Characterization of the Balei Ore Field from a Geophysical Point of View

I. P. Novikov

(Presented by the scientific seminar of the Department of Geophysical Prospecting Methods)

Within the area of the Balei graben two large gold-ore deposits are localized—the Balei and the Taseevo deposits. Almost all types of geophysical investigations conducted over the area of the ore field are subordinated to the task of prospecting for and exploring mineralization of the Balei and Taseevo types. The only exceptions are revision work for uranium raw material and hydrogeological work, which in past years did not receive any substantial development and was connected with the investigation of water-supply sources. Therefore, among the most important geophysical features of the deposit, determining its type from the standpoint of the possibility of applying geophysical prospecting and exploration methods, are those characteristics that facilitate or hinder the solution of the following geological tasks:

1) lithological mapping of the sedimentary rocks of the graben in plan and section;

2) identification and tracing of tectonic dislocations both within the sedimentary rocks of the graben and in the magmatogenic formations of its frame and basement;

3) identification and tracing of ore bodies or at least geological structures favorable for the localization of mineralization;

4) age correlation of geological formations within the graben and its frame.

The conditions determining the possibility of solving these tasks include, first of all, the conditions of occurrence of objects capable of producing geophysical anomalies: the size and shape of the bodies, the elements of occurrence, and the thickness of the overlying strata. Another group of conditions, also of decisive importance, concerns the differentiation of the physical properties of the objects producing anomalies and of the enclosing and overlying rocks. No less important is the consistency in plan and in section (within the prospecting object) and the repeatability of the typical geometric and physical characteristics of these objects (from one object to another). Finally, the presence of stable correlation relationships between the age of geological formations and their petrographic features, at least for similar facies, is of substantial interest.

The ore bodies of the Balei deposit should be characterized as very low-thickness, steeply dipping quartz-vein lodes occurring in the magmatogenic rocks of the graben margins and confined to weakened tectonic zones of crushing. The veins are satisfactorily expressed along strike, but in cross-sections transverse to strike they strongly change their thickness and configuration (the presence of swellings, pinches, transitions into stringers, stringers into veinlets, the presence of a system of closely spaced veins, satellites, apophyses; variable thickness and intensity of the silicification shear zone). The veins differ consistently from the enclosing rocks only in the magnitude of the piezomodulus (by several times). With respect to other properties (magnetism, density, resistivity, radioactivity), the differences between the veins and the enclosing rocks may in individual cases be quite considerable, but they are very unstable, varying widely on both sides of equality, depending on the composition of the veins and enclosing rocks. Detection of such veins is possible only with the aid of logging investigations, petrophysical measurements, and, possibly, very high-precision large-scale surveys in areas where the thickness of the overburden is comparable with the thickness of the ore body. The latter condition is fulfilled at the deposit only very rarely. In a geological-geophysical section typical of Balei, the thickness of the overburden exceeds the thickness of the veins by 5 or more times, which makes prospecting for these ore bodies by field geophysical methods impossible even in the case of the most favorable differentiation in physical properties (except for the piezomodulus). Another reason that hampers the search for veins at Balei is the extreme heterogeneity in physical properties of the deposits covering the outcrops of the ore bodies (chiefly alluvial and eluvial-deluvial deposits), associated with variability in their composition. The differentiation of the properties of deposits of different composition is comparable with the differences in the properties of the veins and the enclosing rocks, and sometimes exceeds the latter; this is especially reflected in the results of electrical prospecting, magnetometry, and radioactive methods, as well as the PEEM method in its ground variant. Changes in the thickness of loose deposits often amount to tens and hundreds of percent within the first tens of meters, which especially complicates the interpretation of electrical-prospecting data and, in part, gravity-prospecting data. Finally, a very substantial difficulty in applying all types of geophysical methods is the absence at the deposit of a stable and typical pattern of anomalies marking the ore bodies.

Mineralization of the Taseevo type, from the geophysical point of view, differs from the Balei type mainly in that the veins at this deposit are localized in terrigenous sedimentary rocks. With respect to the configuration of the ore bodies, conditions at Taseevo are still more complex than at Balei, since the veins, being very unstable both along strike and down dip, are in addition strongly disrupted by post-ore tectonic movements.

