Australian Journal of Earth Sciences
ISSN: 0812-0099 (Print) 1440-0952 (Online) Journal homepage: https://www.tandfonline.com/loi/taje20
Role of faults and folding in controlling gold
mineralisation at Fosterville, Victoria
L. D. Leader , J. A. Robinson & C. J. L. Wilson
To cite this article: L. D. Leader , J. A. Robinson & C. J. L. Wilson (2010) Role of faults and folding
in controlling gold mineralisation at Fosterville, Victoria, Australian Journal of Earth Sciences,
57:2, 259-277, DOI: 10.1080/08120090903552726
To link to this article: https://doi.org/10.1080/08120090903552726
Published online: 17 Feb 2010.
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Australian Journal of Earth Sciences (2010) 57, (259–277)
Role of faults and folding in controlling gold
mineralisation at Fosterville, Victoria
L. D. LEADER*, J. A. ROBINSON{ AND C. J. L. WILSON
School of Earth Sciences, University of Melbourne, Vic. 3010, Australia.
Structural and fluid-flow evolution at the Fosterville gold deposit in central Victoria was influenced by
three distinct stress regimes that each played a role in localising the sulfide-hosted gold deposits.
Major gold mineralisation at Fosterville is younger than at the majority of auriferous quartz-reef-hosted
deposits in the Bendigo Zone, which formed at around 440 Ma, during regional east–west shortening
of the host Ordovician turbidites. In contrast, wall-rock-hosted disseminated, fine-grained auriferous
pyrite–arsenopyrite mineralisation at Fosterville post-dated both the development of chevron-shaped
upright folds and fold-related faults and the emplacement of Early to Middle Devonian felsic dykes.
Here we show that the primary structural control on the distribution of the auriferous sulfides was
related to local reactivation of the west-dipping Fosterville Fault and associated low-displacement
splay faults that truncate a south-plunging syncline. Analysis of slickenline lineations on slip surfaces
indicates most faults initiated as dip-slip, reverse structures during regional east–west compression,
and were subsequently reactivated with sinistral–reverse displacement during later northwest–
southeast compression, coincident with gold mineralisation. Kinematic analysis of conjugate sets of
quartz–carbonate veins that are temporally and spatially associated with the auriferous sulfides at
Fosterville also indicates mineralisation occurred during northwest–southeast shortening. A third stress
regime, characterised by north–south shortening, is inferred to have been active during the late
stages of mineralisation at Fosterville. Late stibnite mineralisation associated with bedding-discordant
faults at Fosterville is similar to that associated with many Devonian-age gold deposits in the
Melbourne Zone to the east. The absence of quartz reefs, the scarcity of visible free gold in primary
ore and the wall-rock-hosted disseminated character of the sulfide ores at Fosterville are atypical of
gold deposits in the Bendigo Zone. However, due to the important role of fold and fault relationships
in localising mineralisation the geometry and structural setting of the Fosterville deposit is similar to
many other goldfields of the Bendigo Zone.
KEY WORDS: deformation, faults, Fosterville, gold, Lachlan Fold Belt, mineralisation, structure, Victoria.
INTRODUCTION
The Fosterville goldfield is located *135 km north of
Melbourne, hosted by quartz-rich Ordovician turbidites
of the Bendigo Zone (Gray et al. 2003) in the western
Lachlan Fold Belt (Figure 1). The majority of gold
deposits in the Bendigo Zone are hosted by quartz veins
associated with low-displacement, steeply dipping reverse faults in the hangingwalls of major intrazone
faults, such as the Whitelaw Fault (Willman 2007).
Fosterville is atypical with gold occurring in solidsolution in arsenopyrite and pyrite grains (Scott 2008)
that are disseminated in the wall-rock around extensive
networks of fault-related quartz–carbonate veins.
Another feature of Fosterville that is atypical of the
Bendigo Zone is the presence of stibnite mineralisation.
Previous structural studies of the Fosterville goldfield have been dominated by observations made within
the oxide zone of old openpits. Wang & White (1993)
concluded that mineralisation at Fosterville occurred
during reverse sinistral reactivation of an earlier
fold-related fault system in a series of shear zones. On
the other hand, King (2004) identified that mineralisation is controlled by a series of splay fault intersections,
without a large shear component, with enhanced gold
grades being controlled by the fault-bedding orientation
relationship. On a regional scale Boucher et al. (2008)
concluded that the Fosterville mineralisation is hosted
by a stratigraphic succession with a similar facies
association to the Bendigo goldfield and that both
goldfields have linked systems of laminated quartz veins
and thrust-faults, hosted by a series of thick shales, that
propagated fold hinges to truncate fold limbs. However,
at Fosterville, the reverse faults are mostly west-dipping
and largely unmineralised, whereas, at Bendigo similar
structures dip both east and west and commonly host
gold. Boucher et al. (2008) attributed the dominance of
west-dipping faults at Fosterville to the fact that the
goldfield sits on the west-dipping limb of a regional-scale
fold. In contrast, the Bendigo goldfield occurs within the
*Corresponding author: ldleader@unimelb.edu.au
{
Present address: CSIRO Exploration and Mining, Australian Resources Research Centre, Bentley, WA 6102, Australia.
ISSN 0812-0099 print/ISSN 1440-0952 online Ó 2010 Geological Society of Australia
DOI: 10.1080/08120090903552726
260
L. D. Leader et al.
Figure 1 Structural zones of the western Lachlan Fold Belt (adapted from Gray et al. 2003). The inferred location of the Selwyn
Block (adapted from VandenBerg et al. 2000) is outlined by the dashed line.
core of a synclinorium consisting of symmetric folds
with subhorizontal enveloping surfaces (Willman 2007).
This paper presents new structural data on the
orientation of faults, associated slickenlines, and veins
observed within and surrounding the Fosterville openpits and underground workings that lie below the oxide
zone where mineralisation is dominated by the primary
sulfides. These observations are used to determine the
kinematic history of faulting at Fosterville and relate
this to the structural evolution of the Bendigo Zone.
This comparison is aimed at highlighting the similarities and differences between the Fosterville goldfield
and other goldfields of central Victoria.
GEOLOGICAL SETTING AND TIMING OF
MINERALISATION
Turbidites of the Lower to Middle Castlemaine Group
(VandenBerg et al. 2000) are the primary hosts to
gold deposits in the Bendigo Zone. The turbidites
underwent deformation associated with the Late Ordovician Benambran (455–440 Ma) and the Late Devonian
Tabberabberan (*380 Ma) Orogenies (VandenBerg
et al. 2000). Deformation resulted in the development of
upright, gently plunging, north–south-trending folds
and associated axial-planar cleavage, which forms the
dominant penetrative deformation fabric in the Bendigo
Zone (Gray et al. 1991; Gray & Willman 1991; VandenBerg et al. 2000; Gray et al. 2003). Other deformation
fabrics developed in the Bendigo Zone include two lesspenetrative fabrics associated with thrusting and
faulting (Schaubs & Wilson 2002), and extensional and
contractional kink bands that formed in response to
north–south shortening (Forde 1991; Forde & Bell
1994; Raine 2005). Following deformation, Late and Early
Devonian granites were emplaced into the turbidite
sequence (Figure 1).
Metamorphic grade in the Ordovician sequences
at Fosterville is within the anchizone (prehnite–
pumpellyite facies) as identified from studies of illite
crystallinity (Wilson et al. 2009). Higher-grade epizonal
(greenschist facies) metamorphic conditions are recorded west of the Whitelaw Fault (Figure 1) and this
change is attributed to the rocks being exhumed as the
edge of the Selwyn Block (Figure 1) moved westward
during the Middle Devonian Tabberabberan Orogeny at
*380 Ma (Wilson et al. 2009). The fluid-inclusion study
by Mernagh (2001) also highlighted differences in the
conditions of mineralisation at Fosterville compared
with other Bendigo Zone gold deposits. In particular,
Mernagh (2001) concluded that the Fosterville goldfield
formed at lower temperatures (*2708C) and at a higher
crustal level (2.7–5.6 km) than other goldfields, which
typically formed at 300–3208C and depths of 5–10 km (Cox
et al. 1995). Illite crystallinity results from sericite in
Fosterville goldfield, Vic.
mudstones at Fosterville also suggest there is no marked
change in metamorphic grade from unmineralised to
mineralised regions (Wilson et al. 2009).
