Cobar_style_deposits_early_thoughts_MichaelOstrowski

Cobar Style Deposits
Orogenic or VHMS
Michael Ostrowski
2011

Table of Contents
1. Cobar Style Deposits-An Overview.....................................................1
2. Deposit Models –An Overview ............................................................3
2.1 Orogenic Deposits..........................................................................3
2.2 Volcanic Hosted Massive Sulphide Deposits..................................3
3. Cobar Style Deposits – An Orogenic Style Variant..............................6
3.1 Structural Control ...........................................................................6
3.2 Tectonic Setting and Metamorphism ..............................................8
3.3 Lithological Control and Alteration..................................................9
3.4 Vein Textures the Key to Classification .......................................11
3.5 Source of Hot Water, Metals and Ligands....................................13
4. Conclusion........................................................................................15
Bibliography..........................................................................................17

1Cobar Style Deposits….Orogenic or VHMS
1. Cobar Style Deposits-An Overview
Cobar style deposits (CSD) are a group of epigenetic and polymetalic deposits
associated with the Silurian-Middle Devonian Cobar Basin, part of the Lachlan
Orogen (Glen & Djomani, 2009) (Lawrie & Hinman, 1998). The Cobar Basin is an a
rift basin with several episodes of extension and compression and the deposition of
thick sequences of sediments including turbidites with deformation and
metamorphism occurring over several stages in the Late Silurian up to the
Carboniferous (Glen, et al., 1996). The Rast and Mount Hope Troughs to the south
are also considered part of the Cobar Super basin with the Canbelego-Mineral Hill
Rift Zone also included by some authors.
CSD ore bodies have a limited strike extent (<300m), extended vertical extent
(>400m), with a northerly plunge and consisting of massive sulphide lenses
sometimes showing banded textures, pipes and quartz vein arrays (Lawrie &
Hinman, 1998) (Stegman, 2001)( refer to figure 1). Other common characteristics
include strong structural control, silicified host rocks, depleted cryptic alteration halos
and similar sulphur and lead isotope ratios between deposits (Lawrie & Hinman,
1998) (Stegman, 2001). (Metal associations for selected deposits can be viewed in
table 1 and deposit overview and classification in Table 4)
Table 1 Metal Associations with Cobar Basin deposits (Adapted from David 2005)
Deposit Basin location Main Metals
exploited
Minor and trace metals
Great Cobar Mine Cobar Trough-Eastern Margin Cu, Au, Pb, Zn, As, Bi
New Cobar Mine Cobar Trough-Eastern Margin Cu,Au Bi, As
Chesney Mine Cobar Trough-Eastern Margin Cu, Au Bi
Peak Mine Cobar Trough-Eastern Margin Cu, Au, Pb,Zn Ag, Bi
Perseverance Mine Cobar Trough-Eastern Margin Cu, Au Pb,Zn,Ag,Bi
CSA Mine Cobar Trough-Eastern Margin Cu,Pb,Zn, Ag Au,Bi,As
Elura Mine Northern Cobar Trough Pb,Zn,Ag Au,Cu,Bi,As
Mt Bobby Mineral Hill Canbelego Rift Zone Au,Cu,Pb,Zn As,Bi
Mineral Hill Mineral Hill Canbelego Rift Zone Au,Cu,Pb,Zn,Ag As,Bi
Wagga Tank Prospect Mount Hope Trough Au,Cu,Pb,Zn. Ag,As,Bi
Hera-Nymagee Mouramba Shelf, Cobar Trough Cu,Au,Zn,Pb
Mount Hope Mine Mount Hope Trough Cu,Pb,Zn, Au,Ag,
Wonawinta Mine Winduck Shelf Ag,Pb,Zn

Figure 1 Tectono-Stratigraphic units of the Cobar Super Basin and mineral
deposits (Adapted from David 2005)