The thickness of the overburden at the Taseevo deposit is also much greater than at the Balei deposit. In all cases the veins prove to be covered by alluvium of the Unda River, the thickness of which often reaches 50 m; moreover, a large part of the ore bodies does not extend directly beneath the alluvial deposits—the mineralization along ore-bearing fractures often dies out at depths on the order of the first tens of meters below the surface of the denudation cut of the conglomerates. As a result, even for vein zones and silicification zones with a thickness on the order of 10 m, the thickness of the overburden exceeds the actual transverse dimensions of the veins by several times. Taking into account that

such thick ore bodies are encountered at Taseevo very rarely, while usually the thickness of the veins does not exceed the first few meters; it should be stated that the deposit is characterized by a five- to tenfold excess of the thickness of the overlying strata over the thickness of the veins.

The differentiation of the properties of the ore bodies and the surrounding rocks at Taseevo is also characterized by features analogous to those at Balei. More distinctly, apparently, differences are manifested in the piezoelectric moduli of vein quartz and enclosing rocks. The variability in the ratio of magnetism, density, radioactivity, and electrical conductivity between the vein-filling material and the Cretaceous terrigenous rocks becomes still more significant as a result of the facies heterogeneity of the latter. In general, there is no need to speak of the stability of the forms of anomalies caused by Taseevo-type mineralization, since not one of the electrical-prospecting or magnetic anomalies known to us in the ore field can reliably be attributed to mineralization, and according to logging data the ore intervals are identified extremely indistinctly.

All this makes it possible to characterize Taseevo-type mineralization as almost inaccessible to direct prospecting from the surface and extremely difficult to detect in drill-hole sections by geophysical methods, owing to the very great depth of occurrence and the unstable differentiation of the physical properties of rocks and ores.

The possibility of prospecting for and tracing steeply dipping tectonic disturbances within the sedimentary rocks at the deposit is unclear, since the methodology of the geophysical work carried out in previous years did not meet the requirements posed by this task. It can only be asserted that this problem appears to be very complex, requiring the application of a complex of methods, and that its solution, if possible, is only for the largest objects.

First, attempts to solve this problem at present are possible only for sufficiently steeply dipping objects (dip angles of 45° and more), since the abundance at the Taseevo deposit of gently dipping boundaries of non-tectonic origin makes the detection of gently dipping disturbances almost impossible.

Second, in surface prospecting one should orient oneself toward detecting disturbances whose dimensions are commensurate with the thickness of the overlying strata. For disturbances of the types of faults and crush zones that are manifested only in the development of more intense fracturing of the disturbed segment, this requirement should be referred to the thickness of the zones themselves; for disturbances associated with displacements of blocks of rocks, to the amplitude of displacement. The detection of disturbances whose characteristic dimensions are 3–4 times smaller than the thickness of the strata overlying them is, under the conditions of the deposit described, a difficult task, although under more favorable conditions the solution of such a problem is entirely possible.

Third, it is possible to search only for disturbances whose origin was in one way or another associated with the emergence of substantial differentiation in the physical properties of geological formations. These include: disturbances that create tectonic contacts of rocks with different properties, significant in strike (in comparison with the depth of occurrence); disturbances along which post-tectonic mineralization develops, substantially altering the physical properties of the corresponding rock zones; and, finally, tectonic zones of crushing and fracturing, possessing increased water saturation and lower density.

In this respect, the different types of disjunctive structures characteristic of the deposit are in differing positions.

Ancient faults, which disrupt only the basement of the graben and the rocks of its margins, in our opinion, by themselves substantially change only the elastic properties of the rock and, in the central part of the graben, can be investigated only by seismic prospecting—the refracted-wave method—which is, of course, very difficult. At present the methodology for tracing them by this method has not been developed. In the near-flank parts they can be successfully investigated by electrical prospecting and gravity prospecting.

In the case where disturbances in the basement are the loci of dikes that differ substantially in magnetic properties from the enclosing rocks, one can apparently attempt to detect them by high-precision, large-scale magnetic surveying, even at depths on the order of a hundred meters.

