On the Occurrence of Gold Mineralization in the Skarns of the Ampalyk Deposit

E. A. Babina

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

Recently, gold mineralization associated with skarns has been attracting attention. Occurrences of gold mineralization in skarns within the Kuznetsk Alatau are few in number. It has been noted that, despite the sharp differences in the geological structure of certain deposits (Olginskoe, Kaliostrovskoe, and others), in the composition and age of the host rocks, skarns, and also parent intrusions, all of them are auriferous, which is of practical interest and attracts the attention of researchers. In studying the Ampalyk deposit, we established the presence of gold in the skarns.

The deposit is confined to the western contact of the northern extremity of the Olginsko–Ampalyk intrusive complex with an effusive-sedimentary sequence of the Sinian and Middle Cambrian.

The rocks composing the Olginsko–Ampalyk complex are represented by a range of transitional varieties of basic and intermediate composition, which at a depth of 500–700 m from the surface are sharply cut off by granites. The effusive-sedimentary sequence is composed of limestones and porphyrites of various composition. The ore-hosting rocks are cut by diverse hornfelses and metasomatic formations. In terms of material composition, among the hornfelses there are distinguished pyroxene, pyroxene-plagioclase, quartz-pyroxene-biotite, and hornblende-plagioclase varieties.

Metasomatic formations—skarns and epidotites—are even more extensively developed. In mineral composition, among the skarns there are distinguished magnesian and calcareous types, of which the latter predominate. The magnesian skarns include forsterite-phlogopite-serpentine and magnetite-serpentine varieties. Among the calcareous skarns there are distinguished pyroxene, pyroxene-scapolite, pyroxene-garnet, garnet, pyroxene-magnetite, and pyroxene-amphibole varieties.

The formation of skarn-ore bodies represents a single multistage process. All ore bodies are situated in the contact zone and have a linearly elongated, lenticular form, which is controlled by the position in space of the skarns and hornfelses and by zones of crushing. The strike of the ore bodies is submeridional, with a steep dip to the northeast of 75–80°.

The ores of the deposit are characterized by a complex composition, caused by diverse mineral associations of ore and non-ore minerals.

On the basis of mineral composition, the following varieties have been distinguished in the ores: magnetite, sulfide–magnetite, arsenic–cobalt-bearing, and polymetallic.

Magnetite ores are predominant. They originated by replacement of skarns. Moreover, the main mass of magnetite was deposited after the deposition of skarn minerals. The deposition of sulfides in this mineral type, as well as in the sulfide–magnetite ores, took place at a later stage and is superimposed.

The polymetallic and arsenic–cobalt-bearing mineralization is associated with still later stages of mineralization. The emergence of these stages is connected with the third phase in the formation of the Olginsko–Ampalyk intrusion.

The entire process of mineral formation of the deposit took place in two epochs: hypogene and hypergene, subdivided respectively into stages and phases. The hypogene epoch includes three stages: 1) contact metamorphism, 2) alkaline metasomatism, and 3) pneumatolytic–hydrothermal. Within the pneumatolytic–hydrothermal stage, 8 phases of mineralization are recognized, characterized by different paragenetic associations of minerals. Since gold is associated with this stage, we shall give a brief description of it.

The earliest phase in this stage is that of magnesian skarns, characterized by the formation of olivine, phlogopite, and hastingsite. Calcareous skarns were superimposed on the magnesian skarns, as evidenced by replacement structures of the former by the latter. In turn, as calcareous skarns formed, postmagmatic solutions altered the enclosing rocks, enriching them in calcium and iron and accompanied by the removal of silica and alkalis. This phase is characterized by the deposition of the minerals pyroxene, garnet, and scapolite.

The phase of magnetite mineralization appeared after tectonic activity in the area, accompanied by zones of crushing and by fractures in the skarns. The iron transported by the solutions was deposited during this phase in the form of magnetite, replacing the skarn minerals.

