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Cobar Mineral Field - CSA, New Cobar, Chesney, New Occidental, The Peak and Perseverance, |
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New South Wales, NSW, Australia |
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Cu Au Zn Pb Ag
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All available as eBOOKS
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sale. No hard copy book more than AUD $44.00 (incl. GST) |
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The Cobar Mineral Field lies within the Lachlan Fold Belt and is located in central New South Wales, ~550 km WNW of Sydney in Australia. In 2018, the field comprised six operating mines CSA, New Cobar, Chesney, New Occidental, The Peak and Perseverance, and seven historic mines, Tharsis, Great Cobar, Dapville, Gladstone, Mount Pleasant, Young Australia and Queen Bee.
Mining in the Cobar field began in 1871 with an erratic production history until 1964, when Broken Hill South Ltd started a modern mining operation at the CSA mine. The operation was acquired by CRA Limited in 1980, and in 1992 by Golden Shamrock Mines. The operation was closed in 1997/8 following its acquisition by Ashanti Goldfields and was reopened in 1999 by Glencore. In March 2022, Glencore agreed to sell the controlling interest to Metals Acquisition Group (MAC). While production from 1965 included substantial quantities of zinc, lead, silver and copper, the CSA Mine more recently has focused on mining copper, with a silver by-product.
The Peak gold ore body was discovered in 1980, with >30 t of Au being mined between 1991 and 2002 when mining ceased. Mining re- commenced in 2006. The Perseverance Au-Cu-Ag-Pb-Zn deposit was discovered in 1994 and production commenced in 2003. New Occidental, which is 3 km north of The Peak, was sporadically mined over the previous century, and after new resources were delineated, recommenced production in 2001. Chesney had also been mined previously but had additional extensions located and was brought back into production in 2009. New Cobar was earlier worked as an open pit and when the open pit ceased in 2004 after producing 1 Mt of ore, was exploited underground.
Geological Setting
The Cobar Mineral Field lies close to the eastern margin the Cobar Basin, a shallow to deep marine, extensional, intracratonic basin that extends for >360 km north-south and 150 km east-west within the Central Lachlan Orogen. The basin has a complex strike-slip geometry where contrasting fault-bounded domains of shallow marine shelf and deep water trough are abruptly juxtaposed (Brown et al., 2017). The sedimentary sequence within the trough is dominated by siliciclastic turbidites, with local, margin-derived felsic volcaniclastic mass flow deposits in the SW of the basin and proximal submarine volcanism in the south. Shelf strata are composed of siliciclastic sedimentary rocks with limestone and local volcanic rocks. Magmatic activity (412.2 to 422.8 Ma; Downes et al., 2016) is interpreted to have accompanied growth faulting along the basin margins. This was represented by abundant felsic volcaniclastics in the south, SW and east, and by S- and I-type granitic plutons in the south and SE, as well as rare quartz-feldspar porphyry dykes (Brown et al., 2017).
The basin developed in two phases, the first of which was syn-rift, marked by rapid deepening and active faulting during a period of sinistral transtension that persisted from the latest Silurian or Early Devonian (~410 or possibly as early as ~420 Ma ) until the late Early Devonian (~400 Ma). During the ~10 to 20 m.y. of its existence, the basin was filled by both syn-rift and late-rift sag phase sedimentary rocks. This was followed by the second phase, a period of basin inversion, dated at ~400 Ma (e.g., Glen, 1991), during which the fault arrays that would later act as fluid pathways for the deposits of the field, were created. At the peak of deformation during this inversion, hot, metalliferous fluids acted as a source for mineralisation which progressed further into the hanging wall with time and becoming increasingly Pb-Zn rich (Kyne, 2014).
The stratigraphy of the Cobar Supergroup and basement of the Cobar Basin, from basement upwards is:
Girilambone Group, which constitutes the basement to the unconformably overlying Cobar Supergroup. It is exposed to the east of the basin, and is composed of Ordovician quartz-arenite, sublithic-arenite, feldspathic arenite, phyllite, siltstone, shale, bedded chert and minor altered mafic to intermediate igneous rocks (Baker et al., 1975; Pogson and Felton, 1978; Felton, 1981).
Late Silurian S- and I-type granitoids in the basement on both the eastern and western margins of the Cobar Basin, coincident with the onset of extension. These include the S-type ~422 Ma Thule Granite and ~419 Ma Erimeran Granite, and the 418 Ma I-type Wild Wave Granodiorite (Glen et al., 1996; Lawrie and Hinman, 1998).
COBAR SUPERGROUP
Kopyje Group - which is of Late Silurian age, and is regionally composed of shallow water sedimentary rocks including siliciclastic sandstones, limestones, polymictic conglomerates and minor siltstones as well as volcaniclastic sedimentary rocks (Pogson and Felton, 1978; David 2005). In the Cobar area it is divided into the:
• Brooklong Formation - the basal unit of the group, which varies from 15 to 120 m in thickness, and is composed of poorly sorted and laterally discontinuous matrix to clast-supported conglomerates containing well rounded to sub-angular clasts of quartzite, sandstone, phyllite and granodiorite (Glen 1994).
• Meryula Formation - which conformably overlies and interfingers with the Brooklong Formation, and comprises a massive to coarsely (10 to 30 cm thick) bedded sub-litharenite, intercalated with laminated bioturbated mudstone (Pogson and Felton 1978; Felton, 1981).
• Baledmund Formation - thinly (0.5 to 8 cm thick) bedded, medium to fine-grained lithic arenite intercalated with mudstone and claystone (David, 2005). Sedimentary structures include cross-bedding, ripple marks and rip-up clasts of mudstone (David, 2005).
The Kopyje Group is considered to have been deposited in a shelf setting, the Kopyje Shelf which extends from Condobolin to Mineral Hill and to the NNW where it passes just to the east of Cobar. It overlies a Girilambone Group basement to the east of the main Cobar Trough, and was largely deposited within an environment ranging from fluvial to shallow-marine outwash fans, characterised by conglomerate and sandstone units, and an offshore facies mainly composed of laminated siltstone and fine-grained sandstone.