Disjunctive disturbances that break the basement of the graben into uplifted and downthrown blocks can probably be mapped by gravity prospecting in those cases where the amplitude of the steps they produce reaches hundreds of meters. However, because the main density boundary, which conventionally marks the basement of the graben, generally does not coincide with the surface of the intrusive-rock relief, these data should be supplemented with the results of other methods, for example vertical sounding. It is also possible that magnetic prospecting and seismic prospecting can be used to solve this problem.

A less complex task is the search for and tracing of tectonic disturbances of the type of faults and zones of crushing that cut the thickness of Mesozoic and Tertiary sedimentary rocks, both those associated and those not associated with visible displacements of rock blocks in the wings.

F. A. Maiorov (1954) assumed that, by the VES method and electrical profiling, such zones would correspond to areas of increased electrical resistivity, since supposedly zones of hydrothermal silicification and freezing should greatly reduce their conductivity. This point of view seems highly debatable. Indeed, it is difficult to imagine that all the numerous fractures of these zones, whose thickness often exceeds the first tens of meters, would be completely healed by vein quartz. As for deep freezing of rocks along such zones, as G. N. Khnyrev (1959) indicated in his work, he did not observe this fact at all. Under these conditions it would be correct to state that the conductivity of such zones is determined by a complex relationship between their water saturation, silicification, and possibly freezing. As a result, in different parts of a zone they may prove to be either very good or relatively poor conductors. For these reasons, one cannot everywhere assume that the zones may have a typical pattern of electrical-prospecting anomalies that would make it possible to distinguish them confidently from other objects and to trace them. Objects producing analogous anomalies at the deposit may be rock areas whose different resistivity is due to natural differences in their porosity and water saturation. The anomaly pattern will also be greatly complicated and obscured by the influence of heterogeneities in the overlying strata.

Under these conditions, electrical prospecting cannot be considered a method for the reliable detection of such zones. Gravity prospecting, however, will be the most effective method.

The possibilities of other methods of exploration geophysics in this direction are limited. Changes in the magnetism of fractured rocks in comparison with massive ones are insufficient for identifying zones of fracturing by magnetic prospecting. Although it is quite probable that the formation of zones and post-tectonic processes caused changes in the magnetic properties of rocks subjected to crushing, there are no grounds to expect that these changes could have caused a transition of the rocks into a neighboring group of magnetism (according to Bersudskii). The weak differentiation of rock properties does not ensure the detection of tectonic zones where the overburden is of considerable thickness. If, for faults cutting granitoids within the margins of the graben and covered by overburden with thicknesses on the order of the first meters, the question of their detection by micromagnetic surveys can still be raised, then in the central part of the graben, with overburden thicknesses on the order of 50 m, solving this problem by magnetic survey is impossible.

The great thickness of loose deposits in the central part of the graben prevents the detection of faults by emanation survey.

The possibilities of solving this same problem by seismic exploration are unclear. There are grounds to expect (B. V. Rogachev, 1955) that the surface of terrigenous sedimentary rocks of Cretaceous and Tertiary age, as well as of volcanogenic sedimentary Jurassic rocks, represents a refracting surface. It is not excluded that, when conducting seismic exploration by the refracted-wave method, it will be possible to identify sectors where waves from this surface disappear, corresponding to zones of fractured rocks. However, the method of investigation has hardly been developed, and the very application of seismic exploration at deposits, as will be said below, is associated with very considerable difficulties.

Zones of crushing and disjunctives within the magmatogenic rocks of the graben margin and in the Jurassic sedimentary complex are apparently more favorable for study by geophysical methods, at least for areas where the thickness of loose deposits fluctuates within a few meters. The question can be raised of tracing them by magnetic survey, gravity prospecting, electrical profiling (for example, with a dipole array), emanation survey, and, finally, seismic exploration, provided that a high degree of detail and accuracy of such work is achieved, requiring the development of a special methodology. In particular, it is apparently possible to map the tectonic contacts of granitoids of different ages (the so-called Borshchovochnyi fault), as well as of Jurassic sedimentary-volcanogenic strata with granitoids. This task can be solved by the combined use of electrical profiling and magnetic-prospecting data with an accuracy sufficient for plotting these faults on maps at scales of 1:25,000 and even 1:10,000, without expending significant volumes of mining and drilling work.

