Results of Geophysical Studies of the Baley Ore Field

I. P. Novikov

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

The Baley and Taseevo deposits are extremely unfavorable types of mineralization from the standpoint of their detection by geophysical prospecting methods.

Geophysical studies within the ore field have been carried out systematically since 1945 and, judging by the expenditures on their performance, have yielded insignificant geological effectiveness. The main reason for their low effectiveness was the incorrect formulation of the geological problem posed for them—direct searches for and tracing of ore bodies (quartz veins). The untenability of posing such a problem will be shown below.

The task of searching for new ore occurrences in the Baley region is still before the geological service at the present time. In beginning an analysis of the possibilities of geophysical methods in solving this problem, we shall dwell on the conditions under which it can be solved. Such conditions include, first of all, the conditions of occurrence of objects capable of producing geophysical anomalies: the dimensions and shapes of bodies, elements of occurrence, and the thickness of the overlying strata. Another group of conditions, also of decisive importance, relates to the differentiation of the physical properties of the objects that produce anomalies and of the enclosing rocks. No less important is also the persistence, in plan and in section (within a single 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 petrophysical features, at least for similar facies, is of substantial interest.

The ore bodies of the Baley deposit should be characterized as very thin, steeply dipping veins occurring in the magmatogenic rocks of the graben margins and confined to weakened tectonic crushing zones. The veins are satisfactorily persistent along strike, but they strongly change their thickness and configuration in sections across the strike (the presence of swellings, pinches, transitions into stringers, branching into veinlets, the presence of systems of closely spaced veins, satellites, apophyses; variable thickness and intensity of the silicification 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, specific resistivity—

...magnetism, radioactivity), as recent determinations of them have shown, the differences between veins and enclosing rocks in particular cases may be very significant. However, they are highly unstable, varying widely on both sides of equality, depending on the composition of the veins and the enclosing rocks. Detection of such veins is possible only with the aid of logging studies, petrophysical measurements, and, possibly, very high-precision large-scale surveys in areas where the thickness of the overlying strata is commensurate with the thickness of the ore body. The latter condition is fulfilled at the deposit only very rarely. In a geologic-geophysical section typical of Balei, the thickness of the overlying strata exceeds the thickness of the veins by a factor of 5 or more, which makes prospecting for these ore bodies by field geophysical methods impossible even in the case of the most favorable differentiation in physical properties (apart from the piezomodulus). Another reason hindering the search for veins at Balei is the extreme heterogeneity, in physical properties, of the strata that cover the outcrops of ore bodies (mainly alluvial and alluvial-deluvial deposits), associated with variability in their composition. The differentiation of properties of deposits of differing composition is commensurate with the differences in the properties of the veins and enclosing rocks, and at times 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 unconsolidated deposits often amount to tens and hundreds of percent over the first tens of meters, which particularly complicates the interpretation of electrical-prospecting and gravity-prospecting data. Finally, a very substantial difficulty in applying all types of geophysical methods at the deposit is the absence of a stable and typical pattern of anomalies marking the ore bodies.

The Taseevskoe-type mineralization differs from the Balei type in geophysical terms 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 even more complex than at Balei, since the veins, being highly unstable both in structure and in dip, are, in addition, strongly disrupted by post-ore and tectonic movements.

The thickness of the overlying strata at the Taseevskoe deposit is also much greater than at the Baleiskoe deposit. In all cases the veins prove to be covered by the alluvium of the Unda River, the thickness of which often reaches 50 m; moreover, a large part of the ore bodies does not come directly beneath the alluvial deposits—the mineralization along the ore-bearing fissures often dies out at depths on the order of the first tens of meters below the surface of the denudation cut of the conglomerates. Consequently, even for vein zones and silicification zones about 20 m thick, the thickness of the overlying strata exceeds the veins’ own transverse dimensions severalfold. Taking into account that such thick ore bodies occur at Taseevo very rarely, while usually the thickness of the veins does not exceed the first 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 ore bodies and surrounding rocks at Taseevo is also characterized by features analogous to those at Balei. Differences in the piezomoduli of vein quartz and enclosing rocks are apparently more clearly expressed. The variability, however, of the ratios of magnetism, density, radioactivity, and electrical conductivity between the material of the vein filling and the Cretaceous terrigenous rocks becomes still more significant owing to the facial variegation of the latter...

... them. 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; moreover, 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 borehole sections by geophysical methods, owing to the very great depth of occurrence and the unstable differentiation of the physical properties of the rocks.

An analysis of the extensive geological–geophysical material on Baley shows that, in prospecting by geophysical methods for deposits of the Baley and Taseevo types, only indirect data can be used—obtained by identifying ore-controlling and ore-hosting tectonic ruptures. The principal methods for detecting and tracing these structures are gravity prospecting and electrical prospecting.

