On the Epochs of Hydrothermal Mineralization in the Sarala Gold-Bearing District

V. I. Bazhenov

(Presented to the scientific seminar of the Department of Geology and Exploration of Mineral Deposits)

The Sarala gold-bearing district is located in the axial part of the Kuznetsk Alatau and is confined to one of the branches of the Kuznetsk–Alatau fault, extending in a north–northwestern direction. This fault is emphasized by a thick belt of dikes of basic, intermediate, and more rarely alkaline composition, as well as by the linear arrangement of exposures of intrusive rocks elongated in a submeridional direction.

The deep-seated fault underwent a long and complex history of formation. Movements along it began as early as the pre-intrusive epoch and continued for a long time, up to the Upper Paleozoic, and it is possible that they also occurred in the Mesozoic. All this determined the complexity and multi-stage character of magmatic manifestations in the district described. On the basis of the study of the relationships of intrusive formations with one another, they are divided into two magmatic complexes: diorite–plagiogranite and granite–granodiorite. The first of these is correlated with the Kundustuyul, and the second with the Martaiga intrusive complex described by M. P. Kortusov [3] for the Mariinskaya taiga.

The emplacement of dikes also occurred repeatedly. Among them, two age groups of vein rocks are quite clearly distinguished: 1) dikes of pre-Devonian age, composed of diabases, diabase porphyrites, gabbro-diabases, hornblendites, and gabbro-diabase porphyrites; 2) dikes of post-Lower Devonian age, represented by conga-diabases, dolerites, spessartites, odinites, and more rarely vogesites and selsbergites, etc.

In connection with such a complex character of magmatic manifestations, a complex character is also established for the development of postmagmatic processes. On the basis of the study of hydrothermal formations and their connections with the magmatic rocks of the district, three epochs of hydrothermal mineral formation are distinguished.

The first, most ancient, epoch of hydrothermal mineralization is represented by numerous quartz veins located within the Sarala ore field. The total number of known veins exceeds 200. These veins occur among rocks of the effusive-sedimentary sequence south of the exposures of intrusive rocks of the diorite–plagiogranite complex. These veins are confined to various systems of fracturing of the enclosing rocks, to small-amplitude faults, and often

their localization is controlled by dikes or sills of igneous rocks (the Andreevskaya, Ivanovskaya, Kaskadnaya, Yubileinaya veins, and some others). The extent of the veins varies; it reaches several hundreds of meters, and in some cases 2–3 km. They are characterized by a multistage ore-forming process. In all, five successive stages of mineralization are distinguished: 1) quartz, 2) quartz–pyrite–arsenopyrite, 3) ankerite, 4) quartz–polymetallic, and 5) quartz–carbonate. The mineralogical composition of the quartz veins of the ore field has been studied more than once and described in the literature [1, 2]. Therefore there is no need to dwell again on their mineralogical characteristics. We shall note only that, in mineralogical composition, the veins belong to typical medium-temperature formations with a moderate content of sulfides, among which pyrite, arsenopyrite, sphalerite, galena, and chalcopyrite are most widely distributed. Two generations of gold are established in the veins: one of them is associated with the quartz–pyrite–arsenopyrite stage, and the second with the quartz–polymetallic stage of mineralization.

Genetically, this main epoch of gold-ore mineralization is associated with the Kundustuyul diorite–plagiogranite complex. Such a connection is supported by the spatial distribution of quartz veins in the southern exocontact of the diorite and plagiogranite intrusion. Within the ore field, manifestations of horizontal zoning of the mineralization are established, changing regularly with distance from the intrusion. Certain mineralogical-geochemical features of the igneous rocks also indicate the genetic connection of the mineralization with the intrusion of the diorite–plagiogranite complex. This conclusion is confirmed by M. P. Kortusov’s inference that the gold content of a number of areas of the Mariinskaya taiga is genetically associated with intrusions of the Kundustuyul intrusive complex.

The second epoch of hydrothermal mineralization is genetically associated with the granite–granodiorite complex. Its manifestations, diverse in genesis, are established in the northern part of the Sarala district. Among them are distinguished: 1) limestone skarns, 2) quartz veins and stockworks, 3) quartz–albite metasomatites, and 4) mineralization of crushing zones.

