Proceedings of the Tomsk Order of the Red Banner of Labor Polytechnic Institute named after S. M. Kirov

Vol. 127, issue 1　　　　　　　　　　　　　　　　　　　　　　　　　　　　　1964

Results and Prospects of Applying a New Method for Prospecting for “Blind” Ore Bodies through the Study of Post-Ore Veins

K. V. Radugin, E. V. Yaroshinsky

1. Xenoliths in magmatic rocks have long been known (since 1893, certain inclusions of Lacroix, and since 1894, the xenoliths and xenocrysts of Sollas). Even earlier, in the middle of the last century, the concept of volcanic breccia had already arisen (in Bonney and Naumann). In 1879, in Kjerulf’s work, mention appeared of assimilation, and from 1857 onward, after Durocher’s work, hybrid rocks were distinguished, one of the forms of hybridism being considered the assimilation of foreign minerals and rocks.

Thus, xenoliths and hybrid rocks have been studied since ancient times. However, as far as is known, they were not used for prospecting purposes. One of the authors of the present work, as early as the beginning of the 1920s, encountered traces of assimilation (first in the Urals and in the region of the Gorlovka Basin, and then in the area of Mt. Orlinaya on the Salair). In 1924 he observed, on the northern slope of Mt. Orlinaya, post-ore vein diabases with specularite, the source of the latter being the neighboring barite-hematite ores. Under the influence of such observations, a simple idea arose and became firmly established: to use them in prospecting for ores at depth.

This prospecting method has now been described in published works (K. Radugin, 1963, et al.). Its essence is the same as that of other methods of prospecting by dispersion halos. Only here we use endogenous dispersion (in magma), rather than exogenous dispersion. It occurs predominantly in the marginal and, to a lesser extent, in the central zones of magmatic bodies that cut the mineral deposit, in two ways: a) mechanically, during the displacement of xenoliths and microxenoliths, and b) in solution, in the magmatic melt, in assimilational or hybrid streams. The upward movement of these xenogenic streams, which is especially characteristic of steeply dipping dikes, leads to the fact that a halo of magmatogenic dispersion may subsequently be observed on the denudation surface. Here it is detected by the usual methods of comparative mineralogical, chemical, spectral (metallometric), heavy-mineral, magnetometric, radiometric, luminescent, and other analyses (using polished sections, thin sections, unpolished sections, and a number of appropriate instruments). Of course, xenoliths and microxenoliths may also be noticed in the field, in particular with the aid of a magnifying glass, or

luminescence, and invisible magnetic xenoliths lying inside the rock—with the aid of known sensitive instruments.

Although the method we propose is based on the same principle of dispersion of the substance of a useful mineral as other prospecting methods (heavy-mineral, metallometric, etc.), doubts were nevertheless expressed as to its effectiveness, on the grounds that the magma of late dikes in the cold level of the earth’s crust is not capable of assimilating an appreciable amount of ore. Against these doubts spoke our comparative chemical analyses of assimilation zones of late veins enriched in manganese (on the Use) and iron (Mt. Topkhan and others) after cutting across carbonate and oxide ores of these metals. The same was indicated by observations of the diagenetic precipitation of their oxides in the marginal zones of late dikes from supersaturated solid solutions. This was also confirmed by observations that established the increased magnetism of the same zones after the assimilation of magnetite (in particular, around magnetite xenoliths).

In order to remove all doubts about the ability of magma to assimilate ores, we carried out experiments (with the assistance of M. Portnyagin, a student of the Tomsk Polytechnic Institute). In an electric furnace, diabase powder was melted without admixture and with admixtures of various amounts of magnetite, siderite, pyrite, hematite, and chalcopyrite. After melting and cooling, the magnetic properties of the resulting tubes of diabase glass were measured (with an M-2 magnetometer). The measurement results are as follows. The control sample of diabase (without admixture of ore minerals) almost did not change its magnetic properties after melting. By contrast, diabase melted with an admixture of magnetite powder became more magnetic, and directly proportionally to the amount of dissolved magnetite admixture. An increase in the magnetism of the diabase glass was also noted in the sample with an admixture of pyrite, which could have passed into pyrrhotite, and also in the sample with an admixture of siderite, which could have partly passed into magnetite (the melting took place with access of air). Finally, we note that a certain, though smaller, increase in magnetism was obtained in the melted diabase also from an admixture of hematite and chalcopyrite. True, into what the latter passed in the melt has not yet been clarified.

