Detecting buried placers by magnetic survey

Abstract

Concentrations of heavy minerals in alluvial deposits and laterites may include those of economic interest such as gold and also magnetite. Computer assisted modelling shows that if the magnetite content is at least 0.5% higher in a long plate of thickness 1 meter, width 10 meters, and depth 10 meters with respect to that of the sediments surrounding the buried plate, the latter is detectable by a detailed ground vertical gradiometerltotal field survey using a sensitive magnetometer (at least 0.1 nT). Testing over post-glacial river deposits in the Eastern Townships of Quebec shows well developed total field and vertical gradient trends with a range of spatial frequency components related to the valley directions rather than the NE-SW trend of the underlying bedrock, indicating that they are due to magnetite concentrations within the river sediments and/or buried channels in the bedrock. High intensity anomalies are observed in areas of ultramafic intrusions where magnetite is more abundant. Power spectra show well developed linear relation ships between the logarithm of the energy and wave number for the sorted river sediments but show a smooth relation over unsorted facial till, (homogeneous distribution of magnetite). These observations reinforce the conclusion that the anomalies observed are due to sources within the river sediments. Contour maps of total field and vertical gradient indicate the areas of primary interest in placer exploration.