Mouramba Group - some 100 to the SE of Cobar, in the Nymagee area, the Kopyje Group interfingers westward into the Mouramba Group which was deposited on a basement of Ordovician turbiditic sedimentary rocks of the Lancefieldian to early Gisbornian Wagga Group (Colquhoun et al., 2005). The Mouramba Group is of Late Silurian to earliest Devonian (Sherwin 2013) and is dominated by the:
• Burthong Formation, which includes a sequence of very fine to medium grained interbedded sandstones and siltstones with minor basal conglomeratic units (MacRae 1987; Pogson 1991). Several volcanic horizons, occurring as polymictic arkosic conglomerate, crystal tuffs and rhyodacite lavas are also mapped (Suppel and Gilligan 1993). Slumping, cross-bedding, graded bedding and soft-sediment deformation is evident in this formation at the Nymagee mine (Paterson 1974), as are current-generated ripple marks and flutes (MacRae 1987), all of which support the interpretation that the Burthong Formation represented an outwash fan to marine shelf sequence (Downes et al., 2016). The Burthong Formation is overlain by, and partially interfingers with the:
• Roset Sandstone, which is mainly composed of crossbedded lithic quartz to quartz-rich sandstone with a thin pebble conglomerate horizon (Suppel and Gilligan 1993; MacRae 1987).
To the south of Nymagee, the Erimeran Granite, an ~40 km diameter irregular mass of Late Silurian to earliest Devonian granites, as described above, obscures the contact between the basement Girilambone to Wagga groups, and separates the Kopyje and Mouramba shelves which diverge to the SSE.
Nurri Group - Late Silurian to Early Devonian in age, developed along the eastern margins of the Cobar Basin, west of the Kopyje and Mouramba shelves, where it is ~3 km thick, and is subdivided into:
• Chesney Formation - which is 100 to 1400 m thick and is composed of clast and matrix-supported conglomerate, interbedded with lenticular sandstone beds. Clasts include quartz sandstone, chert, sub-lithic sandstone, siltstone and intraformational mudstone (David, 2005). The conglomerate beds grade into 1 to 3 m thick lithic greywacke beds, thin, 2 to10 cm quartz greywacke and mudstone lithofacies (Glen, 1987). These rocks are mainly massive to graded with only rare parallel laminations and ripple cross lamination (Glen, 1985).
• Transitional Unit - an informal, but distinctive 60 to 80 m thick unit that passes gradationally upwards from the thick-bedded Chesney Formation sandstone through a succession of thin to medium bedded graded sandstone and siltstone, thinly bedded to laminated calcareous siltstone and mudstone, and then into the Great Cobar Slate. The top of the Transition Unit is defined by the change from well-stratified mudstone and fine-grained siltstone to relatively poorly stratified mudstone with lesser siltstone (Stegman, 2007).
• Great Cobar Slate - which is 900 to 1700 m thick and is composed of dark grey, quartz-muscovite-chlorite slate interbedded with fine-grained siltstone with rare fine-grained quartz siltstone (Russell and Lewis, 1965; Glen, 1985). Where present, bedding is defined by thin, 1 to 2 mm thick, carbonate beds and/or fine-grained siltstone laminations. Glen (1985) suggested the unit represents a mud-drape terminating the Chesney Fan and that it was a 'basin plain facies' to the prograding Amphitheatre Group from the west. The contact with the overlying CSA Siltstone is marked by a 30 m thick transition from relatively massive, grading into poorly to moderately stratified, thinly bedded and then into well stratified and graded mudstone/siltstone sequences. This transition indicates the contact between the two units is conformable, and is evident to the west of the New Occidental deposit as well as in the Perseverance and Peak Mine areas to the south (Stegman, 2007).
Amphitheatre Group - which is ~7 km thick and of Early Devonian age. It is interpreted to be a turbiditic sequence (Glen, 1982) and conformably overlies and interfingers with the Nurri Group. It is divided into the,
• CSA Siltstone - comprises a sequence of carbonaceous siltstones and mudstones with some thin, fine-grained, commonly ripple cross-laminated quartz sandstone beds, and rare, up to 1 m thick beds and lenses of medium bedded, massive to parallel-laminated, medium-grained greywacke to lithic sandstone beds (Glen, 1982; 1987). Siltstone totals ~95% of the unit and is rhythmically interbedded, thin to medium bedded, graded fine- (mud) to coarse-grained. It has a dark grey colour, and contains angular grains of quartz, muscovite, carbonate and minor feldspar, with accessory zircon, tourmaline, anatase and magnetite (Shi and Reed. 1998). Sedimentary structures are abundant within the siltstone, including grading, planar lamination with minor cross laminations, and load, flute, injection, scour, flame and fill marks (Shi and Reed, 1998). Greywacke to lithic sandstone account for the remaining 5% of the CSA Siltstone. It is pale grey with slight variations in grain size and composition (Shi and Reed. 1998). Clasts within the greywacke include 70 to 80% subrounded quartz grains, 20 to 30% white mica and lithic fragments (Shi and Reed, 1998). The greywacke/lithic sandstone facies are poorly sorted with no obvious sedimentary structures. The CSA Siltstone contains as much as 2 to 3% euhedral pyrite concentrated in the base of individual siltstone beds. However, much of the original pyrite appears to have been altered to pyrrhotite and mobilised into the overprinting cleavage. The CSA Siltstone represents deposition of fine-grained turbidites in a less active environment associated with gentle westerly subsidence and marks the onset of the first pulses of the post-rift or sag phase (Glen, 1987; 1990). The steeply dipping unit outcrops over a width of 1.5 to 3 km, with its upper margin defined by the Water Tanks Beds, a zone of boldly outcropping quartzites (Brooke, 1975).
