On the Geology and Metalliferousness of Skarns in the Bolshaya Natalyevka Area

B. D. VASILIEV

(Presented by Professor K. V. Radugin)

The proposed article sets forth, in condensed form, the geology and metalliferousness of skarns, using as an example one of the gold deposits located on the northern margin of the Kuznetsk Alatau, in the basin of the middle course of the Kiya River.

The area is composed of sedimentary and sedimentary-volcanogenic formations of the Lower and Middle Cambrian, intruded by two intrusive complexes of different ages, with each of which gold mineralization is associated.

The stratigraphy of the Cambrian deposits appears to us as follows. The oldest rocks are exposed in the central part of the area, where they make up an anticline of northeastern strike (Fig. 1). Among them, in normal stratigraphic sequence, several rock units are distinguished (from bottom to top):

1) gray marbles interbedded with porphyrites and their tuffs; the thickness of the unit is more than 50 m, and the ratios of the thicknesses of the marble and porphyrite beds within the unit are approximately equal;

2) hornfelsed porphyrites with lenses of andalusite-cordierite hornfels, with thin beds of gray marble; thickness of the unit 30 m;

3) gray banded marbles with rare thin horizons of variegated tuffites and porphyrites; thickness of the unit not less than 150 m;

4) porphyrites, tuffites with horizons of diagenetized porphyrites, red-colored sandstones and conglomerates, with a total thickness of more than 100 m.

A normal section of this sequence was observed by us somewhat farther south along the Kundat River, at the mouth of the Sobolinka stream, where the gray banded marbles in their upper part contain paleontological remains (Collenia, Archaeospongia). The entire sequence is assigned by us, provisionally, to the lower parts of the Lower Cambrian.

Within the limits of the deposit this sequence is folded into a sharp anticline overturned to the northwest, the axis of which plunges gently to the northeast. The limbs of the fold dip steeply, at angles of 70–85°, to the southeast.

In the western part of the area the aforementioned fold is truncated by a major meridional dislocation, west of which manganese-bearing siliceous shales with interbeds of tuffs and limestones are exposed. Everywhere in the area the siliceous shales are conformably overlain by light-gray…

limestones with a rich archaeocyathid fauna of the Upper Kamyshkovka complex (according to P. S. Krasnopeeva) and are undoubtedly of Early Cambrian age.

Farther north, on an erosional surface cutting the above-noted fold, greenstone porphyrites of the Middle Cambrian (the Martayga Formation of V. K. Monich) lie unconformably. In the central part of the area of distribution of the greenstone porphyrites there is an outcrop of Lower Cambrian carbonate rocks, containing the principal skarn-ore bodies of the deposit.

The carbonate sequence, represented in this area by light-gray limestones, calcareous sedimentary breccias, and stromatolitic limestones with Collenia, is folded into small, sharp folds of meridional strike, with steeply dipping limbs, and in age corresponds to the upper horizons of the banded-marble member composing the southern limb of the anticline.

From the south and east the Lower Cambrian carbonate sequence in this area is in contact, along major zones of crushing, with Middle Cambrian greenstone porphyrites, while from the north and west it is unconformably overlain by them.

Thus, in the Bolshaya Natalyevka area, the manifestation of a phase of tectogenesis, presumably at the boundary between the Lower and Middle Cambrian, is clearly established.

The Lower Cambrian rocks composing the lower structural stage are folded into small, narrow, and sharp folds of north–northeastern strike, with steeply dipping limbs and overturning of the folds to the northwest.

Middle Cambrian greenstone porphyrites, composing the upper structural stage, lie unconformably on the erosional surface of the Lower Cambrian structures and are distinguished by more tranquil tectonics.

The occurrence of the Middle Cambrian greenstone porphyrites at various horizons of the Lower Cambrian indicates a prolonged erosion of the Lower Cambrian structures before the eruption of the porphyrites.