Similar experiments will be continued (with an expanded assortment of tested ores) in order to study the interaction of xenoliths and their structures with the melt, in particular, to obtain increased selective corrosion of them along relict tectonic cracks. The aim of the tests may also be to study the interaction of magma and the substance of useful minerals depending on the composition of the magma, on its temperature, and on the duration of contact. Regardless of the results of these future experiments, we consider it even now experimentally proven that, to one degree or another, magma under certain conditions dissolves ore minerals. Of course, the latter may melt in it if their melting temperature is lower than the temperature of the magma, and mix with it. On the other hand, one can imagine such xenoliths (for example, xenoliths of orthoclase in syenite) that will not dissolve in magma, since it is saturated with the corresponding molecules. However, it is rather difficult to imagine a late vein magma that would be saturated with ore substance, and therefore we practically do not take into account the case of insolubility of xenoliths just noted. More often another case will be encountered, when the xenolith entered just before the magma solidified and it had not yet had time to be assimilated to any appreciable degree. But this case is nevertheless not very frequent, and it will not hinder the application of our

method that makes use of all products of interaction between the ore and the post-ore magma.

Thus, we have considered certain doubts as to the applicability of our method, arising from theoretical considerations, and have set them aside. Evidently they do not threaten it. However, the chief test of anything new is in practice. Therefore we have long been studying, in the field and in laboratories, the interaction of magmatic bodies with the surrounding rocks and with the bodies of useful mineral intersected by them. And since 1958, the Department of General Geology of the Tomsk Order of the Red Banner of Labor Polytechnic Institute has organized and is conducting (through postgraduate student E. V. Yaroshinsky) extensive work on testing and applying the method characterized above for prospecting blind magnetite bodies of Gornaya Shoria. The results of this work have in part been published (E. V. Yaroshinsky, 1960). Some of its results are reported below, in the second part, written by E. V. Yaroshinsky on the basis of materials from the Telbes district, especially from the deposits of Northern Argysh-tag and Pravaya Sukharinka.

1. Before turning to these results, it should be recalled that as early as M. A. Usov (1927) drew attention, at the Telbes deposit, to the abundant and very characteristic dissemination of magnetite in diabase-porphyrite post-ore dikes, widely developed also in other analogous deposits. Later, on the floor of an abandoned quarry of the same deposit, in diabase-porphyrite dikes, K. Radugin found and studied xenoliths of magnetite indicating the presence of ores at depth; subsequently these xenoliths and the dissemination of magnetite were studied in detail by E. V. Yaroshinsky (1961) in the same and in the Odrabash quarry. The possibility that dikes may be enriched in magnetite and hematite through the assimilation of enclosing magnetite ores of Shoria was also pointed out by S. S. Lapin (1952) and V. G. Korel (1958). In other districts similar facts have been noted repeatedly, for example by Kh. M. Abdullayev, A. E. Fersman, and others.

The dissemination of magnetite found in post-ore diabase dikes, in our opinion, is of xenogenic origin (from iron ores). This is confirmed by the concentration of magnetite dissemination near the contact of dikes with magnetite ores, and also by the fact that this dissemination appears in the dikes after they intersect the magnetite ore, whereas before it there is none. Such dissemination, like the relatively rare xenoliths of ore, is a direct prospecting indicator, traceable in dikes, apparently, for hundreds of meters from the ore bodies.

How is the distribution of xenogenic magnetite within dikes to be studied? For this purpose a special method^1 of field micromagnetic measurements was applied to samples of diabase porphyrite and enclosing rocks, taken across the strike of the dikes. It is based on generally known data (Nagata et al.).

Of special interest, both theoretically and practically, are the results of measuring the magnetic susceptibility of 1,500 samples of post-ore dikes of diabase porphyrite at the deposits Northern Argysh-tag and Pravaya Sukharinka, where, in 14 boreholes, intersections of dikes at various depths below the ground surface were used (with a total length of dike core of 1,065 m). We have established that diabase porphyrite containing dissemination of magnetite has a relatively high magnetic susceptibility (on average \(2000 \cdot 10^{-6}\) CGS), compared with diabase porphyrite that does not contain dissemination of magnetite (on average \(50 \cdot 10^{-6}\) CGS). Zones of increased magnetic susceptibility are observed, as a rule, in those

^1 This method is set forth in the article “On the question of prospecting for blind iron-ore bodies in Gornaya Shoria,” submitted for publication. The literature is indicated there more fully.