• Lower Amphitheatre Group - which is composed of a poorly outcropping series of mudstone, siltstone and sandstone units (Glen, 1994, David, 2005). Sections of the group have been observed to be interbedded with the CSA Siltstone (Glen, 1985; David, 2005). The sandstones of the Lower Amphitheatre Group are quartzose to quartz-lithic in composition, poorly sorted, and variably, thin to medium (0.5 to 20 cm) bedded, massive or graded (Scheibner and Basden, 1998). Sedimentary structures within the sandstones include planar lamination commonly overlain by cross lamination as well as flutemarks. Locally, sandstones grade into siltstones, but are more commonly interbedded with mudstone and siltstone units (Glen, 1982). Detrital biotite, plagioclase and mudstone fragments are common (Glen 1982). On the northwestern margin of the Cobar Basin, the Lower Amphitheatre Group grades laterally into shallow-water sedimentary rocks of the Winduck Group, whilst in the centre of the basin, the group grades into thicker, mature sandstone beds deposited from larger and reworked fans (Glen, 1992).
• Upper Amphitheatre Group - which is 500 to 8000 m thick and composed of 2 to 20 cm thick, thin to medium bedded sequences of sandstone, siltstone and mudstone units with locally thick-bedded quartz arenite (Glen, 1982; David, 2005). Individual units commonly have sharp basal contacts and an upward gradational progression from either fine-grained sandstone to siltstones or coarse-grained to medium-grained siltstones, with occasional massive sandstone units (Glen, 1982). Sedimentary structures include planar lamination as well as cross and convolute laminations (Glen, 1982). This group is interpreted to reflect the progressive erosion of an emergent block to the west of the basin, a slowing of basin subsidence and the termination of turbidite deposition towards the end of the Early Devonian (Glen, 1982).
Winduck Group - which was deposited in the Pragian Stage of the Lower Devonian, chronostratigraphically equivalent to the upper Amphitheatre Group, and is confined to the western, northwestern and southwestern sections of the Cobar Basin (Glen, 1985). It varies in thickness up to 500 m, partially as a result of thrust repetition (Glen 1987; Scheibner, 1987). It has been deposited within a shallow marine environment, the Winduck Shelf which occurs on the western margin of the Cobar Trough and mirrors the Kopyje Shelf to the east (David, 2005). It has been subdivided into the:
• Buckambool Sandstone - made up of 0.2 to 2 m thick beds of quartz-rich, medium-grained sandstone interbedded with siltstone (David, 2005), with planar laminations, current lineations, cross bedding, ripple cross lamination, and hummocky cross-stratification (Glen, 1985).
• Sawmill Tank Siltstone - composed of a poorly outcropping, unknown thickness of thinly bedded siltstone with lesser sandstone (Glen, 1985, David, 2005). Sedimentary structures include planar laminations, ripple cross lamination and flute and tool marks (Glen, 1985)
• Gundaroo Sandstone - a medium to thick (ranging from 0.1 to 1 m) bedded, quartz-rich, fine to medium-grained sandstone interbedded with siltstone (Stegman, 2007) and local limestone (Glen, 1985). Sedimentary structures include trough cross beds, ripple cross lamination and planar lamination (Glen 1985).
Mulga Downs Group - which is late Early Devonian to Early Carboniferous in age, comfortably and/or unconformably overlies the Mulga Downs Group, and is restricted to the western margins of the Cobar Basin. It is 1300 to 1500 m thick (Rayner, 1961) and primarily composed of quartz conglomerate, quartzite, quartz-rich sandstone, mudstone and shale. It contains multiple lithofacies consistent with deposition in braided fluvial and meandering fluvial, with minor
fluvial-shallow lacustrine, estuarine tidal channel and nearshore lithofacies, contemporaneous with a near-shore braided-delta plain complex (Khalifa and Ward, 2010). It has been suggested to have been deformed in the early Carboniferous (Glen, 1990), and is devoid of quartz veins, igneous intrusions and base metal lodes (Rayner, 1961).
The Cobar Basin sequence is thickest and more structurally complex to the east, thinning and shallowing with time towards the west. Its structure is dominated by local to regional scale north and NW trending upright folds overprinted by north, NE and NNW trending reverse faults (Brown et al., 2017). On the eastern margin of the basin, high strain features are characteristic, including intense subvertical cleavage that accompanies tight folding and parallels regionally persistent NNW trending braided fault complexes (Stegman, 2001). Lower strain environments are found in the central and western parts of the basin with only local sub-vertical cleavage (Glen et al., 1994; David, 2005, 2008).
Structure and Mineralisation
The deformational history of the Cobar Basin and its basement may be summarised as follows (Glen, 1992, 1994; McDermott et al., 1996):
D0 - corresponds to a regional extensional regime that opened the Cobar Basin for deposition and evolved from a syn-rift phase of brittle upper crustal faulting and subsidence followed by post-rift sag phase passive subsidence. The basin was developed over a basement of predominantly Ordovician rocks, the Girilambone Group, that had been deposited in an extensional back-arc basin west of the intraoceanic Macquarie Arc. These basement rocks underwent high temperature-low pressure metamorphism with associated migmatites and large S‐type granite bodies during the Silurian soon after cessation of deposition. An extensive block of these metamorphosed back-arc basement rocks were transported SSE to form the 900 x 150 km Wagga-Omeo Zone as part of the development of the Lachlan Orocline during the Early Devonian Bindian Orogeny. Major partitioning strike-slip faults define the margins of the zone, one of which was the NNW-SSE Gilmore Fault, that forms its eastern margin. Deposition within the NNW-SSE elongated, 300 x 70 km Cobar Basin, which lies within and aver the Wagga-Omeo Zone, was synchronous with the SSE transport of the latter (Fergusson, 2017). The Gilmore fault marks the eastern limit of the Kopyje Shelf that is the eastern margin of the Cobar Basin, whilst a parallel fault complex 20 km to the west, forms the eastern margin of the trough facies of the basin. The stress field imparted by the transport of the Wagga-Omeo Zone along the Gilmore fault produced a sinistral transtension regime concentrated along the eastern margin of the Cobar Basin during the latest Silurian and Early Devonian which opened the basin (Glen, 1991).