40
Ar/39Ar and U–Pb SHRIMP studies have established that there are three broad periods of gold
mineralisation in the Bendigo Zone (Foster et al. 1998;
Bierlein & Maher 2001; Bierlein et al. 2001a). The first
and major period of mineralisation, dated at 455–440 Ma,
coincides with the time of the Benambran Orogeny and
is considered to be a hydrothermal event involving the
circulation of metamorphic fluids through the crust
(VandenBerg et al. 2000; Phillips et al. 2003). This event
is characterised by strong carbonate and sulfide alteration and weak sericite alteration proximal to fault zones
(Gao & Kwak 1997; Phillips et al. 2003). Some 40Ar/39Ar
and U–Pb SHRIMP studies suggest the age of deposits
associated with this event decreases towards the east,
reflecting the eastward migration of deformation and
metamorphism (Bierlein et al. 2001b).
The second period of mineralisation occurred between 420 and 410 Ma and involved new pulses of gold
deposition and the remobilisation of gold into new
structures (Bierlein et al. 2001a). The third event
occurred at 380–370 Ma (Bierlein & Maher 2001). These
second and third events are also characterised by
carbonate and sericite alteration, although they are
distinguished from the first event by elevated concentrations of Mo, Cu, Sb and W. Deposits associated with
these events post-date the emplacement of Late Silurian
to Late Devonian felsic dykes, so are spatially related
and may even be genetically related to magmatism
(Arne et al. 1998; Bierlein et al. 2001a).
Dating of the gold mineralisation at Fosterville has
predominantly been focused on a mineralised felsic
quartz porphyry dyke that intrudes the O’Dwyers Fault
Zone, *1.5 km east of the Fosterville Fault, at Robbins
Hill (Figure 2). U–Pb SHRIMP analyses of magmatic
zircon grains from this dyke yield an age of 388 + 14 Ma
(Arne et al. 1998). Bierlein et al. (2001b) obtained
40
Ar/39Ar ages of 398 + 3 Ma and 381 + 2 Ma. 40Ar/39Ar
analyses of mineralised sandstone and mudstone yield
less-precise ages that range from *400 to *520 Ma
(Bierlein et al. 1999, 2001b; MacCulloch 2005). As noted
by Fu et al. (2007) the available data do not provide a
definitive age for the mineralisation at Fosterville,
although it probably occurred between 400 and 380 Ma.
This indicates that gold mineralisation at Fosterville is
younger than that at the majority of quartz-reef-hosted
deposits of the Bendigo Zone that are associated with the
Benambran Orogeny.
FOSTERVILLE GEOLOGY
The Ordovician turbidite sequence at Fosterville
is extensively covered by Permian glacial deposits,
Quaternary basalts and alluvial sediments (Figure 2a).
These overlying rocks and the paucity and poor quality
of the outcropping Ordovician rocks present difficulties
when attempting to put the Fosterville goldfield into a
regional perspective. However, structural relations at
scattered small outcrops along watercourses and roadsides to the south of the mine lease (Figure 2b, c)
261
indicate that fold geometries are broadly similar to
those elsewhere in the Bendigo Zone (Gray et al. 2003;
Willman 2007).
A series of axial traces are identified along the
Campaspe River (Figure 2b), based on changes in the
younging direction of the sedimentary rocks as well as
changes in vergence relationships between bedding (S0)
and the dominant spaced cleavage S2. These folds plunge
to the south and from cross-section A–A0 (Figure 3a) it is
estimated that the folds have wavelengths that range
from 190 to 325 m.
Along the Sugarloaf Road (Figure 2b), outcrop in
the drainage channels on the sides of the road expose a
syncline and anticline. Angular relationships between
bedding and cleavage in the limbs of these folds indicate that they plunge to the north. Cross-section A–A0
(Figure 3a) indicates that these folds have a wavelength
of *175 m. Changes in the orientation and younging
direction of bedding in outcrops along Axe Creek suggest
the presence of fold closures between the outcrops.
Cross-section A–A0 (Figure 3a) shows that these folds
have a wavelength of *350 m. However, the scattered
nature of the outcrops along Axe Creek as well as the
shorter wavelength of folds observed along Sugarloaf
Road and the Campaspe River suggest that there may be
more fold closures present within this area.
At 277704mE 5929065mN*, on the western side of the
Axedale–Goornong Road a steep west-dipping fault is
exposed in old workings. The fault is defined by a
penetrative west-dipping fabric and a 6 cm-wide fault
gouge that consists of angular pebbles up to 4 cm
suspended in a fine-grained matrix with extensive
iron-oxide alteration. Another fault is exposed in a
nearby road-cutting on the eastern side of the Axedale–
Goornong Road at 277676mE 5928695mN. This fault dips
at *608 to the east and is defined by a 2 cm-wide gouge
zone consisting of angular rock fragments and quartz
pebbles. In both cases the lack of outcrop along the
projected strike of these faults mean that the extent and
importance of these faults could not be determined.
The inferred trace of the Fosterville Fault is shown
on Figure 2b to be *200 m east of the Axedale–Goornong
Road and subparallel to the road. No evidence of the
Fosterville Fault could be found within the area of
the map. However, it is noted that the inferred trace of
the Fosterville Fault is subparallel to a north–south
alignment of scattered old workings, many of which
have been backfilled.
Recent years of below-average rainfall across central
Victoria have resulted in the exposure of good outcrops
of Ordovician rock along the receded shoreline of Lake
Eppalock. Structural data collected from these outcrops
is presented in Figure 2c. This map shows a series of
axial traces that have been inferred from changes in the
orientation and younging direction of bedding. Crosssection B–B0 (Figure 3b) shows that the folds associated
with these axial traces have wavelengths that range
from 250 to 1125 m. The angular relationship between
bedding and cleavage implies that these folds plunge at a
low angle to the south. This is further confirmed by
bedding–cleavage intersection lineations observed on
*All grid references are in the AMG66 datum.
262
L. D. Leader et al.
Figure 2 (a) Geological map showing the location of the Fosterville mine lease and openpits 1–6. (b) Structural map of the
Campaspe River and Axe Creek area [see (a) for location]. (c) Structural map of the Lake Eppalock area [see (a) for location].
Boundaries of lithological units adapted from Wohlt & Edwards (1999a, b) and Parenzan et al. 2001a, b).
Fosterville goldfield, Vic.
263
Figure 3 (a, b) Cross-sections showing the nature of the folding (looking north) in (a) the Campaspe River and Axe Creek area
and (b) Lake Eppalock area. See Figure 2b, c for location. (c) Cross-section at 8350 mN (mine grid) illustrating the nature of
the Fosterville and Phoenix faults and the distribution of mineralisation. (d) Cross-section at 12400 mN (mine grid) showing
the nature of the O’Dwyer’s Fault Zone.
bedding surfaces that plunge up to 158 to the south
(Figure 2c). An exception to these observations occurs in
the southeastern corner of the map where bedding–
cleavage intersection lineations plunge at *58 to the
north (Figure 2c).
MINE-LEASE GEOLOGY
The two main structures associated with gold mineralisation at Fosterville are the Fosterville and O’Dwyers
fault zones (Figure 2a). The Fosterville Fault is a reverse
fault that dips steeply towards the west and comprises
a series of segments that are oriented parallel to the
strike of folds. Wang & White (1993) observed that these
segments are cross-linked by oblique faults that give the
Fosterville Fault an en échelon appearance and an
overall north-northwest (3458) trend. Mineralisation
along the Fosterville Fault was restricted to the footwall
and is in part controlled by a series of low-displacement
west-dipping splays that cross-cut bedding and the
axial surface of folds to link west-dipping beddingparallel laminated quartz veins on adjacent fold-limbs
(Figure 3c). These splays include the mineralised
Phoenix and Harrier Faults, which truncate a doubleplunging syncline in the footwall of the Fosterville Fault
(Figure 3c). The most intensely mineralised portions of
the Fosterville orebody occur south of the fold plunge
reversal in the vicinity of the Falcon Pit (Figure 2).
