On the Methodology of Prospecting for Mineral Deposits

K. V. Radugin

The purpose of this communication is to acquaint readers with a new auxiliary method for prospecting for deposits that are difficult to discover¹. This method, as far as is known, is new, and it is not mentioned in the relevant modern literature \[1, 2, 3, 4, etc.\].

The denudation surface passes, in a certain sense, at a random level with respect to mineral deposits and the bodies that compose them: some of them have been exposed and, to one degree or another, destroyed; others lie tens and hundreds of meters below the loose cover and remain unknown for the time being. How to discover these mineral riches hidden from us constitutes the content of the branch now developing as deep geology.

How great the practical significance is of successfully solving the problem of correct, scientifically substantiated prediction at depth, even only in the area of already known deposits, is shown by the following simple reasoning. Let us suppose that the deposits of interest to us persist well in their extension to depth. But the idea is quite correct and scientifically grounded that if the explored and assessed reserves of mineral raw materials of the USSR are now known down to a certain (and, in different places, different) depth—the reserves of the “first story” of the USSR—then to increase these reserves by 2–3 times means to explore the 2nd and 3rd stories of the USSR’s subsurface. This is precisely what determines the practical and theoretical value of deep geology, which also includes the methodology of prospecting for deposits not exposed beneath overburden. The task of prospecting for them is difficult, but noble. This article is devoted to solving a small part of this task, revealing new paths in this matter.

The theoretical basis and essence of the proposed methodology is simple and indisputable: to use the endogenous dispersion of the substance of a mineral resource in order to discover its unexposed bodies. Thus, in principle, it is the same as in other methods based on the study of dispersion of the substance of mineral resources. But in the proposed methodology the matter concerns a distinctive kind of dispersion—dispersion in a magmatic medium, chiefly in dikes. On the Earth there is a widely manifested series of geological processes that cause the movement of matter against the action of gravity. Such processes include the intrusion and outpouring of magma generated in deep-seated sources. To this same group belong—

…of the movement of liquid and gaseous emanations from magmatic bodies and sources into regions of lesser depths and toward the surface.

If gaseous or liquid solutions move independently of intrusions (for example, as a result of regional metamorphism, or, in general, as a result of the inorganic existence of the Earth), they too must move in the thermodynamic field of the planet from regions of high pressures and temperatures into regions of low temperatures and pressures. Even surface vadose waters that have penetrated to some depth can move from below upward against the action of gravity; they are forced to rise by that same force of gravity according to the “law of communicating vessels.”

In the present article the author has for the time being confined himself to considering the movement of magmatic melts and their interaction with the enclosing masses in the form of dikes. In this process the magma, cutting across a deposit along zones of fracturing, partly captured fragments of the useful mineral, partly assimilated a number of elements¹ of this mineral—and carried all that it had captured into higher horizons of the earth’s crust. If denudation exposes magmatic dikes, then the parts of the deposit captured by it may be discovered:

a) in xenoliths and in the heavy-mineral concentrates obtained from them; b) by comparative spectral and chemical analysis of endocontact assimilation zones and of areas outside these zones; c) by diagenetic coloration resulting from the precipitation, from a supersaturated solid solution, of an excess—for example, by red coloration from hematite; d) in veins and in disseminated colored minerals, i.e., in the products of the inorganic life of assimilation zones enriched in elements of the mineral; e) in local weathering products, especially if they are clearly expressed; f) by comparative measurements, which may reveal the magnetism of rocks of the assimilation zone or its increased radioactivity, higher specific gravity, etc.; g) in some cases by applying luminescent analysis (to detect, for example, scheelite) and X-ray analysis (to detect diamonds), as well as the heavy-mineral concentrate method.

Let us consider methods for all these cases, and first of all the method associated with the search for and study of xenoliths of the useful mineral and of its indicators. Indeed, if the investigator of a given ore field finds a true xenolith of an already known ore, the quality of which, incidentally, can be determined from the ore xenolith, he obtains a very valuable prospecting indication, similar to a boulder of ore in a river. Of course, both this boulder and the ore xenolith do not yet prove the presence of an industrial deposit. But our method is not a method for evaluating the quality and quantity of ores—although it is closely connected with this—but merely a prospecting method, which nevertheless, in combination with other methods, can render a decisive service and at times be the very main condition for a discovery that we have lacked in many cases of difficult searches for ores at depth.