D1 - Basin inversion occurred during the late Early Devonian Tabberabberan Orogeny, imposing dextral transpression and generating a regionally penetrative NE to ENE striking S1 cleavage, dated at ~400 Ma (e.g., Glen, 1991). This led to basin-wide folding and the re-activation and inversion of basin margin extensional faults as well as the initiation of new basin-scale thrust faults. Deformation was partitioned into largely strike-slip plus compressional components along the eastern edge of the basin and into purely compressional components to the south and west (Glen et al., 1994). This event produced both steep folds and the braided fault complexes that form the eastern margin of trough facies of the Cobar Basin and host the Cobar Mineral Field mineralisation. These fault complexes enclose internal fault-bound plates of strongly folded basin sedimentary rocks and represents a linked thrust system merging at depth into a single floor detachment. From deep seismic profiling, this detachment is interpreted to underlie much of the eastern part of the Cobar Basin (Glen et al., 1994). The easternmost strand of the fault complex, the Rookery Fault, is interpreted to have been a syn-sedimentary, west-dipping listric detachment fault during D0 above which the trough section of the basin was initially extended. It was subsequently re-activated as a thrust fault during D1 basin inversion when a series of mainly east dipping short-cut structures (e.g., Chesney Fault) were developed (Glen, 1991). Some other more westerly fault strands are not interpreted as reactivated extensional splay from the deep detachment, but rather as new thrusts developed during inversion (e.g., Cobar Fault; Glen et al., 1994; Glen, 1991, 1994). These fracture systems underwent strike- and dip-slip displacement of as much as 1.5 to 2.5 km (Stegman, 2007).
D2 - was the result of rotation of the stress field from NE-SW to NW-SE transpression, with NE-SW shortening, predominantly affecting the southern and western sections of the basin. Folds initiated during D1 were tightened with the formation of the regional NW-SE trending S2 cleavage. The basin margin faults, including the Great Chesney and Rookery Faults were back-rotated to accommodate thrust ramps further east, steepening them from an initial 60° angles of 70 to 80°. Shortening was further partitioned into folds and NE-SW faults, assisting the tapping, migration and concentration of deep seated fluids along the eastern margin of the Cobar basin (Kyne, 2014). Faults parallel to S1either crosscut S2, or have mutually crosscutting relationships (Kyne, 2014). The differentiation of S1 and S2 is therefore problematic, and may either represent two structural fabrics related to the same deformational event (where they may locally be seen to be mutually crosscutting), or a progression in direction of maximum stress during the same event (Kyne, 2014). Kyne (2014) suggests that the mutually cross cutting relationships between different fault sets (and cleavages) observed to form simultaneously within the stress regime is consistent with an orthorhombic fault array which may be attributed to a three‐dimensional strain field. In this case, D2 is a later phase of D1 and not a separate event. The Cobar deposits were formed late in this D1-D2 event as a result of fluids from deep in the basin being focussed along high strain zones, e.g., via the flat floor detachment below the eastern Cobar Basin, and were transported into dilational traps near the eastern margin of the basin.
D3 - Strike-slip deformation along north-south-trending fault systems along the eastern boundary of the Cobar Basin and development of new basin-scale faults and shear zones. No timing data has been encountered for this event.
The gold, copper and lead-zinc-silver deposits of the Cobar Mineral Field define a north-south to NNW-SSE aligned, >25 km long and up to 1 km wide corridor of shear controlled mineralisation that sub-parallels stratigraphy, but obliquely transgresses the host succession northward from the uppermost Chesney Formation, through the Great Cobar Slate into the overlying CSA Siltstone. This corridor, which encloses multiple parallel to sub-parallel shear zones, is close to the rifted eastern margin of the Cobar Basin trough facies and includes reactivated syn-rift growth structures and reverse faults active during post-rift inversion. The two most significant deposits, the Peak (Au) near the southern end of the corridor, and CSA Cu-(Pb-Zn-Au-Ag) at the northern end, are ~8 km SSE and 11 km NNW of the town of Cobar, respectively. A string of smaller deposits are distributed between and to the south of these, whilst
the Elura-Endeavor Zn-Pb-Ag deposit is hosted by CSA Siltstone some 40 km to the NNW of Cobar.
This fault/shear complex and the mineralised lodes were emplaced following the NE-SW compressional inversion event D1. The location of the deposits was controlled by irregularities within shears of the fault complexes, due to intersections with drag folds, cross faults, competency contrasts and jogs producing steep, ~75 to 80°N plunging pipe-like zones of dilational deformation, elongated parallel to the dominant stretching direction of D1 and D2 strain.
From east to west, the individual shear zones within the corridor; and from north to south along each shear, the associated deposits are as follows:
• Queen Bee Fault which dips east and is only mineralised on the southern extremity of the field, at Queen Bee, ~20 km SE of Cobar. Some 10 km to the north of Queen Bee, this structure is truncated by the major Early Devonian, west dipping, basin margin Rookery Fault;
• Perseverance Fault, an east dipping en echelon southern offset to the Great Chesney Fault which hosts the Perseverance and Peak deposits that are <1 km apart;
• Great Chesney Fault, which also dips east, hosts the New Occidental, Young Australian, Mount Pleasant, Chesney, New Cobar and Tharsis deposits distributed at regular intervals over a strike length of ~6 km;
All of the three preceding faults separate the Chesney Formation and Great Cobar Slate to the east and west respectively;
• Great Cobar Fault which dips west and occurs within the Great Cobar Slate. It is west of the section of the Great Chesney Fault that hosts the Chesney to Tharsis deposits, and hosts the Gladstone (between this an the Great Chesney Fault), Dapville, Gladstone and Great Cobar deposits over a strike length of ~3.5 km;
• Cobar Fault (and the short CSA Fault ~500 m further west) which also dip west and host the CSA deposit ~10 km NNW of Great Cobar. The Cobar Fault follows the contact between the Great Cobar Slate and the CSA Siltstone, while the CSA Fault is within the latter unit.
