ON THE NATURE OF MAGNETIC ANOMALIES AT THE DARASUN GOLD ORE DEPOSIT

G. G. Nomokonova, L. Ya. Erofeev

(Presented by the Department of Geophysical Methods of Prospecting)

In a detailed study of the structure of the magnetic field of the Darasun gold ore deposit, it was established that distinctive linearly elongated anomalous zones occur at this deposit. The curves of the increment of magnetic-field intensity within these zones are complex in configuration and most often have one minimum and two maxima. The intensity difference within these zones reaches several hundred, and sometimes several thousand gammas. The width of such zones varies from several tens of meters to the first hundreds of meters.

To clarify the nature of the existence of such anomalous zones in the magnetic field of the Darasun deposit, special measurements of magnetic-field intensity were carried out and the magnetic properties of the rocks of the deposit were studied.

As a result of studying the magnetic field in the most thoroughly explored parts of the deposit, it was established that anomalous zones of this kind are spatially associated with the emergence beneath alluvium of quartz-sulfide veins, dikes of various composition, and tectonic disturbances composed of mylonitized and cataclastic rocks. However, spatial coincidence alone does not, in essence, allow the anomalous zones to be explained. Indeed, if one assumes that the magnetic inhomogeneities creating the anomalous zones in the magnetic field are the vein rocks, then a substantial discrepancy is found between the observed anomalies and the anomalies that these formations could produce. This discrepancy is manifested in two forms. First, there is a substantial difference in the thicknesses of the disturbing bodies and the vein rocks. According to quantitative interpretation of the field of the anomalous zones, the thickness of the disturbing bodies is several tens of meters, whereas the thickness of the veins and dikes at the Darasun deposit reaches, at most, only a few meters. Second, in the observed anomalous zones there are maxima of intensity which, in magnitude, substantially exceed the field intensity over the enclosing magnetic rocks. Such disturbances in the magnetic field of Darasun cannot be caused by vein rocks, since their magnetic susceptibility and remanent magnetization, with the exception of microgabbro dikes, are several tens of times less than over the enclosing rocks.

It should therefore be assumed that the principal role in the formation of these anomalous zones is played by altered rocks enclosing the vein formations.

To verify this proposition, the magnetic properties of the rocks enclosing the quartz–sulfide veins were studied in detail. Such determinations were carried out in the area of two veins, the thickness of one of which is 10 cm, and of the other, 4 cm. For study, such veins were selected that, in the adit section, were separated from other “disturbing objects” (veins, dikes, rock contacts) by a distance of several tens of meters. The results of determining the magnetic susceptibility of the rocks are given in Fig. 1 as the dependence of magnetic susceptibility on distance to the vein.

Fig. 1. a — graph of \(\Delta Z\) over the Lebedev vein (2), the Main vein (3), and a theoretically calculated vein (1).
b — curve of variation in magnetic susceptibility (4), average magnetic susceptibility of the enclosing rocks (5).

At a distance of 0–5 m from the vein, the magnetic susceptibility increases from the magnetic susceptibility of the vein \((60 \times 10^{-6}\ \mathrm{CGS})\) to the value of the magnetic susceptibility of the enclosing rocks \((1400 \times 10^{-6})\). In the interval of 5–15 meters it exceeds the magnetic susceptibility of the granodiorites by an average of \(400 \times 10^{-6}\ \mathrm{CGS}\). Overall, the change in magnetic susceptibility over the interval 0–15 m is reliably approximated by a pa-

by a second-degree parabola (Fig. 1). It is characteristic that the magnitude of the remanent magnetization changes according to the same law, as a result of which the ratio of remanent magnetization to induced magnetization remains constant and low (0.2–0.3). This makes it possible to assume that the mineralogical changes in the host rocks enclosing the vein occurred at low temperatures.

In the analysis of thin sections and polished sections of rocks from this area, the absence of ferromagnetic minerals in the quartz–sulfide veins was noted; in the host rocks, in the interval 0–5 m, there are fewer iron-bearing minerals than in unaltered granodiorites—near the vein these are predominantly pyrite and arsenopyrite, which farther away are replaced by magnetite. In the interval 10–15 m the magnetite content is higher than in unaltered granodiorites—magnetite is developed here in the mafic minerals during biotitization.

According to the data from determining the magnetic properties (Fig. 1), the curve of change in the vertical component of the magnetic-field intensity was calculated (Fig. 1). Its form and intensity agree well with the curves of change in intensity obtained by field surveying over the Lebedevskaya and Glavnaya veins.

At the Darasun gold-ore deposit, veins, dikes, and tectonic disturbances are rarely encountered separately. Most often they are spatially close together, forming zones which, like individual veins, are accompanied in the magnetic field by the above-described linearly elongated anomalies. Study of the magnetic properties in areas where several zones are developed made it possible to establish that the character of the change in magnetization of the rocks in this case is, in principle, the same as for individual veins.

The magnetic susceptibility of the rocks of the zone is substantially lower than that of the host rocks and averages \(200 \cdot 10^{-6}\) CGS. The remanent magnetization of these formations is \(45 \cdot 10^{-6}\). The distribution law of the magnetic parameters within the zone is bimodal, with modes of \(4 \cdot 10^{-6}\) and \(700 \cdot 10^{-6}\) (for magnetic susceptibility), which is associated with the presence both of dike and vein rocks and of altered host rocks.

The change in the magnetic susceptibility of the host rocks over the interval 0–15 m is also bimodal. One modal value of magnetic susceptibility (\(1000 \cdot 10^{-6}\) CGS) is lower, and the other (\(4200 \cdot 10^{-6}\)) is higher than the average value of magnetic susceptibility of the unaltered rocks.

Thus, the magnetic linearly elongated anomalous zones with increased and decreased values of intensity, in comparison with the host rocks of the granodiorite intrusion at the Darasun gold-ore deposit, are caused by zones of altered rocks spatially confined to dikes, veins, and tectonic disturbances.