Ancient Dioritoid Intrusive Rocks of the Kommunarovsky Ore Field and Some Issues of Their Gold-Bearing Potential

A. F. Korobeynikov, L. G. Osipov

(Presented by Professor A. M. Kuzmin)

Within the Kommunarovsky ore field, vein and stockwork quartz–gold-ore deposits and skarn–magnetite lenses with superimposed gold mineralization are known. The principal stockwork deposits of the ore field are associated with subsill-like intrusive bodies of gabbro–diorite composition, assigned by a number of researchers \[5, 13, 15\] to the oldest Biya intrusive complex. There is no single opinion on the question of the genesis of the gold mineralization. Some researchers \[2, 3\] assign the dioritoid bodies to intrusive formations and genetically connect the gold mineralization with them. Others \[14\] consider them to be metamorphogenic formations that arose through deep reworking of individual horizons of basic effusives, while the gold mineralization, following V. A. Obruchev and D. I. Kalinikov, is genetically linked with the Solgon granodiorite intrusion.

At present, materials from detailed geological-structural study make it possible to clarify certain questions of the magmatism and genesis of the gold mineralization. Therefore, the article shows the relationships of small concordant dioritoid bodies with the effusives enclosing them, distinguishes two phases of emplacement, and gives the general petrochemical and geochemical features of the rocks and their relation to mineralization.

The ore field is situated on the eastern limb of a large anticlinal structure of near-meridional strike and is composed of two heterogeneous sequences: a lower diabase–porphyrite sequence and an upper volcanogenic–sedimentary sequence of presumably Middle Cambrian \[14\] or even Precambrian \[7, 11\] age.

Most of the small dioritoid bodies are confined to the near-contact zone of these heterogeneous sequences and form a peculiar submeridional belt. In the overlying, substantially carbonate sequences, the indicated intrusive bodies have not been established. Both the effusive–sedimentary and the intrusive formations are cut by a large granodiorite massif. In turn, the granodiorites and quartz diorites themselves are cut by dike-like bodies of olivine gabbro and gabbro-porphyrites and of subalkaline diabase porphyrites (teschenites), probably of the Devonian gabbro–syenite intrusion \[6\]. The augite diorites of the Kaliostrovsky area \[2\], which make up the endocontact part of the Solgon massif, belong to formations of the early phase of the granodiorite intrusion.

Subconcordant linear, more rarely dike- and lens-shaped bodies of uralitized gabbro-diorites are usually small in size: up to 0.5–2 km along strike, with a thickness of 20–200 m. Their contacts are sharp on the side of the acid effusives and sedimentary rocks and indistinct on the side of the diabases and diabase porphyrites. No appreciable exocontact effects on the surrounding rocks are observed, which is evidently due both to weak contact metamorphism during the emplacement of these intrusives and to the superposition of subsequent regional and hydrothermal metamorphism. For the most part the small intrusives are concordant with the enclosing rocks, emphasizing the plicative structures of the ore field. The fine fracturing of the dioritoid bodies and of the country rocks is generally the same and evidently reflects the manifestation of the dominant regional deformation.

The gabbro-diorite intrusive complex formed in two successive phases: 1) porphyritic gabbro-diorites and 2) fine-grained gabbro-diorites. The latter, in the form of thin dike-like bodies, cut the porphyritic varieties. The contacts between them are sharp, with indistinctly distinguishable chilled zones and with xenoliths of porphyritic rocks in the endocontacts of the fine-grained gabbro-diorites, with embayments and apophyses of fine-grained rocks in the porphyritic gabbro-diorites. Zones of metamorphic amphibolites are sometimes observed along their contacts.

In addition to the rocks noted, medium-grained gabbro-diabases and diorite-diabases occur; unlike the former, they are less recrystallized and more intensely metamorphosed and schistose. It is quite possible that they belong to subintrusive analogues of the basic effusives. The relationships between the rocks of the first two phases and the gabbro-diabases are unclear.

Both the basic effusives and the gabbro-diorites have undergone intense amphibolitization. A number of investigators [13 et al.] regard these secondary processes as autometamorphic. However, the areal character of the development of secondary alterations in the rocks described and their multistage nature indicate the manifestation not only of autometamorphic processes, but especially of regional-metamorphic and contact phenomena (in connection with the emplacement of the younger granodiorite intrusion).

