NEW DATA ON THE GEOLOGICAL STRUCTURE AND PROSPECTS OF THE ABAKAN IRON ORE DEPOSIT

A. R. LEVERTOV

(Presented by Professor A. M. Kuzmin)

During 1968–1970, the area of the ore field of the Abakan iron ore deposit underwent a fundamental reassessment by the method of structural geological mapping. A new map was compiled, on the basis of which it became possible to predict and identify, by subsequent drilling, new ore bodies, thereby expanding the ore base of the deposit being mined. The work was carried out under the supervision of the author of the present article—a postgraduate student of Professor A. M. Kuzmin.

1. On the history of the formulation of the investigations

The first information about the Abakan iron ore deposit, located on the northeastern spurs of the Kirsinsky Range in the Western Sayan, appeared as early as the middle of the last century, and many researchers made a major contribution to the study of this interesting object. Three times in the period from 1931 to 1958 the deposit was explored by means of core-drilling boreholes and mine workings, and since 1957 it has been operated by the Abakan Iron Mine of the Kuznetsk Metallurgical Combine. The final reserves of the deposit, amounting to 86 million tons of magnetite iron ore, were approved by the State Reserves Commission in 1958. The most recent data on the geological structure and prospects of the Abakan deposit are set forth in the reports of G. G. Savitsky (1955) and A. I. Khodonia (1958). These researchers distinguish two ore bodies at the deposit: the Main Deposit and the Third Ore Body, which occur within the sequence of volcanogenic-sedimentary rocks of the Upper Monok Suite and were formed by contact-metasomatic replacement of marly sandstones and clay shales. These two ore bodies were explored in great detail from an absolute elevation of +690 m to a depth of +165 m, and along strike for somewhat more than one kilometer. On the basis of exploration data and magnetometric surveys, the conclusion was drawn that further study of the ore field was entirely without prospects, since the reserves of the deposit are bounded by the explored limits both along strike and at depth.

At the same time, an opinion on the prospects of the Abakan deposit completely opposite to that of the above-mentioned authors was expressed in 1956 by Professor of the Tomsk Polytechnic Institute A. M. Kuz—

...who indicated the probability of discovering unknown ore masses in the deep horizons of the deposit and proposed testing this prediction by drilling deep boreholes.

Unfortunately, the West Siberian Geological Administration did not treat Professor A. M. Kuzmin’s scientific forecasts with due attention, and the re-evaluation of the ore field was thus delayed by more than a decade.

2. New Data on the Geological Structure of the Deposit

The Abakan iron-ore deposit is located on the northeastern limb of the so-called Khansyn anticline, composed of ancient Lower and Middle Cambrian volcanogenic and tuffaceous-sedimentary formations, bordered on the north and east by a porphyritic fringe of the Lower Devonian. The sequence of ancient rocks extends consistently to the northeast at an azimuth of 10–30°, with steep bedding dips of 65–90° to the southeast. With the overall monoclinal attitude of the rocks, complications of folding are quite clearly observable; and in the area where the rocks of the Upper Monok and Arbat formations are distributed, dips of the beds in the opposite direction, i.e., to the northwest, are not uncommon.

The oldest rocks in the area studied are the volcanic formations of the Lower Monok Formation of Lower Cambrian age. Among the rocks of this formation, which makes up the central part of the anticline, dark-green and gray-green spilites, their clastolavas and tuffs, as well as greenish-gray keratophyres and quartz keratophyres with holocrystalline and cryptocrystalline groundmass are distinguished. A definite regularity is observed in the distribution of these rock varieties: closer to the center, spilites are more widely developed, while along the periphery—keratophyres. Within this volcanic sequence of rocks, rare thin interbeds of dark-gray to black lydite and dark-red jasper are noted.

The lithological-petrographic composition of the overlying Upper Monok Formation, in contrast to the Lower Monok, is characterized by the predominant development of a tuffaceous-sedimentary complex of rocks. Here, in the course of geological mapping, we established agglomerate tuffs of andesite-dacite composition, lapilli-bomb tuffs of intermediate composition, tuff conglomerates, tuffaceous sandstones with subordinate horizons of conglomerates, sandstones, marly sandstones, siltstones, and limestones. This formation is ore-bearing and is being subjected to especially careful study. We succeeded in subdividing the rocks of the Upper Monok Formation into seven fairly persistent members and horizons, among which there are distinguished a horizon of agglomerate tuffs, a horizon of light-gray marbled limestones, and a member of lilac-gray siltstones and medium- and fine-grained sandstones with rare interbeds of limestone. These three horizons of tuffaceous-sedimentary rocks, persistently in the form of two bands each 150–200 m thick, are traced quite clearly in a submeridional direction for more than three kilometers.

