On the Question of Prospecting for “Blind” Gold-Ore Deposits

E. V. Yaroshinskii (TPI)

Prospecting for “blind” deposits of useful minerals is a pressing problem in addressing the question of expanding the country’s mineral-resource base. In this connection, the development of the newest methods for prospecting for difficult-to-discover deposits and the improvement of already known methods acquire great importance.

In prospecting practice, geochemical methods are widely used for searching for hidden deposits by means of primary (endogenic) and secondary (exogenic) dispersion halos of ore elements [2].

The new methodology proposed, as a formulation of the problem, for prospecting for “blind” ore bodies and deposits by xenoliths and products of ore assimilation in later dikes was first put forward by K. V. Radugin [4] and subsequently developed by E. V. Yaroshinskii [5, 6] on the example of skarn-magnetite deposits of Mountain Shoria.

The new methodology is based on the phenomenon of the formation of secondary endogenic areas of dispersion of the substance of a useful mineral or its satellites in the magmatic melt of later dikes. Dikes, cutting across ore bodies (or mineralized zones), borrow part of the ore substance and carry it out to higher horizons, where it can be detected by one means or another. Consequently, dikes are natural messengers from the depths and can play a very substantial role in questions of prospecting for “blind” ore deposits.

The possibility that dikes may borrow ore substance at sites where they intersect ore bodies and mineralized zones was pointed out by Kh. M. Abdullaev [1] and other researchers. Somewhat later this point of view was confirmed by the investigations of K. V. Radugin and E. V. Yaroshinskii. Thus, K. V. Radugin established signs of assimilation by the magma of diabase dikes of carbonate–manganese ores at the Usovskoye manganese deposit in the Kuznetsk Alatau. The results of chemical analyses showed that the manganese content in the central zones of the dikes is ordinary, close to clarke values in diabases, whereas in the endocontact zones this content reaches 6–7% [4].

The first special investigations of later dikes in skarn-

magnetite deposits of Gornaya Shoria were carried out by the author of the article in 1960–1963. Detailed studies of the relationships between later diabase-porphyrite dikes and magnetite ores made it possible to establish regularities in the distribution of borrowed magnetite in the dikes and to develop the main prospecting criteria for blind ore bodies.

Xenoliths of magnetite ores. In the later diabase-porphyrite dikes at the Telbes and Odrabash deposits, xenoliths of magnetite ores were discovered¹. Ore xenoliths were encountered in the endocontact zones and, more rarely, in the central parts of the dikes at different hypsometric levels, beginning at the surface (absolute elevation +340 m) and ending at the bottom of the quarry (absolute elevation +250 m).

All xenoliths were divided by us into macro- and microxenoliths. Macroxenoliths range in size from fractions of a centimeter and larger. Microxenoliths were detected and studied under the microscope. According to the degree of alteration, the xenoliths are divided into three groups.

The first group includes sharp-angled xenoliths of magnetite ore that have not undergone noticeable changes and have preserved their original form. The second group includes xenoliths of more or less rounded form with somewhat smoothed corners and with traces of weak disintegration along the periphery. The third group includes xenoliths with smoothed corners (rounded form), with clear traces of disintegration along the periphery and cracks, which in places leads to the formation of a breccia-like rim around such a xenolith. Small segregations of magnetite arising as a result of the xenolith’s disintegration form a shell around it. Under the influence of the magmatic melt, the material of this shell is often drawn out in the form of tongues and chains, flowing around the larger xenoliths. A considerable development is observed of microxenoliths scattered in the endocontact parts of the dikes and represented by small fragments of magnetite ore with relict tectonostructures (tectonic cracks, quartz and calcite veinlets that do not extend beyond the limits of the xenoliths). Around many xenoliths of magnetite ore, macroscopic observations reveal reaction rims of fine-grained diabase, sometimes lightened and colored by a fine dissemination of diagenetic hematite. Some xenoliths are altogether devoid of such rims and borders. Possibly the latter is due to the capture by the magma of fragments at a moment close to its solidification. Along with xenoliths that are fragments of the enclosing magnetite ores, strongly altered xenoliths of magnetite ores with traces of corrosion or other effects of the magmatic melt were found in the diabase-porphyrite dikes, which makes it possible to regard these xenoliths as having been brought from deeper horizons of the skarn-ore zone.