Turning to the conditions that determine the possibility of applying geophysical methods for distinguishing individual lithological complexes, it should be stated that until recently the solution of this problem was not considered urgent, and little material has been accumulated for its practical solution.

All lithological boundaries characteristic of the deposit, from the geophysical point of view, should be divided into a number of groups, accessible to study to varying degrees.

Tectonic contacts are very widespread at the deposit. The possibility of studying them has already been discussed above. Here it should be added that the clearest of these boundaries (within—

because of the mixing of dimensions and the differentiation of the physical properties of rocks) are objects most favorable for study.

The lithological boundaries of magmatogenic formations in the framing of the graben, which are of a nontectonic character (contacts of intrusion, boundaries of differentiates of magmatic massifs, etc.), given favorable relations between the dimensions of the objects and the thickness of the overlying strata, as shown by the work of the geophysical exploration party of the Baleizoloto combine carried out outside the deposit area, can be mapped with sufficient confidence by magnetic surveying, electrical prospecting, and possibly even metallometry. However, solving this problem has not hitherto been regarded as important from the geological point of view, since no correlation dependence is observed between the lithology of magmatogenic massifs and mineralization.

In the central part of the graben, where the magmatogenic formations of its basement are hidden beneath thick strata of terrigenous rocks, outlining areas of rocks differing in physical properties can be carried out only from changes in boundary velocity, determined from seismic-prospecting data.

The boundaries of facies-distinct loose Quaternary formations are not of substantial interest in the search for mineralization. Loose deposits appear to be separable, according to electrical profiling data, into conducting and high-resistivity varieties, their conductivity being determined by the mutually opposite influences of water saturation and freezing. Reliably linking these facts directly with lithological composition appears difficult. It is possible that magnetic-survey data for areas of undisturbed occurrence of these rocks make it possible to divide loose deposits according to the quantity of magnetic fragments; however, even in this case it will be very difficult to make a lithological tie of the magnetic anomalies.

Of especially great importance at the deposit is the lithologic-facies subdivision of the Jurassic and Cretaceous terrigenous sequences. Their section is extremely variable both in the horizontal plane and vertically. Its subdivision by surface surveys, apparently, is possible only in very coarse detail, which is not of substantial interest at the present stage of study of the deposit.

F. A. Mayorov (1954) assumed that the identification of individual gently dipping boundaries in the section of Cretaceous rocks, distinguished by specific resistivity, is possible from VES-method data. However, there is no doubt that the quantitative interpretation of VES within the graben is greatly hampered by the presence of numerous vertical and steeply dipping boundaries. This gives grounds for doubting the reliability of F. A. Mayorov’s data and the possibility of solving the corresponding problems by electrical prospecting.

The conducting of a magnetic survey will apparently make it possible to judge qualitatively the change in the total thickness of the Cretaceous formations; however, the latter pertains rather to problems of structural mapping, since variations in the thickness of the deposits are determined mainly by the presence of basement highs and depressions.

In borehole investigations the situation for lithological subdivision of the section is much more favorable. The fact that, up to the present time, no method has been developed for identifying in borehole sections rocks differing in material composition and structural-textural features should be attributed only to the narrowness of the applied...

...of the logging operations complex and of the poor study of the petrophysical features of the deposit.

The conditions for carrying out logging investigations at Balei differ fundamentally from the conditions for conducting areal geophysical surveys. In logging it is possible to eliminate or, in any case, greatly reduce the effect of two most important factors that hinder the application of geophysical investigations at Balei.

The first of these is the considerable thickness of loose deposits covering the objects of prospecting. In borehole investigations the same role is generally played by the layer of drilling mud and the walls of the borehole tool, which separate the sensor of the measured quantity from the walls of the borehole. The shielding effect of the latter, of course, is much less than that of alluvium, whose thicknesses reach hundreds of meters. The heterogeneity of the intervening layer also ceases to play an essential role. All this makes it possible to use, for subdividing the lithological section, methods that are almost ineffective in the field variant: electrical, magnetic, radioactive, and nuclear methods.