The mineralised sections of these shear zones parallel lithological contacts, suggesting a stratigraphic influence. They cut progressively higher stratigraphic positions from south to north, overlapping and stepping to the west and north in an en echelon pattern, as indicated above. Together, they define three separate, disrupted, mineralised lines which correspond to zones of increased deformation and shearing, predominantly within slates and sandy slates, and are reflected by persistant lines of high magnetic intensity:
i). the Eastern Line, at and closely below the contact between the Great Cobar Slate with the underlying Chesney Greywacke, which hosts the New Cobar, Chesney, New Occidental, Peak and Queen Bee lodes, the latter well to the south and offset from the main trend;
ii). the Western Line within the Great Cobar Slate in the middle of the field, comprising the Great Cobar, Dapville and Gladstone lodes; and
iii). the CSA Line within CSA Siltstone, containing the CSA lodes. Within this suite of mineralised lines there is a recognisable zonation of metals, with CSA to the north in the CSA Line carrying Cu with abundant Pb and Zn. Further to the south, in the Western Line, Great Cobar has Cu, a little Au and minor Pb-Zn, passing south again to New Cobar and Chesney which are Cu-Au deposits. Continuing this trend southwards, New Occidental is an Au deposit, and then to the Peak which carries Au-Ag, while on the southern limit, the Queen Bee reverts to Cu with Pb-Zn.
The west dipping CSA and Great Cobar faults of the CSA and Western lines host the major Cu (±Zn-Pb-Ag) deposits in CSA siltstone and Great Cobar Slate respectively. These were formed in dilational zones during sinistral strike and east block down dip-slip displacement. Mineralisation was concentrated as semi-massive to massive sulphides with a vertical and horizontal zonation from dominantly lead-zinc in the deposit core, grading outward and or downward to copper. Gold mineralisation occurs within both Cu-rich and Pb-Zn-rich zones, although a greater proportion of the Au deposits are Cu- rather than Pb-Zn rich (Stegman, 2007). The deposits along the east dipping Great Chesney Fault in the northern Eastern Line are localised within dilation sites that formed during sinistral strike-slip displacement. Mineralisation is found immediately to the south of left-step jogs (e.g., New Cobar and Chesney), or where a deflection occurs on, or in close proximity to the contact between Great Cobar Slate and sandstone of the Chesney Formation (e.g., New Occidental). Further south in the Eastern Line, the Peak and Perseverance deposits are associated with the steeply west dipping Peak-Perseverance Fault, and were formed in dilation sites during sinistral strike-slip and east block down dip-slip displacement. Mineralisation occurs in sandstone, siltsone and rhyolite of the Chesney Formation in the footwall of the Great Cobar Slate (Stander and Berthelsen, 2017).
Each of the deposits is composed of a cluster of multiple lodes that occur as flat pipe-like lensoid bodies with steeply plunging long axes elongated in the dominant stretching direction of the enclosing zone of dilational deformation, as described above. Individual lodes/lenses are up to 300 m (averaging 60 to 120 m) in strike length, 6 to 30 m thick and persist over vertical extents of up to 1200 m, with dips that are mostly 70 to 85°E, to sub-vertical, and pitches averaging ~75 to 80°N (Stander and Berthelsen, 2017; Brooke, 1975; Russell and Lewis, 1965). The lenses are in turn composed of veins (generally quartz) and as disseminations in silicified host rock or as structurally controlled massive sulphide lodes. The enveloping zones of deformation that define the deposit vary from 15 to 400 m in width, up to 500 m in strike length, and are strongly cleaved, persisting to depths of as much as 2000 m or more (Russell and Lewis, 1965). Mineralisation occurs throughout these zones, hosted by dark green to black greenschist facies chloritic slate, in contrast to the enclosing, unaltered light, grey-blue slates. The degree of chloritic alteration is proportional to the amount of contained mineralisation (Russell and Lewis, 1965). All of these deposits and their constituent lenses/lodes are discordant to the hosts which are folded, cleaved, thin bedded Early Devonian turbiditic siltstone and slate, and lie at an oblique angle to bedding.
Three primary ore types have been recognised, namely:
i). Siliceous ore, which typically contains 20 to 50% silica in a number of forms, dominated by various quartz vein phases (Stander and Berthelsen, 2017), including brecciated, banded, cherty and granular quartz veins (Shi and Reed, 1998). Disseminated gold and chalcopyrite occur in these veins and in brecciated silicified slates which are characteristic of the Eastern Line, with the exception of Queen Bee. Pyrrhotite, pyrite and minor magnetite, galena, sphalerite, arsenopyrite, native bismuth, bismuthinite and cubanite are also associated with this ore type. At New Cobar and Chesney, chalcopyrite exceeds pyrrhotite, while at the Peak and New Occidental the reverse is the case (Russell and Lewis, 1965). At CSA, envelopes of siliceous ore 2 to 20 m thick surround most of the mineralised lenses. Siliceous ores is often very fine grained and metallurgically problematic (Shi and Reed, 1998).
ii). Siliceous pyritic ore, which comprise chalcopyrite and pyrite in a siliceous gangue as at Gladstone, Queen Bee and some of the CSA lodes, with subordinate arsenopyrite and marcasite, and only traces of gold, magnetite and pyrite (Russell and Lewis, 1965).
iii). Massive sulphide ore is found at Great Cobar, Dapville and CSA, but has marked variations within and between deposits. At Great Cobar, massive pyrrhotite-chalcopyrite-magnetite grades at depth into siliceous magnetite-pyrite-chalcopyrite with common stilpnomelane. At Dapville, massive pyrrhotite-magnetite-chalcopyrite predominates with minor arsenopyrite, cubanite and tetrahedrite, although to the south, pyrite and marcasite take the place of pyrrhotite. At CSA, the Western Lode is principally massive pyrite-pyrrhotite-chalcopyrite; the Eastern Lode is siliceous but alos contains pyrite, pyrrhotite and chalcopyrite (Russell and Lewis, 1965); whilst the intervening Zinc Lode is composed of massive (60 to 90%) sulphide that varies from a variety composed of dominantly pyrite with sphalerite and lesser galena, pyrrhotite and chalcopyrite in a gangue of magnesian chlorite to mainly pyrrhotite-pyrite-sphalerite-galena in black chloritised siltstone (Shi and Reed, 1998). Stander and Berthelsen (2017) suggest instead that these occurrences reflect three massive sulphide associations, i). chalcopyrite-pyrrhotite associated with massive to brecciated magnetite; ii). chalcopyrite-pyrrhotite associated with coarse galena-sphalerite and visible gold; and iii). galena-sphalerite-silver associated with pyrite.