2. Deposit Models –An Overview
2.1 Orogenic Deposits
Orogenic mineral deposits form during periods of compressional or transpressional
deformation at convergent plate margins and in collissional and accretionary orogens
(Groves, et al., 1998). Orogenic deposits are predominantly epigenetic, but some
syngenetic forms are also recognised. While mesothermal or lode gold deposits are
the most widely recognised of orogenic deposits, a wide variety of manifestations
exist including replacement and disseminated styles of mineralisation, mainly
associated with gold. Polymetalic styles of orgogenic deposits are rare with vein style
quartz veins the most abundant mineralisation style. Metals and ligands including
sulphur are predominantly sourced from the deformed and metamorphosed
sequence of hydrated sedimentary rocks with fluid flow driven by strong geothermal
gradients (Groves, et al., 1998) and coupled to deformation and metamorphism
which creates pathways for hot water to flow. Orogenic deposits formed over a
protracted history in the earth’s geologic history. (Refer to Table 2)
2.2 Volcanic Hosted Massive Sulphide Deposits
Volcanic hosted massive sulphide deposits (VHMS) are typically associated with
volcanic rocks in active tectonic zones including continental margins, but mainly
located close to subduction zones and spreading centres in extensional zones
(Robb, 2006). VHMS deposits are recognised as syngenetic deposits, however
feeder zones can be classified as epigenetic style. The underlying hot volcanic rocks
and sub volcanic intrusions provides a heat source which drives the circulation of
sea water which becomes enriched in metals as it passes through the volcanic pile.
Sea water enters the system via transform faults and fractures, where it descends
into the volcanic pile, is heated and then rises to the surface through fractures and
faults with its metal load where precipitation occurs due to rapid temperature change.
Sea water provides much of the reduced sulphur for these systems.
A number of styles of VHMS deposits exist according to associated rocks and metal
endowments including Beshi, Kuroko and Cyprus type, determined by tectonic
setting and associated volcanic rocks (Robb, 2006). (Refer to table 2)

Table 2 Orogenic and VHMS Models- detailed Analysis
Characteristics Deposit Model
Orogenic Style Volcanic Hosted Massive Sulphide.
Form Epigenetic deposits extended
vertical extent, variable strike extent
mainly with quartz vein styles.
Syngenetic-Strataform deposits. Typically
flat to steeply dipping massive sulphide
lenses and quartz rich feeder zones.
Metal
Associations
Au,As,Pb,Zn,Fe,Cu, Sb, Hg,
Mo,W,Sn.
Cu,Zn,Pb,Ag,Au,As,Cd,Sn,Hg,Sb
Host Rocks Sediments and metamorphic rocks
including slates (deformed
metamorphic terranes).
Volcanic rocks including tuffs, fragmental
volcanic rocks, capped by fine grained
sedimentary rocks, also interbedded
sediments
Tectonic setting Orogenic and metamorphic belts
under the influence of a
compressional regime and regional
metamorphism
Continental margins, back arc basins and
extensional zones usually associated with
marine volcanism
Structural
Control
Strong link to deep transcrustal
compressional structures, thrust
faults and a variety of brittle and
ductile deformation structures
Important in water circulation. Growth faults
including rift and caldera related.
Depth and
Depositional
Environment
Deep epizonal to 6km depth,
epizonal from 6-12km and
hypozonal greater than 12km.
Shallow formation usually at the sediment-
sea water interface. Footwall feeder zones
usually sub sea floor at shallow depths.
Metal zonation Strong metal zonations in preserved
systems Proximal Au,Pb,Zn,Cu.
Distal-higher level Sb, As, Hg.
Deeper Cu and distal Pb-Zn in
some deposits.
Typical strongly zoned from deeper/hotter
Cu-Fe, grading out into Cu-Zn, Zn-Pb-Ag,
also Ba-Au.
Enrichment
elements
Ca,S,K, Si, Na, Rb, LILE. Fe,Mn,Ba,K,Mg,S,Si
Depletion
elements
Depletion halos in some deposits
including Na,K,Rb,Sr,Ba, Li.
Na, Ca, inner and outer halos.
Alteration
Proximal
Silica, carbonate (calcite-ankerite
and dolomite) sericite, biotite,
chlorite, albite-orthoclase, hematite
Sericite-chlorite, quartz, feldspar, pyrite
(footwall feeder zones), fuchsite
Alteration distal Chlorite, carbonate spotting,
fuchsite
Chlorite, sericite, carbonate (hanging wall
and cap rocks)
Alteration Styles Quartz-Sericite-Pyrite, Phylic,
Propylitic(carbonate) to moderately
potassic. Generally focussed and
texturally retentive, apart from
proximal zones.
Phylic to Argillic, Quartz-Sericite-Pyrite,
Sodic to Potassic. Chlorite alteration of cap
rocks. Pervasive with focussed zones.
Texturally destructive
Ore Textures Quartz veins, laminated quartz
veins, massive sulphide lenses in
dilation zones. Vein related breccias
deeper in deposits. Predominantly
low sulphidation.
Massive sulphide lenses and stockwork
style quartz veins in feeder zones (footwall).
Massive sulphide lenses commonly banded,
breccias common. High Sulphidation.
Epiclastic breccias associated with mound
collapse.
Hot Water
Geochemistry
Near neutral, CO2 , CH4, low
salinity fluids. Boiling not important.
Reduced to oxidised, acidic, low to
moderate salinity fluids. H2S rich. Boiling in
feeder zones.

