At Waiuta near Reefton there is a remarkable vein of
quartz <1 m thick which, apart from small offsets on late-stage faults, is a>
sheet like body essentially continuous down-dip and along-strike for> 1 km.
The vein strikes NNE and dips steeply west (032/78W). From 1906-1951 the
Blackwater mine produced 740,404 ounces of gold from the vein, called the
Birthday Reef', which was one third of the total production from Reefton
Goldfield and the fourth largest gold mine in New Zealand. Within the mine the
vein is disrupted into a series of northeast-plunging shoots by late-stage
faults. Similar faults, but with larger displacements, were thought in the past
to truncate the vein at the northern and southern ends of the mine. The
implication of this fault dissection model is that offset remnants of the vein
await discovery. This study investigated the geometry of the vein and its
relationship to host-rock structures and late-stage faults. A new model
considers folding and lithological variation in the host-rocks to be fundamental
in controlling the scale and geometry of veining, but does not rule out the
possibility of some late-stage fault dissection. The Birthday Reef truncates
200-400 m wavelength folds in Greenland Group host-rocks at c.10 angle, that
results in variation in the structural properties of host-rocks along strike
every ~1350-2500 m. The spacing of fold hinges in the wall-rock veins is
equivalent to the strike length of historic mining, implying that the Birthday
Reef is probably hosted in a single limb of a fold. Similar relationships
between mineralisation and host-rock folds appear to be present elsewhere in the
Reefton Goldfield (e.g. Capleston and Murray Creek group of mines). Exploration
targets are located one fold wavelength (c.1350-2500 m) further along strike
from the Blackwater mine and relatively barren rock is expected in between the
mine and target areas. Refining the location of folds and different host rock
lithologies, using geophysical data and quality geological mapping, is the key
to reducing exploration risks and discovering further
resources.