Naturally, prospecting value is possessed not only by xenoliths of the useful mineral (for example, of sedimentary ore or of the gold-bearing quartz vein of the Saralinskoye deposit), but also by fragments of a constant or obligatory indicator of the useful mineral, whether a characteristic rock or a mineral. If this connection is inseparable, then the finding of the indicator obliges one to make a scientifically grounded prediction, to test this prediction by geophysical methods, and then to carry out prospecting drilling.

Thus, persistent searches for all kinds of xenoliths, especially ore xenoliths and indicators in the endocontact zones of post-ore bodies, must become

into the practice of geological investigations, first of all in known ore fields, for the discovery of still unknown “blind” bodies.

We pass to the second part of the proposed methodology: the search for deep-seated mineral resources by studying the same endocontact zones, but those no longer containing xenoliths. In this latter case¹ the useful mineral is strongly dispersed. Sometimes it is still noticeable in the coloration of the endocontact zones; more often it does not manifest itself visually. In such a case spectral and chemical analyses of the endocontact zones should be widely used, i.e., precisely those methods that are capable of detecting the most insignificant concentrations of the mineral substance. This is necessary because even industrial concentrations of valuable, rare, dispersed elements may be very small, and in the assimilation zone the concentration decreases sharply. Such circumstances make it difficult to discover very deep deposits. However, even negligible detectable concentrations may, to a certain extent, testify to a possibly industrial concentration of the sought elements in ore-bearing rocks at depth.

Let us give a concrete example of what occurs in an endocontact zone (without xenoliths) as a result of assimilation of ore substance. One very large deposit of carbonate ores of manganese of the okolorifovaya facies is cut by post-ore diabase dikes. In the central part of the dikes the manganese content is ordinary, close to the average manganese content in diabases for the Earth. But in the endocontact zone this content is higher and reaches 6–7% manganese. Incidentally, let us note that here even externally, from the reddish coloration due to anhydrous iron oxide, a considerable assimilation of ore is felt, the ore containing, besides manganese, a certain amount of iron compounds. The diabase, apparently, was saturated with iron, and if the magma was able to dissolve an additional quantity of it, then from the supersaturated solid solution formed after its solidification the excess iron oxide ultimately precipitated in the form of a finely dispersed diagenetic impurity.

The cited example of comparative analysis is taken from the ore body itself. Beyond its limits, in the case where diabase intersects the ore at depth, the picture does not change in principle². Only the content of the sought elements will be lower (other conditions being equal), since the process of dispersion will have had time to go further.

It is clear that the recommended method, if one has in mind spectral analysis without quantitative evaluation, is not applicable for discovering deposits of widespread elements that enter into all or almost all rocks—for example, deposits of iron, phosphorus, titanium, and aluminum. However, chemical analysis (and under certain conditions spectral analysis as well) is capable, in these cases too, of establishing an abnormally high (in comparison with Clarke) content of a widespread element, not characteristic of the given rock. This may lead to the discovery of deposits of iron, manganese, etc.

Thus, the systematic geochemical study of assimilation zones of post-ore dikes and, in general, of magmatic bodies younger than

a useful mineral, must enter into the practice of geological prospecting and geological exploration work.

There is no doubt that this method will make geophysical and drilling operations aimed at the exploration of industrial reserves of mineral raw materials far more effective and purposeful. This method is an essentially necessary element of scientifically grounded prediction at depth.

Next we shall note a third aspect of the proposed methodology for studying endocontact zones for prospecting purposes. This method is based on taking account of the subsequent development of endocontact zones. If the useful mineral element occurs in the endocontact zone in small quantity, it enters into the minerals of the given post-ore vein as an isomorphous or inconspicuous mechanical admixture. It will be just as inconspicuous in aphanites and felsites, i.e., in diagenetized rocks that were once volcanic glass.