Source; (Downes, et al., 2008) (Jiang & Seccombe, 2000) (Lawrie & Hinman, 1998) (Robb,
2006) (Solomon, et al., 2000) (Groves, et al., 1998) (Franklin, 1993).
Figure 2 Pyrrhotite rich massive sulphide Pb-Zn-Ag ore from the Elura Mine
with distinctive rounded to sub angular silicified wall rock clasts.
Temperature Low to hot 150-420C Moderate to hot up to 400C
Role of Intrusive
and volcanic
rocks
Implicated in proving heat source
for driving fluid flow via geothermal
gradient.
Important mechanism for driving circulation
of sea water and resultant geothermal
gradient that transports hot water and
metals to the surface
Source of metals
and ligands
Basement rocks and sedimentary
accumulations where metamorphic
water can strip metals from mineral
phases (i.e. lead from feldspars).
Sulphur sourced from syngenetic
sulphides in sedimentary pile and
magmatic source. Metals
transported as chloride and
bisulphide complexes.
Accumulations of volcanic rocks and lavas
where circulating sea water driven by
geothermal convection and temperature
gradients can strip metals and sulphur from
volcanic rocks. Sulphur mainly sourced from
seawater which also enriches the ore and
alteration zone with metals.
Precipitation
influences
Temperature gradient, reaction with
wall rocks and meteoric water. Fault
valve mechanism plays an
important role in episodic
mineralisation.
Quenching of metal rich geothermal water
on contact with sea water, temperature
change with wall rocks in feeder zones.
Constant supply of metal and sulphur rich
fluids due to circulation.
Deposits Hill End-Sofala(Aus),
Hillgrove(Aus), Ballarat(Aus),
Greenstone Gold deposits including
(Golden Mile (Aus), Homestake
(USA), Kirkland Lake(Canada),
Boddington(Aus), Ashanti(Aus)
Woodlawn(Aus), Hellyer(Aus), Horne and
Noranda (Canada), Askay Creek (Canada),
LaRonde (Canada), Golden Grove (Aus)

3. Cobar Style Deposits – An Orogenic Style Variant
While many authors regard CSD as enigmatic, they are a group of deposits with
many characteristics and attributes that allow for a ready classification as an
orogenic style deposit. Many deposit models rely on a group of common
characteristics, however variability with CSDs set them apart from the more
recognised forms of orogenic deposits such as lode gold or Greenstone belt
deposits. Even though Mississippi Valley Type (MVT) deposits are classed as a
distinct deposit style, MVTs are regarded as an orogenic progeny (Robb, 2006). The
classification of the Mineral Hill deposits as VHMS and a porphyry related systems is
a classic example of the inability of many scientists to recognise this important CSD,
due to the hosting of the deposit in volcanic rocks and variables with the interaction
of oxidised meteoric water in this deposit resulting precipitation on bornite and
magnetite (Morrison, et al., 2004).
3.1 Structural Control
All CSDs share a link to major basin wide structures including north-south faults such
as the Rookery Fault and its splays in the Cobar Basin, and the Gilmore Suture in
the Canbelego-Mineral Hill Rift Zone (Glen, 1987) . North-west and north east
trending transform or tear faults are also linked to CSDs and are important in
influencing the development of dilation and fluid traps when they intersect other
major structures (Glen, et al., 1994). Most CSDs are associated with relatively steep
growth faults linked to flat lying thrust and listric faults and providing a fluid pathway
from the deeper rocks and basement in the Cobar Basin (Glen, 1987) (Stegman,
2001). Deep thrust faults are found associated with many orogenic gold deposits in
the Lachlan Orogen and in the Cobar Basin mineralisation is intimately linked to the
reactivation of these faults during deformation (Glen, 1995). CSDs are characterised
by wide alteration zones caused by protracted fluid flow history. A number of
deposits including McKinnon Tank, Peak and Perseverance are also associated with
anticlinal structures (Stegman, 2001).
Orogenic deposits are characterised by strong structural control that manifests into
many vein related breccias and laminated textures indicating periodic movement of
high pressure fluids along faults. The Structural control in many CSDs is apparent
especially with shear and fault and cleavage hosted mineralisation and later
remobilised styles of mineralisation that form the mineralised envelope along with
multiple quartz vein areas with complex paragenic sequences (Stegman, 2001)

Figure 3 Breccia from the Footwall Breccia, Mineral Hill NSW with silicified
elvan type and chloritised wall rock clasts and disseminated chalcopyrite
Figure 4 Coliform and laminated textured quartz vein with black chlorite
altered wall rock from the New Cobar Deposit.

