On the Nature of the Spatial Variation of Rock Magnetization in the Darasun Ore Field

G. G. Nomokonova

(Presented by Professor D. S. Mikov)

At present, magnetic surveying is being successfully applied at vein gold-ore deposits [2]. However, the available information on the magnetic properties of the rocks of ore fields and on the nature of their spatial variation is often insufficient even for a qualitative interpretation of magnetic anomalies at these deposits. The present work is devoted to the first results of a study of the regularities in the variation of the magnetization of rocks enclosing gold-sulfide mineralization. In this article, the point considered is not the variation in the magnetization of the rocks of the deposit that is determined by the diversity of petrographic rock types, but rather the spatial variability of the magnetization of rocks of the same type, associated with the superposition of secondary processes.

The area of the Darasun deposit is composed of rocks of various intrusive complexes, ranging in age from Upper Proterozoic (?) to Mesozoic. The composition of the rocks is highly varied. Within the ore field there occur rocks from norites and gabbro to leucocratic varieties of granites. The rocks are often hybrid. Processes of secondary hypogene alteration are widely developed. The mineralization is represented by quartz–gold–sulfide veins of Cimmerian age.

The magnetic properties of the rocks and ores of the Darasun ore field were studied from samples that were taken from mine workings and measured with an MA-21 astatic magnetometer, and from measurements of magnetic susceptibility made with a kappameter directly in the mines and adits.

The rocks of the ore field are heterogeneous in magnetization. The statistical distribution laws of magnetic susceptibility for varieties of rocks of the same type most often have two or more modal values. On

Fig. 1, c presents the statistical law of distribution of the magnetic susceptibility of granodiorites from borehole 579. Here one can distinguish at least two populations of granodiorites, differing from one another—those with high and low magnetization. In this case, the statistical heterogeneity corresponds also to geological heterogeneity, since rocks with low and high magnetic character are spatially separated. Graphs of the magnetic susceptibility of granodiorites along the borehole section indicate an alternation of rock intervals with increased and decreased magnetization.

Fig. 1. a) Correlation field of the mean magnetic susceptibility and its standard deviation for plagiogranite-porphyries of borehole 602 (ore field); b) the same for gabbro of borehole 551 (periphery of the deposit); c) statistical law of distribution of the magnetic susceptibility of granodiorites of borehole 579

Another example of heterogeneity in the magnetic properties of rocks of the ore field is shown in Fig. 1, a. The magnetic susceptibility of core from borehole 602, drilled through plagiogranite-porphyries to intersect one of the veins of the ore field, was measured. For every five meters of borehole depth, two numerical characteristics determining the magnetic properties of the rocks of this interval were calculated: the mean magnetic susceptibility and its variance. The correlation field of the mean magnetic susceptibility and the standard deviation (the square root of the variance) gives a very clear representation of the heterogeneity of the magnetization of the plagiogranite-porphyries from borehole 602: the points of the correlation field are grouped into two elliptical regions intersecting in the interval of magnetic-susceptibility values \(400\text{–}600 \cdot 10^{-6}\) CGS and standard deviation \(400\text{–}500 \cdot 10^{-6}\) CGS. For rocks with magnetic susceptibility less than \(400 \cdot 10^{-6}\), the relation between the mean magnetic susceptibility and its variance is positive, whereas for plagiogranite-porphyries with magnetic susceptibility greater than \(600 \cdot 10^{-6}\), a negative li-

linear correlation between these quantities. A similar analysis of rocks located outside the ore field showed that the correlation between the mean magnetic susceptibility and its dispersion for a single rock type is either absent or there is a very weak positive relationship (Fig. 1, б). Therefore, for the plagiogranite-porphyries of borehole 602, at least one type of rock alteration can be indicated; as a result of this alteration the magnetization of the rocks increases and, at the same time, the uniformity of the distribution of magnetite in the rock volume increases, which leads to a decrease in the dispersion of the magnetic susceptibility of the altered plagiogranite-porphyries.

