On the Methodology of Prospecting and Evaluation of Stockwork-Type Gold Deposits under the Conditions of Kommunar (Khakassia)

A. F. Korobeinikov

(Presented by Prof. A. M. Kuzmin)

The geological structure of the Kommunar district has been covered in the literature [1, 2, 6, 13, 14, and others]. Within the ore field there are exposures of rocks of the diabase-porphyrite and volcanogenic-sedimentary sequences, composing the eastern limb of a large anticline of NNE strike. The heterogeneous structure of both sequences determined a two-level structure; moreover, the upper structural level, composed of rocks of the volcanogenic-sedimentary sequence with subconcordant bodies of dioritoids, because of its lithological and mechanical properties proved to be productive for gold. The rocks of the district, gathered into folds, are broken by numerous large pre-ore steeply dipping faults of longitudinal and transverse types (with respect to the strike of the rocks). The spatial relationship of the faults determined the formation of the block tectonic structure of the ore field.

The enclosing sequences are cut by numerous small subconcordant bodies of uralitized dioritoids¹ of the Boi intrusive complex and by the large Solgon granodiorite pluton. It should be noted that the geological boundaries of the ore field have not been precisely established. It can only be said that it encompasses the area of distribution of the volcanogenic-sedimentary sequence; on the west it is bounded by the contact lines of the granodiorite massif and the diabase-porphyrite sequence, on the east by the contact of a substantially shale sequence, and on the north and south it is determined mainly by the distribution of small dioritoid bodies occurring in a large weakened zone of approximately meridional direction.

All known gold-ore deposits of Kommunar are located in the near-contact belt of the large Solgon granodiorite massif and are spatially confined chiefly to small bodies of uralitized diorites and gabbro-diorites, subordinate to the folded structure of the enclosing volcanogenic-sedimentary sequence, presumably of Middle Cambrian age [13]. In most cases the intrusive bodies have a linearly elongated, lens-like, and more rarely stock-like—

... in plan. Against the general background of approximately homogeneous enclosing rocks, these dioritoid bodies proved to be a very important structural element, which, by virtue of their rigidity, caused anisotropy of the deforming medium. The latter, to a certain extent, affected the manifestation of fracture tectonics, and this, in turn, influenced the distribution of gold mineralization within the ore field. The ore bodies are located either inside the diorite massifs or tend toward the contacts of the latter.

By their structural-morphological features, all known deposits of Kommunar fall into three main groups: 1) ore bodies formed in single tectonic fractures (vein type); 2) ore bodies formed in systems of closely spaced fractures (stockwork type)¹ and 3) ore bodies of combined type, including both quartz veins and the stockworks accompanying them.

The choice of prospecting and exploration methodology, in addition to the morphology and structure of the ore bodies, is substantially influenced by the material composition of the ores and the character of metal distribution in them [5]. Within the Kommunar gold-ore district, among the known deposits the following mineral associations are clearly distinguished (according to data from 70 ore bodies): 1) quartz-pyrite-arsenopyrite-pyrrhotite, 2) quartz-pyrite-chalcopyrite, 3) quartz-chalcopyrite-tetradymite, 4) quartz-chalcopyrite-sphalerite-galena with a poor gold content, and 5) quartz-carbonate with pyrite and sphalerite, containing no gold. The most widespread are the first two groups of mineral associations, which constitute up to 70–80% of the total number of ore bodies. According to the classification of N. V. Petrovskaya [6], two types of productive associations are clearly distinguished in the ores of Kommunar: 1) variably productive, including vein quartz of the second generation with pyrite and pyrrhotite, and 2) constantly productive, including the quartz-pyrite-chalcopyrite and quartz-pyrite-chalcopyrite-tetradymite associations with native gold. However, no clear connection of native gold with sulfides of the first types of mineral associations is observed; moreover, the sulfides themselves in the ore bodies do not exceed 0.2–1% of the total mineral composition, and only with copper-bismuth mineralization is a clear connection established. Often quartz veins and veinlets enriched in sulfides turn out to be poorer in gold than vein quartz devoid of sulfides. All this indicates a certain spatial independence in the distribution of sulfides and gold. The highest metal contents, as a rule, occur in the thinner quartz veinlets composed of fine- and medium-grained grayish-white quartz. The largest accumulations of gold aggregates mostly tend toward the selvage parts of quartz veinlets, or appear as separate nests, coatings, and veinlets along fractures in vein quartz and enclosing rocks. Native gold often penetrates along the finest cracks into the country rocks as well. In large quartz veins the distribution of gold is predominantly pocket-like, whereas in stockworks it is columnar. The metal content in veins and stockworks is highly uneven. The gold-enriched sections of quartz veins and stockworks—ore pockets and columns—have varied shape and size, and their location and configuration are directly dependent on the degree of tectonic fracturing of the enclosing rocks.

