Deciphering deformation in ultramafic intrusions via magnetic fabric (AMS) and palaeomagnetic studies, Savannah Ni-PGE camp, NW Australia

Abstract

Knowledge of the internal architecture of mafic-ultramafic intrusions is critical to efficient targeting of magmatic Ni-PGE mineralisation. However, the visual homogeneity of such rocks in drill-core, coupled with poor outcrop in many areas can make their structure difficult to decipher using standard techniques. In this study we use measurements of anisotropy of magnetic susceptibility (AMS) and remanent magnetisation to examine the structural history of four adjacent, temporally equivalent, ultramafic intrusions from the east Kimberley Province, NW Australia. Whilst temporally equivalent, the intrusions have contrasting shape including bladed dyke, sill and stock geometries, and are hosted in rocks of contrasting rheology, including mafic granulite and meta-sedimentary migmatite. Early palaeomagnetic signatures present at Savannah and Savannah North originally had shallow NNE- to ENE-oriented inclinations, consistent with acquisition at low palaeolatitudes at ca 1800 Ma. However, these primary magnetisations were subsequently rotated during the ca. 1810 Ma Mount Stafford Event and/or ca. 1400–1200 Ma Yampi Orogeny, demonstrating that significant deformation occurred post-emplacement. Magmatic fabrics observed in AMS data for Savannah, Savannah North and Dave Hill all show evidence of NE or SW rotation during the same events. However, whilst there are similarities in the implied shortening directions across all four intrusions, they each preserve a unique deformation style, which is attributed to the unique shape of each intrusion coupled with the contrasting rheology of their respective host rocks. Each intrusion preserves a common dominant palaeomagnetic signature which is consistent with a high-grade metamorphic overprint at ca. 1200 Ma during the latter stages of the Yampi Orogeny. The lack of subsequent rotation of this dominant palaeomagnetic signature demonstrates that the majority of deformation pre-dates the Yampi event. This study demonstrates that even where tectonic conditions are similar, that intrusion shape and host rock rheology exert strong control on their deformation style.