The Fosterville Goldfield is located in the Bendigo Zone of the Lachlan Fold Belt. The host rock lithologies in this zone are dominated by a sequence of folded and faulted Ordovician turbidites which were subsequently deformed in the Late Ordovician (450-430Ma) Benambran Orogeny. The sediment pile was deformed under east-west compression resulting in the formation of north-south folds. As this process continued and fold limbs steepened, a series of west-dipping reverse faults progressively developed. This generation of faults is interpreted to have a listric geometry and were likely conduits for ascending mineralised fluids.
There are two broad periods of gold mineralisation indicated by radiogenic dating across the western Lachlan Fold Belt. The first of these is associated with quartz vein hosted mineralisation and is concentrated from 425Ma to 400Ma. This period and style of mineralisation extended from Stawell in the west to Bendigo - Wattle Gully in the east. Fluid inclusion studies indicate that this style of mineralisation formed at temperatures between 320°C and 400°C and at depths of 5-10km (Roberts et al, 2003).
The Bendigo Zone was intruded by two granitic suites during the early Devonian and again in the late Devonian. These events appear linked to the second phase of mineralisation which occurred between 380Ma to 365Ma and extended from Ballarat in the west to the Woods Point - Walhalla belt in the east. The Fosterville mineralisation appears to have formed during this phase. Mineralisation from this second phase can manifest in a range of styles from quartz-carbonate vein hosted free gold through to sulphide hosted refractory gold in association with arsenopyrite, pyrite and stibnite (Roberts et al, 2003).
Deep weathering and erosion in the late Tertiary resulted in the development of a regional laterite profile with weathering locally to 50 metres depth.
The Stawell Goldfield is located in the western Stawell Zone of the Lachlan Fold Belt Figure 7-1. The Stawell Zone is a belt of predominantly deformed meta-sedimentary rocks representing the lower parts of the Cambro-Ordovician Lachlan Fold Belt stratigraphy bound to the west by the Moyston Fault and to the east by the Coongee Break.
Interpretations from the Victorian Geological Survey present a thin skinned tectonics model where the Moyston Fault is an east dipping basal detachment which has juxtaposed higher metamorphic grade rocks of the Stawell Zone against lower grade Cambrian rocks of the Delamarian Glenelg Zone. The west dipping Stawell Fault, Coongee Break and other parallel west dipping faults represent back thrusts from the Moyston Fault. These back thrusts have progressively emplaced deeper stratigraphy against shallower stratigraphy with a generally west over east sense. An apparent anomaly in this sequence is the presence of deeper magnetic stratigraphy in the Stawell-Wildwood corridor. Vandenberg et al. 2002 interprets that the Pleasant Creek Fault, to the west of the Stawell Fault, actually dips east and has an east over west sense - similar to the Moyston Fault. The Stawell-Wildwood corridor therefore represents a significant structural high in an up-thrown block of deeper stratigraphy between the Coongee Break and Pleasant Creek Fault.