Summarizing personal observations and materials of previous studies [1, 2, 13, etc.], one may draw certain general conclusions about prospecting

¹ We also assign to the stockwork type of gold mineralization the skarn-magnetite lenses of the Kaliostrov deposit with superimposed gold mineralization.

criteria and indicators for ore gold: 1) the mineralization is genetically associated with a granodiorite intrusion and is located in the near-contact zones of the Solgon intrusive body; 2) among the enclosing strata of the district, the areas most favorable for localization of mineralization are those where a volcanogenic-sedimentary sequence is developed, with numerous small subconcordant bodies of amphibolitized dioritoids. These dioritoid bodies may be regarded as productive. A favorable factor is the heterogeneity of the composition of the enclosing sequence and the presence in it of horizons of massive rocks (porphyrites and their tuffs), interbedded with carbonaceous-siliceous schists, along which, during tectonic deformation, large interstratal fracture cracks arose, accommodating quartz-sulfide veins; 3) of decisive importance in the emplacement of ore bodies are large longitudinal faults, usually passing along linear dioritoid bodies, which determined the formation of systems of feathering tectonic fractures accommodating gold mineralization. The diorite bodies situated in the hanging walls of the indicated faults prove to be the most enriched in metal. Also favorable for the localization of mineralization are junctions of large longitudinal faults and sublatitudinal zones of tectonic fracturing. Among the contact-metasomatic formations, the ore-hosting bodies are skarn-magnetite lenses formed in zones of controlling longitudinal faults, in which, owing to repeated tectonic movements along the indicated disturbances, numerous feathering fractures arose that subsequently localized gold-ore mineralization; 4) the gentle contacts of the granodiorite pluton undoubtedly played a role both in the emplacement of metasomatic processes of a somewhat regional character and in the distribution of gold mineralization over the area. Probably the gentle contacts of the intrusive body are a criterion of no small importance for prospecting blind ore bodies and especially gold-bearing skarn-magnetite lenses; 5) of all the postmagmatic processes accompanying gold mineralization, silicification and actinolitization are the accompanying ones. Therefore, the presence in the rocks of products of the named processes may serve as a very reliable prospecting indicator for gold in the indicated district. A second prospecting indicator is secondary dispersion halos [10] surrounding the deposits.

On the basis of the indicated prospecting criteria and indicators, the following scheme is proposed for carrying out prospecting and evaluation work in the Kommunar district, namely: geological-structural mapping, the principal task of which is to establish the general geological structures and especially the large ore-controlling faults, usually of submeridional strike, associated with areas and zones of increased tectonic fracturing of dioritoid rocks. Study of the morphology of small intrusions, their fracturing, silicification, and actinolitization. On the basis of the study of fracture tectonics, maps of rock fracturing can be compiled which, taking account of other geological factors, will be predictive for the search for gold-ore bodies. In addition, it is necessary to re-examine all vein deposits of the district from the standpoint of finding stockworks, and especially those of them which are located in the near-contact belts of diorite massifs and horizons of effusive rocks, to which a number of researchers [2] drew attention in their time.