For the rocks of the ore field there is a definite spatial regularity in the change of magnetization near veins, which may be called zoning. In the general case, three zones can be distinguished. The first zone of altered rocks directly adjoins the ore body. The rocks of this zone have a comparatively well expressed hydrothermal alteration and low values of magnetic susceptibility and residual magnetization (the first tens of \(10^{-6}\) CGS). The thickness of this zone usually lies within ten meters; however, it may be considerably greater (30 m along one of the sections of the Novo-Kuznetsovskaya vein). The transition to the second zone is very sharp. The magnetic susceptibility of the rocks of the second zone exceeds the average magnetic susceptibility of practically unaltered rocks, sometimes by a factor of 2–3. A very low dispersion of magnetization is characteristic, which corresponds to a very uniform distribution of magnetite in the rock. The third zone, external with respect to the ore body, is characterized by reduced magnetization relative to unaltered rocks. The transition from the second to the third zone is usually gradual. The magnetic susceptibility of the third zone has great irregularity and heterogeneity in its values. The magnetic-susceptibility curve usually has a sawtooth character.

The zoning revealed is fully manifested around small isolated veins. Branching, apophyses, and the close position of several veins disturb it.

Figure 2 gives plots of the magnetic susceptibility of the core from borehole 601. In the depth interval 350–400 m there are three closely spaced quartz-sulfide veins. In this case the described zoning is fully observed for the two outer veins. For the first vein, the first magnetization zone is in the depth interval 350–353 m, the second is 338–350 m, and the third is at a depth of 316–338 m. For this vein, an absence of symmetry of the zoning with respect to the middle of the vein is characteristic; this is connected with the close position of the second vein to it, as a result of which the second zone is practically absent, and the third spatially coincides with the third zone of the second vein. The zoning of magnetization is also disturbed between the second and third veins. The described zoning of magnetization of the enclosing rocks is observed, moreover, for zones of hydrothermally altered rocks without vein filling (Fig. 2, borehole 618).

In Fig. 3, а a plot is presented of the magnetic susceptibility of granodiorites across one of the veins of the deposit, and the boundaries of the identified magnetization zones are shown approximately. Along this same section the density of the rocks was determined and their petrographic composition was studied.

The first zone is characterized by the complete absence of dark-colored petrogenic minerals. Sericite (20–25%) and quartz (35–70%) are widely distributed; carbonate is rarely present. Ore material amounts to as much as 5% and is represented by pyrite. In composition, the rocks of the first magnetization zone of this section correspond to beresites. The rocks of the second zone

less altered. The bulk of the rock consists of primary minerals: feldspars, quartz, biotite. Hornblende is rarely present; in most cases it has been replaced by biotite. Chlorite, epidote, and carbonates, in turn, develop after biotite. Ore matter is often confined to segregations of mafic minerals. Rocks of the third zone differ from rocks of the second by the complete absence of hornblende, a smaller amount of biotite and ore matter, and a greater development of chlorite and epidote.

Fig. 2. Graphs of the magnetic susceptibility of core from boreholes 601 and 618 (the plotted values are the mean magnetic-susceptibility values from three kappameter measurements); 1 — quartz diorites, 2 — gabbro-amphibolites, 3 — quartz-sulfide vein, 4 — zone of hydrothermally altered rocks, 5 — volcanic glass.

As is evident from the description, the second and third magnetization zones are represented by rocks propylitized to varying degrees, with the rocks of the third zone being close in composition to granites. On the basis of a comparison of the petrographic composition and magnetization of the rocks enclosing the vein, an attempt may be made to explain the causes of such a change in the magnetization of rocks near veins of the Darasun ore field.

The early process of hydrothermal alteration of rocks at the deposit—propylitization—although it develops in time before the veins are filled with ore material and occupies considerable areas, uses the same fractures that were subsequently filled with ore [6]. Therefore, propylitized rocks are most often spatially associated with ore bodies.

In the process of propylitization, during the decomposition of dark-colored minerals and the formation of chlorite, magnetite is released, and the rocks acquire additional magnetization. Further cooling of the hydrothermal solutions increases the dissociation of hydrogen sulfide in water [1], and magnetite may be partially replaced by pyrite. The association chlorite, magnetite, and pyrite is characteristic of the early stages of hydrothermal alteration of certain vein deposits [3]. A later process of hydrothermal alteration of rocks—beresitization—leads to a significant decrease in the magnetization of rocks: iron is either bound in pyrite or is completely removed.

