Experience from Studies of a Non-Ore Magnetic Anomaly
Abstract Generated abstract
This paper examines the Dzheb magnetic anomaly in the Krasnokamensk group of magnetite deposits in the Eastern Sayan, where field intensity and geological setting initially suggested possible magnetite mineralization. The study combines trenching and drilling with measurements of magnetic susceptibility and remanent magnetization in oriented rock samples, followed by calculation of theoretical magnetic fields from the observed section. The results show that hornfelsed and skarnized effusive rocks possess magnetic properties sufficient to account for the anomaly, while pits and two boreholes encountered no ore. The case demonstrates that relatively simple geophysical measurements and field modeling can help distinguish barren magnetic anomalies from ore-related ones.
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Experience of Investigations at a Barren Magnetic Anomaly
R. V. Terekhova
(Presented by Professor D. S. Mikov)
In the area of the Krasnokamensk group of magnetite deposits in the Eastern Sayan, magnetic surveys have revealed a number of anomalies, some of which owe their origin to rocks possessing elevated magnetic properties, but not to ores.
Over the past ten years, the Dzheb magnetic anomaly has several times attracted the attention of geologists. The area of the anomaly along the 2000-gamma isoline is 0.77 sq. km; the magnetic-field intensity at the epicenters usually does not exceed 5000–7000 gammas, and only at one point does it reach 19,300 gammas. On the northwestern flank of the anomaly a negative field is observed, with an intensity of up to 500 gammas. In terms of its dimensions and the volume of the magnetic field, the Dzheb anomaly resembles the anomaly of the Odinochnoye deposit, whose reserves are estimated at approximately 80 million tons.
The geological setting in the anomaly area is considered favorable for the formation of a magnetite deposit. Between two intrusive massifs of granitic and granitoid composition lies a sequence of strongly altered metamorphosed effusive rocks, to which the epicentral part of the magnetic anomaly is confined. The effusives are cataclased, schistose, and penetrated by veinlets of quartz–epidote composition. The effusives are characterized by hornfelsing and enrichment with finely disseminated magnetite, which in places forms small nests and interbeds.
Table 1
| Rock name | No. of samples | Magnetic susceptibility, in CGS units | Residual magnetization, in CGS units | Induced magnetization, in CGS units | Theoretical field, in gammas |
|---|---|---|---|---|---|
| Albitophyres | 20 | 0 | 0 | 0 | 0 |
| Granites | 25 | 0.00007 | 0.00007 | 0.00008 | 50 |
| Hornfelsed effusives | 26 | 0.00900 | 0.00475 | 0.00800 | 5000 |
| Hornfels from the epicenter of the anomaly, where the field intensity is 19,500 gammas | 3 | 0.02681 | 0.04150 | 0.04245 | 26000 |
The study of the rock of the anomaly was begun by driving several pits and trenches in its epicentral part. No mineralization was encountered. A study of the magnetic properties of samples taken from pits in the epicentral part of the anomaly showed that the effusive rocks, especially the hornfelsed ones, possess high magnetic susceptibility and an even higher remanent magnetization (Table 1). Rocks having such a high intensity of magnetization can produce anomalous fields with an intensity of up to 20,000 gammas and more. Nevertheless, a borehole was sunk beneath the epicenter of the anomaly; it passed through 350 m of hornfelsed effusive rocks with high magnetic properties.
Fig. 1. Magnetic plan of the Dzheb anomaly:
1 — isolines of the positive intensity of the vertical component of the magnetic field, in millioersteds; 2 — zero isoline of the magnetic field; 3 — negative isolines; 4 — points of collection of oriented samples and the theoretically possible magnitude of the magnetic-field intensity, in millioersteds, calculated for the half-space; 5 — the exploration line along which the section is given
Despite the absence of ore in the section of the first borehole, interest in the anomaly did not diminish, and its study continued. Almost the entire area of the anomaly within the 2000-gamma isoline was covered by a network of pits. Measurement of the magnetic properties of oriented rock samples from the pits showed that the albitophyres are practically nonmagnetic, the granites are weakly magnetic, and the hornfelsed effusive rocks from the region of fields up to 5000 gammas possess increased magnetic susceptib—
...susceptibility and significant remanent magnetization, which is entirely consistent with the observed fields (Fig. 1). In addition, a theoretical curve was calculated from the section along the profile located in the plane of borehole No. 1. The theoretical curve coincided with the observed one, and no ore bodies could be expected from the magnetic data.
Fig. 2. Section along the exploration line through the epicenter of the Dzheb magnetic anomaly:
1 — observed curve of the vertical component of the magnetic-field intensity; 2 — theoretical curve selected from the known section; 3 — nonmagnetic boundaries; 4 — weakly magnetic porphyrites; 5 — very weakly magnetic brecciated effusives; 6 — nonmagnetic tuffs; 7 — skarnized effusives with magnetization intensity 0.005 CGS; 8 — skarnized and hornfelsed effusives with magnetization intensity 0.012 CGS.
Despite the negative results of the geophysical work, a second borehole was assigned behind the first. This time, on geological grounds, ore bodies were expected at a depth of 200–300 m. The borehole was drilled to a depth of 500 m, but encountered no ore (Fig. 2). This once again proved the non-ore nature of the Dzheb magnetic anomaly and confirmed the conclusion reached on the basis of analysis of the magnetic field, measurement of the magnetic properties of rock samples, and calculation of theoretical fields.
Thus, the use of the simplest geophysical methods of investigation can sometimes substantially help in resolving the question of the geological nature of magnetic anomalies.