Investigating the Significance of Magnetic Fabrics Preserved in Hydrothermally Altered Rocks

Anisotropy of Magnetic Susceptibility (AMS) and Anisotropy of Magnetic Remanence (AMR) are critical petrofabric tools commonly applied in investigating the evolution of volcano-magmatic, tectonic, and surface process systems. These highly sensitive techniques can distinguish multiple magnetic fabrics within individual samples, shown to be crucial in assessing archives of emplacement and deformation in intrusions where magmatic and tectonic processes occur concurrently or successively. They have also been used to understand magmatic processes within layered igneous complexes associated with the concentration of economic mineral phases. However, the application of AMS and AMR is hindered by the mineral phases that dominate magnetic properties and their susceptibility to hydrothermal alteration, potentially overprinting pre-existing petrofabrics. Despite the impacts of hydrothermal alteration being a well-known occurrence, the mechanisms and extent to which magnetic fabrics can be modified remains poorly constrained, raising concerns about the reliability of interpretations in studies involving hydrothermally altered rocks.

Our recent work assesses the significance of magnetic fabrics preserved in a hydrothermally altered fault zone that crosscuts a granitic pluton. Data were collected from unaltered granodiorite peripheral to the fault, the fault damage zone and the fault core to assess the impact of hydrothermal alteration on magnetic fabrics associated with magmatic and tectonic processes. Magnetic and hyperspectral data were used to characterise alteration distribution and intensity by quantifying changes in hydrous silicate and iron oxide phases. AMS and AMR fabrics were then measured and interpreted as either magma transport, tectonic, or hydrothermal alteration fabrics with context from field and petrographic data.

Our integrated hyperspectral-magnetic approach defines three alteration zones. Onset of hydrothermal alteration is identified from a subtle removal of white mica and low coercivity iron oxides (titanomagnetite) and the growth of new, high coercivity iron oxides (hematite) alongside chlorite and epidote. As alteration intensity increases, titanomagnetite and white mica are removed entirely, with hematite, epidote and chlorite becoming dominant in the system. In step with the changes in oxide and hydrous silicate mineralogy, we observe changes to AMS and AMR fabrics, with partial to complete destruction of tectonic and magmatic fabrics observed with increased alteration intensity. As these precursor fabrics are destroyed, they are replaced by a sub-vertical petrofabric defined by the alignment of hematite, interpreted as a product of hydrothermal fluid transport.

We demonstrate a threshold to alteration intensity, above which precursor petrofabrics are obliterated and replaced by fabrics associated with hydrothermal alteration. We envisage these results being highly informative in studies seeking to examine tectonic and mineralisation processes using rock magnetic methods.