Limestone skarns form deposits and lenses in the exocontact of granites with carbonate rocks, and also with plagiogranites of the older diorite–plagiogranite complex on the eastern slope of Mt. M. Ararat. More rarely, bodies of infiltration skarns occur along zones of disjunctive disturbances. By composition, garnet, garnet–pyroxene, pyroxene, wollastonite, and tremolite–wollastonite skarns are distinguished; in addition to the minerals listed, amphiboles, epidote, scapolite, quartz, calcite, and occasionally rare disseminations of pyrite and chalcopyrite are established in them. The genetic connection of the skarns with the granite–granodiorite complex raises no doubt, owing to the close spatial relationship of these formations with the indicated intrusion.

Quartz veins, rather numerous, are characteristic of the northern part of the district described. They occur in granites, and also in the rocks enclosing the granites: carbonate–terrigenous deposits of the Glavstan suite and diorites of the Kundustuyul complex.

The veins consist of massive white coarse-grained sulfide-free quartz. In quartz veins occurring in the southern exocontact of the granites among diorites on the southern slope of Mt. B. Ararat, together with quartz, albite is noted in the form of large crystals and, in places, radiating epidote.

In the southeastern contact of granites and granodiorites among the diorites of the Kundustuyul complex, a zone of strongly silicified rocks was encountered, often penetrated by a large number of small veinlets forming stockworks. No sulfide mineralization associated with this hydrothermal occurrence has been found.

Quartz–albite metasomatites are located only near the quartz stockwork described above, on the northeastern slope of Mt. B. Ararat. They are likewise controlled by a fault zone. These formations arise as a result of replacement of diorites and consist of a fine-grained aggregate of quartz and albite, often with spots of relict diorite minerals. Their connection with the granite intrusion is confirmed by the fact that the metasomatites are controlled by fault structures that cut the granites.

A mineralized fault zone was encountered north of the settlement of Glavstan. It extends along a hydrothermally altered dike of igneous rocks occurring among bituminous limestones of the Glavstan Formation. The ore is a tectonic breccia of the altered dike, cemented by medium-grained gray carbonates. Sulfide mineralization is confined to the breccia cement. Microscopic study of the zone established light, low-iron sphalerite, galena, pyrite, and chalcopyrite. We regard the connection of the mineralization with the granites as provisional.

In any case, in mineralogical and geochemical features this occurrence differs markedly from the ores of the productive quartz veins of the ore field by the presence of low-iron sphalerite, the minor role of iron and copper sulfides, and the absence of arsenopyrite and vein quartz.

The gold content in all the described occurrences of hydrothermal mineralization associated with the granite–granodiorite complex is very low. Usually it does not exceed 1–2 g/t, in individual cases rising to 3–5 g/t. At the same time, the high silver content is noteworthy, reaching 30–50 g/t in the quartz stockwork and mineralized crushing zone, and locally 215 g/t. Probably the occurrence of molybdenum mineralization that we discovered on the left side of the L. Sarala River is also connected with “this” same intrusive complex; it is confined to beresitized granites in the form of disseminated molybdenite in altered granites and in quartz veinlets.

The third epoch of hydrothermal mineralization was weakly manifested. It was established in zones of faults among deposits of the Byskhar Series of Lower Devonian age, extending along the contact of dikes of post–Lower Devonian age. The mineralization is expressed as silicification of the rocks and the formation of small veins and veinlets composed of white drusy quartz. No sulfide mineralization has been found in them. The gold and silver contents are very low.

Thus, in the Sarala district three different-aged epochs of hydrothermal mineralization have been established, of which only the earliest is characterized by fairly rich sulfide and gold mineralization. The weak gold content of the hydrothermal formations of the second epoch of mineralization may be due to some regeneration of gold from more ancient deposits as a result of their assimilation by granite magma.

REFERENCES

1. Bazhenov V. I. Mineralogy and conditions of formation of the columnar mineralization of the Andreevskoe deposit. Izv. TPI, vol. 90, 1958.

1. Bylinnikov A. Ya. Gold-ore formations and gold-bearing provinces of the Altai–Sayan mountain system. Tomsk, TSU Press, 1948.

2. Kortusov M. P. Paleozoic intrusive complexes of the Mariinsk taiga (Kuznetsk Alatau). Vol. I. Lower Paleozoic intrusive complexes. Tomsk, TSU Press, 1967.