Relationships of Magnetic Anomalies with Sulfide-Quartz-Carbonate Veins at One of the Gold-Ore Deposits of Transbaikalia

L. A. Zashchinskii

(Presented by Prof. D. S. Mikov)

The effectiveness of applying electrical prospecting within a complex of geophysical methods for searching for mineral deposits associated with sulfide-quartz-carbonate veins occurring in masses of crystalline rocks (magmatogenic or metamorphogenic) was convincingly substantiated by N. A. Nikiforov, using as an example one of the gold-ore deposits of Transbaikalia [1].

However, the recommendations of the above-mentioned investigator on the use of magnetic prospecting have not yet been reflected in the practice of subsequent work, even at the deposit described in the article cited above. For example, the possibility of direct searches for ore bodies of the above-named type by magnetic anomalies is underestimated. The reason for this is the unclear nature of the relationship between magnetic anomalies and mineralization, which creates difficulties in distinguishing promising—ore-bearing—anomalies. Continuous checking of anomalies usually proves to be ineffective and leads only to discrediting magnetic prospecting as a most important prospecting method.

Below we shall somewhat supplement N. A. Nikiforov’s ideas concerning the nature of magnetic anomalies and their relationship with mineralization, using as an example the deposit described by him [1].

The ore bodies of the deposit are steeply dipping veins (dip angles \(60^\circ\)—\(90^\circ\), rarely \(45^\circ\)) occurring in crystalline magmatogenic rocks and confined to disjunctive disturbances of the fault type, which in turn are often accompanied by zones of crushing whose transverse dimensions reach the first tens of meters. The thickness of the veins themselves, together with the zone of near-vein alterations, usually does not exceed half a meter and only rarely, in places of branching, reaches 2—3 meters. The thickness of loose eluvial-deluvial deposits covering the outcrops of the ore bodies varies from 0 to 5 meters and averages \(2.0\)—\(2.5\) m. The composition of the ores is essentially sulfide-quartz-carbonate, with vein minerals often predominating over sulfides. In the zones of near-vein alterations, bleaching of the rocks, their silicification, and kaolinization are observed.

At the deposit, acidic dikes and lamprophyre veins are widely developed, and the dikes sometimes accompany the ore bodies.

According to their magnetic properties, the rocks developed in the area of the deposit may be divided into three groups:

1. Acidic formations (volcanic glasses, quartz and quartz-free porphyries, felsites, leucocratic granites, aplites, and pegmatites) are practically nonmagnetic. The magnetic susceptibility of these rocks is \(10—100 \cdot 10^{-6}\) CGS.

2. Rocks of intermediate basicity (diorites, granodiorites, and gabbro-diorites) have susceptibilities within the range \(500—3000 \cdot 10^{-6}\) CGS.

3. The greatest susceptibility (up to \(10000 \cdot 10^{-6}\) CGS) is found in basic and ultrabasic rocks (olivine gabbros, peridotites, gabbro, gabbro-diabases, and amphibolites).

The ore material is usually practically nonmagnetic (susceptibility on the order of \(10—100 \cdot 10^{-6}\) CGS). An exception is pyrrhotite ores, with susceptibility up to \(400 \cdot 10^{-6}\) CGS, which occur extremely rarely.

Some ore bodies of the deposit are marked (Fig. 1) by anomalies of the natural potential (SP), with an intensity of up to \(-250\) mV. The SP isolines on the plan have, in this case, the forms of more or less elongated ovals, the long axes of which, being parallel to the outcrops of the ore bodies, are as a rule displaced from the latter by several tens of meters. Moreover, no regular displacements toward the dip or rise of the ore bodies are observed. It should be noted that by no means all veins, and not along their entire strike, are marked by SP anomalies, which is probably connected with different physicochemical conditions in anomalous areas and areas of the normal field, as well as with shielding effects of loose deposits. Thus, although SP anomalies under the conditions of the deposit are a reliable prospecting criterion for sulfide mineralization, they nevertheless do not make it possible to detect not only all but even the majority of the ore bodies.

In the ore field of the deposit, the outcrops of veins sometimes spatially coincide with the axes of electrical-profiling anomalies (symmetric, dipole, combined, and median-gradient). At the same time, irregular displacements of anomaly axes from the outcrops of ore bodies are also observed (Fig. 1). The practice of checking these anomalies has shown that they are usually caused not by ore veins, but by water-saturated zones of crushing, within which mineralization is sometimes localized. Since at the deposit there are veins not associated with such zones, and since the number of barren disjunctives is many times greater than the number of ore-bearing ones, electrical-profiling methods reveal a very considerable number of “false” anomalies and therefore do not provide reliable indications for prospecting mineralization.

