The changing microstructural arrangement of graphite during deformation and hydrothermal alteration of amphibolite-facies mylonite, Alpine Fault, New Zealand.

Abstract:

Graphitisation in a convergent plate boundary setting, such as the Alpine Fault, New Zealand, is associated both with fault weakening and orogenic gold mineralisation. Previously, these processes have been investigated in rocks that experienced mineralisation at maximum of greenschist-facies conditions. However, metals are most mobile at upper greenschist- to amphibolite-facies. We examine the microstructural record of mobilisation of graphite at these conditions due to dislocation and diffusion creep in the Alpine Fault zone and as a function of varying shear strain magnitude.

We have mapped graphite distribution across a strain gradient in samples, recovered from Deep Fault Drilling Project (DFDP) boreholes, by using reflected light and scanning electron microscopy. Raman spectrometry was used to determine the degree of maturity of the carbonaceous material. In the schists and protomylonites, graphite occurs as very fine (1-5µm), dusty grains, dispersed as inclusions in the main mineral phases (quartz, anorthite, muscovite, biotite). Further into the mylonite zone, the modal proportion of graphite increases and it forms clusters and trains, aligned with the foliation. In the brittlely-deformed rocks (cataclasites and gouges on or near the fault principal slip zone) graphite is most abundant (<50%), occurring as clusters and shear plane parallel trains.

We infer shear deformation under both ductile and brittle conditions concentrates the graphite. Independent evidence demonstrates fluid transport and consequent alteration was most important in the brittlely deformed rocks (Sutherland et al., 2012, Geology 40, 1143; Schleicher et al., in press. N.Z.J.Geol&Geophys). We thus infer hydrothermal enrichment caused graphite remobilization, re-deposition, and enrichment in structurally controlled microstructural sites. We will discuss implications of these microstructural and mineralogical changes for strain localisation and deformation-induced permeability.