Xenogenic streams. Magnetite borrowed by the magmatic melt of the dikes from the enclosing magnetite ores is distributed in the dikes in the form of dispersion halos; in transverse-vertical section, the latter have the form of streams, which we have called “xenogenic streams.” The detection of xenogenic streams and their tracing in the dikes were carried out by means of magnetometric measurements of dike samples taken from outcrops and from drill-core.

Magnetometric investigations established that the endocontact zones of dikes containing xenogenic magnetite have elevated—

...appreciable magnetism in comparison with other parts of this same dike.

A general idea of the connection between the increased magnetism of dikes and magnetite ores was provided by the results of magnetometric investigations of post-ore dikes of diabase porphyrite in the Telbes-Odrabash, North-Arlyshtag, Pravo-Sukharinsk, and other ore fields. It has been established that the magnitude of the magnetic susceptibility of diabase porphyrite varies both along the strike and down the dip of the dikes. Thus, within the skarn-magnetite deposits themselves, the diabase porphyrites have, on average, a magnetic susceptibility of \(2000 \cdot 10^{-6}\)—\(3000 \cdot 10^{-6}\), while beyond their limits, in the very same dikes, the magnetic susceptibility averages \(50 \cdot 10^{-6}\) CGS, i.e., it corresponds to the “normal magnetic background” of diabase porphyrites.

A detailed study of xenogenic streams in dikes of diabase porphyrite was carried out using core material from boreholes at the North-Arlyshtag skarn-magnetite deposit. This deposit has been explored. A steeply dipping “blind” ore body, cut by several dikes of diabase porphyrite, has been contoured (Fig. 1). The upper boundary of the ore body lies at a depth of 100 m, and the lower one at 460 m from the surface. The dikes were studied in intervals located below the ore body, within the ore body itself, and above the ore body. In the upper intersection of the dike, above the ore body (180 m up the dike), a xenogenic stream with a magnetic susceptibility equal to \(3000 \cdot 10^{-6}\) CGS was discovered, situated in the central part of the dike. In the middle intersection of the dike above the ore body (90 m up the dike), two xenogenic streams were discovered—zones of increased magnetism with \(\chi = 1500 \cdot 10^{-6}\)—\(2000 \cdot 10^{-6}\) CGS, situated near the contacts of the dike. In the lower intersection, below the ore body (70 m down the dike), throughout the entire forty-meter interval of the dike the diabase porphyrite is practically nonmagnetic (\(\chi = 30 \cdot 10^{-6}\) CGS)—xenogenic streams are absent.

Thus, xenogenic streams were found in the dike only above the intersection of the ore body, while below the ore body they are absent. Mineralogical studies and chemical analyses confirmed an anomalously high content of magnetite (“ore iron”) in the xenogenic streams.

The results of studying the relationships of post-ore dikes of diabase porphyrite with magnetite ores in Gornaya Shoria made it possible to draw the following conclusions:

1. Enrichment of post-ore dikes of diabase porphyrite with xenogenic material from magnetite ores occurs in the process of interaction between magma and ores at the places where they intersect.

2. The borrowed xenogenic material is distributed in the dikes in the form of dispersion areas elongated in the direction of magma movement. Areas of xenogenic material are projected onto the plane of vertical and horizontal sections of the dikes in the form of xenogenic streams.

3. The extent of xenogenic streams in dikes in the direction of the ascending movement of magma is measured in many hundreds of meters.