Under these conditions, facts of spatial coincidence with the outcrops of ore bodies and with the axes of SP anomalies of narrow, linearly elongated minima of the vertical component of the magnetic field \(\Delta Z\), with intensities from \(-1000\) to \(-1500\) gammas, are of serious interest (Fig. 1).

Analysis of Fig. 1 makes it possible to establish the confinement of the anomalies to areas of hydrothermal mineralization. The axes of the SP and \(\Delta Z\) minima, as well as the ore bodies, coincide to a significant degree with one another or are continuations of one another, which cannot be the result of accidental causes.

N. A. Nikiforov [1] assumed that \(\Delta Z\) anomalies associated with mineralization are caused by different contents of magnetic minerals in the ore and in the enclosing rocks. However, from such positions, unless one assumes significant residual magnetization of the rocks in the near-

in the vein zone can be explained only by comparatively weak anomalies. Thus, for the area in Fig. 1, calculation by the formula

\[ z_m = 4 \varkappa Z_0 \operatorname{arctg}\frac{m}{2h}, \]

where \(m\) is the thickness of the ore body together with the zone of wall-rock alteration,
\(h\) is the depth of its occurrence,
\(\varkappa\) is the excess magnetic susceptibility of the enclosing rocks,
\(Z_0\) is the normal value of the vertical component of the magnetic field,
\(z_m\) is the maximum intensity of the expected anomaly; for values \(\varkappa = 2000 \cdot 10^{-6}\) CGS; \(\operatorname{arctg}\frac{m}{2h} = 0.25\), most optimal for our conditions, gives a maximum anomaly value on the order of 250 gammas.

To explain the observed anomalies with an intensity of 1000–1500 gammas, it seems natural to assume the presence of zones of rocks accompanying the ore bodies and having residual magnetization opposite to the modern induced one. During the introduction of ore-bearing

Fig. 1. Plan of geophysical anomalies of one of the areas of the ore field of the deposit: 1 — axes of anomalies of symmetrical profiling; 2 — veins; 3 — isolines of the natural electric potential field; 4 — isodynamic lines (field intensity 500 gammas and higher); 5 — isodynamic lines (field intensity below zero); 6 — granodiorites; 7 — granosyenites; 8 — amphibolites; 9 — quartz diorites. Fields with intensity below −1000 gammas are hatched.

emanations these rocks in the wall-rock zone could have been heated above the Curie point and, during subsequent cooling, could have become magnetized under the influence of the earth’s field of past epochs or by one of Néel’s mechanisms [2]. This assumption is supported by the fact that

the configuration and intensity of the \(\Delta Z\) minima is well maintained in areas of rocks with differing magnetic susceptibility (Fig. 1).

Finally, the occurrence of magnetic anomalies with an intensity of 1000–1500 gammas, in the absence of significant differentiation of the magnetic properties of the enclosing rocks and ore formations in areas where sulfide mineralization is developed, may also be explained by other causes. The form and intensity of the anomalies of the natural electric potential over the ore bodies of the deposit, as well as the ratio of the specific electrical resistivities of the enclosing rocks and ores, make it possible to assume the flow, along the veins downward from the surface, of electric currents with a density of \(10^{-5}\)—\(10^{-7}\) a/mm\(^2\). With an inclined direction of the currents and of the vein plane, an anomaly of the vertical component of the Earth’s magnetic field may be observed on the ground surface, caused by an effect determined by the Biot–Savart law. Calculation shows that anomalous fields of this kind may exist even in the case where, at the ground surface, owing to the shielding action of loose deposits, no noticeable SP anomaly is present.

All of the foregoing makes it possible, in our view, to draw the following conclusions.

Magnetic prospecting, under conditions similar to those described above, is an important method for the search for sulfide mineralization. Especially successful results from its application should be expected in areas where ore formation was accompanied by remagnetization of the enclosing rocks. Therefore magnetic surveying, as the cheapest and most highly productive method, should supplement—and, under conditions of intense interference (shielding of natural potentials by sediments, complicated grounding conditions, wandering points, etc.), replace—SP surveying. In this connection, calculations of magnetic anomalies caused by natural currents under various geological and geophysical conditions are of serious interest.

In carrying out further geophysical work at the deposit described by N. A. Nikiforov [1], magnetic anomalies with intensities of \(-1000\) gammas and more should be considered the most promising. From the standpoint of developing a rational methodology for prospecting mineralization of an analogous type, it is necessary to continue, at the indicated deposit, work to determine the nature of the magnetic anomalies, to carry out surveys of the three components of the magnetic field, and to investigate the physical properties of the rocks and ores.

REFERENCES

1. N. A. Nikiforov. Geophysical methods as a means of studying and prospecting for gold-ore deposits. Izv. AN SSSR, ser. geogr. i geofiz., No. 4, 1945.

2. A. N. Khramov. Paleomagnetic Correlation of Sedimentary Strata. Gostoptekhizdat, 1958.