THE ROLE OF SPESSARTITE DIKES IN THE LOCALIZATION OF GOLD MINERALIZATION AT THE CENTRAL DEPOSIT

A. M. Mityushin (ZapSibZoloto Trust)

The Central gold-ore deposit is located in the northern spurs of the Kuznetsk Alatau. The geological structure of the ore field is discussed in the author’s work [1]. Dikes of various composition are extensively developed.

Dikes of acidic rocks occur rarely within the ore field. Their strikes are highly varied: northeastern, northwestern, and latitudinal. They are represented by aplites, pegmatites, granite-porphyries, and aplite-like rocks. They occur in mine workings at elevations of 87 and 420 m (“Krasnaya” adit), and 190 and 270 m (“Yubileinaya” adit). The thickness of the dikes does not exceed 0.20 m, rarely 0.50 m; along strike the dikes have not been traced. Investigators of the deposit assign the dikes of acidic rocks to pre-ore formations. The relationship of these dikes with other structures in the workings has not been observed. The dikes have no significant importance at the deposit.

Dikes of intermediate and basic rocks of various composition, termed melanocratic, are very widely developed at the Central deposit. They have a northwestern and, more rarely, meridional and northeastern strike, dipping to the southwest and northwest at angles of 50–70°. Among them, dikes of spessartites, microdiorites, and porphyrites are significantly widespread.

Spessartite dikes are of great importance in the structure of the ore field. At the deposit, the dikes have been exposed by mine workings at elevations of 355 and 460 m. In the Yubileiny sector, the spessartite dikes have a dip azimuth of 205–250°, with an angle of 60–80°.

Within the intrusive body, some dikes were traced from the surface. The traced extent along strike reached the first kilometers. The thickness of the dikes in the workings ranges from the first centimeters to 2 m, occasionally 4 m. The contacts of the dikes with the enclosing rocks are sharp and uneven. Usually the spessartite dikes bear traces of significant cataclasis. Fractures in the spessartite are filled with veinlets of calcite, more rarely quartz, and sometimes sulfides. Along the contacts, traces of tectonic movements with slickensides and striations are observed. At the contact of the dikes with granodiorites, a tectonic clay of greenish-gray color is usually developed. On some levels of the Yubileiny ore node, dikes Nos. 1, 2, and 5 are ground down to a greenish-gray clay.

In the Central ore field, spessartite dikes (the western and eastern dikes 1 and 2) have the following attitude elements: dip azimuth 280–330°,

angle of 10–65°. The entire system of mineralized fractures, quartz veins, and fractures filled by spessartite dikes is bounded on the east by the third eastern dike, and on the west by the third western dike. These two bounding dikes are elongated in a northeasterly direction, have considerable extent, a consistent thickness, and even contacts with the enclosing rocks. The third western dike is composed of microdiorites; the third eastern dike consists of crumpled, schistose, bleached rocks with quartz veinlets. The entire ore area of the sector is divided by four spessartite dikes into a series of blocks arranged stepwise relative to one another. The displacement amplitude varies from several meters to several tens of meters and reaches 30 m and more.

The ruptural disturbances that passed along structures filled by spessartite dikes are classified by investigators as reverse-strike-slip faults with a predominance of reverse-fault elements.

Hydrothermal alteration of the spessartites at places of junction with the ore bodies is expressed by slight bleaching of the rocks.

In the Yubileinyi ore node, dikes of microdiorites have been encountered in mine workings along the Dmitrievskaya vein at the 190 m level. Elements of occurrence: dip azimuth 70–90°, angle 45–60°. Macroscopically, the microdiorite dikes are little distinguishable from spessartites. The thickness of the dikes does not exceed 1.5 m. The microdiorites also bear traces of cataclasis; the contacts of the dikes with the enclosing rocks are sharp and uneven. Hydrothermal alteration of the microdiorites is expressed by bleaching.

In the northern part of the deposit, microdiorite dikes are known in the Dorozhnyi ore node. The ore fractures of the Dorozhnyi node extend to the northeast, dipping to the southeast. They are broken by disjunctive disturbances filled by microdiorite dikes.

Porphyrite dikes have been encountered in mine workings along the Altaiskaya vein and the Dmitrievskaya vein. Elements of occurrence: dip azimuth 90–110°, angle 65–70°. The thickness of the dikes is 1.0–3.0 m. The contacts of the dikes with the enclosing rocks are sinuous, often welded. Macroscopically, the dikes are dark-gray dense rocks with a cryptocrystalline groundmass texture. In the porphyritic segregations, plagioclase phenocrysts are up to 2–3 mm across. Like the microdiorite dikes, the porphyrite dikes are displaced by quartz veins. The intersection of the dikes by the Altaiskaya vein has been established at the 138 and 190 m levels.

According to a number of investigators (N. A. Rozanova, V. I. Bazhenov), the dikes listed above are pre-ore formations.