But then there begin and proceed, one after another, phases of the evolutionary and stepwise development of the rock. From the solid solution a fine dusting of metal may precipitate and color the endocontact zone. This was the case in the example cited above of the endocontact zone of diabases, reddened by finely disseminated hematite.

Another form of the inorganic life of a rock is the movement of solutions and their redeposition (in fissures or in disseminated form, i.e., already in a visible form) of ore minerals at the expense of previously inconspicuous elements. Thus, in the endocontact zone noted above, such a large quantity of manganese was assimilated that an environment so saturated with it resulted that the direction of changes in the subsequent inorganic life was quite definite: these processes created disseminated manganese minerals and veins with manganese minerals. Among the disseminated minerals in the endocontact zone of the diabases, spessartites, rhodonite, and bustamite were identified. Manganese, previously invisible, now appears before us in the endocontact zone in visible and tangible form. Such is the fourth aspect of the methodology under consideration.

There is perhaps no need to speak of the fifth and sixth varieties of the methodology of using magmatic minerals and weathering products of assimilation zones for prospecting. But one should dwell on the magnetometric study of assimilation zones.

Thus, in studying the assimilation zones on Mt. Topkhan in the Kuznetsk Alatau with the aid of Logachev’s instrument, an anomaly was detected in the air identical to that over the large Tashtagol deposit. Meanwhile, on Mt. Topkhan there is no magnetite at the surface, or almost none. The metasomatism characteristic of the magnetite deposits of Shoria, however, is widely developed there. On Mt. Topkhan there are dikes of labradorite porphyrite, in the endocontact zone of which this rock is so magnetic that individual specimens exert a very strong influence on the compass needle, although not even a grain of magnetite can be seen in them under a magnifying glass. If the compass is moved along the vertical contact plane of the dike, then 25–50 cm upward, downward, or to the side, the magnetic needle turns through 360°. If one moves away from the dike by several paces, one enters an almost normal (?) terrestrial field. Specimens from the center of the dike do not affect the compass. Chemical analysis of specimens from the marginal zone of the dike showed an increased iron content in comparison with its central part. The author explains the increased iron content in the marginal part of the porphyrite dike, as compared with the central part, by the formation of an endocontact assimilation zone enriched in dispersed magnetite, captured by the magma as it passed through the ore body. The cause of the large-

of the magnetic anomaly detected by an aircraft are, of course, not these fields, but the deeply lying mass of magnetite ores, whose assimilation created the endocontact zones described above.

Some practical procedures associated with the method described are as follows. If a solid ore body that ends blindly at depth is cut by post-ore dikes exposed by erosion, then we can determine its area in projection onto the ground surface or onto a horizontal surface. To do this, systematic sampling of the post-ore dikes is carried out (at intervals of 5–10 m or other intervals), until we reach such parts of the endocontact zones of these dikes where there is no enrichment in useful components of the ore body lying at depth. In this way, points or boundaries of traces of mineralization in a given dike are established in the endocontact zone. For other dikes, other points of the same boundary are established. By connecting these points, we obtain the general outline and approximate area of the ore body (or bodies) in projection onto the ground surface. Then it is necessary to take into account the direction of laminar motion of the magma along the contact of the dike (from the oriented elongated crystals of the endocontact zones). This is a second procedure, useful in prospecting for bodies at depth, since the movement of magma does not always proceed strictly from below upward, vertically. Knowing the direction of movement of the magma, we can more accurately direct a borehole along the contact of the dike.

The third procedure is as follows. Suppose that exploration by a trench has exposed a meridional ore body (for example, of magnetite) dipping to the west. To intersect such an ore body, we usually lay out a borehole, placing the collar of the borehole to the west of the ore body. But with depth the ore body may become vertical and then acquire a dip to the east; clearly, the borehole laid out in this case will not intersect the body being explored. If, however, before drilling, we study the endocontacts of a neighboring post-ore dike, this study may show, for example, the presence of enrichment of the assimilation zone on the eastern side of the magmatic body. Consequently, application of the proposed method makes it possible to avoid an error in laying out the borehole.

