Gold-bearing veins have formed at a wide range of depths
throughout the uplift history of the Otago Schist. The earliest gold
mineralisation occurred during the later stages of Mesozoic metamorphism. This
mineralisation was characterised by diffuse wall rock alteration with
greenschist facies metamorphic or retrograde metamorphic mineral assemblages,
and gold occurs in this altered rock. Quartz veins formed in .steeply dipping
extensional fractures associated with this mineralisation type strike north to
northeast. The Macraes mine exploits this deposit type. The present northwest
strike of the mineralised zone at Macraes is a result of post-mineralisation
deformation of the schistosity-parallel mineralised shear zone which hosts
northeast striking extensional veins. This combination of schistosity-parallel
shear zone and steeply dipping northeast striking veins may exist elsewhere in
Otago, and this deposit type is most prospective.
Steeply dipping quartz veins formed during
post-metamorphic un-roofing of the Otago Schist strike northwest to west and
occur as clusters of near-parallel structures in east and central Otago. These
veins consist mainly of massive quartz, although some prismatic quartz occurs
also. Breccia fragments in veins are common and these are generally silicified,
with some gold. Wall rock alteration is negligible, and vein margins are sharp.
A set of middle Tertiary veins occurs in northwest
Otago. Most of these also strike northwest and dip steeply, but some strike
northeast. These veins contain abundant ankeritic carbonate as well as quartz.
The carbonate pervades adjacent wall rock as veinlets, but gold mineralisation
is confined to veins. Quartz is commonly prismatic and vuggy, and contains gold,
which is commonly coarse grained especially in silicified breccia zones.
Associated veins contain stibnite. This vein type formed close to the
paleosurface (>2 km).
Mineralised veins in Marlborough currently strike northwest, but these have been
rotated from northeast strike during Alpine Fault displacement of Marlborough from Otago.
Some of these veins were formed under late metamorphic conditions and are
similar to the north to northeast striking veins in Otago. As in Otago, this
type should be considered to be most prospective if they have associated
schistosity-parallel shears.