The distribution of gold in the ore bodies of the deposit is highly uneven; unevenness is observed both along the strike and down the depth of the veins. Many investigators, as well as mine geologists, in studying the Central deposit have noted to one degree or another the uneven character of the distribution of sulfide mineralization and gold mineralization in the veins of the deposit (E. A. Gukovsky, A. Ya. Bulynnikov, F. N. Shakhov, D. A. Timofeevsky, S. S. Mikhailov, N. A. Rozanova, V. I. Bazhenov, A. M. Mityushin, and others).

In all known commercial veins, enriched ore shoots of varying size and quality are outlined. Along the Tsentralnaya vein there are several such shoots. An example of an enriched ore shoot is the thick zone of mineralization at the first western dike, traced along a raise for several hundred meters. The length of this shoot varies from several tens of meters on individual levels to 100 m and more at the 420 m level. The richest blocks in terms of content are confined to it.

An enriched zone occurs at the 3rd eastern dike. The structural conditions here are more complex, since the ore zone is enclosed between two veins and is linked with them diagonally.

Along the Kazanskaya vein, higher enrichment and gold content are likewise observed near the dike, and on both sides of it. Here there are no very high contents over a restricted area; rather, it is distributed more evenly over a considerable area.

A particularly characteristic association of enriched ore shoots with tectonic elements is manifested in another ore node—the Yubileiny node, situated south of the Central node. Here several ore shoots are traced along the Lotereinaya, Kavkazskaya, and Altaiskaya veins.

Along the Lotereinaya vein there is an enriched ore shoot, traced along strike for a distance of more than 300 meters and enclosed on its flanks between the Severnaya vein on the west and dike No. 1 on the east. Moreover, the Severnaya vein itself is enclosed within a microdiorite dike, strongly fractured and locally displaced.

A large enriched ore shoot, both in size and in quality, is located in the western flank of the Kavkazskaya vein on the eastern side of dike No. 2. This spessartite dike, in composition, is distinguished by considerable thickness, persistence of the elements of occurrence, considerable fracturing, and brecciation. On all levels a general regularity is traced—the deterioration of commercial ore mineralization and the decrease in gold content with distance from the dike eastward. A sizable enriched ore shoot was identified along the Altaiskaya vein in the footwall of dike No. 5.

From the examples cited it follows that the association of enriched ore shoots and zones with spessartite dikes is unquestionable. It should be noted that enriched ore shoots do not gravitate toward all dikes at the deposit. Thus, at the Yubileiny ore node, in the eastern flank, mine workings at levels 138 and 190 m exposed microdiorite dikes that are broken and displaced by veins. At these dikes, no increase in enrichment or gold content is observed.

Another feature is the uneven distribution of mineralization and gold content within the enriched shoots and zones themselves. As N. A. Rozanova correctly noted, a decrease in gold content is observed along the strike of the ore veins from west to east within blocks enclosed between dikes, from the footwall of the more western dike to the hanging wall of the next, eastern one. This feature is well expressed in the veins of the Yubileiny node of the deposit, which have a latitudinal trend. It is less clearly manifested in the veins of the Central ore field. Here, in individual shoots, high enrichment and gold content are observed both on the footwall and on the hanging-wall sides of the dikes.

The most complex and strongly mineralized ore nodes (Central, Yubileiny, etc.) are located at places where two or more systems of fractures are developed and intersect. It is precisely the places of junctions and intersections of several fracture systems that are the most favorable for the localization of a large number of commercial veins. Most often these are areas where fractures of northeastern strike intersect with fractures of latitudinal strike. It may be assumed that along the continuation of the tectonic zones to which the Tsentralnaya, Severnaya, and other veins are confined, new ore nodes with a large number of veins may be expected at the places where they intersect fractures of latitudinal strike.

Of interest in this respect at the deposit may be the large dike bodies by which the major commercial veins are bounded. We still know about the dikes only as much as the mine workings allow us to judge. The dikes are exposed by mine workings

insignificantly, and only in cases where they intersect during searches for displaced parts of veins. Along the continuation of dikes, where other fractures join them, one may also expect the presence of new nodes, new veins, and vein zones.

In connection with the above, it is advisable to conduct prospecting and exploration work in directions across the strike of known ore structures, i.e., to investigate the areas between known ore nodes along the continuation of tectonic zones, dikes, and junctions. Along with known structures, the presence of new structures, new systems of fractures, and their intersections and junctions is entirely probable; these may contain new commercial ore bodies.

The evaluation of dike formations must be approached from new positions, bearing in mind that commercial mineralization is concentrated near them. Dikes, especially those of melanocratic composition and, in particular, spessartite dikes, are extensively developed at the deposit and have considerable length, probably extending from one ore node to another.

REFERENCES

1. Bazhenov V. I., Mityushin A. M. Principal features of the structure of the Central gold-ore field. In: Geology of Gold-Ore Deposits of Siberia, Publ. House of the Siberian Branch of the Academy of Sciences of the USSR, Novosibirsk, 1967.