On the use of magnetics and gravity to discriminate between gabbro and iron-rich ore-forming systems

Abstract

If the major rock-forming minerals are divided into three categories, and, if the density, magnetic susceptibility, and the proportions of each category are known, then the density and susceptibility of a mixture of the three categories can be determined. Conversely, if the physical properties of a mixture are known along with the physical properties of each of the three components, then the proportions of each component can be calculated. This reasoning can be used to superimpose category-percentage contours onto a density-susceptibility scatter diagram. Such a diagram can be used to relate petrophysical measurements on a rock specimen to its mineral category content, the veracity of which can be appraised by a geologist with a hand lens. Several hundred qualitative tests suggest that the approach is valid. The analysis is independent of scale, and can be applied to the bodies of a density-susceptibility model developed to simulate exploration data. Gabbro with magnetite plots along a specific locus on the combined phase/scatter diagram and can often be distinguished from denser accumulations of hematite and/or sulphides. Case histories for a number of economic and non-economic density-susceptibility anomalies give support for the usefulness of the method.