Lower greenschist
quartzo-feldspathic schists within the N-S trending Earnslaw Synform of the
Upper Wakatipu area are pervaded by late-stage
quartz filled extensional and extensional shear fractures, with quartz-scheelite
lodes infilling normal faults. Exposed veins/lodes mapped over at least 1000
km2 define a paleoflow system developed during late metamorphic
exhumation of the schist. Both veins and lodes have consistent angular
relationships to the schist foliation independent of their location on the
Earnslaw synform. Unfolding the synform leads to a regional subhorizontal
foliation and a systematic set of subvertical quartz veins striking NNW
extending over a depth interval >5 km, striking parallel to a set of normal
faults hosting the quartz-scheelite lodes. The structures define an extensional
regime with s1
sub-vertical, s3
oriented WSW-ENE, a low differential stress, and suprahydrostatic fluid
pressures.

The
quartz-scheelite lodes (<2 m thick) occupy faults which cross-cut the quartz>
veins, and are traceable for a few hundred metres, at most. The lodes consist of
laminated and/or massive quartz, slivers of crushed and folded schist, random
and discontinuous lenses of scheelite with clay selvages separating the lode
from relatively undisturbed wallrock schist. Textural evidence suggests that
slip developed incrementally with total displacement on faults during lode
formation on the order of a few ten's of metres.

The
distribution of Glenorchy veins demonstrates broad scale hydrothermal flow
through low-permeability schist, channelled within dilatant mesh style
structures comprising interconnected shear and extension fractures. Flow through
the lodes was likely derived from the greater surrounding area via the
interlinked network of fractures and focused into the low displacement shears.
Available evidence suggests the system developed together in a uniform
extensional stress regime toward the bottom half of the seismogenic
regime.