Turbidite-hosted, mesothermal lode gold deposits in
Paleozoic metasedimentary rocks of the Buller Terrane, western South Island have
a similar geological history and genesis to the Paleozoic lode gold deposits ofVictoria, Australia and Nova Scotia, Canada. In all three regions the gold
deposits are hosted by interbedded meta-sandstone and meta-argillite that have
been folded and regionally metamorphosed to greenschist facies, and locally in
Nova Scotia to
amphibolite facies. The gold deposits are typically narrow, structurally
controlled quartz veins associated with dilatant zones in reverse faults or
fold-related fractures. The principal vein types include: bedding parallel or
concordant veins, discordant veins, saddle reefs, and stockwork veins.
En-echelon and "ac" type veins are sometimes also gold-bearing in Nova Scotia. The
contrasting competency of the host meta-argillite and meta-sandstone beds is an
important factor in vein formation, particularly for the bedding parallel veins,
which are typically developed within meta-argillite beds or along the contacts
between meta-argillite and meta-sandstone. Veins found within meta-sandstone
units are generally discordant.
The deposits in
Victoria are well known for their bedding-parallel veins,
saddle reefs (
Bendigo
) and leather jacket reefs (Ballarat).
Bedding-parallel veins are also common in
Nova
Scotia
, along with a few examples of
saddle reefs. In contrast, vein formation probably occurred later in the
deformation history of the Reefton and Lyell goldfields, with most veins being
bedding discordant and more typically aligned with the axial plane cleavage of
the folds rather than bedding planes. The quartz lodes in one deposit,
Globe-Progress, strike approximately perpendicular to the bedding and fold axes.
No saddle reefs have been recorded from the Buller Terrane. Mesothermal vein
morphology reflects the amount of fmite strain, the strain rate, competency
contrasts, lithotypes, and the confirming pressures to which the system was
exposed. On the basis of comparable depositional depths and rheologies of the
host rocks, a controlling factor influencing the development of different vein
styles (and timing) might be finite strain, as exemplified by the tightness of
folding in the host rocks. Folding appears to be tighter in the Victorian rocks
than in either the Buller Terrane or Nova Scotian
rocks.