On the Interpretation and Evaluation of Weak Magnetic Anomalies

G. K. Avten'ev, Z. A. Myshko

(Presented by the Department of Geophysical Prospecting Methods)

As applied to magnetite deposits, weak magnetic anomalies should be understood as anomalies with an intensity of no more than 3–5 thousand gammas. Such anomalies—on average in area, shape, and intensity—may be caused both by deeply buried ore bodies and by shallowly buried ore-bearing rocks. Distinguishing such anomalies from one another is a pressing but difficult task. Its solution is especially important in the geological evaluation of prospective iron-ore reserves both at known deposits and in new anomalous areas.

At magnetite deposits of the skarn and hydrothermal types, an increase in magnetite content is observed from the periphery toward the ore body. Ore bodies are most often confined to the contact zone of rocks that usually possess different magnetic properties; the contact zone itself, as a rule, represents a band of metasomatically altered rocks with magnetization different from that of the adjoining rock masses.

In the geological interpretation of magnetic anomalies in such areas, it becomes necessary to isolate and exclude the anomalous field from zones of heterogeneously magnetized host rocks in order to obtain residual anomalies corresponding to excess masses, which in many cases may be caused by ore or ore-skarn bodies.

Solving this problem is very complicated and is possible only when the law of distribution of the magnetization of the host rocks is known; this law is determined by petromagnetic studies of samples. To establish the presence of a linear variation in the magnetic properties of rocks, A. A. Lotachev [1] recommends calculating the horizontal gradient of the vertical component; its character serves as a criterion for the presence of a zone with a linear increase in magnetization, and from the gradient graph it is possible to estimate the parameters of this zone—its thickness and depth of occurrence.

This article presents some results on the interpretation of comparatively high-intensity anomalies of the Muryuk group (northwestern spurs of the Kuznetsk Alatau). Taking into account the geological setting and the magnetization of the rocks and ores, it proved possible here to isolate residual anomalies and evaluate their industrial prospects.

The Muryuk group of anomalies is confined to the left bank of the upper course of the Muryuk River. The magnetic anomalies are localized in two zones,

elongated as belts in a NNW direction. In the western zone, three anomalies are clearly contoured, of which the largest and most thoroughly studied is considered here. Discovered by aeromagnetic survey in the basin of the Staraia Olenevka River, this magnetic anomaly was studied in detail by ground magnetic-prospecting work and explored by pits and core-drilling boreholes that exposed magnetite ores. The total length of the anomaly is 2.4 km; its width along the 1500-gamma isodynamic lines is 200–300 m. The mean azimuth of the anomaly’s strike is 320°.

Within the anomaly, three local epicenters of elliptical cross section are distinguished by closed isolines of 2000–2500 gamma; the field intensity in them is 3000–12000 gamma. In the SSE part of the general anomalous zone, a sharp crowding of isolines is observed, with an increase of the horizontal gradient to 500–1000 gamma per meter. The anomaly is characterized by positive values of \(\Delta Z\); negative fields are absent. The \(\Delta Z\) graphs taken across the strike of the anomaly are characterized by asymmetry: the difference in the marginal values of \(\Delta Z\) is 500–1000 gamma (Fig. 1).

As a result of the mining work carried out (A. E. Gavrilov, A. I. Mostovskii, 1964), it was established that the magnetic anomaly is confined to the contact of plagiogranites of the Urgadat massif with plagioclase porphyrites of the Mundybash Formation. The porphyrites at the contact are hornfelsed; in certain areas these rocks are scapolitized and enriched with disseminated magnetite. This dissemination is maintained along the strike and covers a belt 50–160 m wide. Within this belt, intervals up to 20 m thick and more are distinguished, with an industrial iron content of up to 30–65%. The average content of total iron in the enriched rocks is not less than 10–15%.

The magnetization of rocks and ores (induced—\(I_i\) plus residual—\(I_r\)), according to measurements of 560 samples selected from pits and boreholes, varies within wide limits. The average total magnetization intensity of unaltered porphyrites is \(3000 \cdot 10^{-6}\) CGS. Here and below, calculations were carried out using the simplified formula

\[ I_{\mathrm{av}} = I_{i\mathrm{av}} + I_{r\mathrm{av}} \tag{1}, \]

where \(I_{\mathrm{av}}\) is the average total magnetization intensity;
\(I_{i\mathrm{av}}\) is the average value of the induced component of magnetization, calculated without allowance for the demagnetizing factor;
\(I_{r\mathrm{av}}\) is the average value of residual magnetization.

The magnetic field from such rocks may reach 1900 gamma. Rocks of granitoid composition in contact with the porphyrites have a magnetization intensity averaging \(1800 \cdot 10^{-6}\) CGS, i.e., almost 1.5 times lower. Accordingly, the magnetic fields from such rocks do not exceed 1200 gamma. The magnetization of hornfelsed rocks in the zone of the immediate contact, within 10–20 m, decreases to 0.001 CGS; with distance it increases to 0.01–0.25 CGS. The width of the zone of elevated values of magnetization intensity reaches 50–160 m. Beyond these zones the magnetization decreases and characterizes unaltered rocks. Where several closely spaced ore interbeds are present, the noted regularity is not manifested. The ores themselves possess the highest magnetization. Their average magnetization intensity is 0.07 CGS, which, with bed thicknesses of 10–20 m and a depth of occurrence of the upper edge of 10 m, may produce an anomalous effect of 14,000 gamma.