Can the quantity and quality of a useful mineral be evaluated by means of the method described? The proposed method (i.e., the qualitative one) is only a method of prospecting for deposits. The method merely helps to find deposits that would otherwise be difficult to discover, and it must be applied in combination with all other methods effective in the given case. However, if we have an ore xenolith, then we may also have the first basis for judging the quality of the ore.

Can one judge, by means of our method, the depth of occurrence of a body of useful mineral? Again we emphasize that an exact answer to this question about depth is a matter for exploration, and in some cases a preliminary solution is a matter for geophysical investigations. Here, however, we outline the following way of improving the method for a very approximate estimate of the depth of occurrence of a body of useful mineral.

Suppose that the minimum industrial content of the useful component is 0.5%. We know the direction of movement of the magma in the endocontact of the post-ore dike. If we have succeeded in establishing the gradient ($\Gamma$) of change in content along this direction (for example, from 0.01% to 0.02% over 10 m), then we roughly and tentatively determine a depth of 0.5 km, where the content reaches 0.5% in the endocontact zone, i.e., where an ore body with the minimum industrial content may appear. In the same way we can roughly estimate the maximum depth, if we proceed from the maximum content. Of course, the gradient of change in content ($\Gamma$) must be determined at a series of points and strictly at the same distance from the contact (in order to obtain a more reliable—

figures). Finally, the depth of the ore body is determined if it is cut by at least two subsequent dikes with nonparallel magma flow.

Thus, is the proposed method of practical interest? Yes—at least whenever it is applied by a geologist capable of recognizing the useful mineral of interest, or even a companion mineral of the useful mineral. In the Sora area post-ore dikes enriched in copper green are known. If this copper enrichment is selectively confined precisely to the endocontact zone of the dikes, then the meaning of this may prove quite definite. It is not the result of distant migration of copper compounds in the weathering crust, but the result of oxidation and carbonatization within it of primary copper compounds, predominantly in the place where they occur. If this is so, it is an indicator of possibly practically valuable copper ores at depth, whose scale—given the specific nature of the example—is probably much greater than the scale of the ores being mined, whose presence we are obliged to verify. Of course, copper minerals are a very good signboard above copper mineralization, one that is hard to pass by without noticing. Other minerals are often much more difficult to notice. However, the degree of difficulty in diagnosing the sought minerals and ores is another matter (with its own specific methodology), and the evaluation of the proposed method should not depend on it.

Thus, the author advances a new method for evaluating materials for organizing searches for deposits on the basis of relatively weakly expressed signs of secondary (mechanical) mineralization, predominantly of dike rocks—signs that at first glance might not have attracted the investigator’s attention as an element in the possible organization of the searches themselves. This is the main point in our method (and not only in it, but also in hydrothermal dispersion). If weakly mineralized rocks of such a genesis—as noted above, i.e., if they are products of inorganic life of the endocontact zones of post-ore veins, if they are a product of dispersion rather than concentration—then they become the Ariadne’s thread that may lead, by subterranean paths, to the discovery of industrial deposits, including the largest ones.

Thus, the mineralogy of dispersed inclusions and veinlets of the endocontact zones of magmatic rocks must be studied persistently. One must search in them for useful minerals and their obligatory, inevitable companions. A fundamental reassessment of weakly mineralized rocks should be made if the present attitude toward them is incorrect. In those cases where a specific analysis, such as the one carried out, shows that the process of mineralization proceeds with dispersion of the useful mineral, nonindustrial mineralization may be the trace of more significant concentrations, including an indication of a large industrial deposit on whose threshold of discovery we stand.

The experimental development and verification of the method outlined above, undertaken by us, has not yet advanced sufficiently. The task is to test in initial trials, by dissolving magnetite in a melt (for example, of labradorite porphyrite or diabase), how the magnetic properties of the glass change depending on the amount of dissolved magnetite (as well as pyrite, hematite, siderite, pyrrhotite, etc.). A second series of experiments aims to determine how lightning strikes affect the magnetism of rocks: 1) if there is no assimilation of magnetite, 2) if it is present, and 3) if there is a magnetite deposit in the depth of the tested sample. These experiments at the first stage have yielded fundamentally important, encouraging results. But the accumulation of observations and the development of experiments in other directions are required. A summary of the experimental results will be given in another communication.

