On the Gold Potential of Skarns in the Northern Part of the Sarala Ore Field
Abstract Generated abstract
This paper evaluates the gold potential of skarn occurrences in the northern Sarala ore field of the Kuznetsk Alatau, where known gold mineralization is mainly represented by gold-bearing quartz veins. Using geological observations, channel sampling of bedrock and dumps, and placer concentrate sampling, the authors classify the skarns into garnet and pyroxene-bearing contact-metasomatic types and wollastonite or tremolite-wollastonite infiltration types, then compare them with productive skarns elsewhere in the Sayan-Altai region. Despite mineralogical similarities to gold-bearing skarns at other deposits, sampling showed generally low gold contents, mostly no more than 1 to 2 g/t, with only isolated values of 3.5 to 5.0 g/t and no gold in concentrates. The authors infer that these skarns are linked to a younger granite-granodiorite intrusion that was likely poor in gold, unlike the older diorite-plagiogranite complex associated with the ore field’s principal gold-bearing quartz veins.
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ON THE GOLD POTENTIAL OF SKARNS IN THE NORTHERN PART OF THE SARALA ORE FIELD
V. I. Bazhenov, S. K. Knysh
(Presented by the seminar of the Department of Geology and Exploration of Mineral Deposits)
The Sarala ore field, located in the axial part of the Kuznetsk Alatau, is one of the oldest gold-mining sites in Western Siberia. Geologically, the ore field occurs within a complex effusive-sedimentary sequence composed of basic, intermediate, and acidic effusive rocks, their tuffs, and various, predominantly clastic, varieties of sedimentary rocks. From the east, this sequence is adjoined by dark bituminous limestones, distinguished as the Glavstan Formation of Upper Proterozoic age.
In the northern part of the ore field, intrusive rocks are exposed, forming several massifs. Geological mapping of this area made it possible to subdivide the intrusive massifs and to distinguish among them two magmatic complexes. The first, older, magmatic complex includes the diorite–plagiogranite intrusion of Mount B. Ararat. Here the diorites compose a slightly elongated, latitudinally oriented intrusive massif. The plagiogranites represent a later intrusive phase. These rocks form a body elongated in a latitudinal direction along the southern boundary of the gabbro-diorite massif. Smaller bodies of plagiogranitic composition are also known in the southwestern contact of the diorite intrusion. To this same magmatic complex we also assign the plagiogranites exposed on the summit and on the eastern slope of Mount M. Ararat.
The second granite–granodiorite complex includes the intrusive massif of Mount M. Ararat, as well as a number of smaller intrusive bodies elongated in a submeridional direction on the left side of the Levaya Sarala River. This complex is younger than the diorite one, since the rocks of the latter are distinctly hornfelsed and often skarnified in the exocontact of the granite–granodiorite intrusion.
The only type of gold mineralization in the Sarala ore field is gold-bearing quartz veins. They consist of quartz with moderate contents of sulfides and gold. Among the sulfides, pyrite, arsenopyrite, pyrrhotite, sphalerite, galena, chalcopyrite, pale ores, and other minerals are the most widespread. Genetically, the gold mineralization of the ore field is apparently associated with the ancient gabbro-diorite–plagiogranite magmatic complex.
The discovery in the Kuznetsk Alatau of a number of gold deposits of contact-metasomatic genesis drew attention to the possible gold potential of skarns also in the northern part of the Sarala ore field.
areas where their occurrences had been noted in the past, but had not been subjected to systematic study. The present article presents certain data concerning their gold content, based on the results of sampling of bedrock outcrops and dumps, as well as placer sampling carried out for the purpose of a general assessment of the gold content of individual areas bearing skarn manifestations.
Within the area under consideration, the skarn bodies are spatially associated with the contacts of the granite–granodiorite intrusion, which indicates a genetic connection with the young magmatic complex. More rarely, they are confined to zones of faults at some distance from the intrusive contacts. In composition, all the occurrences discovered belong to the type of calcareous skarns. According to mineralogical composition and the ratio of the principal rock-forming minerals, all skarns may be divided into two groups:
- Garnet, pyroxene–garnet, garnet–pyroxene, and pyroxene skarns.
- Wollastonite and tremolite–wollastonite skarns.