Along three calculated profiles of high-precision survey (levels 1, 4, and 8 m), run over the above-noted epicentral areas, we

a quantitative interpretation was carried out, taking into account the magnetic properties of the rocks from pits and boreholes. At each point where a pit had been sunk, the limiting value of the magnetic field produced by the enclosing rocks was calculated. The calculation was made by the formula

\[ \Delta Z_{\text{res}}=\Delta Z_{\text{obs}}-2\pi I_{\text{av}} \tag{2}, \]

\(\Delta Z_{\text{res}}\) — the value of the vertical component with a correction for the magnetic background of the rocks;

\(\Delta Z_{\text{obs}}\) — the observed values of the magnetic field, reduced to the normal level.

The residual anomaly obtained along calculation profile 1 for the level of 8 m, with an intensity of 4000 gammas, is satisfactorily compensated by four bodies with the following parameters (Fig. 2):

Here \(h_1\) and \(h_2\) are, respectively, the depths to the upper and lower edges of the disturbing body; \(2b\) are their horizontal thicknesses (in meters); \(I_{\text{av}}\) is the mean total intensity of magnetization; \(\alpha\) is the angle of dip (to the southwest); \(2L\) are the dimensions of the disturbing bodies along strike.

The calculation of \(\Delta Z\) from bodies 1 and 2 was made with K. F. Tyapkin’s palette [4] with \(L=5\) cm (the dimensions along strike were taken as 200 m), and for bodies 3 and 4 with \(L=2\) cm (accordingly, the dimensions along strike were taken equal to 40 m). In selecting the contours of bodies 1 and 2, data on the intensity of magnetization from borehole 20 were used. The magnitude of the calculated values

Fig. 1. Geological-geophysical plan of the Murokskaya anomaly (after E. N. Tshunel, 1962). 1 — porphyrites; 2 — granodiorites; 3 — established geological boundary; 4 — pits (and their numbers), from which rock samples were taken for determination of magnetic properties; 5 — column-drilling boreholes and their numbers (after materials of the Surazhskii GPE, 1963); 6 — calculation profiles of the detailed magnetic survey with an M-17 magnetometer at levels 1, 4, and 8 m (Tshumak S. N., Belogolov V. T., 1963); 7 — isanomals of the vertical component of magnetic-field intensity in tens of gammas.

Fig. 2. Example of the geological interpretation of a weak magnetic anomaly along calculated profile No. 1. 1 — curve $Z_a$, observed at a height of 8 m above the ground surface; 2 — residual curve, calculated by formula (2) for the 8 m level; 3 — theoretical curve from the assumed objects 1, 2, 3, 4, reduced to the observation level (the object numbers are shown by numerals in circles); 4 — calculated curve $Z$ for the same level from the enclosing rocks; 5 — deluvial deposits; 6 — porphyrites; 7 — granodiorites; 8 — magnetite ores according to mining data (A. E. Gavrilov, 1963); 9 — zone of intense disseminated magnetite, established or inferred from drilling data (A. E. Gavrilov, 1963); 10 — magnetite ore bodies inferred from geophysical data; 11 — zones of increased disseminated magnetite inferred from geophysical data (non-ore objects); 12 — trenches and their numbers, core-drilling borehole and its number.

of the principal parameters of objects 3 and 4 was checked against the nomogram of G. G. Orlov [2]. With \(2x^{0}=40\ \text{m}\), for different body shapes we have the following range of variation of the calculated parameters: \(h_{1}=4\text{–}14\ \text{m}\), \(2в=6\text{–}28\ \text{m}\), \(2l=19\text{–}34\ \text{m}\), \(h_{2}=23\text{–}48\ \text{m}\), \(\alpha=55\text{–}60^\circ\).

Objects 3 and 4, in terms of magnetization intensity, correspond to ore bodies. The calculated parameters of the objects agree satisfactorily with the data from shafts 462 and 452. Thus, the main field of the northern part of the anomaly is explained by magnetic rocks exposed by borehole 21. Similar calculations were carried out for profiles 2 and 3.

Using the method of A. M. Polonsky [3], we also estimated reserves from the digital plan of the magnetic survey at a scale of 1:2000, taking into account the magnetic background of the rocks.

As a result, the lower limit of possible iron-ore reserves was obtained, equal to 13 million tons.

Analyzing the results of the calculations performed, the following conclusions may be drawn:

1. The principal background field of the anomaly, previously taken to be the anomalous field of deep-seated ore objects, is explained by the rocks of the zone of increased dissemination of magnetite. Dipping to the southwest, these rocks create an elevated magnetic field in the southwestern part of the Muryuk anomaly area.

2. The principal epicentral parts of the anomaly are of ore origin. The ore bodies are represented by steeply dipping lenses, separated from one another, and having limited dimensions along strike and dip.

3. The maximum prospective ore reserves for the anomaly are estimated at 29 million tons.

REFERENCES

1. Logachev A. A. Magnetic Prospecting. “Nedra,” Moscow, 1968.

2. Orlov G. G. Nomogram for determining the depths of occurrence of deposits from the intersection points of derivative curves taken at different heights. Transactions of the Mining-Geological Institute, Ufa Branch of the Academy of Sciences of the USSR, issue 30, 1957.

3. Polonsky A. M. On the estimation of reserves of iron-ore deposits from magnetic-survey data. Izvestiya of the Academy of Sciences of the USSR, No. 1, 1963.

4. Tyapkin K. F. Interpretation of gravitational anomalies caused by geological objects finite along strike. Gosgeoltekhizdat, part II, 1963.