Three main alteration-mineralisation stages are recognised in the Cobar Mineral Field (Stander and Berthelsen, 2017; Kyne, 2014):
• Early pervasive silicification producing a ubiquitous pervasive bleached and cherty appearance, mostly replacing feldspar clasts and grains, followed by an iron-rich chlorite (largely pyrochlore) and quartz, mainly vein phase, with associated magnetite and gold. Chlorite may persists for up to 500 m laterally from ore at CSA.
• Minor quartz, mainly as veining and breccia fill, with associated copper and remobilised gold; and
• Late Mg-rich chlorite with associated lead-zinc mineralisation, and a gangue that includes carbonate minerals.
These three stages are interpreted to have coincided with the following hydrothermal fluids reflected in fluid inclusions (Stegman, 2007; Kyne, 2014) although Stander and Berthelsen (2017) only recognise two:
• an early-stage, gold-rich, high temperature and weakly saline fluid, with limited ability to transport base metals, but responsible for the strong Fe-enrichment and K-Rb-Ba depletion associated with Fe-chlorite alteration and white mica destruction in the wall rock;
• a second transitional fluid, initially responsible for K-Na-Rb-Ba-Mg enrichment with associated stilpnomelane and Fe-Mg chlorite alteration; and
• a final more base metal-rich and slightly more acidic Mg-rich hydrothermal fluid stage producing more chlorite and other Fe-Mg-rich silicate alteration as well as rare talc. This stage was responsible for the formation of the Cu-ores. During this stage an overall reduction in temperature of the hydrothermal fluid was followed by the deposition of late-stage galena-sphalerite, pyrrhotite-pyrite greenalite, Fe-clay and carbonate. This stage is not interpreted to have introduced any new gold, but partially remobilised that of the first stage.
Isotope studies have been conducted in an attempt to identify the source of metals in the Cobar Mineral Field deposits, as follows:
• Lead isotope studies by Suppel, et al., (1990), Hinman (1992), Dean (1991), Carr et al. (1995), Lawrie and Hinman (1998) and Stegman (2007) have been ambiguous and inconclusive. They produced relatively homogenous values suggestive of i). metal a source from a large-scale, Palaeozoic age, relatively isotopically homogeneous crustal lead reservoir containing minor amounts of mantle-derived mafic material, or ii). the effect of large-scale homogenous magmatic and/or hydrothermal processes (Stegman, 2007). The same data sets also found deposits within the basement Girilambone Group had lower Pb207/Pb204 and Pb208/Pb204 than those within the Cobar deposits. Lawrie and Hinman (1998) suggested the Au-rich deposits (Peak and New Cobar) tend to be less radiogenic than those associated with predominantly base metals (CSA and Endeavour), with the less radiogenic Pb representing ore fluids derived from metamorphic basement rocks while the more radiogenic Pb represents basinal fluids. Sun and Seccombe (1998) interpreted the lead values at Elura/Endeavour to indicated a likely basement granite source, leached by hydrothermal fluids associated with basin formation. Stegman (2000) concluded that Cobar Mining Field Pb is largely derived from a large-scale, relatively homogenous Palaeozoic source extending across the Lachlan Orogen, that small Pb ratios variation between the different Cobar deposit styles represents subtle variations in the source, and that the most likely lead isotopic source signatures found within the Cobar Mining Field was the large scale basement I-type granites.
• Sulphur isotopes data have been assembled and studied from across the Cobar Mining Field and at Elura/Endeavour, including by Rayner (1969: CSA); Sun (1983: CSA and Endeavour); Brill (1988: CSA); Sun and Seccombe (2000: Endeavour); and Stegman (2007; New Occidental and others). These data reveal the δ34S ratios for the various deposits within the Cobar Mining Field range from +4 to +12‰. As with the Pb isotope data, the conclusions drawn from data set have also be inconclusive. On a broader rage of deposits, Jiang and Seccombe (2000) showed that δ34S values across the Cobar Mining Fields do not have significant isotopic compositional variation between ore-types, vein types, or relative levels within the different deposits suggesting the presence of a homogenous sulphur source, as well as a possible common sulphur fractionation process. Largely based on sulphur isotope data from New Occidental Stegman (2007) concluded that the sulphur source must be from basement-derived sedimentary sulphide having precluded the Cobar Basin strata itself as a possible source.
Stegman (2007) undertook a fluid inclusion study at New Occidental, and calculated the mineralising fluids had maximum temperatures of 350 to 400°C, and that country rock temperatures at the time of ore formation were 50 to 100°C cooler than he core of the deposit. These fluid inclusions and others of the
Cobar deposits generally have low salinities (<10 wt.% NaCl equiv.). Stegman (2007) and Kyne (2014) showed that the mineralising fluids were mostly pH neutral (chlorite-stable), H2O-CO2-
NaCl-CH4 hydrothermal fluids. In addition the mineral assemblage within the deposits is not particularly oxidised or reduced and not extremely acid or alkaline in character. However, Stegman (2007) and Kyne (2014) have recognised the widespread anomalous presence of fluorine within the Cobar Mineral Field with alteration minerals carrying high F values as follows - apatite (4.3 wt.%), biotite (2.0
wt.%), Fe-chlorite (0.1 wt.%), Fe-Mg-chlorite (0.5 wt.%), greenalite (2.0 w.t%, muscovite (1.0 wt.%) and stilpnomelane (1.0 wt%). Kyne (2014) suggests that in the absence of Cl, F may have acted as ligand during transport of metal in the hydrothermal fluid (c.f., Kipushi in the DRC; Chabu, 1996).