3.2 Tectonic Setting and Metamorphism
CSDs predominantly formed during periods of basin inversion or in simple terms a
compressional regime, after periods of extensions. The Cobar Super basin, including
the Canbelego and Mineral Hill trough are extensional basins that began to form in
the Silurian and filled with sediments during the Late Silurian and well into the
Devonian (Glen, 1987). The change from extension to compression regimes resulted
in the development of basin wide westerly dipping thrusts and shallow reverse faults
while NW and SW striking transform of tear faults were related to extensional periods
(Glen, et al., 1994).
The Cobar Goldfield just to the east of Cobar (refer to figure 1) is characterised by a
bend in the north south structures. In simple terms this bending results in intense
stress and breaking of the rocks in this interpreted high strain zone, providing a
pathway for fluids travelling along deep thrust faults and in providing dilation in a
variety of settings from intersection faults, thrusts and where faults propagate many
splays. The broken rocks control is an important aspect to CSDs and when linked in
with compressional regimes is a strong argument for classifying CSDs as orogenic
style deposits.
Figure 5 Periods of extension and contraction associated with the Cobar Super
basin including mineralisation stages, fluid chemistry and temperature and
relative ages (Adapted from Giles and Marshall 2004)

The main mineralising stage in many CSDs has been associated with regional
deformation and the development of cleavage and metamorphism in the Cobar
Basin which occurred 395-400 Ma (Glen, 1992) (refer to figure 5) and is consistent
with orogenic style deposits. The role of extension in the formation of CSDs is far
from certain due to inconsistencies in dating tectonic episodes (Stegman, 2001),
however periods of extension may be associated with the recharge of water in the
basin later to be expelled during compression and further sedimentation as the
Cobar Basin was still a shallow sea well into the Devonian and later. While
metamorphism and cleavage development can overprint earlier VHMs deposits the
general consensus that CSDs are syndeformational is well established (Stegman,
2001).
3.3 Lithological Control and Alteration
CSDs are predominantly hosted in highly altered finer grained siltstones and
sediments and sandstones metamorphosed to upper greeschist facies (Glen, 1987)
(Lawrie & Hinman, 1998), however some are associated with volcanic rocks
including an intrusive rhyolite at the Peak, and contacts with tuffs at Shuttleton and
sequences of interbedded tuffs and sediments at Mineral Hill and Canbelego (David,
2005) (Stegman, 2001)( refer to Figure1 and 9). While some authors such as David
(2010) regard many deposits in the Mount Hope-Rast and Canbelego-Mineral Hill
troughs as being VHMS , little evidence exists to support this theory. VHMS deposits
precipitate metals and sulphur on contact with sea water, wet sediments and
chemical interaction with wall rocks (Robb, 2006), which is not the case with CSDs.
An important aspect of CSDs is the absence of substantial chemical wall rock
precipitation mechanisms and a strong open space filling (dilation) . The
mechanisms for ore precipitation are inconsistent with VHMS deposits and rely on
possible mixing of two distinct fluids ( basinal and basement derived), the lack of
evidence to support interaction with meteoric and connate waters, inferred steep
temperature gradient linked to steep dipping faults and uplift of the eastern part of
the Cobar Basin during mineralisation (Solomon, et al., 2000) (Glen, et al., 1996) and
the role of dilation.