Intrusive formations in the deposit area are represented by two complexes of different ages. Outcrops of rocks of the Martayga granodiorite intrusive complex are widespread; its age is considered by some investigators to be Caledonian [4], by others—Salairian [1, 6]. In our opinion, a Cambrian age of the intrusion is more probable.

The Martayga intrusive complex is represented by quartz monzonites, quartz diorites, diorites, syenites, and granodiorites.

Fig. 1. Geological map of B. Natalyevka (schematized). Compiled by B. D. Vasiliev.
1 — greenstone porphyrites ($Cm_2$); 2 — siliceous shales ($Cm_1$); 3 — porphyrites, tuffites, sandstones ($Cm_1$); 4 — gray banded marbles ($Cm_1$); 5 — hornfelsed porphyrites, hornfelses ($Cm_1$); 6 — gray marbles, porphyrites, tuffs ($Cm_1$); 7 — gabbro intrusion ($D?$); 8 — granodiorite intrusion (Martayga complex); 9 — garnet skarns; 10 — lens No. 1 (tremolite skarns); 11 — diabase dike; 12 — contacts of unconformable occurrence; 14 — magmatic contacts; 15 — dysjunctives.

Almost all outcrops of rocks of the Martaiga intrusive complex are subordinate to the folded structure of the enclosing Cambrian strata and have a distinct north-northeastern orientation. In certain areas, the contacts of the intrusive bodies are accompanied by diopside-garnet skarns with superimposed ore mineralization.

The intrusion of ilmenite gabbro, identified at the deposit in 1958, is younger. The intrusion occurs in the northern part of the area as a small (400 × 60 m) linear body composed of olivine gabbro-norites, gabbro-diorites, and gabbro.

The intrusive body is elongated in a latitudinal direction and cuts not only the porphyrites of the Middle Cambrian, but also the monzonites of the Martaiga intrusive complex. By analogy with the known young ilmenite-bearing gabbro intrusions in the Kuznetsk Alatau, the age of the intrusion is apparently Devonian (the Patyn complex).

The gabbro intrusion is accompanied by skarns: under the action of skarn-forming solutions, limestones are replaced mainly by diopside-tremolite skarns, and porphyrites mainly by garnet skarns. The skarns of the gabbro intrusion, as a rule, carry superimposed gold-copper-bismuth mineralization.

The diopside-tremolite composition of the skarns, intense dolomitization, serpentinization, and talcification accompanying the skarns indicate a considerable introduction of magnesium prior to ore deposition.

Gold mineralization is genetically associated with two intrusive complexes: the Martaiga granodiorite complex and the younger gabbro complex; of greatest interest are the compositionally distinctive ores connected specifically with the gabbro complex.

The gold-bearing nature of the Martaiga granodiorite complex was established long ago and was noted by all investigators of the Mariinsk taiga. The gold mineralization of the Berikul and Central deposits is genetically linked with this complex [3]. Gold-arsenic mineralization of the Berikul type, characterized by high contents of pyrite, arsenopyrite, galena, and sphalerite in the ores, also occurs at the deposit described here in the form of small quartz-sulfide veins in the northern part of the area. Pyroxene-garnet and magnetite-garnet skarns accompanying the outcrops of rocks of the Martaiga intrusive complex rarely carry superimposed gold-sulfide mineralization.

A distinctive gold-copper-bismuth mineralization is genetically and spatially associated with the gabbro intrusion; it is localized both within the gabbro massif itself (quartz veins with bismuthinite, chalcopyrite, and native gold) and in the enclosing rocks immediately near the contact, in diopside-tremolite skarns. At present, the greatest practical value is represented by light-colored diopside-tremolite skarns with superimposed gold-copper-bismuth mineralization.

Bodies of diopside-tremolite composition have a lens-like form and are located in the carbonate sequence beneath the cover of Middle Cambrian porphyrites. Where bodies of porphyritic composition have been affected by skarnization, the latter have been transformed into garnet skarns. Using this feature, it is possible to detect blind skarn-ore bodies beneath the porphyrite cover in areas of bleaching and skarnization of the porphyrites.