The O’Dwyers Fault Zone is 50–80 m wide and
bounded by north–south-trending, steep west-dipping
reverse faults that are cross-linked by lower displacement northwest–southeast-trending fault structures
(Figure 3d). Mineralisation within this zone was restricted to dilational sites at the intersections of fold
hinges and northwest–southeast-trending faults (Davis
2006). Felsic quartz porphyry dykes have propagated
along the axial surface of folds and as such also intrude
the O’Dwyers Fault Zone. These felsic dykes are
mineralised only in the vicinity of the fault zone and
appear to post-date much of the shearing as they are not
significantly offset by the fault zone (King 1998; Davis
2006).
Figure 4 presents a series of cross-sections through
the openpits adjacent to the Fosterville Fault, and
illustrates the nature of the geology in the mine lease.
At Daley’s Hill (Figure 4a) bedding predominantly dips
steeply (60–708) to the west except within Zone 1 where
bedding dips 708 to the east. Within the sequence of westdipping units there are a number of bedding-parallel
faults that occur either along bedding contacts or in
the upper portion of mudstone units. Slickenline lineations and extensional quartz veins associated with
these faults indicate a southwest over northeast sense
of movement. Other faults at Daley’s Hill truncate the
west-dipping bedding and have transport directions
that vary from southwest over northeast to northwest
over southeast. Intersection relationships between S0
and S2 at Daley’s Hill indicate that folds (F2) plunge
south at 10–308.
John’s Pit (Figure 4b) is located in the hinge region
of a gently south-plunging syncline with a rounded
hinge and interlimb angle of 508. Numerous quartz veins
and faults are developed in the core of the syncline.
A west-dipping bedding-parallel fault, with an extensive
hangingwall stockwork zone, occurs in the eastern
limb of the syncline 20 m east of the axis. Slickenline
lineations associated with this fault indicate a northwest over southeast sense of movement. The western
limb of the syncline is truncated by the west-dipping
Harrier Fault with a northwest over southeast movement sense. Bedding–cleavage intersection lineations in
the immediate hangingwall of this fault indicate that
folding plunges 58 north.
The Harrington’s Hill Pit is centred on a structurally complex fault-bound zone (Figure 4c, Zone 3)
associated with the immediate hangingwall of the
Harrier Fault. This zone occurs in the centre of the
pit and consists of numerous west- and east-dipping
faults and veins that truncate and offset bedding.
Younging directions and bedding orientations indicate
that these fault-bound blocks have undergone significant rotation. Due to the lack of vertical exposure it
cannot be determined whether this rotation is purely
due to faulting or due to the presence of faulted-out fold
closures within this zone. To the east and west of the
264
L. D. Leader et al.
Figure 4 Cross-sections, viewed looking north, illustrating the structural geology of the openpits in the southern area of the
mine lease. (a) Daley’s Hill. (b) John’s Pit. (c) Harrington’s Hill. (d) Sandhurst Pit. See Figure 2a for location of openpits.
Adjacent to the sections are equal-area stereographic projections showing the orientation of the principal faults and veins.
Fosterville goldfield, Vic.
fault-bound zone the geology consists of west-dipping
interbedded sandstone and mudstone that form limbs
of south-plunging synclines. To the west of the faultbound zone, bedding has been offset by moderately
east-dipping reverse faults that have a northeast
over southwest sense of movement. At 1902mE (mine
grid) there is a change in the younging direction
from west to east that is marked by a steep westdipping fault with a southwest over northeast sense of
movement.
Three structural zones defined by differences in the
plunge of folds, bedding orientations and younging
directions are exposed in Sandhurst Pit (Figure 4d).
The westernmost zone (Zone 1) is characterised by westdipping and west-younging bedding and north-plunging
bedding–cleavage intersection lineations. Bedding in
the central zone youngs to the east but dips both steeply
west (i.e. overturned) and to the east. Bedding–cleavage
intersection lineations plunge gently to the south.
Moderately to steeply west-dipping faults in this zone
have associated subhorizontal extension vein arrays
implying reverse displacement. An upright, tight to
isoclinal south-plunging syncline occurs in the easternmost zone (Zone 3).
MESOSCOPIC STRUCTURAL FEATURES
Overprinting relationships between structures observed
in outcrop, drillcore and thin-section have determined
the chronology of structural development and the
geometry and kinematics of these structures have led
to the recognition of three different stress regimes
at Fosterville. The first stress regime involves east–
west compression that produced the folding and initial
faulting during the Benambran Orogeny. A second
stress regime involving northwest–southwest compression is associated with fault reactivation and mineralisation. Northwest-dipping and southwest-dipping
conjugate faults provide evidence for a third stress
regime that is characterised by north–south-oriented
compression. The different stress regimes and the
associated structures are discussed below.
S1 and early carbonate spots
The earliest fabric (S1) developed in the Ordovician
sedimentary rocks at Fosterville is a bedding-parallel
foliation defined by strongly aligned fine-grained micas
and generally only recognisable in thin-section. S1
is similar to early bedding-parallel foliations developed in Lower Paleozoic strata elsewhere in Victoria
(Schaubs & Wilson 2002).
An early generation of carbonate spots (Figure 5c, d)
that pre-date the main penetrative S2 cleavage are
superimposed on optically isotopic clots and clasts of
carbonaceous matter as well as calcium phosphate
aggregates, interpreted as fossils by McKnight (2004).
The deposition of the carbonate spots has recently
been attributed to be an interaction of a methanerich fluid with seawater sulfate along bedding
planes (Dugdale et al. 2009) prior to the main deformation D2.
265
Folds and cleavage associated with D2
D2 was characterised by the development of upright
folds, termed F2, with north–south-trending axial surfaces and interlimb angles of 25–608. These folds plunge
to the north and south at angles of up to 308 having
wavelengths that range from 200 to 1200 m (Figure 3).
Associated with these folds is an axial planar cleavage
(S2) that is best developed in the hinge zone of the folds.
In sandstones it is a poorly developed spaced cleavage
defined by anastomosing subparallel planes up to 2 cm
apart (Figure 6a). In hand-specimen the S2 cleavage is a
well-defined spaced cleavage with penetrative parallel
planes spaced up to 1 cm apart in siltstone and less than
0.5 cm apart in the mudstones (Figure 6b). The S2
cleavage has a convergent fanning geometry and is
commonly diffracted, by up to 208, as it passes from a
sandstone into a mudstone. The orientation of the S2
cleavage implies that the folds formed as a result of east–
west-directed compression.
Bedding-parallel veins and slickenline lineation
patterns
Bedding-parallel quartz–carbonate veins, which range
in thickness from a few centimetres to a few tens of
centimetres, are commonly developed on the limbs of F2
folds (Figure 4). The veins have a laminated texture
defined by ribbons of quartz–carbonate separated by
slivers of wall-rock (Figure 7). The bedding-parallel
veins mostly occur in mudstone or at contacts between
mudstone and sandstone (Figure 7a). Similar veins are
common in chevron-folded sub-greenschist facies turbidite sequences elsewhere in the Bendigo Zone (Jessell
et al. 1994; Fowler 1996) and are considered to be an
integral part of the flexural slip folding mechanism
proposed by Ramsay (1974).
Microstructural observations of trails of equidimensional, elongate quartz grains and elongate wall-rock
inclusions parallel to the bedding-parallel vein margins
at Fosterville (Figure 7b) imply that the veins formed
during folding. Further evidence for the development of
the veins after the earliest stages of folding is the
numerous inclusions of angular cleaved wall-rock fragments observed within the veins (Figure 7c).
