ON THE GENESIS OF GOLD IN OXIDIZED ORES OF THE LEBEDINSKY DEPOSIT

P. F. STOLBOVA

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

The Lebedinsky deposit is a zone consisting of gold-bearing skarn bodies. The zone is elongated along a tectonic disturbance that cuts the contact of granitoids with the enclosing rocks. The latter are represented mainly by amphibolitized andesite-basalt porphyrites. In them, areas of disseminated and veinlet-disseminated sulfide mineralization with an elevated gold content are observed. In the surroundings of such rocks, on the northern continuation of the zone, two more gold-ore bodies have been discovered. They are located near the watershed part of the relief along the continuation of the tectonic disturbance and are separated only by a small pinch. They are called the Upper ore bodies. Unlike the ores of the other bodies of the zone, represented by skarns with coarse gold of productive gold-bismuth-telluride mineralization, the ores of the Upper bodies are products of an oxidation zone with fine “ochred” gold.

At present in Altai, the question is being discussed of the possibility of accumulation of gold released from the sulfides of transformed enclosing rocks. In this connection another question has arisen—what is the source of the gold at this deposit: the sulfidized enclosing rocks, or the gold-ore mineralization accompanying the skarns? Forecasts of mineralization at depth depend on its solution.

To resolve the question, all available material from mine workings and the modern open pit was studied. On the basis of the results obtained, a geological section was compiled, a brief description of which is given below. The gold-ore mineralization is also characterized.

The upper part of the section is composed of eluvial-deluvial deposits. Their clastic material is represented by the surrounding amphibolitized andesite-basalt porphyrites and their cataclastic and epidotized varieties. This material has been partly displaced downslope from the watershed. The thickness of the deposits is 1.5–2.0 m.

Lower in the section are boudinaged rocks with slickensides, with traces of disintegration, and with clear signs of cataclasis and mylonitization. The original products of the cataclasites and mylonites are the enclosing rocks, partly skarned and saturated with veinlet prehnite-albite and potash-feldspar mineralization. The latter is characteristic of the outer aureoles of the skarn bodies of the zone.

Without a sharp boundary, the rocks pass into clay-like brownish-

greenish formations. Their structures resemble porphyritic ones, and the segregations are represented by a white brittle mica, apparently pyrophyllite. The nature of the distribution of the porphyritic segregations and their size leave no doubt that the original products were the surrounding porphyrites. Among these formations, occurring as patches of various sizes (from several tens of centimeters to meters in diameter), there are very brittle cataclased pyroxene-garnet skarns and ocherous fine-grained magnetite formations. The thickness of the formations is about three meters.

As patches and lenses among the rocks described, merging approximately at one level, there occur ochres and flowstone and cellular formations of brown iron ore. They consist of hydrogoethite, goethite, hematite, and hydrohematite. In places they are enriched in copper carbonates and carry incrustations of black manganese oxides. The thickness of the alteration zone ranges from several tens of centimeters to 2.5 meters.

In areas with an intense manifestation of brown iron ores, zones of loose or compacted porous scree appear. Their thickness is 0.2–0.5 m. They usually have a fine-grained structure and a layered texture. The latter is due to the alternation of differently colored interlayers—white, brick-red, gray, and brown—reflecting changes in the composition of the individual layers. The scree zone is composed mainly of fragments of gray and milky-white quartz and quartz with brown ocherous incrustations, hematite, malachite, covelline, and hydrogoethite. Often the composition of the scree is more complex. Along with quartz fragments, fragments of garnets and pyroxenes are found here, as well as skarns, magnetite ores, and epidotites. There is very little sulfide material here. Only relics of chalcopyrite occur, replaced by chalcocite and copper carbonates; pyrite occurs surrounded by iron oxides and hydroxides; and tellurobismuthite occurs in replaced montanite.

The oxidized rocks are industrially valuable ores, since their metal content is only slightly lower than in other skarn bodies of the zone. In the scree zone the gold content increases by more than twofold. This circumstance indicates a possible migration of gold from the overlying rocks. The distribution of gold, especially in the brown iron ores, is uneven, similar to the distribution of gold in the skarns, and is evidently related to the position of the primary ore shoots.

From the zones of quartz scree, the composition of the heavy-mineral concentrates was analyzed. Garnets and magnetites predominate in them; tetradymite, tellurobismuthite, bismuthinite, and, of course, gold are present.