According to electrical- and gravity-prospecting data, the Jurassic and Cretaceous sedimentary formations developed within the graben, in comparison with the rocks of its margins, are marked respectively by reduced and low values of resistivity and of the gravity field. At the boundary of the sedimentary formations and the Paleozoic granitoids, in most cases an increase in the gradient of resistivity and gravity is observed. Over almost the entire area of the graben, the magnitude of these gradients is several times less than on its margins.

On maps of resistivity and of the gravity field, as well as on geoelectric sections, the various lithological varieties of both sedimentary and magmatogenic rocks do not stand out to any appreciable degree. The only exceptions are the Paleozoic granites (the so-called borshchovka intrusion) and bodies of small intrusions developed on the southern periphery of the graben.

Within the area of the reduced resistivity field, a series of linearly elongated zones of relatively low resistivities is clearly distinguished. As a rule, the zones are not isolated in character, but usually are connected either with one another or with areas of the same values of apparent resistivity. The length of some zones is 1–2 km or more.

In the central part of the graben, at the junction of the linearly elongated zones, are located all the ore zones and veins of the Taseevo and Southern sectors of the Baley deposit that have been explored to date.

According to gravity-prospecting data, a gravity minimum spatially coincides with the Taseevo and Southern sectors of the Baley deposit, reaching its greatest intensity here. A similar gravity minimum occurs east of the deposit (the eastern block of the graben). Within the main part of the sedimentary sequence of the graben, the gravity field increases from the center of the graben in all directions and reaches maximum values on its margins.

In addition to local gravity anomalies (second-order anomalies), within the area under study third-order anomalies are outlined in very broad contours, having the form of linearly elongated zones. For reliable interpretation of these anomalies, and also in order to exclude from the observed gravity field the regional background, to obtain information on the structures of the sedimentary complex of the graben, and to compare the electrical-prospecting data with the gravity-prospecting data, which reflect geological features mainly in Mesozoic—

…formations, the gravity field was recalculated by us into the second derivatives of the gravitational potential.

The recalculation was carried out with the aid of an M-20 electronic digital computer.

The results of recalculating the gravity field into the field of second derivatives showed the following picture. Against the background of relatively calm fields, areas and zones of positive and negative values of the second derivatives stand out clearly. The fields that are negative in their form and dimensions differ sharply from the positive ones. For the most part these are linearly elongated zones of meridional, northeastern, and northwestern strike. The length of the zones amounts to hundreds of meters and reaches several kilometers. As with the zones of reduced resistivity, the zones of negative values of the second derivatives are usually conjugated with one another. One of the nodes of their intersection is spatially associated with the Taseevskoe gold-ore deposit.

The indicated decreases in the gravity field and the resistivity field within the Mesozoic formations of the graben are due to the following causes.

The mineralogical densities of the sedimentary rocks are close to one another. Changes in their density, both between different petrophysical groups and within individual groups, are caused mainly by changes in the porosity of the rocks. Sedimentary rocks are usually characterized by high porosity in comparison with other types of rocks, and their porosity varies within a fairly wide range. The porosity and density of rocks are interrelated and depend on the conditions of formation of the rocks, as well as on the intensity of such geological factors manifested in them as geostatic pressure, folding, oscillatory movements expressed in the formation of intensely crushed and fractured rocks, and physicochemical processes.

Analysis of the geological and geophysical materials available for the Baley district shows that, within the area of the graben, mainly the last two factors are manifested. Thus tectonic fracturing, crushing, and cataclasis of rocks, as already noted, are manifested everywhere over the area of the graben and its margins, and especially within deposits, individual ore occurrences, and areas with increased gold-quartz mineralization. Fracturing is manifested both in the presence of macrofractures and in the presence of microfractures. Only part of the fractures is filled with cement; the larger part of them is open, “gaping,” filled mainly with water and, to a small extent, with gas. There is also a large amount of evidence indicating that, in most such areas, the weathering crust developed selectively. The transformation of rocks in the weathering crust is usually characterized by a decrease in the density of these rocks caused by physicomechanical and geochemical processes. The former are characterized by strong crushing of the rocks, an increase in their fracturing and porosity, and intensified penetration of groundwater into fractures and pores. From the geochemical point of view, a well-developed weathering crust is characterized by the formation and broad development within it of dispersed-colloidal systems with a large specific surface, i.e., with high microporosity and low density.

The superposition and aggregate of the described geological factors so markedly caused a decrease in the density of the rocks that it became possible, by means of high-precision gravimetric surveying with subsequent transformation of its results, to map tectonic zones whose transverse dimensions are commensurate with the thickness of the overlying unconsolidated Quaternary deposits.