Quartz–carbonate slickenline lineations on the bedding-parallel veins are variably oriented (Figure 8a).
However, it is evident that when contoured, the slickenline lineations have two dominant orientations (A and
B) that are geometrically related to the intersection of
the west-dipping and east-dipping vein sets. Orientation
A, also recognised by Wang & White (1993), is centred
about 678!2968 and is subperpendicular to the fold axis,
defined by the intersection of the mean orientation of
the east-dipping and west-dipping veins. This implies
that the majority of the west-dipping bedding-parallel
veins formed during folding, in response to east–west
compression.
Orientation B, centred about 728!0808 (Figure 8a), is
not subperpendicular to the fold axis, defined by the
intersection of the east-dipping and west-dipping vein
sets suggesting that these veins are not associated with
folding. However, east-dipping bedding-parallel veins
266
L. D. Leader et al.
Figure 5 Photomicrographs illustrating the relationships between bedding (S0), the S1 and S2 cleavages and the early
generation of carbonate spots. (a) S2 is oriented at an oblique angle to S0. Area outlined by rectangle indicates the field of view
in (b). Crossed polars. (b) S1 is subparallel to S0 and overprinted by S2. Crossed polars. Drillhole SPD 085, 145.6 m. (c, d) Early
carbonate spots warp and deflect the S2 cleavage implying that the spots formed prior to the development of the S2 cleavage.
Crossed polars. Drillhole SPD 213B, 917.4 m.
with slickenline lineations at an oblique angle to the
fold axis are observed on fold limbs where the opposing
fold limbs contain west-dipping bedding-parallel veins
that are interpreted to be associated with folding. Thus,
it is likely that the east-dipping bedding-parallel veins
also formed during folding, with the obliquity between
slickenlines and the local fold axis reflecting the noncoaxial nature of the folds as demonstrated by Dubey
(1982). On a doubly plunging fold the slickenline
lineations are perpendicular to the fold axis on the
culminations and depressions of the fold, whereas in the
limbs, the slickenline lineations will have a variety of
orientations dependent upon their position in the
structure (Figure 8b).
Faults and slickenline lineation patterns
At Fosterville faults that cross-cut bedding have
displacements of 100–200 m and are predominantly
west-dipping (Boucher et al. 2008) in which either
hangingwall or footwall bedding is at an oblique
angle to the fault (Figure 9a). Typically these faults
(Fosterville, Phoenix and Harrier) have a comparable
morphology to the bedding-parallel veins, in that they
have a laminated texture but are characterised by a
greater extent of brecciation (Figure 9b). It is interpreted that these faults have developed during the late
stages of D2. As reactivation of the bedding-parallel
veins continues, there is a point where the F2 folds
become so tight that they lock-up. Beyond this point,
further reverse movement along the bedding-parallel
veins results in their propagation across the F2
fold hinges, linking up- and down-dip, with beddingparallel veins on the same limb of two separate
but adjacent folds to form the bedding-discordant
faults. As documented by Wang & White (1993) these
faults have slickenline lineations that are oriented
subperpendicular to the local fold axis and a second
set of lineations that are oblique to the fold axis
(Figure 10a). Slickenline lineations oriented subperpendicular to the fold axis plunge steeply towards the
west (Figure 10b) and are indicative of the sense of
Fosterville goldfield, Vic.
267
Figure 6 Cleavage in turbidites at Fosterville. (a) Cleavage in the sandstones is generally a poorly defined spaced cleavage.
Grid reference 277704mE 5929065mN. (b) Cleavage within the mudstone is a well-defined spaced cleavage. Grid reference
277218mE 5929652mN. Pencil is 15 cm long.
movement along the bedding-parallel veins during the
late stages of folding by flexural slip and the sense of
movement along faults during the early stages of
faulting.
Slickenline lineations oriented at an oblique angle to
the fold axis plunge at a lower angle to the northwest
(Figure 10b). Numerous observations of the inclined
slickenline lineations in close association with mineralisation were made in the openpits and underground at
Fosterville. One such example is a west-dipping fault
observed in the Phoenix 5040 RL ore drive in which a
large portion of the striated fault surface was mineralised (Figure 10c). Northwest-plunging striations on
this and other fault surfaces at Fosterville are interpreted to indicate the displacement direction during
mineralisation.
Conjugate sets of small displacement, northwestand southwest-dipping faults are developed in the
hangingwalls of the Phoenix and Harrier Faults. Stepped
slickenline fibres and gentle-dipping extensional quartz–
carbonate veins (Figure 11) associated with these faults
indicate a reverse sense of displacement. Although
overprinting relationships with the reactivated late D2
bedding-discordant faults were not observed these faults
are considered to be representative of a later deformation event due to the different shortening direction
inferred by the north- and south-plunging slickenline
lineations. The hangingwall and footwall of these faults
host extensional vein sets (Figure 11b) and significant
arsenopyrite mineralisation.
Arsenopyrite, pyrite and carbonate in
wall-rocks
The majority of arsenopyrite and pyrite grains are
disseminated in the wall-rock adjacent to beddingdiscordant faults. Figure 12 shows that pyrite and
arsenopyrite grains cross-cut the S2 cleavage. Quartz
strain fringes, which also overprint the S2 cleavage, are
locally observed on the margins of pyrite and arsenopyrite grains (Figure 12a, c). In some instances the
suture line that marks the boundary between strain
fringes on adjacent faces of the sulfide grains, is
parallel to S2 and in other instances the suture line is
at an oblique angle to S2 (Figure 12b, d). Collectively
these relationships indicate mineralisation post-dates
development of the S2 cleavage.
A second generation of carbonate spots occur in the
zones of auriferous pyrite and arsenopyrite development (Figure 13). The majority of these carbonate
spots overprint S2 indicating they were deposited postS2. However, some of the spots are elongated parallel
to and truncated by S2 suggesting they either belong
to the earlier generation or were locally truncated
by further dissolution along S2 during mineralisation. Carbonate ribbons (Figure 13a, b) also occur
268
L. D. Leader et al.
Figure 7 Laminated bedding-parallel quartz–carbonate vein observed in a mudstone unit. Grid reference 282007mE
5924420mN (a) Vein in outcrop. Pencil is 15 cm long. (b) Photomicrograph illustrating elongate to equidimensional grains
oriented subparallel to vein margin. Cross-polarised light. (c) Photomicrograph showing inclusions of cleaved wall-rock
fragments suggesting the veins developed during folding or after folding. Cross-polarised light.
Figure 8 Nature of slickenline lineations. (a) Equal-area, lower-hemisphere stereographic projections of slickenline lineations
on laminated bedding-parallel veins. Slickenline lineation orientations have been contoured and form two loci (A þ B) that
are geometrically related to the mean orientations of the veins that are plotted as great circles. The white circles represent
lines within the planes of the veins that are perpendicular to the intersection of the east-dipping and west-dipping veins.
Slickenline lineations on west-dipping bedding-parallel veins are subperpendicular to the intersection, whereas lineations on
east-dipping bedding-parallel veins are not. (b) Schematic diagram of a doubly plunging fold. Slickenline lineations that form
as a result of flexural slip will only be perpendicular to the fold axis at the culminations and depressions of the fold.
Elsewhere along the structure slickenline lineations arising from flexural slip will be oriented at an oblique angle to the
fault.
in fractures that both parallel and cross-cut the S2
cleavage. Some carbonate ribbons are truncated and
overprinted by S2, and other ribbons overprint quartz–
carbonate veins that cross-cut S2. In some instances
sulfides are observed to nucleate on the carbonate
spots (Figure 13c, d) suggesting that the post-S2
carbonate spotting is pre- and or syn-mineralisation
and therefore possibly the result of an early stage of
the hydrothermal
mineralisation.
alteration
responsible
for
gold
Quartz–carbonate veins associated with gold
mineralisation
Mineralised zones adjacent to the bedding-discordant
faults are characterised by stockwork arrays of
Fosterville goldfield, Vic.