It was noted that precisely these lithological varieties of rocks, under conditions of intense pre-ore crushing, are most readily subjected to ore replacement, with the formation of industrial ore bodies. Thus, the main ore deposit is located in the western belt of development of these rocks, while the third ore body is in the eastern belt.

The section of ancient rocks is completed by essentially sedimentary formations of the Arbat Series of Middle Cambrian age, represented...

pinkish-gray conglomerates, greenish-gray gravelites, and interbedded gray sandstones and siltstones. A general tendency is observed for the size of the clasts of the terrigenous material of this suite to decrease from bottom to top. Within the sandstones and siltstones of the Arbaty Series are concentrated the noncommercial ore bodies of the so-called Alekseevka deposit, the detailed study of which we have planned for the future.

The intrusive formations of the ore field are represented by two bodies of different ages: the Keni intrusion of pyroxene diorites of Lower Cambrian age and the Abaza intrusion of albitized plagiogranites of post-Lower Devonian age, together with their numerous dike derivatives. Both intrusive bodies are exposed at some distance from the ore bodies (the first to the west, the second to the east), and most researchers associate the formation of the ore bodies of the Abakan deposit with the emplacement and development of these intrusions.

The disjunctive tectonics of the ore field is very complex. There is no doubt that the ore process was preceded by very extensive pre-ore crushing, which involved almost the entire range of rocks of the Upper Nemona Suite. This is confirmed by the broad manifestation of cataclasis and mylonitization when approaching the ore bodies.

We are inclined to connect the post-ore crushing of the rocks, which created the block structure of the ore field, with the manifestation, at the boundary of the Cambrian and Devonian rocks, of a rather strong fault that is traceable in the mine quarry, as well as in boreholes and trenches along the left bank of the Rudnaya Kinya River. The dip of the fault plane of this dislocation is rather steep, 75–80° to the northwest. The existence of this fault is associated with a whole network of other dislocations, which completely or partially cut the ore bodies.

The study carried out of the ore field of the Abakan deposit has led us to the following conclusions:

1. Within the area of the ore field there are at least two ore zones (structures): western and eastern, composed of agglomeratic tuffs, sandstones, siltstones, and limestones, which are most readily subject to ore replacement.

2. These ore-bearing structures extend persistently along strike and dip, and do not pinch out, as had been believed on the basis of previous studies.

3. The process of ore formation is preceded by pre-ore crushing of agglomeratic tuffs, sandstones, and limestones, and the ore-formation process itself is accompanied by intense chloritization, actinolitization, epidotization, albitization, and silicification of the replaced rocks. Since all the factors listed above do not disappear with depth at the Abakan deposit, the mineralization must extend to great depths, and the ore bodies of the deposit cannot be exhausted by those previously known.

3. Expansion of the Prospects of the Deposit

The very first boreholes drilled to verify the conclusions reached uncovered, at deep levels of the Abakan deposit, new ore bodies of considerable thickness and with a high iron content.

The eastern ore-bearing structure was drilled by borehole No. 3 on exploration line 21 and is currently continuing to be drilled by borehole No. 12 on exploration line 27, located 200 m north of line 21.

exploration line, and by borehole No. 16 in exploration line 14, located 200 m south of the 21st exploration line. A positive result was obtained in all three drilled boreholes. Thus, borehole No. 3 intersected a new ore body in the interval 612–715 m; borehole No. 12 traced the same body northward in the interval 855–913 m (the borehole is still being drilled and has not yet emerged from the ore), while borehole No. 16, according to interim magnetic logging data, is approaching the ore body and should enter it at a depth of 850 m.

Thus, in the eastern ore-bearing structure, at absolute depths from +50 to −200 m, we have identified and traced over a strike length of 400 m a new ore body with an average thickness of 50–70 m and an average iron content of 43.2%.

The western ore-bearing structure was drilled by two deep boreholes: No. 11 in the alignment of the 21st exploration line and No. 15 along the 27th exploration line, located 200 m north of the 21st.

Borehole No. 11 drilled through an ore interval 224 linear m long, from a depth of 612 to 836 m, while borehole No. 15 entered the same ore body at a depth of 832 m. The geological data available to us show that in the western ore-bearing structure the ore body, as in the eastern one, has its upper edge at absolute elevations of +50 m, but the body itself is larger (average thickness 100–120 m) and has a higher iron content: on average 46.7%.

The set of drilling operations carried out allows us to conclude with confidence that, at the deep levels of the Abakan deposit, there are ore bodies whose reserves are comparable with the reserves of ores explored at the upper levels and may possibly exceed them. These, in general terms, are the new geological data and prospects of the Abakan iron-ore deposit based on the results of the latest work in 1968–70.