Limits of application. Of course, the proposed method is conceived only as an element of an integrated research methodology, as a modern and rapidly applicable supplement to known prospecting methods. Like any other, it has strictly limited bounds of application.

In its present form, the proposed methodology is intended only as a prospecting methodology, and, moreover, primarily in conjunction with known bodies of the useful mineral. It should be applied in geological-prospecting surveying, in prospecting and especially in exploration, chiefly before drilling and simultaneously or almost simultaneously with geophysical investigations, which makes it possible to check and confirm the forecast made by this method. The task of geophysics is to specify the depth coordinates of blind ore bodies¹. The task of the proposed methodology is to indicate the plan coordinates where geophysical and drilling work should first be placed on “blind” bodies of useful minerals. Our methodology clearly indicates in what direction work should be carried out at depth: of course, along the contacts of magmatic bodies having, at the contacts, an elevated content of the useful component, along the flow line of the magma of the latest dikes.

However, in addition to this, the methodology we are developing even now directs the researcher toward the study of xenoliths; among them there may be fragments of ores and endocontacts of zones that may prove to be enriched in finely dispersed ore substance. What has just been said makes it possible, as noted above, to delineate the area of possible mineralization not exposed by erosion, while native xenoliths at the same time make it possible to obtain a first idea of the quality of the ores themselves. All these scientifically substantiated data can provide sufficient grounds for undertaking prospecting-and-exploration work at depth.

Further tasks. In light of the proposed methodology, even now a number of serious questions and tasks arise that are important from both theoretical and practical points of view. But the principal tasks are as follows. First, it is necessary to carry out a thorough experimental development of the method using various specific minerals and useful minerals. Second, an attempt should already be made during exploration work, with the aid of the recommended method, to discover by traces blind ore bodies that would then be intersected by a borehole.

Since 1952 the proposed methodology has been at the stage of practical testing and introduction into practice. The author hopes that the discovery, with its aid, of new reserves of useful minerals, the discovery of new ore districts and deposits, is not far off; then the time will come for more insistent recommendations to use, for prospecting purposes, the immense extent of endocontact zones. For nature itself, in the most accessible form, gives us what billions of rubles have to be spent on in drilling.

Together with verification, the methodology itself, as it develops, rises to a higher level. In order for this to happen sooner, the author asks his comrades to help him in evaluating the method and especially in using it in practice, in testing it, and in making proposals connected with improving the method; for this he would be very grateful. Information on successful application of the method is especially important.

References

1. Ginzburg, I. I. An Attempt at Developing the Theoretical Foundations of Geochemical Methods of Prospecting, 1957.

2. Korochev, A. V., and Polyakov, V. E. Manifestations of hypogene mineralization as indicators of concealed ore mineralization. Soviet Geology, No. 34, 1948.

3. Smirnov, V. I. Geological Foundations of the Prospecting and Exploration of Ore Deposits. Moscow University Press, 1954.

4. Ushakov, N. D. On the methodology of prospecting for “blind” skarn and ore bodies. Exploration of the Subsurface, No. 2, 1952.

1. The problem of determining depths is also solved by our method: 1) approximately, as noted above; 2) very approximately by analogy with known neighboring bodies (but the latter is a matter of tectonic criteria); 3) occasionally more accurately, if there are two series of later dikes with two directions of flow. Hence follows the necessity of studying the flow structures of endocontact zones and taking oriented samples from them. ↩↩↩↩

2. Some would suppose that assimilation of ore occurs only if the magma is not moving. But it also occurs during movement: around xenoliths that are in relative rest, and at the contact. The latter is proved, for example, 300 m outside the body of manganese ores, in dikes that cut marbles and contain in the endocontact zone an elevated manganese content. The presence of movement of magma in the dikes does not require proof: it is visible in the fluidal structure and so forth. ↩