4. Xenogenic streams in dikes are arranged quite definitely; their position is directly dependent on the degree of distance from the place of their origin (the direct intersection of the dike with ore). In dike intervals located within ore bodies, xenogenic streams are recorded in their endocontact zones. As the dikes move away from the ore body, xenogenic streams gradually shift toward the central part of the dike. The spatial position of xeno-

Fig. 1. Xenogenic jets in diabase-porphyrite dikes cutting magnetite ore bodies. Severnye Argysh-Tag deposit, exploration section 1. (Based on materials of the Kazakh Expedition of WSGU, 1960, with additions by E. V. Yaroshinsky)

1 — marbles; 2 — quartz-chlorite-carbonate rocks; 3 — porphyrites; 4 — quartz porphyries, diorites; 5 — ore bodies; 6 — diabase-porphyrite dikes (a) and xenogenic jets (b); 7 — magnetic-susceptibility graphs; 8 — magnetite dissemination (a) and leucoxenized ilmenite (b).

mineralized veins in dikes can serve as a criterion of the relative depth of occurrence of a “blind” ore body.

The application of the new method for prospecting for “blind” ore bodies is possible only on the condition that post-ore dikes are present in the area under study (ore field, deposit). This method is auxiliary (it does not claim universality) and must be used in combination with other geological methods (geological-structural, petrological, mineralogical, geophysical, etc.).

The question raised in the article concerning the development of a methodology for applying it to the search for “blind” gold-ore deposits may be substantiated by the following considerations:

first, the results of applying the new prospecting method at the skarn-magnetite deposits of Gornaya Shoria show that, given the appropriate conditions (intersection of ore bodies by dikes at depth), this method, in a certain modification using high-precision methods for diagnosing elements (the activation-analysis method, analysis of trace elements, etc.), can be applied in searching for blind deposits not only of ferrous, but also of nonferrous, rare, and precious metals;

second, numerous cases are known of the presence of post-ore dikes at many gold-ore deposits of the Kuznetsk Alatau (the Saralinsky district, the Tsentralny mine, etc.) and in other regions of the country;

third, taking into account the genesis of gold-ore deposits and, accordingly, the characteristic complexes of mineral associations of different types of gold ores (Petrovskaya, 1960), the structural-geological and morphological features of gold-ore deposits, and other factors, it may be assumed that ultimately it will be possible to develop the principal prospecting criteria for “blind” gold-ore deposits that will facilitate the most rapid discovery of new deposits.

LITERATURE

1. Abdullaev Kh. M. Dikes and mineralization. Gosgeoltekhizdat, 1957.

2. Ginzburg I. I. Geochemical prospecting for “blind” ore bodies. International Geological Congress, XXI session, reports of Soviet geologists. Problem 2, Gosgeoltekhizdat, 1960.

3. Petrovskaya N. V. On certain regularities in the distribution of ore columns and mineralogical criteria for prospecting for concealed zones of rich ores (on the example of certain gold-ore districts). In: “Problems of the study and methods of prospecting for concealed mineralization.” Gosgeoltekhizdat, 1963.

4. Radugin K. V. On the methodology for prospecting for deposits of useful minerals. Proceedings of TPI, vol. 121, Tomsk, 1963.

5. Radugin K. V., Yaroshinsky E. V. Results and prospects of applying a new method for prospecting for “blind” ore bodies by studying post-ore veins. Proceedings of TPI, vol. 127, issue 1, Tomsk, 1964.

6. Yaroshinsky E. V. Certain features of the interrelations of post-ore dikes with magnetite ores of the Severo-Argishtan deposit in Gornaya Shoria. In: “Materials on the geology and useful minerals of Western Siberia,” Tomsk, 1964.

1. A collection of xenoliths of magnetite ores discovered in the later diabase-porphyrite dikes of the Telbes and Odrabash deposits is kept in the Mineralogical Museum of the Siberian Branch of the Academy of Sciences of the USSR. ↩