Figure 6 (left) Clasts of silicified sediment (Elvan) in mineralised quartz vein
from the New Cobar mine. Figure 7 (right) Angular elvan clasts with
chalcopyrite, galena, red and dark sphalerite and quartz from Perseverance.
Figure 8 (left) Black chlorite altered wallrocks associated with massive
chalcopyrite and pyrrhotite vein with elongated quartz clasts from a high strain
zone Perseverance mine Cobar. Figure 9 (right) altered tuffs at the Mayday
open cut Gilgunnia.
The majority of CSDs are associated with strongly silicified siltstones and
sandstones forming a chert like rock referred to as Elvan (Binns & Appleyard, 1986)(
refer to Figure 5 & 6). Elvan is a Cornish mining term mainly used to describe fine
grained aplite rocks notes for their hardness. Rather than being synergetic chemical
sediment deposited during sulphide deposition in a VHMS setting, elvan still retains
relic sedimentary textures similar to non silicified host rocks (Binns & Appleyard,
1986), supporting the classification of CSDs as epigenetic deposits.

The presence of elvan and ore bodies in CSDs is also associated with depletion
halos indicating high fluid flow rather than diffusion (Robertson & Taylor, 1987),
which supports an epigenetic model rather than a VHMS based model. These halos
to CSDs are characterised by depletion in base metals and alkali metals including
lithium, aluminium and titanium associated with the breakdown of feldspars and
sericite. While VHMS system, especially in felsic rocks are characterised by strong
sericite alteration, its presence in CSDs is rare.
VHMS deposits are typically associated with wide zones of alteration enriched in
base metals some alkali earth elements and are not associated with dilation which is
significant in the majority of CSDs. (Binns & Appleyard, 1986) suggest that the
depletion halos associated with CSDs occurred prior to or during the early stages of
metamorphism indicating an early priming stage of fluid. This is very apparent at the
Elura Mine where an early silicification of wall rocks took place prior to mineralisation
(Jeffrey, pers.comm., 2010) and later mineralised fluid flow phases occurred.
The inferred coarse clastic and volcanic sediments located below finer grained
turbidites and sediments and interbedding of contrasting lithologies in the Cobar
Basin (Glen, et al., 1994) (Stegman, 2001) is an important source and control for hot
water migration from the deeper regions of the Cobar basin. The finer grained
sediments act as an effective cap to coarse rocks at depth with deep thrust faults
acting as effective permeable zones focussing fluids into steeply dipping faults at
higher levels that have also undergone uplift.
3.4 Vein Textures the Key to Classification
VHMS deposits are associated with quartz vein feeder zones along with strong
sericite, chlorite, and silica and pyrite alteration of wall rocks. Quartz veins are
predominantly massive to vuggy, copper and iron sulphide rich with intensely altered
wall rocks and large volumes of pervasively altered rocks. Orogenic style deposits
are characterised by distinctive quartz veins often with low sulphide levels, vein
breccias and slivers of wallrock in distinctive laminated vein textures with focussed
wall rock alteration halos.
CSDs do not have quartz vein rich feeder zones but do have extensive envelopes of
quartz vein arrays associated with the ore zones (Glen, 1987). Breccias are common
in the deeper portions of many deposits such as Peak, CSA and other deposits of

the Cobar Goldfield (Stegman, 2001)(refer to figure 2,3 and 5). Distinctive laminated
and colloform vein textures occur in the New Cobar and New Occidental Mines and
the Mineral Hill Mine and indicated a protracted fluid flow history and strong
structurally focussed fluids consistent with orogenic hydrothermal systems (refer to
figure 4 and 10) . These textures are largely absent from VHMS systems that
typically occurred in active magmatic/volcanic environments associated with volcanic
successions such as the Mount Reed Volcanics in Tasmania (Solomon, et al., 2000)
and usually lack the fault valve control to fluid flow. While epiclastic breccias and
conglomerates are common in VHMS systems they are related to sedimentary
processes rather than vein and brittle deformation processes in CSDs and orogenic
deposits.
Figure 10 Laminated and colloform vein textures from the Southern Ore Zone,
Mineral Hill Mine NSW.
While banded sulphide textures are typical of most VHMS deposits, their presence in
deposits such as Elura reflect the deposition of sulphides in dilational voids and the
development of foliation. The low pyrite content of CSDs is also at odds with most
VHMs deposits where it is usually the dominant sulphide and iron sulphides
comprise up to 90% of sulphide content (Franklin, 1993). The presence of Pb-Zn
lenses in many CSDs does not support the metal zonations as seen in VHMS
deposits due Pb-Zn lenses found at variable locations in many CSDs. While some
argue a clear Pb-Zn zonation exists within the Cobar Basin, and at some deposits
such as CSA and Elura Cu and As increase with depth, there appears to be a
common Pb-Zn stage in most CSD s as well as strongly anomalous As, Au and Bi.