Taking into account the factors that the Kommunar gold-bearing district is situated in a covered area, where more than 80% of the surface is occupied by kurums and deluvial-alluvial formations, often reaching considerable thickness (from 2 to 7 m), at the first stage of prospecting-

...of the work, the most reliable prospecting method is boulder tracing, including piece sampling of deluvial fragments of vein quartz and of hydrothermally altered and mineralized rocks. These boulder trains often form secondary dispersion halos (mechanical dispersion halos according to A. P. Solovov [11]). A rather thick cover of deluvial deposits almost completely precludes observations on bedrock outcrops and necessitates the application of a number of measures for observing the deluvium. By means of boulder tracing, areas of gold mineralization are contoured which are subsequently subjected to detailed evaluation work. In contouring them, the main attention is paid to determining the direction and distance of transport of the fragmental material down the slopes of the bald mountains, taking into account the nature of the relief. Boulder tracing is carried out during the period of route traverses over the area under study. The direction of the routes is chosen mainly across the dominant strike of the geological structures, with the first routes being laid out at the foot of the slopes, which makes it possible to give an initial assessment of the prospects of a given area. During the period of route traverses, much attention must be paid to examining eluvial-deluvial deposits in order to identify enrichment of a particular area with skarn-magnetite, silicified, actinolitized, and sulfidized formations associated with gold mineralization. In doing so it is necessary to clearly distinguish in the field vein quartz of different generations, since the productive quartz is generally grayish-white, medium-grained quartz of the second generation.

Boulder tracing is accompanied by panning sampling for gold. Panning sampling, as experience in the district shows, gives positive results, especially in areas where ore bodies with relatively coarse native gold are developed (with sizes of 0.05–0.5 mm). Panned-concentrate samples are taken from test pits 0.5–1 m deep [3, 9] and from all surface workings driven during the survey. From shafts, one channel sample is taken from the deluvium for the entire depth of the shaft, the dimensions of the channel not exceeding 10×20×200 cm [4, 9]. In the case of considerable overburden (more than 3 m) or the presence of a layer of kurums, samples are taken from the deluvial material of the near-bedrock part of the shaft. First of all, areas where zones of hydrothermal alteration and skarn formation are developed, as well as intrusions of diorites, gabbro-diorites, etc., are subjected to panning sampling. The first lines of test pits, from which panned-concentrate samples are taken, are laid in the deluvium in accordance with the relief, i.e., at the foot of slopes, usually along the valleys of streams and gullies. Initially two lines are run, with a distance of 100 m between them and 40 m between pits. During detailing, the distance between pits is reduced to 20–10 m. The sample size is 0.02 m³ (the capacity of two trays). All stages of work in panning sampling are set out in greater detail in a number of manuals [3, 4, 9, 10, 11]. The samples are washed to a gray concentrate; from the material obtained, after it has been examined for gold, a sample is taken for gold spectrochemical analysis.

At the same time, channel samples (3×5×100 cm in size) are also taken from outcrops of hydrothermally mineralized rocks, as well as from surface workings that have exposed dioritoids. The position of the channels depends on the direction of veining and generally has a NNW direction.

As the first experimental work of gold spectrometric surveying, carried out by the geophysical party of the Krasnoyarsk Geological Administration in 1960 in the Kommunar district and yielding...

positive results, especially in areas of gold-bearing skarn-magnetite lenses, it should be widely applied. This survey successfully supplements panning and substantially increases the effectiveness of prospecting and exploration work [7, 8].

Taking into account the fact that the gold deposits of Kommunar, at least in significant quantities, contain associations of heavy-metal sulfides, it is possible to recommend the application of the hydrogeochemical prospecting method, which has been used quite successfully in a number of regions [12]. Hydrogeochemical sampling should include the following set of components accompanying gold mineralization: As, Sb, Ag, Cu, Bi, Te, Mo, Pb. This method should undoubtedly yield results in areas where major tectonic zones are developed and where groundwater discharges to the surface are observed.