269
Figure 9 Nature of faults. (a) Exposure of the Harrier Fault, a late D2 west-dipping fault with footwall bedding oriented
subparallel to the fault and hangingwall bedding oriented at an oblique angle to the fault. Mine grid reference 1875mE
7180mN. (b) West-dipping faults that cross-cut bedding are commonly brecciated consisting of angular wall-rock and quartz–
carbonate fragments. Mine grid reference 1930mE 6340mN.
Figure 10 Slickenline lineations are commonly observed on the surfaces of the bedding-discordant faults. (a) Slickenline
lineations on the surface of the west-dipping Phoenix Fault. LS1 plunges steeply to the west and LS2 plunges at a lower angle to
the northwest. Pencil is 15 cm long. (b) Equal-area, lower-hemisphere stereographic projection of poles to bedding planes.
Poles have been contoured in order to calculate the statistical fold axis. The LS1 slickenline lineations are oriented
subperpendicular to the fold axis implying that they are associated with folding. The LS2 slickenline lineations post-date LS1
lineations and are oriented at an oblique angle to the fold axis implying that they formed under a different stress regime to
that which was present during folding. (c) Sulfidic slickenline lineations observed on a west-dipping fault surface in the
Phoenix 5040 RL ore drive. The northwest-plunging slickenline lineations on this and other west-dipping fault surfaces are
interpreted to record fault slip during mineralisation and reflect a different stress regime to that which existed during
folding.
quartz–carbonate veins (Figure 14a). The veins are
mostly developed in sandstone, truncate S2 and locally
contain angular fragments of cleaved wall-rock (Figure
14b). Photomicrographs show that quartz grains have
morphologies that range from irregular to equidimensional to elongate. Elongate quartz grains are aligned
subperpendicular and obliquely to the vein margins and
contain well-developed inclusion bands (Figure 14c–e).
Irregular and equidimensional quartz grains exhibit
undulose extinction, subgrains and deformation lamellae (Figure 14f). Carbonate grains typically replace
quartz (Figure 14c, d). Two species of carbonate,
possibly siderite and magnesite, are unevenly distributed along the margins of, and within, veins. The grains
of one species of carbonate have clear well-defined
curved margins and a cleavage that is locally overprinted by arsenopyrite (Figure 14g). The grains of the
second species of carbonate have diffuse margins and a
fuzzy texture with no cleavage development. Pyrite and
arsenopyrite grains are commonly distributed along the
margins of quartz–carbonate veins and occasionally
randomly distributed within veins where they occur
adjacent to carbonate grains (Figure 14b).
Quartz–carbonate veins in the ore zone at Fosterville
have four dominant orientations (A–D) that are common
to both the west-dipping and east-dipping limbs of
F2 folds (Figure 15a) indicating they post-date folding.
Orientation A dips steeply south and also occurs
regionally, away from the ore zones (Figure 15b).
Orientations B and C dip gently to the southeast and
gently to the west, respectively. Veins of these orientations do not occur regionally so it is inferred that these
veins are associated with mineralisation. Orientation D
dips steeply to the west, parallels the S2 cleavage, and
is similar to orientation E (Figure 15b), which occurs
away from the ore zone. Mutual cross-cutting relationships of orientations B, C and D suggest that these veins
are all broadly coeval.
Microstructures indicate vein-opening directions
ranged from low angles (Figure 14c) to subperpendicular
270
L. D. Leader et al.
Figure 11 (a) Equal-area, lower-hemisphere stereographic projection showing the average orientation of conjugate northwestand southwest-dipping faults with slickenline lineations. (b) Southwest-dipping fault with gentle northwest-dipping
extensional vein arrays in hangingwall and footwall.
Figure 12 Strain fringes on the margins of sulfide grains. (a) Pyrite grain with strain fringes overprinting S2 cleavage. Crosspolarised light. Drillhole SPD 221, 368.5 m. (b) A closer view of the strain fringes shows that the suture line is oriented at an
oblique angle to S2. Crossed polars. Drillhole SPD 221, 368.5 m. (c) Arsenopyrite grain with strain fringes overprints S2
cleavage. Crossed polars. Drillhole SPD 085, 146.2 m. (d) A closer view of the strain fringes shows that the suture line is
oriented subparallel to S2. Cross-polarised light. Drillhole SPD 085, 146.2 m.
to the vein margins (Figure 14d, e). Closely spaced,
parallel trails of wall-rock-derived inclusions, indicating crack–seal vein growth, are preserved locally in
shallow-dipping veins (Figures 14e). This implies that
these veins were initially antitaxial extension veins
with subsequent growth occurring during extensionalshear reactivation. The steep-dipping veins have quartz
and carbonate grains that are oriented at low angles to
the vein margins. Thin dark seams within carbonate
grains (Figure 14c) represent inclusion trails of the
Fosterville goldfield, Vic.
271
Figure 13 Photomicrographs of late D2 carbonate spots and ribbons. (a) Carbonate spots overprint S2 and are truncated by S2.
Carbonate ribbons are oriented parallel and obliquely to S2 cleavage planes. Cross-polarised light. Drillhole SPD 125, 452.7 m.
(b) Carbonate ribbons overprint quartz–carbonate veins that cross-cut S2. Plane-polarised light. Drillhole SPD 145, 218.3 m. (c)
Pyrite grains nucleate on carbonate spots implying that the carbonate spots are a precursor to mineralisation. Area outlined
by rectangle indicates the field of view in (d.) Plane-polarised light. Drillhole SPD 213C, 615.7 m. (d) Magnified view of pyrite
grains nucleating on carbonate spots. Reflected light.
initial vein microstructure and record dilation at a
relatively low angle to the vein margin implying a
greater component of shear during vein growth.
Quartz–stibnite veins
Quartz–stibnite veins (1–30 cm wide) also truncate S2
cleavage at Fosterville and are emplaced along the
Fosterville and Phoenix Faults. Figure 14h shows a
wall-rock fragment with a well-developed S2 cleavage
overprinted by arsenopyrite and pyrite that is surrounded by stibnite. This suggests that the stibnite
mineralisation post-dates S2 development and arsenopyrite–pyrite–gold mineralisation and is contemporaneous with late movement on the bedding-discordant
faults.
Late crenulation cleavage
S2 is locally overprinted by one or more locally developed spaced crenulation cleavages (S3) (Figure 16a). The
S2 cleavage has also undergone ductile deformation to
form D3 kink bands (Figure 16b). In thin-section the S3
cleavage is deflected by pyrite (Figure 16a) and the D3
kink banding deforms post-S2 carbonate spots (Figure
16b) implying that the development of these fabrics is
syn- to post-mineralisation. However, these fabrics have
a very limited distribution and are only observed in a
small number of thin-sections. For this reason, the exact
nature, orientation and relationship of this fabric with
other structures has not been fully established, although
it does have similarities to late fabrics observed at
Bendigo by Schaubs & Wilson (2002) and Raine (2005).
Paleostress analysis of faults, lineations and
veins
The P–T dihedra method (Angelier 1984) is used to
establish fields of possible maximum (s1) and minimum
(s3) principal stress orientations that were active at the
time of folding and initial faulting and at the time of
mineralisation. The method involves plotting the P
(compressional) and T (tensional) dihedra of each of
the faults on separate stereonets and then overlaying
the stereonets to determine the areas of compatible P
dihedra and T dihedra (Figure 17a). The boundaries of
272
L. D. Leader et al.
Fosterville goldfield, Vic.