The second factor—the variegation of the petrophysical features of the section—is also of little significance in borehole investigations. In field work the effect of this factor is expressed mainly in the impossibility of establishing the association of one or another type of anomaly with definite kinds of geological objects, since the total number of anomalies is usually small and they are located in geologically poorly studied areas. In logging the situation is directly the opposite. Examination of characteristic diagrams recorded at the deposit convinces one that the amount of information obtained as a result, for example, of logging an average borehole by the SP method, considerably exceeds the amount of information provided, for example, by electrical profiling along a profile of equal length. In other words, the number of confidently distinguished anomalies on an SP diagram is much greater than on an electrical-prospecting graph. In addition, it is always possible to carry out a sufficient number of logging measurements in boreholes with a well-studied geological section and to develop statistical criteria for assigning a definite aggregate of logging reference points to one or another geological object. The application of these criteria in the study of intervals with poor core recovery, and also for interborehole correlation of beds, will of course substantially increase the reliability of interpretation. We believe that borehole investigations make it possible, in addition to other objects, to distinguish quartz veins in borehole sections; however, determination of the useful component in them is hardly possible.

The possibility of age correlation of geological formations within the Balei graben on the basis of petrophysical data is not yet clear. For the present one can only state that at the present time certain facts have been revealed that speak in favor of the assumption that this problem can be solved on the basis of petromagnetic investigations. These include: the presence in some horizons of the Balei suite of remanent magnetization with an orientation different from the orientation of the present magnetic field; the presence in the Balei suite of reversely magnetized varieties of rocks; and, finally, the presence of specific features of magnetization (the ratio of remanent magnetization to induced magnetization, the values of magnetic susceptibility, and the orientation and intensity of remanent magnetization) characteristic only of certain types of magmatogenic and sedimentary rocks of the district. The first of these features—the preservation by some types of rocks of a magnetization different from the present one—makes it possible to raise the question...

The fact that some of them have preserved the orientation of the vector of residual magnetization that arose at the moment of rock formation makes it possible to carry out age correlation. The second feature—the presence of reversely magnetized rock varieties—is closely connected with the role of hypothetical inversions of the Earth’s magnetic field in Cretaceous time. Establishing a regular connection between reversely magnetized rocks and particular stratigraphic horizons would make it possible to carry out their narrow relative dating. Finally, the third feature—the specific magnetization characteristics of certain formations—may be brought into correspondence with the conditions of their origin. And if the latter, over the course of the history of formation of the area’s present geological aspect, were repeated sufficiently rarely, then these features can also be used for age correlation.

It must be emphasized, however, that the generally weak petrophysical study of the deposit does not permit one to state with confidence that at least one of these methods of correlation can definitely be used in practice. At present one should say only that a solution to this problem is not excluded, and many features of the deposit are favorable to it.

Summarizing all that has been said about the typical features of the deposit as a whole, the following should be stated:

1. The ore bodies of the Balei and Taseevka deposits are favorable for prospecting by field geophysical methods at scales of 1 : 2,000—1 : 10,000.

2. Problems of lithological mapping at the deposit can be satisfactorily solved only at a degree of detail corresponding to the requirements for geophysical maps and sections at a scale of 1 : 25,000—1 : 50,000.

3. The application of geophysical methods within the Balei graben is most effective in solving problems of tectonic mapping.

4. Almost all geological features of the deposit, for reliable determination, require in the overwhelming majority of cases the application of several methods differing in their physical basis.

5. In detailed prospecting work (scale 1 : 2,000 and larger), the only effective method for studying mineralization of this type is the electrical profiling method (in its underground variant); however, the quality of detailed prospecting work will suffer if geophysical methods for documenting sections (logging, petrophysical methods) are not applied.

LITERATURE

F. A. Maiorov. Summary report on the work of the Balei geophysical party for 1954. Archives of the “Baleizoloto” Combine.

N. V. Petrovskaya et al. Geological structure, mineralogy, and features of the genesis of the gold-ore deposits of the Balei ore field (Eastern Transbaikalia). Transactions of the TsNIGRI Institute, no. 45, 1961.

B. V. Rogachev. Seismic investigations in the Balei district, 1955 (report on topic 416 for 1954). NIGRIZoloto. Archives of the “Baleizoloto” Combine.

G. I. Khnyrev, O. A. Loginova. Report on the hydrogeological work of the GRE of the “Baleizoloto” Combine for 1954–59. Archives of the “Baleizoloto” Combine.