Fig. 3. Graphs of magnetic susceptibility (a) and density (b) of granodiorites across the space of one of the veins of the deposit; 1—average magnetic susceptibility of granodiorites, 2—regression line of the density of rocks of the third zone.

Thus, the magnetization of the rocks enclosing the vein depends substantially on the degree of their hydrothermal metasomatism: rocks of the initial stage of alteration may be either more or less magnetic relative to unaltered rocks, whereas further alteration of the rocks leads only to a decrease in their magnetization. If an increase in the magnetism of rocks occurred at the very beginning of hydrothermal metasomatism, then the zone of increased magnetization should be external with respect to the ore body, and beyond it unaltered rocks should be located. In our case, however, the external zone is—

a zone with low values of rock magnetization. Some explanation for this arrangement of the zones is provided by the graph of rock density along the section studied (Fig. 3, b).

The principal, widely distributed heavy element in the rocks and ores of the deposit is iron; therefore, the introduction or removal of iron should be reflected in the density of the rocks. Along the section studied, the distribution of rock-density values is quite regular. The density maximum falls on the vein. Here the iron occurs in sulfides. In the first magnetization zone distinguished by us, the rock density decreases to \(2.6\ \mathrm{g/cm^3}\). Here iron is present in the pyrite of the beresites. The second magnetization zone is characterized by a higher rock density than that of unaltered granodiorites. This indicates that, for the formation of secondary magnetite, not only the iron of decomposing mafic minerals was used, but also introduced iron.

For the third zone, from the density graph one may note a decrease in density toward the ore body. Since the rock density varies within small limits and its decrease toward the vein is weakly expressed, a statistical analysis of a possible trend was carried out to confirm the assumption of a regular behavior of the rock density in the third magnetization zone [4].

The probability of a random distribution of density values about the median is 0.286. Since the probability is not very small, the hypothesis of the absence of a trend cannot be rejected. At the same time, the rank correlation coefficient is 0.66, which indicates the possibility of a linear decrease in density toward the vein. In addition, the probability of a random distribution of density values relative to the regression line found by the method of least squares was also calculated. The probability obtained was 0.09, which is considerably less than the probability of a random distribution of density values about the median. On the basis of these studies it can be stated with confidence that the density of the rocks of the third zone decreases toward the vein. Thus, the decrease in the magnetism of the rocks of the third zone is connected not only with the transfer of iron into the nonmagnetic fraction, but also with its removal from the rocks, probably into the second zone and the vein. The possibility of borrowing iron during ore formation from the enclosing rocks is currently assumed for many deposits, including Darasun [5].

From the foregoing, the following conclusions may be drawn:

1. The rocks of the deposit are heterogeneous in magnetization. One of the reasons for this is the hydrothermal alteration of the rocks.

2. The influence of the hydrothermal process on the magnetization of rocks is polar: it can both increase and decrease magnetization.

3. For ore-hosting rocks, a zonality of their magnetism is characteristic, associated with the movement of iron during the hydrothermal process of rock alteration and with the staged character of this process.

REFERENCES

1. Betekhtin A. G. Hydrothermal solutions, their nature and the processes of ore formation.—In: Fundamental Problems in the Study of Magmatogenic Ore Deposits. Publishing House of the Academy of Sciences of the USSR, 1953.

2. Erofeev L. Ya. Magnetic surveying under the conditions of the Darasun gold-ore deposit.—In: Geology and Exploration of Mineral Deposits of Transbaikalia. Chita, 1968.

1. Kreiter I. V. Stages of near-ore metasomatism at a pyrite-polymetallic deposit in Western Transbaikalia.—In: Geology and Mineral Resources of Transbaikalia. Chita, 1967.

2. Miller R., Kahn D. Statistical Analysis in the Geological Sciences. “Mir,” Moscow, 1965.

3. Sakharova M. S., Nekrasov E. M. Mineralogical-geochemical characteristics and conditions of localization of new types of gold-sulfide ores of the Darasun deposit.—Geology of Ore Deposits, 1969, No. 3.

4. Shakhov F. N. Geology of Vein Deposits. “Nauka,” Moscow, 1964.