273
Figure 15 (a, b) Equal-area, lower-hemisphere stereographic projections of poles to planes of quartz–carbonate veins. (a) Veins
in the ore zone at Fosterville have four dominant orientations (A, B, C and D) that are common to both the west-dipping and
east-dipping limbs of F2 folds. (b) Away from the ore zones vein orientations B, C and D are not observed suggesting that these
veins are associated with mineralisation. (c) Determination of the principal stress orientations associated with the conjugate
set of gently dipping veins indicates s1 plunges to the northwest.
the dihedra are defined by the fault plane and a plane
perpendicular to the fault normal and the slickenline
lineation (Ramsay & Lisle 2000). For a set of reverse
faults s1 will lie within the region defined by the P
dihedra and s3 will lie within the region defined by the T
dihedra. Possible s1 and s3 orientations determined
from the analysis of P–T dihedra for slip surfaces
associated with (i) flexural slip folding and fold-related
faulting and (ii) later gold mineralisation are shown in
Figure 17b, c, respectively. The limits of the field for
possible s1 orientations were used as constraints for the
grid search stress inversion program SLICK.BAS (Lisle
1999) that analyses fault and slickenline orientations to
determine the principal stress orientations, the stress
shape ratio [f, where f ¼ (s1 – s2)/(s1 – s3)] and the angular
deviation between the measured slip direction with the
calculated slip direction. The results of the stress
inversion are presented in Table 1. The s1 orientation
with the lowest angular deviation is considered to be
the best fit to the fault data. This means that a s1
orientation of 28!0788 is associated with folding and
initial faulting, and that a s1 orientation 248!3158 is
associated with mineralisation. The different s1 orientations for folding and mineralisation imply that there has
been a rotation of the stress field after folding and that
movement along the north–south-trending faults during
mineralisation involves a greater sinistral strike-slip
component.
Application of the P–T dihedra method to the northwest- and southwest-dipping conjugate faults yields two
subfields of possible s1 orientations (Figure 17d). Using
the limits of these fields as constraints for SLICK.BAS, a
preferred s1 orientation of 78!0008 (Table 1) is derived,
implying that after or during the late stages of mineralisation there has been a further rotation of the stress
field resulting in north–south-oriented compression.
Further evidence of a northwest–southeast-oriented
stress field at the time of mineralisation is provided
by the orientation of quartz–carbonate veins in the
ore zone. Determination of the principal stresses associated with the conjugate set of gentle east-dipping and
west-dipping veins (Figure 15c) indicates that the veins
formed in response to northwest–southeast-oriented
compression. However, the gently dipping antitaxial
extension veins with subsequent growth occurring
during extensional-shear reactivation suggest that
there was either a change in the s3 orientation or vein
orientation relative to the stress field during or prior to
the early stages of vein formation and mineralisation.
The northwest–southeast-oriented stress field is also
consistent with the formation of the steep west-dipping
veins that record a large component of shear.
CONTROLS ON MINERALISATION
An important structural control on gold mineralisation at Fosterville is the development of the beddingdiscordant faults that propagate across fold hinges to
link, up- and down-dip, with bedding-parallel veins on
3
Figure 14 Quartz–carbonate veins. (a) Stockwork arrays of quartz–carbonate veins. Diameter of coin is 25 mm (b) Angular
wall-rock fragments with a well-developed S2 cleavage are found as inclusions in quartz–carbonate veins. Arsenopyrite (asp)
and pyrite (py) grains occur along vein margins. Plane-polarised light. Drillhole SPD 091, 188.7 m. (c) Carbonate replacement
of quartz grains. Inclusion trails oriented at a low angle suggest dilation occurred at a relatively low angle to vein margin.
Plane-polarised light. Drillhole SPD 085, 182.3 m. (d) Elongate quartz and carbonate grains oriented at a high angle to vein
margin. Crossed polars. Drillhole SPD 151, 247.5 m. (e) Elongate quartz grains, with locally well-developed inclusion bands,
are aligned subperpendicular and obliquely to vein margins. Crossed polars. Drillhole SPD 169, 279.4 m. (f) Quartz grain with
deformation lamellae. Crossed polars. Drillhole SPD 091, 188.6 m. (g) Arsenopyrite grains overprint cleavage of carbonate
grains indicating deposition of arsenopyrite occurred after the deposition of carbonate. Plane-polarised light. SPD 045,
272.5 m. (h) Wall-rock fragment with a well-developed S2 cleavage surrounded by quartz–stibnite mineralisation. The S2
cleavage is overprinted by arsenopyrite–pyrite mineralisation inferring that stibnite mineralisation post-dates arsenopyrite–pyrite mineralisation. Plane-polarised light, composite image of parts of twelve separate micrographs. Drillhole SPD 213C
615.5 m
274
L. D. Leader et al.
Figure 16 (a) S2 cleavage crenulated by S3. Plane-polarised light. Drillhole SPD 045, 272.5 m. (b) D3 kink bands result from
ductile deformation of the S2 cleavage. Plane-polarised light. Drillhole SPD 145, 218.3 m.
Figure 17 (a) Illustration of the P–T dihedra method. (a) P–T dihedra of two individual faults of a different orientation are
overlain to determine common regions of P (compressional) dihedra and T (extensional) dihedra. The s1 orientation will lie
within the region of common P dihedra and s3 will lie within the region of common T dihedra (adapted from Angelier 1984).
(b–d) Equal-area, lower-hemisphere projections of the results of the P–T dihedra method for (b) folding and early faulting, (c)
faulting associated with mineralisation and (d) late faulting, indicating the regions of possible s1 and s3 orientations. The
orientations of the principal stress axes of s1, s2 and s3 determined from the SLICK.BAS software program (Lisle 1999) are
also shown.
equivalent limbs of adjacent folds (Boucher et al. 2008;
this study). Mineralised zones are generally better
developed where bedding, in either the hangingwall
or footwall, is obliquely intersected by the beddingdiscordant faults (e.g. footwall to Fosterville Fault and
hangingwall to Phoenix Fault in Figure 3c). Wang &
White (1993) noted that significant gold mineralisation
occurs where the Fosterville Fault is intersected by
low-displacement bedding-discordant faults such as
the Phoenix Fault. These low-displacement faults occur
across the Bendigo Zone and are regularly associated
with extensive quartz veining that commonly hosts
nuggetty gold mineralisation (Cox et al. 1991a; Willman
2007). However, at Fosterville the arsenopyrite–pyrite–
gold mineralisation occurred in hangingwall and footwall rocks, and not in veins developed along the faults.
In contrast, stibnite is largely restricted to quartz veins
along bedding-discordant faults and, more rarely, in
association with bedding-parallel veins.
The bedding-discordant faults typically have two sets
of slickenline lineations. One set is associated with
folding and is indicative of east–west compression. The
second set, indicative of reverse sinistral strike-slip
during northwest–southeast-oriented compression, is
Fosterville goldfield, Vic.
275
Table 1 Results of the SLICK.BAS stress inversion of faults and slickenline lineations associated with folding and initial faulting, and
with mineralisation and late faulting.
Field
Search azimuth
Search plunge
s1
s2
s3
f
Deviation
Initial faulting
Mineralisation
Late faulting
1
2
3
4
5
6
2328–3248
08–58
08!2958
28!2058
888!258
0.00
3.98
0528–1458
08–108
08!1188
28!2088
888!0288
0.00
4.08
2488–3448
08–458
248!3158
308!2108
508!0778
0.40
6.58
0878–1648
08–108
08!1418
218!2318
698!0518
0.20
6.68
2978–0268
08–308
78!3578
168!2658
738!1108
0.45
4.18
1308–2158
08–208
08!1808
148!2708
768!0908
0.35
6.38
SLICK.BAS (Lisle 1999) assumes that the slip direction on the fault is parallel to the resolved shear stress and that movement along
faults has occurred in a homogeneous stress field. The program uses possible s1 orientations to predict the direction of shear stress
on each of the fault planes; the predicted slip direction of the faults is then compared with the slickenline lineation orientations. The
final s1 orientation will be the orientation that best fits the fault data (Ramsay & Lisle 2000). Field 1 has a lower angular deviation
than Field 2 so that the s1 orientation associated with folding and initial faulting is 08!2958. The s1 orientations associated with
mineralisation and late faulting are 248!3158 and 78!3578, respectively
associated with arsenopyrite–pyrite–gold mineralisation. Wang & White (1993) also suggested most mineralisation at Fosterville is related to the reactivation
of faults during a phase of sinistral transpression.
This sense of movement is not commonly associated
with gold mineralisation in the Bendigo Zone where
the timing of mineralisation overlaps that of folding
(Willman 2007).