CSA Mine
The CSA deposits lies within the hanging-wall of the major, steeply west-dipping, north-south to NNW-SSE trending Cobar Fault hosted within the CSA Siltstone. The deposit is composed of 5 main structurally controlled 'systems', each a separate cluster of ore lenses that range from 13 to 200 m strike length, 5 to 80 m in width, and 200 to 1200 m in vertical extent (Shi and Reed, 1998; Kyne, 2014).
Kyne (2014) recognised two major cleavage groups at CSA, one corresponding to the regional S2 cleavage and a second found locally that has varying orientations and is termed SX to avoid timing implications. A subset of SX was found to represent an early cleavage and was designated S1. Stretching lineation within this cleavage pattern has a down-dip orientation suggesting it was formed during a dip-slip regime with very little transpression. S1 and S2 are each parallel to one of the two fault systems that co-intersect in the immediate deposit area, the Cobar and the WNW-ESE trending Plug Tank faults and suggest the presence of orthorhombic fault arrays. Structural analysis shows east-west compression acted upon the Cobar and Plug Tank faults, producing dilation along the intersections of these faults in the overlying orthorhombic fault array allowing mineralising ore fluids to be focused into the observed pipe-like ore lenses.
The CSA deposit comprises five main ore systems/deposits which all strike approximately north and dip steeply to the east, grossly sub-parallel to the cleavage orientation. Mineralisation is present as a number of vein complexes or as sub-massive to massive sulphide bodies, known as lenses, defined by a sufficient density of veining to attain ore grades. The massive ore has 70 to 90% sulphides. Most lenses have a steep northerly plunge parallel to the dominant lineation with strike extents rarely exceeding 80 m and thicknesses of 6 to 20 m, but persisting down plunge for several hundred metres. A broad halo of chloritisation surrounds the individual lenses with a lateral extent of up to 50 m, commonly with associated silicification.
The five systems are the (after Shi and Reed, 1988; Kyne, 2014):
Western System - comprising relative small en echelon lenses with average strike lengths of 45 m, widths of 7 m and plunge extents of ~200 m, which are developed over a northerly strike length of around 300 m. These lenses are composed of high grade zinc-lead ores, the 'Massive Sulphide with Galena and Sphalerite' ore style described below composed of Pb-Zn rich sulphide lenses of galena, sphalerite, pyrrhotite with minor chalcopyrite that are often banded and are largely restricted to chlorite-rich shears that anastomose through the core of the lens. The average grade of the lenses was 6% Zn, 3% Pb, 2% Cu, increasing in Cu with depth. High grade pods of Cu-rich mineralisation are associated with zones of strong quartz veining with chalcopyrite, pyrrhotite and pyrite. The local hosts are chloritised (Mg rich), silicified and quartz veined, with pervasive silicification.
Eastern System - which is located ~200 m ENE of the Western System. It consists of more than 4 lenses, each of 50 to 80 m in length and variable widths averaging 10 m. Each comprises a number of vein systems which average 3% Cu and belong to the 'Semi-massive Sulphide with Quartz' ore style described below. Mineralisation is predominantly chalcopyrite, pyrrhotite in quartz and/or pyrite within intensely cleaved chloritic siltstone. The top of the ore zone is below 250 m depth. A subsidiary CZ zone between the Eastern and Western Systems averages 10% Zn, 1% Pb, <1% Cu.
QTS North - is ~ 150 km NE of the Eastern System. It is a blind system which occurs approximately 100 m east of the Eastern System. It comprises a series of lenses that are <100 m in length and average 10 m in width, with a northerly strike and steep east dip. All of the lenses are Cu rich, averaging 5% Cu, with no significant Pb or Zn. The mineralogy is predominantly chalcopyrite and pyrrhotite. Cu grade increases to the east, with the westernmost being similar in grade to those of the Eastern System. Ore lenses are predominantly composed of Semi-massive Sulphide with Chlorite with limited cores of Massive Sulphides
QTS Central - which is located 300 m south of QTS North and 1200 m below the surface. It comprises two main lenses hosted within closely bedded sandstones and siltstones. The sandstone is not generally mineralised, but is heavily silicified, while the siltstone is mineralised and chloritic. The principal mineralised lens has a strike length of 5 to 200 m and 450 m vertical extent. Massive sulphide feeder structures occupy the core of the two
main lenses, although the system is more chlorite-rich than in QTS North and QTS South.
QTS South - is also a blind system, ~700 and ~600 m SSE of QTS North and the Eastern systems respectively. It comprises several sub-parallel, north-striking, up to 200 m long and 8 to 20 m wide lenses that dip steeply east. It is Cu rich with subsidiary pyrrhotite and isolated pods of galena and sphalerite on the extremities of the system.
Geochemical analysis at CSA (Kyne, 2014) suggests Se, Cd, Fe, Mn, Sn, Tl and Ge are enriched and Ba, K Na and Rb are depleted within 100 m of ore lenses, while depletion in Sr and Na occurs up to 500 m from mineralisation. Fe-Mg-rich chlorite alteration is recognizable within 10 m of ore lenses. Dating of detrital zircon (206Pb/238U) and hydrothermal monazite suggest the host was deposited before 404 Ma, while mineralisation occurred at 388±4 Ma (Kyne, 2014). These ages are consistent with the age of mineralization currently expected for the Cobar Basin during the Tabberabberan Orogeny (Glen, 1994).
The ore 'systems' that constitute the CSA deposit fall within an enveloping broad halo of fault controlled veins, some of which are independent of mineralisation, while others may occur up to 800 m distant, and others still are restricted to distances of 500 and 100 m of ore. The veins of the halo will be described below, followed by the main ore types that make up the ore lenses.
VEIN HALO - Kyne (2014) recognised a paragenesis of eleven distinctive veins and a strongly silicified, discrete fault breccia that together make up a halo enveloping, and cutting the ore lenses of the CSA deposit. The degree of deformation of these veins varies with relative age, with early types strongly folded and/or boudinaged while later generations are only brittly deformed or lack deformation entirely. The bulk of the veins are sub-vertical and parallel to the regional cleavage (S2) produced during the D2 deformation compressional event. Late stage veins, however, are sub-horizontal and most likely reflect a later stage compressional regime. The vein types recognised and described by Kyne (2014) are as follows:
D1 - Stage 1
• VA - Bedding-parallel Quartz - composed entirely of massive and milky quartz and devoid of mineralisation, these veins are parallel to bedding, 0.1 to 0.5 cm thick and are highly deformed and dislocated by subsequent deformation. They are rare and not restricted to areas of increased stress and were formed pre-deformation, but later were 80% syn-tectonic recrystallised.