Evaluation work on the areas of discovered ore occurrences constitutes the second stage and includes both surface mining work and the drilling of shallow inclined and horizontal holes with small-size rigs. In carrying out surface exploration work, great attention should be paid to the study of fracture structures, the character of vein filling of fracture zones, the distribution of the useful component, etc. The principal method of evaluation is channel sampling of outcrops, trenches, and prospect pits. The placement of surface workings and of channel samples in them and in outcrops takes into account the complex of geological-structural conditions of localization of mineralization and especially the direction of quartz veinlet development at each ore occurrence individually. However, as experience in the district shows, the most rational placement of surface workings is along lines in a south-southwest direction, approximately across the strike of the dominant structures, whose position is refined after the first data are obtained. The distance between lines should not exceed 20–40 m and depends on the scale of mineralization and the smallest dimensions of ore bodies. The placement of channel samples in workings and outcrops, in the general case, is north-northwest, approximately across the strike of the dominant quartz-ore veinlet development. In evaluating ore occurrences at the surface, trenches should be preferred to prospect pits. However, at the first stage of work, lines of prospect pits should be sunk (at intervals of 10–20 m), which, if necessary, are connected into main trenches. In evaluating ore occurrences at depth, it is recommended to drive series of horizontal and inclined holes using small-diamond drilling rigs, which is especially favorable on sections of steep slopes that are often covered by a thick (up to 3–5 m) layer of kurums. To set up the rigs, small adits (“set-ins”) up to 5–7 m long are driven through the full thickness of the deluvial deposits. In many parts of the ore field (the eastern slope of the Saktychul, Podzvezdny, and Podoblachny barren summits), owing to the presence of significant areas of kurums, this method is the only acceptable one for prospecting and evaluating ore sections. From one “set-in” a fan of holes is drilled. The first horizon of holes is 20 m below the surface level. If one takes into account that the core of the holes is just as representative as a sampling channel with a cross section of 3 × 5 cm, provided that all core material goes into the samples and the core recovery is not less than 70%, then the data obtained from drilling will be quite sufficient for the final evaluation of the identified ore occurrences. All the more so because sampling data for stockwork ores from the core of short-meterage holes and from mine workings show fairly good agreement in analytical results. Thus, when carrying out prospecting and evaluation work in areas with sharply dissected relief, geomorpholo-

graphic factors, such as the steepness of the slope and the degree of their talus cover, the latter sometimes making it impossible to carry out shaft and trench work because of the sliding of the talus. In this case, the use of small-diameter diamond drilling with compact rigs is the most rational.

Proceeding from the conditions of localization, the morphological features of stockwork ores, and the experience of exploration work at the Kommunar deposits, it is recommended that drilling operations be conducted on a broad front. First of all, boreholes should be laid out along lines following productive subplutonic diorite massifs, in order to discover blind ore bodies and to evaluate known deposits at depth. From this point of view, drilling should be carried out at known vein deposits occurring in the near-contact parts of dioritoid bodies and now worked out or possessing noncommercial reserves, with the aim of discovering possible stockworks accompanying them. In this case, the main direction of prospecting and exploration work is determined by large ore-controlling faults of approximately meridional trend. It is necessary to evaluate all areas of dioritoid bodies that are, to one degree or another, spatially connected with these disturbances. In this case, borehole lines are placed along the controlling faults in areas where massive and brittle rocks of the type of diorites, diabases, and porphyrites are developed; the distance between boreholes at the first stage of work should be no less than 200 m. Such an orientation will sharply reduce the volume of drilling work and will give the most tangible results in the search for blind ore bodies. The depth of the boreholes averages 200 m and depends on the depth of occurrence of productive horizons—dioritoids—as well as on the upper boundary of the northeastern contact of the granodiorite pluton.