Skarns of the first group constitute the most numerous group. In all cases they are situated in the exocontact of the granite–granodiorite intrusion and are formed through the skarnification of various types of host rocks: contact hornfelses, hornfelsed plagiogranites of the gabbro–diorite–plagiogranite magmatic complex, and also carbonate rocks of the Glavstan Formation. These rocks are widely distributed on the eastern, northern, and southeastern slopes of Mt. Ararat. They occur as fairly extensive lenses, and also compose deposits irregular in form.
Macroscopically, they are rocks of fine-grained to medium-grained structure. Their texture is most often mottled, caused by the uneven distribution of the principal rock-forming minerals. Less commonly, they are characterized by a massive texture, which is usually typical of monomineralic garnet and pyroxene skarns; the general color of the rocks is brown or greenish, associated with the predominance of one or another mineral.
The principal minerals of the skarns are garnet and pyroxene of the diopside–hedenbergite series. Amphiboles, epidote, plagioclase, wollastonite, scapolite, and sphene occur as more or less significant admixtures. The last two minerals occur in skarns formed at the expense of plagiogranites. Typical hydrothermal minerals—quartz and calcite—are in some cases rather abundant, making up as much as 10% of the skarns, although their content is usually considerably lower. Dissemination of limonitized pyrite is occasionally observed in the skarns.
Skarns of the second group occur on the left side of the Levaya Sarala River, within the area of distribution of carbonate rocks of the Glavstan Formation. They are situated in zones of faults at a distance from intrusive contacts, representing typical infiltration formations. They apparently compose small vein-like bodies and lenses, traceable as chains along the zones of faults. Macroscopically, wollastonite and tremolite–wollastonite skarns are light-colored rocks with medium- to coarse-grained structure. The principal mineral of the skarns is wollastonite. Pyroxene, tremolite, and calcite are present in them as impurities. In the bedrock outcrops of the skarns described along the Levaya Sarala River, disseminated pyrite, chalcopyrite, and coatings of copper green were established.
The similarity of the geological structure of the area described with the geolo-
The geological structure of skarn-type gold-ore deposits of the Sayan–Altai folded region served as a premise for prospecting for gold-bearing skarns within the Sarala district as well. The similarity of the mineralogical features of the Sarala skarns to the gold-bearing skarns of other deposits is noteworthy. It should be emphasized that the wollastonite and tremolite–wollastonite skarns of Sarala, which contain disseminated pyrite and chalcopyrite, are very close to skarns of analogous composition at the Sinyushenskoye and Natalyevskoye deposits, where they are the most productive in terms of gold content. Garnet and garnet–pyroxene skarns are close in composition to the gold-bearing skarns of the Mayskoye and Kaliostrovskoye deposits.
However, the channel sampling of the skarns distributed over the area described yielded negative results. Gold contents usually do not exceed 1–2 g/t, and only in isolated samples from garnet and garnet–pyroxene skarns on the eastern slope of Mt. Ararat do they rise to 3.5–5.0 g/t. Analysis of the sampling results showed a complete absence of any dependence of gold contents on the intensity of the manifestation of hydrothermal mineralization in the skarns. Concentrates collected in areas of skarn development also showed an absence of gold.
Thus, the work carried out showed very weak gold mineralization of the contact-metasomatic formations in the northern part of the Sarala ore field. This fact may be explained by various causes.
The gold mineralization of the Sarala ore field is genetically related to an ancient intrusion of the diorite–plagiogranite magmatic complex. Such a genetic relationship is supported by the spatial proximity of the gold-bearing quartz veins to this intrusion, which makes up Mt. Ararat. At the same time, a certain horizontal zonality of mineralization is observed, manifested in a change in the mineralogical composition of the quartz veins with increasing distance from the intrusion. Some geochemical features of the volcanic rocks of the district also support a genetic connection between the gold mineralization and the ancient intrusion.
At the same time, as noted above, the skarns of the northern part of the Sarala ore field are genetically related to a younger intrusion of granite–granodiorite composition. It may be assumed that this intrusion was practically barren with respect to gold mineralization. Evidence for this may be the persistently low gold contents in the skarns and in the quartz veins associated with this intrusion, as well as the absence of gold in concentrates from the area of its distribution. The weak gold mineralization of the skarns described is possibly due to some regeneration of gold as a result of assimilation by granitic magma of gold-ore deposits associated with the more ancient intrusion.