The gold is fine-grained, dust-like, film-like, and 25% of it is in a reddish-brown coating. Rare gold grains exceed 0.2 mm in size. In the concentrate, on the basis of criteria proposed by V. M. Kreiter, V. V. Aristov, and others [1], hypogene (primary) and hypergene (secondary) gold are distinguished. Their comparative characteristics are given in the table.

Primary gold in the relatively coarse part of the concentrate makes up about half of it. The true value of the ratio of primary to secondary gold will be somewhat different, since the gold of the finer fraction is for the most part apparently hypergene.

Below the scree zone, crushed epidotized host porphyrites with patches of skarns are found. The latter merge into a single large skarn body on the southern continuation of the oxidation zone. According to data from individual mine workings, industrial concentrations of gold in these rocks are absent. This fact made forecasts for depth pessimistic, until we discovered abundant gold-bismuth-telluride mineralization in the underlying skarns. Taking into account

Table

Characteristics of gold from the oxidation zone

of its very uneven distribution, after supplementary exploration the underlying skarns were assigned to the ore body.

The skarns of the Upper ore body are characterized by a fine-grained structure and a bimineral composition. The skarns consist of diopside and garnet with 30–35% andradite molecule content. The skarns are accompanied by lenses and spots of magnetite formations, as well as veinlet-disseminated, successively forming mineralization: quartz-sulfide, gold-telluride, carbonate-sulfide. These skarns, unlike the skarns of other bodies of the zone described by us earlier [2], are characterized by a lesser tendency toward monominerality and by a very fine-grained structure. The mineral composition of the later formations is the same as in the other ore bodies, but its distribution is somewhat different. If in other skarn bodies they appear as veins, spots, nests, and lenses, here fine-veinlet, reticulate, disseminated textures predominate.

The gold in the skarns described, unlike the coarse-grained, lumpy gold of other skarn bodies, is film-like, fine-grained, barely reaching sizes of 0.2–0.5 mm. Its habit is octahedral, complicated by cube faces. On the octahedron faces a sculptural, concentrically zonal pattern is observed, repeating the outlines of the (111) face. Film gold reflects the structure of the cavity that it fills. It is usually a platelet with imprints of octahedron and rhombododecahedron faces. Hook-shaped and wire gold occurs. Gold is constantly associated with bismuth tellurides and occurs in intergrowths with tetradymite and tellurobismuthite. The gold fineness is 880; the content of impurity elements is as follows: Ag > 1%, Bi, Te, Sb, Hg—0.03, Cu, Pb—0.003.

Comparison of gold from the skarns with hypogene gold of the oxidized ores shows their almost complete identity. This may indicate that the source of the gold in the oxidized ores could have been mineralization that also carries gold in the skarns.

In other mineral associations neither macro- nor microscopic gold occurs. However, in pyrite and arsenopyrite of the quartz-sulfide, higher-temperature association, as also in other ore bodies, radioactive analysis has detected gold in elevated quantities. Apparently, this is dispersed gold.

The material presented makes it possible to arrive at certain conclusions. Thus, the location of the oxidized ores near the watershed, within the zone of tectonic crushing, was a favorable condition for the development of the oxidation zone. The cataclasites and mylonites undergoing oxidation are products of the crushing of intruding porphyries, with signs of near-skarn alteration, and of skarns. Thus, the gold-bearing sulfide mineralization could have been associated with processes accompanying skarn formation. The source of gold in the oxidized ores could have been both disintegrating sulfides and gold–telluride mineralization. The accumulation of elevated gold concentrations in the seepage zone indicates migration of some part of the gold from the upper horizons. However, the presence of ore bodies at greater depth is not excluded, since skarns with gold-ore mineralization occur there. The continuation of the tectonic disturbance northward, and finds there in the deluvium of skarns and brown ironstones with elevated gold content, make it possible to suppose that gold-bearing oxidation zones are also present there.

REFERENCES

1. Kreiter V. M., Aristov V. V., Volynsky I. S., Krestovnikov A. N., Kuvichinsky V. V. Behavior of Gold in the Oxidation Zone of Gold-Sulfide Deposits. Gosgeoltekhizdat, 1958.

2. Stolbova N. F. General features of the geology of the Lebedinsky skarn–gold-ore zone. New Data on the Geology and Mineral Resources of Altai Krai. Biisk, 1968.