The skarn bodies are generally arranged parallel to the contact of the gabbro intrusive body.

Thus, for example, lens No. 1 is a small lens-shaped body of diopside-tremolite composition, cut in the upper

in its own part by several small lenses of magnetite. With respect to the folded structure of the enclosing limestones, lens No. 1 has a crosscutting position. The strike of the lens is northwest; the dip is gentle, at an angle of 45° to the northeast, with a slight inclination to the east.

Lens No. 1 represents only part of a larger skarn body situated in the hanging, displaced wall of a normal strike-slip fault. The plane of the displacement of the normal strike-slip fault dips steeply southward and is confined to a thin diabase dike, which in turn cuts and displaces the skarns.

Having a coarse-prismatic and radial-rayed structure, the diopside-tremolite skarns are, in the bulk of their mass, gold-bearing. They carry an uneven fine dissemination of chalcopyrite and, to a lesser degree, bismuthine and native gold. Quartz, in the form of small irregular grains, is extremely rare in the skarns.

On the lying side of lens No. 1, the skarns are accompanied by barren quartz-sulfide ores, among which four generations of quartz with corresponding assemblages of ore and vein minerals have been distinguished.

Gold in the ores is closely associated with bismuthine and chalcopyrite; it occurs in the native state both in the skarns and in the quartz veins.

The disjunctive tectonics at the deposit are complex and multiphase. Sub-latitudinal disjunctives controlling the skarns and mineralization should be especially distinguished. However, their identification is associated with great difficulties. Post-ore tectonics was manifested above all in the formation of latitudinal strike-slip faults filled by rocks of diabase magma and, following this, in the occurrence of displacements along the plane of the normal strike-slip-fault type dike.

Subsequently, the principal zone of meridional-strike crushing is renewed or arises; it passes along the western margin of the area and has been traced for a distance of more than 5 km. It should be assigned to first-order displacements.

Large disjunctives of southeastern and northeastern strike, with vertical displacement amplitudes of more than 100 m, have been assigned to second-order displacements. They are subsidiary with respect to the first-order displacement.

A series of southeastern-striking displacements, characterized by insignificant amplitudes and rapid flattening of the displacement planes with depth, has been assigned to third-order displacements (the dip angle of the displacement plane changes from 70° to 40° over 30 m down the dip).

In conclusion, it is necessary once again to emphasize that gold mineralization in the area is genetically connected with two intrusive complexes. Accordingly, two gold-ore formations are distinguished: 1) a gold-arsenic formation, genetically connected with the Martaiga granodiorite complex, and 2) a gold-copper-bismuth formation, genetically connected with a younger (D?) gabbro intrusion.

The gold-bearing bodies are quartz veins and skarns with ore mineralization superimposed upon them.

REFERENCES

1. Ananyev A. R. The Kozhukhovsky granite-diorite massif in the northwestern spurs of the Kuznetsk Alatau. Scientific Notes of TTU, No. 14, 1950.

2. Bulynnikov A. Ya. Gold-ore formations and gold-bearing provinces of the Altai-Sayan mountain system. Proceedings of TTU, geological series, vol. 102, 1948.

3. Dembo T. M. Geological structure and gold mineralization of the northern part of the Kuznetsk Alatau. Proceedings of Nigrizoloto, issue 19, 1952.

1. Zenkova A. A. and Kazakevich Yu. P. On the question of the age of the monzonite–syenite intrusion in the northwestern part of the Kuznetsk Alatau. Bulletin of ZSGU, No. 5, 1940.

2. Krasnopeeva P. S. Archaeocyathid and archaeocyathid-trilobite horizons of the Cambrian of the Altai-Sayan region. Materials on the Geology of Western Siberia, issue 61, 1958.

3. Monich V. K. Geological sketch of the Berikul district. Proceedings of the Zolotorazvedka and Nigrizoloto Trust, issue 7, 1937.