Interactions between
structure and lithology in the Hyde-Macraes Shear Zone controlled the style of
gold mineralisation at the Macraes mine. Large volumes of rock in some mine pits
have most gold disseminated in hydrothermally altered schist, rather than in
quartz veins. Mineralisation was initiated in the latter stages of metamorphism.
Pods of massive schist rich in quartz and feldspar are surrounded by more
fissile schist consisting of interlayered micaceous and quartzofeldspathic
schist. Later shears have been partly controlled by the micaceous schist, and
anastomose around pods of more massive schist. Micaceous schist was replaced by
pyrite and arsenopyrite, accompanied by recrystallisation of phengitic muscovite
under lower greenschist facies conditions. Gold occurs in sulfides and with
microcrystalline quartz formed during minor silicification. Massive schist is
locally mineralised along sulfidic microshears which have phengitic muscovite
largely illitised under sub-greenschist facies conditions. Mineralised massive
and micaceous schists were overprinted by cataclastic shears, with addition of
graphite and more sulfides. Mineralisation occurred during a transition from
ductile to brittle deformation regimes as the schist was uplifted in the late
Jurassic.