The high-temperature sulfide phase is separated from the magnetite phase by a tectonic break, expressed by brecciation of magnetite and its replacement by sulfides. After the deposition of the main mass of skarn minerals, magnetite, and high-temperature sulfides, a new intrusion of magma occurred (the third phase of the Olginsko–Ampalyk intrusion), with the development of alkaline granites. The concluding stage of this intrusive phase was accompanied by low-temperature mineralization.

Native gold is not recorded in all varieties of ores. Of greatest interest in this respect are the magnetite, arsenic–cobalt-bearing, and polymetallic varieties. In the former, gold is associated with magnetite and arsenopyrite, and in the latter—with galena, molybdenite, and bismuthinite.

In polished sections, it was possible to establish two generations of gold, differing in their paragenetic association of minerals and in the time of deposition within the overall skarn–ore process. The first generation of gold tends toward pyroxene–magnetite skarns and, in time, appeared simultaneously with the formation of skarn–magnetite ores and is connected with the high-temperature phase of the pneumatolytic–hydrothermal stage. Native gold occurs within grains of magnetite. Under the microscope it was established that the gold forms small rounded, isometric inclusions ranging in size from 0.015 to 0.25 mm. The inclusions tend toward the boundaries of magnetite grains, forming with them structures of mutual boundaries. During structural etching with aqua regia, the outlines of the boundaries of gold grains with magnetite are revealed. The outlines of the grains are slightly

sinuous, almost mutual, allotriomorphic. In two polished sections it was possible to observe similar relationships of gold with arsenopyrite.

The first generation of gold was deposited after the deposition of the skarn minerals—pyroxene, garnet, and scapolite—of the first generations, but earlier than the main mass of high-temperature sulfide minerals. At the same time, no direct connection between gold and sulfides was noted; this is confirmed by spectral and chemical analyses of monomineral fractions of sulfide minerals, in which gold is absent from the first generations of sphalerite, pyrite, pyrrhotite, and chalcopyrite. In chemical and spectral analyses of magnetite, and more rarely of arsenopyrite, gold is present where the iron and cobalt contents are elevated. However, in magnetite aggregates where gold was not found in polished sections, it was recorded by chemical analyses and probably occurs in a dispersed state.

The second generation of gold is associated with polymetals, where it is associated with galena and molybdenite. Here gold is observed in the form of small grains of droplet-like, irregular, and irregularly rounded form, ranging in size from 0.07 × 0.3 mm to 0.09 × 0.42 mm.

Structural etching reveals a fine-grained structure with serrated boundaries of individual grains, which corrode grains of bismuthine, galena, and molybdenite. In the latter two minerals, observed as thin veinlets in arsenopyrite, the gold appears as isolated grains of irregular form, sometimes penetrating deep into the arsenopyrite grains.

The second generation of gold is lower-temperature relative to the skarn minerals, magnetite, high-temperature sulfides, and the gold of the first generation. This correlates well with the final stage of the pneumatolytic-hydrothermal stage—low-temperature sulfides—and almost simultaneously with altaite and silver. Spectral and chemical analyses of monomineral fractions of low-temperature sulfides established the presence of gold.

In conclusion, it may be inferred that both generations of gold are characterized by different paragenetic associations of minerals separated from one another in time. The different paragenetic association of minerals of the two generations of gold can be explained by the composition of the postmagmatic solutions: the first were richer in alkalis, and their acidity, as silica was removed from the enclosing rocks, gradually increased (Korzhinskii, 1953); the second were more acidic.

The first generation of gold has limited development and is associated with different phases of the Olginsko-Ampalyk intrusive complex; the second is more widely developed and is associated with low-temperature sulfides, and genetically with the third phase of the indicated intrusion.

It is superimposed with respect to skarns, magnetite, and high-temperature sulfides. The identified association of native gold with magnetite, arsenopyrite, bismuthine, galena, and molybdenite is of practical interest. Therefore, in the study and exploration of skarn iron deposits, it is necessary to keep in mind the possibility of finding rich gold-bearing ores.

REFERENCES

1. D. S. Korzhinskii. Essay on metasomatic processes. In: Fundamental Problems in the Study of Magmatogenic Ore Deposits. Publishing House of the Academy of Sciences of the USSR, 1953.