On the Prospects for the Rudny Cascade Magnetite Deposit at Depth Based on Magnetic Survey Data

R. V. Terekhova

(Presented by Prof. D. S. Mikov)

The Rudny Cascade magnetite deposit was discovered in 1933 by the topographer I. Elyukov, who detected a magnetic anomaly in the middle course of the Kanzyba River (Eastern Sayan). In the subsequent study of the deposit, magnetic surveys of various scales played a significant role. By the present time, fifteen ore bodies, extending beneath the overburden, have been explored from the surface and at depth. These bodies correspond to intense magnetic anomalies with field intensities at the epicenters of more than 10,000 gamma.

In carrying out an analysis of the magnetic field within the limits of the deposit and quantitative calculations of theoretical fields from ore bodies known from exploration data, it turned out that the observed fields were not fully explained. Weak magnetic anomalies with intensities of 2,000–2,500 gamma on the flanks of the known bodies did not always find an explanation.

In the Central sector of the Rudny Cascade deposit, within the limits of ore body IX (Fig. 1), different investigators explained the defect of ore masses in different ways.

The diorites widespread in the Central sector possessed appreciable magnetism. It was assumed that they accounted for the elevated fields on the flanks of the anomaly. However, study of the magnetic properties of the rocks and ores of the sector showed that the diorites can create fields no higher than 600–800 gamma.

Another explanation for the elevated fields was sought in the influence of boulder ores formed through the destruction of primary deposits. But negative forms of relief are not so widespread as the weak anomalies, and the effect from a horizontally lying, thin layer of boulder ores beneath the overburden is not very large and does not fully account for the observed effect.

The most correct proved to be the supposition concerning the ore nature of the weak magnetic anomalies on the flanks of ore body IX. Verification by drilling revealed a number of blind ore lenses making up ore zone XII (Fig. 2). Further tracing of the ore zone was carried out with the aid of borehole geophysical methods. At present it has been established that the zone extends to depths of 600 m and more, and along strike is traced for the first hundreds of meters. The discovery of this zone significantly increased the reserves of the deposit.

It is quite possible that in the area of other ore bodies, beneath weak magnetic anomalies, blind ore bodies may also be present, since the elevated field occupies almost the entire area of the Central sector.

In the Western sector of the Rudny Cascade deposit, quantitative calculations also revealed a deficit of magnetic masses, and, according to magnetic-survey data, the sector is considered underexplored; however, because the expected reserves are small, this forecast has not been checked by drilling.

Between the Central and Western sectors, a 600 m deep borehole was drilled, which was laid out in order to solve certain structural problems. In particular, this borehole addressed the question of the connection between the ores of the two sectors at depth. It should be noted that the magnetic field

Fig. 1. Section through ore body IX at the initial stage of exploration.
1 — observed curve of the vertical component of the magnetic field; 2 — theoretical curve from ore body IX; 3 — theoretical curve from ore body IX and from boulder ores; 4 — ores in primary occurrence; 5 — boulder ores; 6 — skarns; 7 — limestones; 8 — diorites; 9 — deluvium.

does not have an anomalous character here and does not indicate the presence of significant ore bodies at depths down to 600 m. Therefore this test did not yield positive results.

In the Eastern sector, the greater part of the ore bodies has been explored completely. The underexploration of some ore bodies due to shallow

Fig. 2. Section through ore body IX and ore zone XII according to the latest data. 1 — observed curve of the vertical component of the magnetic field; 2 — theoretical curve from the section; 3 — ores; 4 — skarns; 5 — albitophyres; 6 — amphibolites; 7 — limestones; 8 — diorites; 9 — deluvium.

Visible annotations in the figure: boreholes 594, 69, 68, 67, 64, 63, 62, 61, 570, 574, 596, 551; depth mark 740 m; scale 0–200 m.

Fig. 3. Section through ore body XIV in the area of a weak magnetic anomaly.
1 — observed curve of the vertical component of the magnetic field; 2 — theoretical curve from the known ore body; 3 — theoretical curve from the known body and from the inferred additional ore bodies; 4 — outlines of the inferred additional ore bodies according to the variant with the smallest possible depths; 5 — outlines of the inferred ore body according to the variant with one deeply buried ore body, from which the theoretical field at the surface would amount to 2000–2500 gammas near the epicenter; 6 — ores; 7 — skarns; 8 — sandstones; 9 — diorites.

...of the occurrence of ores, according to magnetometric data, was taken into account during exploration. With respect to the prospects of the Eastern sector at depth, the question was resolved by drilling a single borehole 580 m deep, laid out approximately in the center of the sector without regard to magnetic data. Between the collar of the borehole and the point of projection of the bottomhole onto the surface there is a small local anomaly, which could correspond only to a magnetic body of very small volume and shallow occurrence. This local anomaly is located in a field with an intensity of less than 500 gammas, and no weak anomalies of significant size, which would indicate deeply occurring ore bodies, are observed here. As a result, the borehole did not encounter ore. At the same time, on the western flank of the Eastern sector there is a weak anomaly occupying a large area and resembling the anomaly of the Central sector, where the ore nature of an anomaly of similar character was confirmed by drilling.

The study of the magnetic properties of samples of rocks and ores from the Eastern sector showed that most of the enclosing rocks are practically nonmagnetic or weakly magnetic. Diorites proved to be the most magnetic, but they can create fields with an intensity of 600–800 gammas. The observed fields, however, amount to 2000–2500 gammas. It was also noted that, in their physical properties, the diorites are fairly homogeneous, and no difference is observed between diorites sampled in the normal field and within the anomaly. In addition, the contours of the weak anomalies are not spatially related to the contours of the diorite massif. Consequently, the entire body of information indicates the possible ore nature of the weak magnetic anomalies on the western flank of the Eastern sector (Fig. 3).

Thus, it should be considered that drilling deep boreholes at the junction of the Central and Western sectors and in the center of the Eastern sector does not resolve the question of the prospects for mineralization at depth. At the Rudny Kaskad deposit, blind mineralization of the type of ore zone XII can be expected only within weak magnetic anomalies of considerable area throughout the western flank of the Eastern sector.

As numerous studies in many iron-ore provinces show, when evaluating the prospects for magnetite mineralization at depth, it is necessary first of all to analyze geophysical data on weak magnetic anomalies, which sharply increases the effectiveness of deposit exploration.