Gabbro-diabases and diorite-diabases are most widespread in the Solgon area [9] and constitute small linear, lens-shaped, or irregular stock-like bodies in plan. A completely gradual transition from gabbro-diabases to diabase porphyrites through diorite-diabases and diabases is frequently observed. In thin sections they show a porphyritic texture with a prismatic-granular and diabase texture of the groundmass. Porphyritic segregations 2–3 mm in size are often represented by zoned plagioclase No. 56–22 (up to 25%), along which albite No. 5–9 develops, by green hornblende, actinolite (up to 15%), and rare quartz. The main mass is composed of a fine-grained aggregate of andesine-labradorite No. 49–52, amphiboles, and chlorite-serpentine, which belongs to dzhensinite. Among the accessory minerals apatite, sphene, and magnetite are widespread, and pyrite and chalcopyrite occur rarely.

Porphyritic gabbro-diorites are the most widespread within the ore field. These are greenish-gray rocks with numerous large (up to 5–8 mm) phenocrysts of greenish-white plagioclases. Under the microscope a typically porphyritic texture with a prismatic-granular groundmass is established. The phenocrysts are represented by tabular, rarely zoned labrador-bytownite.

No. 52–75 (up to 45%), intensely saussuritized, and amphibole (up to 20–30%). Three varieties of amphibole have been encountered: uralitic green hornblende, blue-green hornblende, and actinolite. Pyroxene is exceptionally rare, since it is almost always replaced by uralitic amphibole, and only occasionally are relics of diopside observed in the centers of uralite grains. Among the later minerals, chlorite is developed, replacing all amphiboles, as well as quartz and albite. Accessory minerals are apatite, sphene, magnetite, leucoxene, and pyrite.

Fine-grained gabbro-diorites are greenish-gray, fine-grained, compact rocks consisting of plagioclase and amphibole. In thin sections the rock texture is gabbroic or diabasic and granoblastic, the latter appearing as a result of intense amphibolization. Mineral composition: plagioclase No. 52–67, less commonly 47–32 and 22–20, in the form of saussuritized polysynthetically twinned grains (up to 60%), and green hornblende (up to 30%) with the rarest relics of pyroxene. Of the later minerals, tremolite-actinolite, chlorite, albite, and quartz are developed. Accessory minerals are magnetite, apatite, sphene, leucoxene, and pyrite.

Diabases and diabase porphyrites of the effusive sequence are represented by greenish-gray, fine-grained, compact rocks of typical greenstone appearance. Under the microscope, plagioclase No. 51–66, up to 65%, green hornblende and actinolite, up to 25%, are identified. Relics of pyroxene grains 0.05–0.3 mm in size, replaced by uralite, are sometimes found. The rock texture is ophitic or pilotaxitic. Among accessory minerals, apatite, sphene, leucoxene, magnetite, and pyrite have been identified.

The results of recalculating chemical analyses of rocks of the gabbro-diorite subintrusive complex and the diabase-porphyrite formation of the ore field are shown on the petrochemical diagram of A. N. Zavaritsky (Fig. 1). The chemical analyses are given in Table 1.

In chemical composition, all rocks of the diabase-porphyrite formation belong to the group of alkali-poor rocks of the class undersaturated in silica. They are characterized by somewhat reduced alkalinity ($a:c = 0.8–1.8$), predominance of sodium over potassium ($n = 61–95.5$), increased iron content ($\mathrm{Mg}:\mathrm{Fe} = 1:1.2–1:6$), and low titanium content.

By contrast, quartz porphyries and keratophyres are distinguished by oversaturation in silica ($n = 40.5$), a certain excess of alumina, low magnesia and iron content, and they are characterized by both potassic (6–21) and sodic (407) specialization.

The rocks of the subintrusive gabbro-diorite complex (gabbro-diabases, diorite-diabases, gabbro-diorites, and diorites) belong to the series of rocks slightly undersaturated in silica. If, however, one considers the change in rock composition from phase to phase, then in general a certain tendency is outlined toward an increase in their acidity and alkalinity from the early phase (24–14) to the late one (502).

In comparison with the effusives, the gabbro-diorites are characterized by somewhat increased alkalinity, as indicated by the deviation of the vector field to the right of the line $CB$ in the $ASB$ plane (Fig. 1). At the same time, sodium predominates over potassium ($n = 65–89.0$). The somewhat increased alkalinity of the subintrusive rocks relative to the effusives is probably due not so much to the process of differentiation of a single basaltoid magma as to metamorphic phenomena.