An early silicification stage is inferred to have occurred at many CSDs (Stegman,
2001). At New Occidental and New Cobar these stages are associated with oxide
phases such as wolfram (later replaced by scheelite) cassiterite and magnetite. At
Mineral Hill clasts of colloform silica are often found in mineralised veins and
massive sulphide lenses and is very similar to textures found at New Occidental and
New Cobar (refer to figure 4 and 10). Intense chlorite alteration of wall rocks is a
common in many CSDs that are also characterised by more subtle chlorite alteration
within the halos of these deposits (refer to figure 8), reflecting early pervasive fluid
flow and later mineralised focussed fluid flow focussed by both structure, fracturing
of silicified rocks forming dilation and cleavage hosted mineralisation where chlorite
alteration is intense.
3.5 Source of Hot Water, Metals and Ligands.
(Binns & Appleyard, 1986), argue that mineralisation at the CSA mine formed rapidly
after sedimentation, deformation and metamorphism in the Cobar Basin. This
suggests that an early source of fluids was derived from the dewatering sedimentary
sequence at depth. Water was potentially trapped under the finer grained turbidites
and focussed along thrust faults and shallow steeply dipping growth faults and as
diagenesis, basin fill and deformation continued these fluid conduits continued to
transport hotter water from the deeper sediments and basement rocks. These basin
derived fluids are supported by fluid inclusion data from many deposits including
from the CSA (Giles & Marshall, 2003), who also found evidence of a basement
sourced ore forming fluid. The two fluid sources are supported by fluid inclusion and
isotope data from many CSDs.
Sulphur isotope data from many Cobar Basin deposits shows a restricted range.
Recent S isotope data from the Nymagee Mine range from 6.5% to 10.2% with those
from the Hera deposit in the range 4.4% to 7.4% (Page, 2011). Most CSDs fall in the
range at Nymagee and point to a number of sulphur sources including the Silurian
felsic volcanics, possible magmatic input and an input from a reduced seawater
source (Downes, et al., 2008). The intrusion of granites and mafic magmas into the
basement of the Cobar Basin and potential crustal thinning (Glen & Djomani, 2009),
would be an important driver of geothermal gradients and fluid migration.
Lead isotope data falls within the Silurian VHMS and Devonian granites window
(refer to figure 11) and regarded by many authors as being a homogenous source.

However some variation in lead isotope data, especially from more recently studied
deposits such as the Peak and Hera suggest that lead was derived from multiple
sources reflecting leaching from the deeper Silurian felsic volcanic derived
sediments that filled the Cobar Basin early in its history and a deeper basement
source including granites and a potential dense mafic intrusive body (Page, 2011)
(Jiang & Seccombe, 2000) (Glen & Djomani, 2009).
Much of the fluid inclusion data shows that the hot water responsible for
mineralisation evolved over time in a number of CSDs studies including The Peak,
CSA and Elura (David, 2008) (Giles & Marshall, 2004) (Jeffrey, 1994) (Jiang &
Seccombe, 2000). The data indicates changing fluid geochemistry from early low
salinity, CO2 rich and cooler water which evolved to hotter, saline and CH4 rich
water. An interesting multiple paragenesis of fluid stages is inferred to have occurred
in many CSDs including an early oxidised phase and possible magmatic input.
Isotope and fluid inclusion data for the Hera deposit suggest some degree of
magmatic input (Page, 2011), and also implicated in other CSDs especially the Peak
(Jiang & Seccombe, 2000).
Figure 11 Lead Isotope data for deposits of the Cobar Basin (Adapted from
Stegman 2001)
The early oxidised fluid was probably associated with a shallow metamorphic derived
fluid that was also enriched in CO2 and metals such as W are typical of other
orogenic deposits such as Hillgrove NSW. As metamorphism and deformation
intensified deeper and hotter fluids were the main source of metals and ligands that
precipitated into dilations and jogs formed through the movement of faults and

