Aeromagnetics, geology and ore environments, I. Magnetite in igneous, sedimentary and metamorphic rocks: An overview

Abstract

Aeromagnetic (AM) surveys continue to play a prominent role in mineral exploration. The function of AM surveys is to map the distribution of magnetic minerals in the bedrock. Rock magnetic properties originate from the opaque minerals, of which the most abundant is magnetite. Interpretation is the drawing of inferences about the geology and ore potential of a given region from AM survey data; but it is hampered by a lack of clearly stated principles relating magnetic minerals to geology and to ore environments. This article, the first of a series, is a culling of materials from the geological literature which relate to the production and annihilation of magnetite in various geological environments. It begins with a description of the FeTi oxide minerals (the primary opaque oxide minerals in all rocks) and the factors which determine their induced and remanent magnetizations. The role of deuteric oxidation is of particular importance: rocks which have had different cooling histories will also have different magnetic properties, even if they have similar chemical compositions. The magnetic properties of rocks are determined by the partitioning of iron between the oxides of iron and titanium on the one hand, and (Fe, Mg) silicates on the other. The partitioning is controlled primarily by oxygen fugacity and temperature, and only secondarily by bulk chemistry. Biotite plays a special role in determining the amount of magnetite that can form during magmatic emplacement and also during subsequent metamorphism. Primary basaltic magmas differentiate along two different trends, depending upon whether oxygen levels are maintained externally, or whether they are allowed to drop. Magmas which follow the first trend produce primary Fe;Ti oxides and silica-rich end members. These give the calc-alkaline series, which are relatively deficient in magnetite. Thr second differentiation trend leads to primary Fe-silicates and FeO-rich end members which are silica-deficient. Rocks which follow this trend are relatively magnetic. The two trends correspond with quite different tectonic environments. The highest magnetic susceptibility values in either series should be found in rocks of intermediate composition approximating that of dacite or diorite, unless there has been alteration. During progressive regional metamorphism secondary magnetite can be produced at all stages, but mostly it develops under rather low-grade conditions by the conversion of hematite, or under high-grade conditions from the breakdown of hydrous (Fe, Mg) silicates such as biotite and hornblende into simpler orthosilicates. The magnetite-producing capacity of a rock during metamorphism is determined by attributes which are largely inherited from the premetamorphic state, of which the most important are total iron content, oxidation state, and degree of silica saturation. Other influencing factors include magnesium, aluminum and carbon content. Magnetite is destroyed during granitization and by low-temperature alteration. In interpreting AM survey data, close attention should be paid to all of the local factors that might have an influence upon magnetite, including mineralogy, chemical composition and metamorphic grade. Magnetite in metamorphic terrains often reflects the premetamorphic lithostratigraphy; hence magnetic interpretation maps should not be constrained to resemble maps of the existing geology.