The magnetic stratification of layered mafic intrusions: Natural examples and numerical models

Abstract

Mafic–ultramafic intrusions commonly display a prominent petrographic and geochemical layering attributed to differentiation in a magma chamber. This layering occurs at scales from several hundreds of meters in some large intrusions, down to scales of a few millimeters. Layering-forming processes are numerous and include, both internal processes, such as crystal settling, double diffusive convection, flow segregation and compaction, and external processes such as magma-recharge, -extraction or -mixing. Mafic magma chambers may operate as a closed system or as an open system. Several recent investigations have been conducted on borehole cores through the Insizwa sill (South Africa), the Great Dyke (Zimbabwe), and the Bushveld Complex (South Africa). We present an overview of magnetic results on these intrusions. The magnetic properties of mafic rocks are dominated by the presence of magnetite in most layers. Pyrrhotite, the only ferrimagnetic sulfide, is generally restricted to sulfide-rich reefs (Merensky, J-M, Böhmke) or to massive sulfide deposits at the base of the intrusion. All intrusions display distinct variations of their magnetic properties (anisotropy of magnetic susceptibility, magnetic remanence, magnetic saturation, high field magnetic susceptibility) across the layering. The significance of these magnetic zones varies depending on the nature of variations from zone to zone. For example, a drop in magnetic susceptibility does not necessarily correspond to a new magma batch, unless it is also associated with a change in magnetic anisotropy parameters. In the studied cases, all open systems, the magnetic susceptibility and the total iron content (estimated from high field measurements) increase towards the top of these intrusions suggesting a general pattern of iron enrichment. A numerical petrologic model for a tholeiitic magma predicts magnetic susceptibility variations across a statically differentiated, closed system intrusion (no convection). The effects of sequential magma recharge and discrete magma extraction are investigated from the magnetic properties point of view. Conversely, these models can be used to interpret the magnetic stratification pattern in natural examples and test for its possible causes.