry color stand out clearly. These are areas with numerous fragments of milky-white quartz, enriched in goethite and hematite. These are probably oxidized relics of quartz-sulfide mineralization in the skarns. It is precisely these areas that are most enriched in gold.

The gold here is fine-grained, dust-like. The dimensions of individual gold particles do not exceed 0.1 mm. Their form is rounded, droplet-like. The color of the gold is often obscured by brown coatings of iron hydroxides.

All these data indicate that exogenous processes can mobilize the gold of sulfides in the oxidation zone and, under conditions of accumulation, create industrial concentrations of it.

LITERATURE

1. S. S. Il’enok. Magmatism and the connection with it of endogenic deposits. In the collection Geology of Gold Deposits of Siberia, Siberian Branch of the USSR Academy of Sciences (in press).