Tracing Late-Stage Fluid Migration within Intrusions via Magnetic and Spectral Characterisation

Hydrothermal alteration and geofluid transport in magmatic systems plays a crucial role in the development of ore deposits, the systematics of geothermal resources and the structural stability of volcanic edifices. Characterising the type, intensity and distribution of alteration associated with geofluid pathways is therefore critical to understanding how essential resources form. However, alteration is routinely classified on the basis of highly subjective evaluations made by individual geologists or on single semi-quantitative datasets such as hyperspectral core analysis. Similarly, the role of alteration in controlling the distribution of strain is poorly constrained within magmatic systems. This study adopts a semi-quantitative approach to characterising hydrothermal fluid alteration using a novel combination of hyperspectral and magnetic analysis to efficiently characterise the silicate, oxide and sulphide mineralogy of a hydrothermally altered granitoid shear zone and its impact on strain development.

 

The monzodioritic Fand Pluton, NW Ireland, is a late Caledonian intrusion crosscut by a NE-SW shear zone in its eastern periphery. Field observations across the ≈10m wide shear zone show partitioned strain development, with ≈0.5m wide bands of heavily sheared and foliated granite interspersed between regions of strongly altered yet relatively undeformed granite. Alteration systematically intensifies toward the core of the shear zone, from a partial alteration of the host intrusion to a complete destruction of original rock texture.

 

Lab analysis aims to quantitatively evaluate the type and intensity of alteration across the shear zone and evaluate if zones of high strain systematically map to zones of high or low alteration. Hyperspectral reflectance data were collected using airborne multispectral and handheld hyperspectral instruments to characterise hydrous mineral phase assemblages within each alteration type. Magnetic characterisation experiments including hysteresis, first order reversal curves and temperature dependent susceptibility were combined to characterise the ferromagnetic mineral assemblage.  Anisotropy of magnetic susceptibility and anhysteretic remanent magnetisation were measured to determine the distribution of strain across the shear zone, evaluating the role of alteration intensity in the observed partitioning of strain.

 

Our results outline a multi-disciplinary method of mapping late-stage fluid transport within igneous intrusions, identifying pathfinder signatures and fabric parameters, linking them to alteration intensity from distance from fluid pathways.