A Report of the Magnetic Survey at the Miao-erh-kou Iron Mine, South Manchuria

Abstract

It is not necessary to review the conclusions that have been presented, but the importance of some of the problems may be emphasized to advantage. (i) The magnetic survey at the explored area of the Miao-erh-kou iron mine was confronted with a considerable difficulty owing to the unfavourable conditions: rich ore bodies are scattered in the lean ore deposit; the surface and underground workings are both considerably extended in the Main and Ling-nan shoots ; floats are scattered here and there ; besides, the topography is very steep. Therefore the lines of the equal magnetic anomaly were much disturbed, and the interpretation of the results was very difficult. (ii) The largest magnetic anomaly in the vertical intensity at Miao-erh-kou is 4.45 I and it is by far larger than that at Kiirunavaara, Kursk, and Caribou. The reason why such a great anomaly was found in the area, remains uncertain and may be a subject of scientific inquiry for the future. (iii) The relation between the area delineated by the lines of equal vertical anomaly and the horizontal area of the upper part of a deposit depends upon many factors: the shape, size, attitude, depth below the surface, magnetic property, and cause of magnetisation of the deposit, etc. Therefore, it is clear that the relation cannot be determined as a whole. In the explored area the outline of the deposit resembles the shape delineated by the 70° dip curve; the position of ore shoots, which have been explored before or are now worked, is clearly shown by the 80° dip curve; and the horizontal area exploited in the Ling-nan shoot corresponds to about 50 per cent of the area encircled by the 80° dip curve. (iv) The author found six points where rich ore bodies might probably exist. There may be no shoots equal to the Main shoot, but one of them may be somewhat larger than the Ling-nan shoot and the other may be equal to, or smaller than the latter. (v) Perhaps the Nashi-no-ki shoot, on which much hope was placed on account of its pretty outcrop, may not be workable as a rich ore body. (vi) There may be no room for the further horizontal development in the Main and Ling-nan shoots. (vii) The deposit lying on the upper stratigraphical horizon may be expected to extend further below the 1cth level, while the vertical extension of the deposit lying on the lower horizon is obscure. The range of the vertical continuance of ore shoots cannot be determined by the magnetic survey alone. The Main shoot may, however, extend to a considerable depth. As the probability of the vertical extention of the Ling-nan shoot is supposed to be less than that of the Main shoot, we must quickly advance the underground prospecting in the former. (viii) As the result of effort, a party was able to make observations at the average rate of 6.84 acres per 8-hour shift, though the topographical condition of the explored area: was very unfavourable. (ix) For the investigation of deposits of magnetite a less sensitive type of instrumtent, such as the Thalén-Tiberg magnetometer serves the purpose enough. It is necessary to balance the accuracy of the topographical survey, magnetic measurement, and plotting.