structures. Periods of extension may have provided a water recharge to the basin as
seen in some of the isotope evidence. While fluid geochemistry differs from the more
recognised orogenic “lode gold” deposits in respect to CO2, salinity, metal
associations and an variability fluid sources, the strong structural control and links to
deformation and metamorphism strongly support CSDs being classed as orogenic
variants.
4. Conclusion
Cobar style deposits are structural controlled deposits associated with dilational
voids and associated structures that formed during periods of deformation in the
Cobar Super basin. Unlike the more recognised forms of orogenic deposits including
lode gold, SCDs are polymetalic deposits often consisting of massive sulphide
lenses incorrectly identified as VHMS due to such things as banded sulphides and
the incorrect linking of deposits to volcanic rocks and the mis-identification of
chemical sediments.
CSDs are clearly epigenetic and not directly related to volcanism or volcanic rocks
ruling out a VHMS classification even though metals and sulphur were largely
derived from early volcanic fill of the basin and potential early VHMS deposits. The
tectonic setting is also at odds with VHMS deposits with CSDs located in thick
sedimentary accumulations that were being deformed and uplifted at time of
deposition with precipitation of metals linked to the filling of structural voids,
temperature change linked to steeply dipping faults and the lack of conclusive
chemical precipitation mechanisms related to wall rock interaction.
Difficulties in precisely dating periods of extension and compression within the Cobar
Basin may have also incorrectly linked some mineralising episodes to extensional
periods. The majority of CSDs, their structural links and ore textures are consistent
with deposition during periods of compression and metamorphism and the periodic
release of high pressure hydrothermal fluids via the fault valve mechanism and
strongly supported by ore and vein textures. While some doubts exist in regards to
the composition and source of various isotopes this does not negate the strong
overriding controls to mineralisation. Therefore I propose that Cobar Style Deposits
share a common classification based on a range of sometimes variable attributes
(refer to Table 4)

Table 4 Cobar Style Deposit Classification
Characteristic Comments Exploration Context
Host Rocks Variable ranging from fine grained turbidites,
sandstone and volcanic rocks. Permeability
contrasts important with formation of strong
silicification.
High relief associated with
silicification. Look for
contacts between silicified
and fine grained chloritised
sediments
Structure Strong structural control. Deposits located in high
strain zones, steeply dipping faults and where
faults intersect. Link to deep thrust faults. Faults
are the main permeability pathways for
mineralisation.
Mapping of faults, deep
thrust faults and cross
cutting transforms faults
vital. Extension of known
faults?
Metal Associations Variable paragenic sequence with Cu-Au-Bi, Pb-
Zn-Ag, early Fe-W-Sn, multiple overprinting
episodes, As in most deposits.
Discrete anomalies
including strong Bi. Au, As
and Cd also important
pathfinders.
Metal Zonations Increasing Pb-Zn in the northern part of the basin.
Many deposits contain discrete Pb-Zn lenses
often forming higher and outer layer lenses.
Increasing Cu-As with depth
Strength of Cu against
Pb/Zn may indicate level
within a system
Tectonic association Mineralisation occurring in periods of compression
associated with basin inversion and
metamorphism.
Deformed and
metamorphosed rocks
important
Intrusive Association No direct link apparent but potentially important as
a source of fluids in the basement and partly
supported by isotope data. Potentially important in
initial silicification and geothermal gradients
Nearby and concealed
granites need to be
evaluated, especially late
Silurian and Devonian age.
Isotopes Support a metamorphic –basinal fluid model.
Initial fluid flow with mineralising events
associated with moderate depth metamorphic
fluids with later deeper fluids evolving during
deformation
Test isotopes in structures
intersected during drilling
Fluid Inclusions Support the metamorphic-basinal fluid model with
initial cooler CO2 rich fluids and later hotter CH4
and hydrocarbon rich fluids.
Strong structural control
Alteration Initial strong silicification of permeable wallrocks
forming depletion halos and silicified wall rocks
which are an important factor in later dilation.
Finer grained rocks strongly chloritised. Some
deposits occurring on contact with silicified rocks
and finer grained chloritised sediments.
Discrete Mg rich chlorite
associated with the cores of
some deposits. Fe rich
chlorite and stilpnomelane
occur with deposits.
Deposit Geometry Limited strike extent linked to strong structural
control and dilation.
Strong conductors with
electrical geophysical
techniques.

