The
Fosterville Goldfield has a modest past production (0.3 Moz), but is now
emerging as a significant goldfield (2.6 Moz resources, including 1.1 Moz
reserves) with implications for exploration within the orogenic gold provinces
of the Lachlan Fold Belt in Australia and the Buller Terrane on the West Coast
of New Zealand. Fosterville is located 20 km east of the 22 Moz Bendigo Goldfield and is
geologically similar to other Victorian goldfields, however the nature of the
gold mineralisation differs significantly. Unlike other Victorian goldfields
there is no primary free gold at Fosterville. The gold occurs in as sub-SEM
resolution particles, in fine grained arsenopyrite and pyrite crystals
disseminated through the wall rocks in fault controlled quartz-carbonate vein
stockworks. The Fosterville Goldfield comprises several subparallel fault
systems striking 340 over an exposed strike length of 28 km. The Fosterville
Fault system comprises the west dipping (~75), reverse Fosterville Fault and
several less steeply west dipping splay faults formed in the footwall. The splay
faults extend through east dipping beds from the Fosterville Fault across a
syncline hinge into stratigraphically controlled positions on the west dipping
limb of the syncline. The predominant control on gold mineralisation is the
intersection of the bedding on the east dipping limb of the syncline and
faulting which, is in turn controlled by the orientation of bedding in the west
dipping limb of the syncline. The mineralised shoots are thus subparallel to the
plunge of the syncline and show a plunge reversal about a culmination in the
footwall syncline. This provides a very powerful exploration tool with bedding -
cleavage intersection lineations measured in outcrop and oriented core
indicating the plunge of mineralisation. Mineralisation is also, albeit less
commonly, controlled by local variations in strike and dip of the host fault and
at metre scale by host lithology. As the faults and mineralisation are
stratigraphically controlled, detailed stratigraphic interpretations are
essential to provide control on structural interpretations for resource
modelling. Compared to other Victorian goldfields, the Fosterville Goldfield
formed later (385 Ma to 360 Ma cf 440 Ma), at lower temperatures and at
shallower depths (fluid inclusion studies of Au associated veins suggest vein
formation at ~270C (range 140C to 385C) and at depths of 2.6 - 5.7
km).