Distribution of Gold in the Skarns of the Natalyevskoye Deposit

B. D. Vasil’ev (TPI)

The Natalyevskoye gold-ore deposit is located in the northern part of the Kuznetsk Alatau, in the basin of the Kiya River. The geological structure of the deposit, and the features of its mineralogy and structure, have been covered in a number of works [1—5].

The skarn-ore bodies of the central part of the deposit occur in dolomitic marbles, without connection to contacts of intrusive massifs; they have a lens-like form and are controlled by a system of rupture disturbances. Under the action of skarn-forming solutions, skarns of various composition arose in different rocks: in dolomitic marbles—leucodiopside skarns; in tuffaceous rocks and porphyrites—brown garnet skarns and skarnoids.

The most typical ore body of the central sector is lens No. 1. It is composed almost entirely of leucodiopside–tremolite skarns, and the degree of tremolitization of the leucodiopside gradually increases from top to bottom, up to complete replacement of the leucodiopside by tremolite. In the upper part, lens No. 1 is cut by small lenses of magnetite skarns.

Hydrothermal mineralization is superimposed on all types of skarns and skarnoids; in places it extends beyond the skarns and creates zones of silicified limestones with ore mineralization, while in the footwall of the upper part of skarn-ore lens No. 1 quartz and quartz-calcite lenses with sulfides and gold are distinguished.

The principal ore mineral, apart from gold, is chalcopyrite; pyrite, arsenopyrite, bismuthinite, molybdenite, and very rarely scheelite, sphalerite, and galena occur in smaller amounts. Ore minerals, usually in amounts of 5—6%, form disseminations in the skarns; however, in the footwall of lens No. 1, in the quartz-calcite lenses, areas of massive sulfide ores, mainly chalcopyrite, were encountered. The fineness of the gold ranges from 773.5 to 845.9 and, on average from four determinations, is 802.3.

The degree of gold content of the various ore types of the central sector of the deposit is shown in Table 1, from which it is evident that the hydrothermal quartz-sulfide and quartz-calcite ores proper are distinguished by a high gold content. Gold is associated with bismuthinite and chalcopyrite, and the gold content in massive sulfide ores is an order of magnitude higher than the average. Low-temperature quartz-ankerite...

Table 1

Average gold content in different types of ores of the central sector of the Natalyevskoye deposit

… ores are weakly gold-bearing, occur rarely, and are of little significance for the deposit as a whole.

Of the skarns of the central sector, the most gold-bearing are the magnetite skarns. They are sometimes devoid of gangue minerals; gold and chalcopyrite are disseminated in a fine-grained aggregate of magnetite. Preferential deposition of gold on magnetite crystals is also characteristic of the ores of the southern sector [3]. Leucodiopside–tremolite skarns are gold-bearing practically throughout their entire volume and constitute the main mass of industrial ores. They form a highly porous prismatic or fibrous aggregate; ore minerals and gold fill the interstices, producing a more or less uniform dissemination and, locally, causing banding of the ores. Garnet skarns after porphyrites are less gold-bearing, mainly where they have a coarse-crystalline structure and, accordingly, are well permeable to hydrotherms. Skarnoids after porphyrites are distinguished by high viscosity and density. They contain sparse dissemination of sulfides. The gold content in them is an order of magnitude lower than in garnet skarns. The gold-bearing character of silicified limestones is extremely uneven and is directly dependent on the chalcopyrite content in the ores.

A special type of ore in the central sector is represented by mineralized dike diabases occurring within the skarn-ore bodies. Diabases that underwent intense tremolitization served as concentrators of gold mineralization: the gold content in them is always an order of magnitude higher than in the surrounding leucodiopside–tremolite skarns (Fig. 1). As one approaches such a dike, the gold content in

Fig. 1. Influence of a pre-ore diabase dike (black) on the distribution of gold in the skarns of lens No. 1
(vertical axis—content scale in conventional units)

in the skarns increases and reaches its maximum value in the dike itself.

Thus, the principal industrial type of ore in the central part of the Natalyevskoye deposit is leukodiopside–tremolite skarn. Comparatively small bodies of magnetite and garnet skarns, quartz and quartz–calcite lenses, as well as pre-ore diabase dikes (only within the skarn bodies), despite their small size, are highly productive. Of lesser interest are zones of silicified limestones and low-temperature quartz–ankerite lenses.

According to existing views, the distribution of gold within skarn-ore bodies is extremely uneven and is rarely amenable to analysis. To reveal regularities in the distribution of gold in lens No. 1 of the Natalyevskoye deposit with depth, the average gold contents in ores of the principal type—leukodiopside–tremolite skarns—were calculated from sampling data from exploration and production workings over six mined levels. The results of the calculation are summarized in Table 2 and shown in the graph (Fig. 2), from which

Fig. 2. Distribution of gold in diopside–tremolite skarns of lens No. 1 vertically (solid line) and taking into account the mineralized diabase dike (dashed line).

it follows that the gold content in the skarn-ore body of leukodiopside–tremolite composition increases regularly from bottom to top, and in the upper part of the lens it is an order of magnitude higher than in the lower part.

Table 2

Distribution of gold in the diopside–tremolite skarns of lens No. 1 by individual levels

Considering that the highly productive magnetite, quartz, and quartz–calcite lenses are also located in the upper part of the main skarn–ore body, it must be acknowledged that the degree of gold content in the upper parts of the skarn–ore bodies increases to an even greater extent in comparison with the lower parts. This general tendency is somewhat disrupted by the presence of pre-ore dikes in the skarns (see the dashed line in Fig. 2).

REFERENCES

1. B. D. Vasil’ev. On the geology and metalliferous character of the skarns of the B. Natal’evka area. Izvestiya TPI, vol. 120, 1961.

2. B. D. Vasil’ev. Intrusive formations of the Natal’evskoye deposit area in the Mariinsk taiga. Izvestiya TPI, vol. 121, 1963.

3. B. D. Vasil’ev. Gold in the skarns of the Natal’evskoye deposit. Materials on the geology and mineral resources of Western Siberia. TGU Publishing House, 1964.

4. B. D. Vasil’ev. Elements of the structure of the Natal’evskoye gold-ore deposit. In: Geology of Gold-Ore Deposits of Siberia. Novosibirsk: Nauka Publishing House, 1970.

5. V. A. Vrublevskii. Gold-bearing skarns of the Natal’evskoye deposit in the Mariinsk taiga. Uchenye zapiski TGU, No. 36, 1960.