intervals of dikes that are located above the places where they intersect ore bodies. In dikes that do not cut ore bodies, however, weak magnetic susceptibility is recorded. To confirm this, let us consider an example of the intersection, by a diabase-porphyrite dike, of a blind ore body (Fig. 1) at the North Argysh-tag deposit (exploration section II). According to geological-exploration data, the dike was cut by boreholes in two places: above the ore body by borehole 500 (upper intersection) and beneath the ore body by borehole 481 (lower intersection). In the upper intersection of the dike, near its hanging and footwall sides, two zones of increased magnetism were found, with maximum values of magnetic susceptibility equal, respectively, to \(1990 \cdot 10^{-6}\) CGS and \(2450 \cdot 10^{-6}\) CGS. In the remaining parts of this interval of the dike—in the endocontact zones and in the central part—the magnetic susceptibility averages \(40 \cdot 10^{-6}\) CGS. In the lower intersection, located beneath the ore body, the diabase porphyrite has a magnetic susceptibility on the order of \(30 \cdot 10^{-6}\) CGS. In the diabase porphyrite having increased magnetic susceptibility, abundant disseminated magnetite was established. In addition, leucoxenized ilmenite is uniformly dispersed everywhere in the diabase porphyrite.

Thus, the discovered zones of increased magnetic susceptibility in the upper intersection of the diabase-porphyrite dike (above the ore body) are due to the presence of xenogenic (assimilated) magnetite borrowed by the dike from the intersected ore body, which occurs between boreholes 500 and 481 (Fig. 1).

As for the xenogenic dissemination of magnetite in dikes, it is dispersed in elongated zones. In the plane of a cross-section (relative to the strike of the diabase dikes), these zones have the appearance of jets; therefore we call them xenogenic (in particular, assimilation) jets. They arise at places where diabase magma comes into contact with magnetite ores and, with increasing distance from the latter, are gradually “pushed back” from the contacts of the dikes toward their central parts. The spatial position of xenogenic jets in dikes can be used to determine the relative depth of occurrence of blind ore bodies.

Along with studying the features of the relationships between post-ore dikes and known (from geological-exploration data) magnetite ore bodies, we conducted searches for new, still unknown blind ore bodies. As a result, very encouraging results were obtained. Thus, in the area of the North Argysh-tag deposit (exploration section II), three predicted bodies were outlined, occurring within the drilled ore zone but not intersected by boreholes.

On the northeastern flank of the Pravo-Sukharinskoye deposit (exploration section XVI), facts were established that indicate the potential of the lower, undrilled horizons (600 and more meters from the surface) for blind ore bodies. According to our data, there are five predicted ore bodies, of which three correlate fairly well (at elevations of 60 and 120 m) with ore bodies identified by drilling on the neighboring exploration section XV.

Summing up the study of post-ore dikes, the following conclusions may be drawn: 1) enrichment of post-ore dikes of diabase porphyrite with assimilated and xenolithic magnetite occurs where diabase magma comes into contact with magnetite ores; 2) xenogenic magnetite is distributed in the dikes in the form of jets extending through the dikes for many hundreds of meters in the direction of movement of the magmatic melt; 3) the position of xenogenic jets in dikes (near the contacts or in the central part) indicates the rela-

Fig. 1. Xenogeneic stringers in a diabase-porphyrite dike cutting a magnetite ore body. Northern Argyshtag deposit.
1—diabase-porphyrite dikes (a) and xenogeneic stringers (b);
2—magnetite ore;
3—magnetic-susceptibility graphs;
4—inclusions of magnetite (a) and leukoxenized ilmenite (b).

comparatively shallow or, conversely, comparatively deep occurrence of blind ore bodies.

Thus, such xenogenic jets, detected in dikes, are direct indicators of magnetite ores and can be used with some success in prospecting for blind iron-ore bodies (and deposits) of the contact-metasomatic type.

3. Conclusion. As is evident from the above, according to the preliminary results of applying the new method for prospecting for “blind” ore bodies, the prospects for its use are favorable. It indicates possible new ore bodies even after drilling. However, the main immediate task ahead remains such a forecast of new “blind” ore bodies made with its aid, which would then be confirmed by boreholes and similar workings. After this task has been solved, boundless prospects for practical application will open before our new method.

LITERATURE

1. A. G. Volodin and V. G. Korel. On certain prospecting indicators of ores in the Kondoma district (Gornaya Shoriya). Bulletin of ZSGU and NTGU, No. 1, 1958.

2. S. S. Lapin. Structural-genetic features of some iron-ore deposits of Gornaya Shoriya (Candidate dissertation). Novosibirsk, OF ZSFAN, 1952.

3. K. V. Radugin. On the methodology of prospecting for mineral deposits. Izv. TPI, vol. 121, 1963.

4. M. A. Usov. The Telbess iron-ore district. Izv. Sib. Otd. Geol. Kom., vol. 4, issue 5, 1927.

5. E. V. Yaroshinsky. On the methodology of prospecting for “blind” bodies of magnetite deposits (abstracts of a report). In: Interuniversity Student Geological Conference. Leningrad State University Press, 1960.