The
Hyde Macraes Shear Zone is a mineralised low-angle shear within the Otago Schist
traceable for ~30 km with a NW strike. It is been actively mined along 12km of
this strike, producing >6 Moz of gold. The shear zone consists of greenschist
facies rocks with a mineralogy only subtly different from that of the host
schist. Pervasive fluid flow has occurred along grain boundaries and within
microshears which have been enhanced by the deposition of hydrothermal graphite.
We model the development of these microshears using a threedimensional numerical
code. For an inhomogeneous initial rock with lithostatic fluid pressures and a
dynamic permeability deformed under dextral non-coaxial flow, deformation is
distributed into weak shear bands. Graphite deposition and associated
reaction-softening has a dramatic effect on the development of model shear
zones. Graphite is deposited into a shear band, weakening it and further
localising the deformation into that shear band, producing an anastomosing and
interconnected shear zone structure such as observed in the HMSZ. The degree of
graphite precipitation and graphitic-induced weakening are varied in the models.
More graphite precipitation with a low degree of reaction-softening results in
more diffuse shear bands. In contrast less graphite deposition but with a high
degree of reaction-softening more leads to the development of more focused shear
bands. Graphite deposition has clearly controlled the formation of the HMSZ
structure, and is an important indicator of potentially mineralised structures
elsewhere.