Gold mineralisation at Fosterville is temporally
and spatially associated with stockwork arrays of
quartz–carbonate veins. A conjugate set of gently
east-dipping and west-dipping veins indicate they
formed during compression in a northwest–southeastoriented stress field. The stockwork veining is typically
restricted to sandstones and is characteristic of tensile
hydraulic fracture that results from high fluid pressure
and high fluid flux (Cox et al. 1991b). Tensile hydraulic
fracturing is considered to be an important aspect of the
mechanism for gold emplacement across the Bendigo
Zone (Cox et al. 1991b; Cox 1995).
As well as structural controls there are also chemical
controls on the distribution of gold mineralisation at
Fosterville. Pre-existing Fe-bearing minerals within the
wall-rocks and extensive carbonate (siderite/ankerite)
alteration, forming spots and ribbons, provide a significant source of iron for the deposition of arsenopyrite
and pyrite (Scott 2008).
Late stibnite mineralisation, associated with bedding-parallel veins and bedding-discordant faults
at Fosterville, is atypical of the Bendigo Zone but
commonly observed in the Melbourne Zone (e.g. at
Costerfield: McCarthy et al. 2008). However, the concentration of gold within stibnite is significantly lower at
Fosterville. At Costerfield gold concentrations within
stibnite commonly exceed 100 ppm (McCarthy et al.
2008), whereas as at Fosterville gold concentrations
range from 1 to 10 ppm (Allwood 2003). A lack of
kinematic data associated with the stibnite mineralisation at Fosterville means that it cannot be attributed to
either the northwest–southeast- or north–south-oriented
stress fields.
Northwest-dipping and southwest-dipping faults are
present in the mineralised zones at Fosterville and
indicate a north–south-oriented compressional stress
field. The hangingwall and footwall of these structures
host significant arsenopyrite, although the control
that these faults exert on gold mineralisation is currently unclear. It may be that the implied north–south
movement of these faults is the result of a localised
reorientation of the stress field during northwest–southeast shortening due to stress permutations arising
from variations in fluid pressure (Cox 1995) and/or
material strength properties (Hu & Angelier 2004).
However, it is interesting to note that in other Victorian
gold deposits there is emerging evidence for a change
in the far-field stress (Miller et al. 2006) being associated
with the late stages of mineralisation. Fold and
fault related structures indicative of north–south
shortening have been recognised in the Melbourne
Zone (Gray & Mortimer 1996), the Bendigo Zone (Raine
2005) and the Stawell Zone, and attributed to deformation during the Tabberabberan Orogeny (Miller et al.
2006).
CONCLUSIONS
Structural observations within and surrounding the
Fosterville openpits and underground workings indicate that the rocks hosting the gold mineralisation
have been influenced by three distinct stress regimes.
An initial east–west-oriented stress field was active
at the time of folding and initial faulting. A rotation of
this stress field, to a northwest–southeast orientation,
resulted in sinistral reactivation of faults and the
formation of stockwork arrays of quartz–carbonate
veins during mineralisation. A third stress regime,
characterised by north–south-oriented compression, is
inferred to have been active during the late stages of
mineralisation.
The disseminated nature of the gold mineralisation
and the orientation of the stress field during mineralisation at Fosterville make it atypical of the Bendigo
Zone, although there are similarities with other Bendigo
Zone gold deposits. These include the association
of low-displacement bedding-discordant faults and
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L. D. Leader et al.
extensive quartz veining with mineralisation. The
inferred mechanism of gold emplacement facilitated by
tensile hydraulic fracturing resulting from high fluid
pressures and high fluid flux is also similar to other gold
deposits in the Bendigo Zone.
ACKNOWLEDGEMENTS
We wish to thank Perseverance Corporation for their
help in initiating this project and Northgate Minerals
Corporation for permission to publish this work.
The project was funded by Northgate Minerals Corporation, the Predictive Mineral Discovery Cooperative
Research Centre and ARC Linkage grant (LP0882157).
In particular, we thank Neil Norris, Chris Reed, Simon
Hitchman, Nathan Phillips, Stewart Govett, Darren
Stephens, Ian Holland and the geological team at the
Fosterville Gold Mine for their on-site assistance and
advice. Clive Willman and Rob Scott are also thanked
for their careful reviews of this paper.
REFERENCES
ALLWOOD K. 2003. Fosterville Gold Project, April 2003 Resource
Estimate. Report for Perseverance Exploration Pty Ltd (unpubl.).
ANGELIER J. 1984. Tectonic analysis of fault slip data sets. Journal of
Geophysical Research 89, 5835–5848.
ARNE D. C., BIERLEIN F. P., MCNAUGHTON N., MORAND V., RAMSAY W.
R. H. & WILSON C. J. L. 1998. Absolute timing of gold mineralisation in central and western Victoria: new constraints from
SHRIMP II analysis of zircon grains from felsic intrusive rocks.
Ore Geology Reviews 13, 251–273.
BIERLEIN F. P., ARNE D. C., FOSTER D. A. & REYNOLDS P. 2001b.
A geochronological framework for orogenic gold mineralisation in central Victoria, Australia. Mineralium Deposita 36,
741–767.
BIERLEIN F. P., ARNE D. C., KEAY S. M. & MCNAUGHTON N. J. 2001a.
Timing relationships between felsic magmatism and mineralisation in the central Victorian gold province, southeast Australia.
Australian Journal of Earth Sciences, 48, 883–899.
BIERLEIN F. P., ARNE D. C., REYNOLDS P. & MCNAUGHTON N. J. 1999.
Victorian Gold—timing relationships and emplacement. Final
report on geochronology to AMIRA for P478, pp. 1–26.
BIERLEIN F. P. & MAHER S. 2001. Orogenic disseminated gold in
Phanerozoic fold belts: examples from Victoria, Australia and
elsewhere. Ore Geology Reviews 18, 113–148.
BOUCHER R. K., HITCHMAN S. P. & ALLWOOD K. J. 2008. Stratigraphic
controls on structure and mineralisation in central Victoria 3:
Fosterville. Australian Institute of Geoscientists (AIG) News 93,
6–9.
COX S. F. 1995. Faulting process at high fluid pressures: an example
of fault behaviour from the Wattle Gully Fault, Victoria,
Australia. Journal of Geophysical Research 100, 12841–12859.
COX S. F., ETHERIDGE M. A., CAS R. A. F. & CLIFFORD B. A. 1991b.
Deformational style of the Castlemaine area, Bendigo–Ballarat
Zone: implications for evolution of crustal structure in Victoria.
Australian Journal of Earth Sciences 38, 151–170.
COX S. F., SUN S-S, ETHERIDGE M. A., WALL V. J. & POTTER T. F. 1995.
Structural and geochemical controls on the development of
turbidite-hosted gold quartz vein deposits, Wattle Gully Mine,
central Victoria, Australia. Economic Geology 90, 1722–1746.
COX S. F., WALL V. J., ETHERIDGE M. A. & POTTER T. F. 1991a.
Deformational and metamorphic processes in the formation of
mesothermal vein-hosted gold deposits—examples from the
Lachlan Fold Belt in central Victoria, Australia. Ore Geology
Reviews 6, 391–423.
DAVIS C. 2006. Structural controls on mineralisation within the
Robbins Hill area, Fosterville, Victoria. BSc thesis, University of
Melbourne, Melbourne, (unpubl.).
DUBEY A. K. 1982. Development of interlayer slip in non-cylindrical
flexural slip folds. Geoscience 3, 103–108.
DUGDALE A. L., WILSON C. J. L., LEADER L. D., ROBINSON J. A. &
DUGDALE L. J. 2009. Carbonate spots: understanding the relationship to gold mineralisation in central Victoria, southeastern
Australia. Mineralium Deposita 44, 205–219.
FORDE A. 1991. The late orogenic timing of mineralisation in some
slate belt gold deposits, Victoria, Australia. Mineralium Deposita
26, 257–266.