To clarify and trace large faults, a complex of geophysical investigations should be applied, and first of all emanation surveying, which has shown positive results in experimental tracing of large fault zones.

In addition to purely prospecting-and-exploration boreholes, it is necessary to drill separate structural-prospecting boreholes 400–500 m deep along the eastern boundary of the ore field. Their drilling is dictated by the need to clarify the general geological structure of the ore field and to refine the position of the lower boundary of the productive volcanogenic-sedimentary sequence, which constitutes the upper structural stage of the ore field.

Thus, prospecting work in areas where stockwork mineralization is distributed includes a complex of geological-structural investigations, scintillometry, and heavy-mineral sampling in combination with gold-spectrochemical and hydrogeochemical methods. Evaluation work on discovered ore occurrences and blind ore bodies presupposes surface sampling and the drilling of series of short horizontal and inclined boreholes.

REFERENCES

1. A. Ya. Bulyannikov. Gold-ore formations and gold-bearing provinces of the Altai-Sayan mountain system. Proceedings of the Tomsk State University, vol. 102, Tomsk State University Publishing House, 1948.
2. D. I. Kalinnikov. The Podlunnyi Golets—the base of Bolshoi Kommunar. Collection of Works of Zolotorazvedka, issue 2, 1936.
3. B. M. Kosov and N. M. Ostromenetskii. Tin. Evaluation of deposits during prospecting and exploration, issue 2, Gosgeoltekhizdat, 1949.
4. S. A. Muzylev et al. Methodological guide to geological surveying and prospecting. Proceedings of VSEGEI, Gosgeoltekhizdat, 1954.

1. N. V. Petrovskaya. The significance of productive mineral associations for prospecting and exploration of gold-ore deposits. Exploration and Protection of Mineral Resources, No. 2, 1955.

2. V. A. Obruchev. Report on the geological examination of the Bogomdarovannyi mine of the Ivanitskys in the Achinsk Mining District. Selected Works, vol. 2. Publishing House of the Academy of Sciences of the USSR, 1963.

3. N. I. Safronov et al. Spectro-gold-metric surveying as a method for prospecting gold-ore deposits not accompanied by halos (dispersion trains). Publishing House of VITR, Experience Exchange Series, issue 1, 1957.

4. N. I. Safronov et al. Experimental-methodological work on gold-metric surveying in Eastern Transbaikalia. Soviet Geology, No. 7, 1958.

5. A. P. Sigov. Heavy-mineral prospecting. Proceedings of the Ural Scientific Research Institute for Geological Exploration and Investigation of Mineral Raw Materials, issue 4, 1939.

6. V. I. Smirnov. Geological Principles of Prospecting and Exploration of Ore Deposits. Moscow State University Press, 1957.

7. A. P. Solovov. Provisional instructions for metallometric surveying. 1951.

8. P. A. Udodov, I. P. Onufrienok. Experience in hydrochemical investigations in the territory of the mountain massifs of Western Siberia. In: Geochemical Prospecting for Ore Deposits in the USSR. Proceedings of the First All-Union Conference on Geochemical Methods of Prospecting for Ore Deposits. Gosgeoltekhizdat, 1957.

9. N. A. Fogelman, A. E. Shabalovskii. Conditions of localization of stockwork deposits within the Kommunar ore field, on the eastern slope of the Kuznetsk Alatau. Proceedings of NIGRIZoloto, No. 21, 1956.

10. Ya. S. Edelshtein. Preliminary report on geological investigations carried out in the Achinsk gold-bearing district, 1907. Geological Investigations in the Gold-Bearing Region of Siberia, issue VII, St. Petersburg, 1909.

NOTED MISPRINTS

1. The author, unlike P. A. Fogelman [13], adheres to the viewpoint of A. Ya. Bulynnikov [1] and D. I. Kalinnikov [2] that the dioritoid bodies are intrusive formations. ↩