Magnetic petrology of igneous intrusions: implications for exploration and magnetic interpretation

Abstract

Magnetic petrology integrates rock magnetism and conventional petrology in order to define the processes that create, alter and destroy magnetic minerals in rocks. By relating magnetic mineralogy, bulk magnetic properties, petrology and geochemistry to observed magnetic anomalies an understanding of the geological factors that control magnetic signatures is obtained, which can be used to improve geological interpretation of magnetic surveys. The magnetic properties of igneous intrusions, and hence the magnetic anomalies associated with them, reflect bulk rock composition, redox state, hydrothermal alteration and metamorphism. These geological variables are in turn controlled by tectonic setting, composition and history of the source region, depth of emplacement and nature of wall rocks. The fundamental control on magnetic mineralogy and bulk magnetic properties is partitioning of iron between silicate and oxide phases, which is strongly influenced by oxidation ratio. This paper reviews and synthesises information on relationships between the chemistry, mineralogy and metallogenic associations of igneous intrusions and their magnetic properties. Although links between magnetic properties and broad rock names are tenuous, refined rock classification enables magnetic properties to be predicted with reasonable confidence. Oxidised, magnetite-series, and reduced, ilmenite-series granitoids have quite distinct metallogeny. Cu, Mo and Au are associated with oxidised granitoids and Sn with reduced granitoids. Fractional crystallisation, which has a distinctive magnetic expression, plays an important role in generating magmatic-hydrothermal ore deposits. Hydrothermal alteration profoundly affects magnetic properties, in a generally predictable fashion. Implications for interpretation of magnetic anomalies associated with igneous intrusions and recognition of magnetic signatures of potential intrusive-related ore deposits are adduced.