In general, the chemical composition of the gabbro-diorites of Kommunar is completely analogous to that of the rocks of the Biya complex [12] and falls on the same variation line in the $ASB$ and $CSB$ planes. By compar-

Table 1

Chemical analyses of rocks of the gabbro-diorite complex and effusives of the Kommunarovsky ore field

Gabbro-diorite intrusive complex: 24—14—gabbro-diorite of the Kalistrovsky area; 6—1—gabbro-diorite of the Podoblachny area (borehole 6); 231-a—diorite-diabase of the Solonovsky area (borehole 231); 546—porphyritic gabbro-diorite of the Kalistrovsky area (outcrop 546); 502—gabbro-diorite porphyrite (outcrop 502); 1198—melanocratic gabbro-diorite from the Podzvezdnaya adit (outcrop 1198) (second phase); diabase-porphyrite formation: 8—46—pyroxene diabase porphyrite (borehole 8, Podoblachny adit); 7—8—dark-gray diabase porphyrite tuff (keratophyre, outcrop 407, Kalistrovsky area); 6—9—quartz keratophyre (borehole 6, Podoblachny adit); 14, 15, 16, 17, 18—gabbro-diorites of the Boiskoe intrusive complex [9].

1) Chemical analyses were performed in the central laboratory of the Krasnoyarsk Geological Administration.

Fig. 1. Petrochemical diagram by the method of A. N. Zavaritsky [4, 18] for rocks of the effusive and gabbro-diorite subintrusive complex of the Kommunarovsky ore field. The list of chemical analyses is given in Table 1.

compared with the average types of similar rocks according to R. Daly [18], they have a somewhat more acidic composition (Fig. 1).

Comparing the rocks of Kommunar with certain effusives and “small intrusions” of gabbro-diorites of Lower Cambrian age in the Eastern Sayan [19], we arrive at the conclusion that both are almost entirely analogous in their conditions of occurrence and chemistry. Thus, all the rocks being compared are characterized by sodium specialization, increased iron content, and moderate basicity.

The geochemical characterization of the rocks described is given on the basis of the results of eighty semiquantitative spectral analyses of samples from the rocks and accessory magnetite (Fig. 2). The most typical trace elements of diabases, diabase and plagioclase porphyrites are titanium, nickel, vanadium, cobalt, chromium, copper, zinc, and zirconium in amounts from 0.002 to 0.1%. It is of interest that accessory magnetite from the diabases of the Solgon—

Fig. 2. Diagram of the distribution of trace elements in effusives and gabbro-diorites of the Kommunar ore field:
1 — content up to 0.001; 2 — up to 0.005; 3 — up to 0.01; 4 — up to 0.05; 5 — up to 0.3%.

—sky area contains lead, molybdenum, and tin in amounts up to 0.001%.

An entirely different picture is observed in quartz porphyries and keratophyres, which are very poor in microelements. In them only titanium, vanadium, copper, and zirconium have been established in amounts up to 0.005%, which is probably due to deep differentiation of the initial basaltoid magma.

In gabbro-diabases, gabbro-diorites, and diorites the following trace elements are widespread: titanium, nickel, vanadium, cobalt, copper, and zinc in amounts from 0.003 to 0.2%. Very characteristic elements are titanium, vanadium, and copper, present in amounts of 0.03–0.2%, and the chromium content is very low. Molybdenum and tin are present in insignificant amounts (up to 0.001%); they are usually recorded in accessory magnetite. In contrast to the effusives, in the gabbro-dio-

olivine appears in the rocks, and there is no zircon. The latter circumstance is not entirely clear. Comparing the geochemical specialization of the effusive and intrusive rocks of the Kommunarovsky ore field, we find that both are very characteristically marked by the presence of molybdenum and a reduced chromium content. In this they differ from similar formations of the Eastern Sayan [19].