Bibliography
Binns, R. & Appleyard, E. C., 1986. Wallrock alteration in the Western System of the
CSA Mine, Cobar, New South Wales, Australia.. Applied Geochemistry, Volume 1,
pp. 211-235.
David, V., 2005. Metallogenesis of the Cobar region, central New South Wales..
Australian Journal of Mineralogy, December, 11(2), pp. 55-62.
David, V., 2008. Structural-geological setting of the Elura-Zn-Pb-Ag massive
sulphide deposit, Australia. Ore Geology Reviews, Volume 34, pp. 428-444.
Downes, P., Seccombe, P. K. & Carr, G. R., 2008. Sulfur- and lead isotope
signatures of orogenic gold mineralisation associated with the Hill End Trough,
Lachlan Orogen, New South Wales, Australia. Mineralogy and Petrology, Volume 94,
pp. 151-173.
Franklin, J., 1993. Volcanic-associated Massive Sulphide Deposits. In: R. Kirkham,
W. D. Sinclair, R. I. Thorpe & J. M. Duke, eds. Mineral Deposit Modelling. Geological
Association of Canada, pp. 315-334.
Giles, A. & Marshall, B., 2004. Genetic significance of fluid inclusions in the CSA Cu-
Pb-Zn deposit, Cobar, Australia.. Ore Geology Reviews, Volume 24, pp. 241-266.
Glen, 1995. Thrusts and Thrust-Associated Mineralization in the Lachlan Orogen.
Economic Geology, Volume 90, pp. 1402-1429.
Glen, R., 1992. Thrust, Extensional and strike slip tectonics in an evolving
Palaeozoic orogen - a synthesis of the Lachlan orogen of southeastern Australia.
Tectonophysics, Volume 214, pp. 341-380.
Glen, R. A., 1987. Copper- and Gold Rich Deposits in the Deformed Turbidites at
Cobar, Australia: Their Structural Control and Hydrothermal Origin. Economic
Geology, Volume 82, pp. 124-140.
Glen, R., Clare, A. & Spencer, R., 1996. Extrapolating the Cobar Basin model to the
regional scale. Devonian basin-formation and inversion in western New South
Wales.. In: W. G. Cook, et al. eds. The Cobar Mineral Field-1996.. Melbourne:
Australian Institute of Mining and Metallurgy, pp. 43-83.
Glen, R. & Djomani, Y., 2009. Geophysical Evidence for 'Blind' Magmatism
Associated with the Devonian Rifting, Lachlan Orogen, New South Wales.. ASEG
Extended Abstracts, pp. 1-5.
Glen, R. et al., 1994. Structure of the Cobar Basin, New South Wales, based on
seismic reflection profiling. Australian Journal of Earth Sciences, Volume 41, pp.
341-352.

Groves, D. et al., 1998. Orogenic gold deposits: A proposed classification in the
context of their crustal distribution and their relationship to other gold deposit types.
Ore Geology Reviews, Volume 13, pp. 7-27.
Jeffrey, S., 1994. A Structural, Geophysical, Isotopic and Geochemical Appraisal of
the CSA Deposit, Cobar, Australia: Implications for the Deformation of the Cobar
Basin and Mineral Potential. s.l.:Unpublished Thesis (PhD), University of Tasmania
Hobart..
Jiang, Z. & Seccombe, P. K., 2000. Source of Ore-Forming Components at the Peak
Mine, Cobar, NSW. -Evidence from Isotopic Studies.. In: K. G. McQeen & C. L.
Stegman, eds. Central West Symposium Cobar 2000 Extended Abstracts.. s.l.:s.n.,
pp. 41-45.
Lawrie, K. & Hinman, M. C., 1998. Cobar-style polymetallic Au-Cu-Ag-Pb-Zn
deposits. AGSO Journal of Australian Geology and Geophysics, 17(4), pp. 169-187.
Morrison, G. et al., 2004. Age and Setting of the Mineral Hill Au-Base Metal
Deposits. In: Tectonics to Mineral Discover Deconstructing the Lachlan Orogen.
GSA Abstracts.. Orange(New South Wales): Geological Society of Australia, pp. 83-
93.
Page, D., 2011. Geology of the Hera (Pb-Zn-Au) and Nymagee (Cu) deposits, New
South Wales.. s.l.:Unpublished Thesis (Honours), University of Wollongong.
Robb, R., 2006. Introduction to Ore Forming Processes. s.l.:Blackwell Publishing.
Robertson, I. & Taylor, G. F., 1987. Depletion Haloes in Fresh Rocks Surrounding
Cobar Orebodies, NSW., Australia: Implications for exploration and Ore Genesis..
Journal of Geochemical Exploration, Volume 27, pp. 77-101.
Solomon, M., Groves, D. I. & Jaques, A. L., 2000. The Geology and Origin of
Australia's Mineral Deposits. s.l.:Centre for Ore Deposit Research and Centre for
Global Metallogeny.
Stegman, C., 2001. Cobar Deposits: Still Defying Classification. SEG Newsletter,
January, Volume 44, pp. 1,15-26.

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