FORDE A. & BELL T. H. 1994. Late structural control of mesothermal
vein-hosted gold deposits in central Victoria, Australia—mineralisation mechanisms and exploration potential. Ore Geology
Reviews 9, 33–59.
FOSTER D. A., GRAY D. R., KWAK T. A. P. & BUCHER M. 1998.
Chronology and tectonic framework of turbidite-hosted gold
deposits in the Western Lachlan Fold Belt, Victoria: 40Ar/39Ar
results. Ore Geology Reviews 13, 229–250.
FOWLER T. J. 1996. Flexural slip generated bedding-parallel veins
from central Victoria, Australia. Journal of Structural Geology
18, 1399–1415.
FU B., FAIRMAID A. M. & PHILIPS D. 2007. Geochronology of gold
deposits in the western Lachlan Fold Belt, Victoria: a review.
Geoscience Victoria Gold Undercover Report 6.
GAO Z. L. & KWAK T. A. P. 1997. The geochemistry of wall
rock alteration in turbidite-hosted gold vein deposits, central
Victoria, Australia. Journal of Geochemical Exploration 59, 259–
274.
GRAY D. R., FOSTER D. A., MORAND V. J., WILLMAN C. E., CAYLEY R.
A., SPAGGIARI C. V., TAYLOR D. H., GRAY C. M., VANDENBERG A.
H. M., HENDRICKX M. A. & WILSON C. J. L., 2003. Chapter 2:
Structure, metamorphism, geochronology, and tectonics of
Palaeozoic rocks. In: Birch W. D. ed. Geology of Victoria, pp.
15–71. Geological Society of Australia Special Publication 23.
GRAY D. R., GREGORY R. T. & DURNEY D. W. 1991. Rock-buffered
fluid–rock interaction in deformed quartz rich turbidite sequences, eastern Australia. Journal of Geophysical Research 96,
19681–19704.
GRAY D. R. & MORTIMER L. 1996. Implications of overprinting
deformations and fold interference patterns in the Melbourne
Zone, Lachlan Fold Belt. Australian Journal of Earth Sciences 43,
103–114.
GRAY D. R. & WILLMAN C. E. 1991. Deformation in the Ballarat Slate
Belt, central Victoria, and implications for the crustal structure
across southeast Australia. Australian Journal of Earth Sciences
38, 171–201.
HU J-C. & ANGELIER J. 2004. Stress permutations: three-dimensional
distinct element analysis accounts for a common phenomenon in
brittle tectonics. Journal of Geophysical Research 109, B09403,
doi:10.1029/2003JB002616.
JESSELL M. W., WILLMAN C. E. & GRAY D. R. 1994. Bedding parallel
veins and their relationship to folding. Journal of Structural
Geology 16, 753–767.
KING S. 2004. Geology of the Harrington’s Hill to Daley’s
Hill area with special reference to Daley’s Hill pit
geology. Report prepared for Perseverance Exploration Pty Ltd
(unpubl.).
KING S. 1998. Structural controls on gold mineralisation at the
Fosterville Gold Project, Victoria (with special reference to the
Daley’s Hill and O’Dwyer’s South pits). Report prepared for
Perseverance Exploration Pty Ltd (unpubl.).
LISLE R. J. 1999. SLICK.BAS—a grid-search method of stress
inversion, software program. In: Ramsay J. G. & Lisle R. 2000.
The techniques of modern structural geology. Volume 3: Applications of continuum mechanics in structural geology, pp. 805–807.
Academic Press, San Diego.
MACCULLOCH J. 2005. 40Ar/39Ar constraints on the timing of gold
mineralisation in the Fosterville, Nagambie and Walhalla
deposits. BSc (Hons) thesis, University of Melbourne, Melbourne
(unpubl.).
MCCARTHY M., FOWLER A. & MARSHALL S. 2008. Augusta Gold–
Antimony Mine. In: Dominy S. ed. Narrow Vein Mining
Conference Ballarat 2008, pp. 145–151. Australasian Institute of
Mining and Metallurgy, Carlton.
MCKNIGHT S. W. 2004. Detailed description of carbonaceous matter
occurring in slates submitted by Perseverance Corporation Pty
Ltd. Report prepared for Perseverance Mining Pty Ltd. (now
Northgate Minerals Corporation) (unpubl.).
Fosterville goldfield, Vic.
MERNAGH T. P. 2001. A fluid inclusion study of the Fosterville
Mine: a turbidite hosted gold field in the Western Lachlan Fold
Belt, Victoria, Australia. Chemical Geology 173, 91–106.
MILLER J. McL., WILSON C. J. L. & DUGDALE L. J. 2006. Stawell gold
deposit: a key to unravelling the Cambrian to Early Devonian
structural evolution of the western Victorian goldfields. Australian Journal of Earth Sciences 53, 677–695.
PARENZAN M. A., EDWARDS J. & SLATER K. R. 2001a. Huntly and part
of Kamarooka 1:50 000 geological map. Geological Survey of
Victoria, Melbourne.
PARENZAN M. A., EDWARDS J. & SLATER K. R. 2001b.
Bendigo1:50 000 geological map. Geological Survey of Victoria,
Melbourne.
PHILLIPS G. N., HUGHES M. J., ARNE D. C., BIERLEIN F. P., CAREY S. P.,
JACKSON T. & WILLMAN C. E. 2003. Chapter 13: Gold. In: Birch W.
D. ed. Geology of Victoria, pp. 377–433. Geological Society of
Australia Special Publication 23.
RAMSAY J. G. 1974. Development of chevron folds. Geological Society
of America Bulletin 85, 1741–1754.
RAMSAY J. G. & LISLE R. 2000. The techniques of modern structural
geology. Volume 3: Applications of continuum mechanics in
structural geology. Academic Press, San Diego.
RAINE M. D. 2005. Polyphase deformation and the structural controls
on economic gold occurrences within the Bendigo gold field,
central Victoria, Australia. PhD thesis, James Cook University,
Townsville (unpubl.).
SCHAUBS P. M. & WILSON C. J. L. 2002. The relative roles of folding
and faulting in controlling gold mineralisation along the
Deborah Anticline, Bendigo, Victoria, Australia. Economic
Geology 97, 351–370.
277
SCOTT R. J. 2008. Microstructural, isotopic and sulfide trace element
constraints on gold mineralisation at Fosterville: an overview of
P923 research and major findings. In: Controls on the formation
and sulfide trace element signatures of sediment-hosted gold
deposits. CODES, University of Tasmania, Hobart, Final Data
Compilation AMIRA P923 (DVD).
WANG G. M. & WHITE S. H. 1993. Gold mineralisation in
shear zones within a turbidite terrane, examples from central
Victoria, S.E. Australia: structural setting and controls on
mineral deposits. Ore Geology Reviews 8, 163–188.
VANDENBERG A. H. M., WILLMAN C. E., MAHER S., SIMONS B. A.,
CAYLEY R. A., TAYLOR D. H., MORAND V. J., MOORE D. H. &
RADOJKOVIC A. 2000. The Tasman Fold Belt System in Victoria.
Geological Survey of Victoria Special Publication.
WILLMAN C. E. 2007. Regional structural controls of gold
mineralisation, Bendigo and Castlemaine goldfields, central
Victoria, Australia. Mineralium Deposita 42, 449–463.
WILSON C. J. L., MCKNIGHT S. W., DUGDALE A. L., RAWLING T. J.,
FARRAR A. D., MCKENZIE M. J. & MELLING W. D. 2009.
Illite crystallinity and the b-spacing values of white
micas and their implications for gold mineralisation in
the Lachlan Orogen. Australian Journal of Earth Sciences 56,
1143–1164.
WOHLT K. E. & EDWARDS J. 1999a. Avonmore 1:50 000 geological map.
Geological Survey of Victoria, Melbourne
WOHLT K. E. & EDWARDS J. 1999b. Heathcote 1:50 000 geological map.
Geological Survey of Victoria, Melbourne.
Received 20 March 2009; accepted 2 December 2009