Thus, the effusive and intrusive formations of Kommunar, in terms of mineralogical-petrographic composition, petrochemistry, and the composition of trace elements, are characterized by approximately identical indicators (Figs. 1, 2; Table 1). Considering the conditions of occurrence of subbed intrusive bodies and horizons of basic and intermediate effusives, we come to the conclusion that both took part in folding; i.e., the gabbro-diorite complex is synorogenic. All this makes it possible to regard the small bodies of gabbro-diorites as possibly comagmatic with the effusives of the diabase-porphyrite formation, which are derivatives of a single basaltoid magma according to the classification of Yu. A. Kuznetsov [10], especially since, spatially, they are associated with effusive-sedimentary sequences. The data we have set forth confirm the statements of researchers [5, 13, etc.] that the Boi intrusion characterizes the geosynclinal stage in the formation of the mobile belt of the Kuznetsk Alatau.

The gold-ore deposits of Kommunar are situated in the contact zone of a large granodiorite intrusion and are often spatially confined to small bodies of uralitized gabbro-diorites. Along with this, gold-bearing quartz veins and veinlets are observed in rocks of the volcanogenic-sedimentary sequence (the Yanvarskaya vein), in granodiorites and diorites of the Solgon intrusion (the Ust-Saktyshulskaya, Malo-Berezovskaya, and other veins), and in skarn-magnetite lenses of the endo- and exocontacts of the granodiorite massif (the Kaliostrovskoye deposit). The confinement of gold occurrences to small linear intrusions is conditioned by the distinctive geological-structural and physicochemical conditions of localization of mineralization. The varied character of the manifestation of fracture tectonics within the ore field caused the formation of a definite type of ore. Along the linear dioritoid massifs, longitudinal ruptures arose, sometimes with systems of subsidiary fractures, in which both single quartz–gold-ore veins and quartz-veinlet (stockwork) ores with poor sulfide mineralization were formed. Constant joint fractures in the rocks of the volcanogenic-sedimentary sequence caused the formation of quartz-sulfide veins with tellurium-bismuth mineralization. In skarn-magnetite lenses, gold mineralization was localized in those cases where intramineralization fracturing developed in them, controlling the penetration of metal-bearing solutions.

As already noted, subbed bodies of gabbro-diorites are intruded and metamorphosed by rocks of the granodiorite intrusion. Dikes of diorite porphyrites, lamprophyres, and fine-grained diorites, which are schlierens of the granodiorite intrusion, sometimes contain xenoliths of rocks (tonalites) of the parent intrusion and, at the same time, are themselves cut by quartz–gold-ore veinlets. In addition, in dike fine-grained diorites, often intensely hydrothermally altered, an elevated gold content is almost constantly recorded. Younger olivine gabbros, gabbro-porphyrites, and teschenites cut and metamorphose the rocks of the granodiorite intrusion and the ore bodies (Fig. 3), and themselves contain no gold mineralization.

Contact iron-ore deposits, which sometimes contain superimposed gold mineralization (the Kalustrovskoye deposit), are genetically connected with the granodiorite intrusion, which we have considered separately.

Gold in all types of deposits is free and of high fineness: assays range from 920 to 960. Spectroscopic studies of gold particles from all ore types, carried out in the laboratory of Tomsk Polytechnic Institute by A. D. Glazunova, showed the presence of silver and copper in approximately equal amounts (up to 2%).

Thus, the data we have presented confirmed and provided additional evidence for the genetic connection of the gold mineralization with the granodiorite intrusion, while the frequent confinement of mineralization to ancient subplastic gabbro-diorite bodies is essentially spatial, determined by their favorable physicomechanical and chemical properties. Along these dioritoid bodies there arose numerous systems of tectonic fractures that proved accessible to auriferous solutions, while the increased iron content of these rocks probably accelerated the processes of gold precipitation. Consequently, the physicomechanical and chemical properties of the dioritoid rocks were decisive in shaping the fracture structure of the ore field, and subsequently the mineralization as well.

Fig. 3. Intersection of quartz ore veinlets (2) by teschenite (3); 1—diorite of the Solgon intrusion.

It may be assumed that the principal amount of gold was introduced by hydrothermal solutions from the common magmatic chamber that gave rise to the granodiorite pluton, following the formation of dikes of fine-grained diorites. It is not excluded that the solutions that took part in the formation of the gold deposits of the district could have borrowed part of the gold from the host rocks during their passage through them. Similar points of view have been discussed repeatedly in the literature [1, 8, 16, 17].

Thus, only those small dioritoid massifs of Kommunar that occur under favorable geostructural conditions prove to be gold-bearing. The discovery of small gabbro-diorite bodies in tectonically weakened zones near exposures of the granodiorite intrusion may serve as an additional criterion for gold-ore mineralization in the district.

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