Gravity and Magnetic Methods
Over thirty years ago geophysical prospecting for iron ore deposits in China began and has resulted in significant progress in all aspects. This paper presents a summary of the experiences gained in China. Field procedure has changed from prospecting along irregular grids to conducting systematic geophysical surveys over a large iron mining district. To improve the measurement of magnetic parameters, a static magnetic laboratory was established. The anisotropy of magnetic susceptibility of iron ore has been studied and model studies of orebodies carried out using both isotropic and anisotropic magnetic susceptibility. In China, the study of magnetic anomalies has gone through three stages; strong anomalies in the 1950s, weak and smooth anomalies in the 1960s, and complicated magnetic anomalies at the end of the 1970s. New iron ore deposits were discovered in every stage.
Strong lightning-induced remanence (Q-5) was found for the near-surface parts of banded iron formation in the Qianan mining district. The Q decreases to about one tenth this value in the deeper parts of the iron formation.
The magnetic, gravity, and induced polarization methods were tried in the search for hematite ores but no universally acceptable approach was found. Each area must be analyzed individually.
An example from the Chengchao iron deposit in Hubei province shows the need for initial interpretation to be followed by drilling and reinterpretation in several successive steps. At each step the residual anomaly defines a target for further drilling. Much additional tonnage was added using this process.
Scale model studies using anisotropic magnetic susceptibilities were particularly useful in studying synformal banded iron formations. Guides to interpretation were developed from these studies.
The Zhaocun deposit in Shanxi province, occurring in mountainous terrain, is of a complex synformal type. The interpretation of the ground magnetic anomaly over it was greatly aided by model studies and by removal of the topographic effect.
Figures & Tables
“This is the first collection of technical papers providing a general picture of exploration geophysics in China. Many case histories are included, plus some theory and technical developments.”