• VB - Early Barren Quartz - sub-vertical, entirely milky, quartz veins that crosscut VA, but were emplaced prior to D2 deformation. They range from 0.5 to 3 cm in thickness.
D1-2 transition - Stage 2
• VC - Folded Quartz ± Sulphides - highly deformed quartz-rich veins that locally contain sulphide minerals. They are closely associated with VD, but are folded whilst the latter are only boudinaged. They are interpreted to have begun growing before D2. VC and VD are found in zones of increased strain, becoming more prevalent in areas surrounding ore lenses. They are composed of quartz, including coarse grained recrystallise varieties, with accessory chalcopyrite, pyrrhotite, chlorite and rarely calcite. They range from 0.8 to 4.5 cm in thickness and are mildly anastomosing.
• VD - Boudinaged Quartz ± Sulphides - which are interpreted to have begun forming before D2 and continued growing throughout the early stages of that event. They are dominantly composed of quartz, but also contain coarse grained syn-tectonic recrystallised variants, with accessory chalcopyrite, pyrrhotite and chlorite, with trace cubanite (CuFe2S3), sphalerite, pyrite and rare calcite. They range from 1.0 to 4.0 cm in diameter, with characteristic sausage shaped symmetrical boudins. When present, sulphides generally occur around the edges of boudins and/or concentrate in the neck zone pressure shadows.
D2 - Stage 3 - the veins of this stage are directly coeval with the main ore zone mineralisation styles described later.
• VE - Massive Quartz ± Sulphides - which formed syn-D2, sub-vertical, associated with mineralising fluids, and generally found flanking major ore lenses. They may contain minor chalcopyrite, galena, pyrite, pyrrhotite and sphalerite, but lack of cubanite. Gangue includes quartz, chlorite and plagioclase. Quartz varies from 0.5 to 2 cm open-space fill clear quartz to massive, milky white to dirty inclusion-filled varieties. Chlorite occurs within 10 cm of the vein boundaries but is <30 &mico;m across. A moderate amount of plagioclase is sporadically present in veins that are >30 cm wide, accompanied by minor 1 to 2 cm biotite books with minor calcite and chlorite. These veins range from 6 cm to 3 m in thickness.
• VF - Barren Quartz - while barren, these veins always occur with VE, VG and VH veins in a mutually cross-cutting relationships, with all four sometimes occurring directly beside ore lenses and/or are the main constituent of an ore lenses. They are composed entirely of milky quartz but are not texturally banded. The veins have primary open fill textures with quartz grains growing inward toward the vein centre, perpendicular to the walls, with multiple generations of quartz growth. The veins range from 1 to 3 cm in thickness.
• VG - Quartz + Sulphides - which with the other veins from VE to VH occur directly beside ore lenses and/or are the main constituent of those lenses. They vary from 2 to 6 cmin tickness. Often, there is a gradation in vein dominance from VF to VH towards ore lenses in their proximity. Whilst the bulk of these occur as sub-vertical veins, in zones of intense veining they frequently form stockwork-like patterns, and contain very high grade mineralisation. VG veins specifically contain up to 50% sulphides, but generally carry 10 to 50% chalcopyrite, with lesser pyrrhotite, cubanite and sphalerite, trace native bismuth and a silver-rich sulphide. The gangue comprises quartz that is generaly milky, with chlorite and minor calcite. Stilpnomelane is often found at the contact between sulphides and gangue, accompanied by intense alteration. Cubanite commonly occurs as lamellae or blebs within chalcopyrite, but only when VE veins are present within intensely altered zones. In VG veins, cubanite can occur without stilpnomelane but where stilpnomelane occurs cubanite is always present.
• VH - Sulphides - distributed as for the VE to VH veins described above. They range from 0.5 to 6 cm in thickness. VH veins also occur as precursors to ore lenses, generally increasing in intensity close to a lens, with a variety of compositions. They may contain >95% sulphide, sometimes also including <5% quartz and/or chlorite. These veins may contain ≥95% chalcopyrite, but often include ≥10% cubanite, but most commonly are a mixture of 50 to 80% chalcopyrite and 20 to 25% pyrrhotite. They often contain minor galena and sphalerite and traces of native bismuth, galena and a silver sulphide (acanthite). Close to ore zones, these veins are rimmed by stilpnomelane and chlorite.
D2 - Stage 4
• VI - Calcite + Quartz - which occur at the end of D2, and truncate all earlier sets, but mutually crosscut VJ. Both sets are still sub-vertical and are best developed within intensely altered and high strain zones, although they can occur in zones of negligable alteration and strain, but are only <1 cm thick. They comprise >50% deformed calcite with 10 to 20% or more quartz, trace muscovite and <0.1% microcline or Na plagioclase, and vary from 0.2 to 1.5 cm in thickness. Larger veins have a zebra like striping resulting from a unique banding produced by alternating layers of quartz and calcite.
D2-3 transition Stage 4
• VJ - Chlorite + Pyrite veins accompanying the VI variety, representing the last of the mineralising fluids. However, unlike VI veins, these only occur in moderate to intensely altered and strained zones. They comprise <70% chlorite and <50% pyrite, with small quantities stilpnomelane, biotite and muscovite in the most chlorite-rich varieties. The pyrite is a late phase which is subhedral to
euhedral, with little zonation, a moderate amount of brittle deformation and thickness of from 1 to 3 cm.
D3 Stage 5
• VK - Late Barren Quartz - are the last vein set, cross-cutting all structural features within the deposit are except late low angle brittle faults. They are 1 to 2 cm thick, sub-horizontal, and were emplaced during a late stage compressional regime. These veins are devoid of any sulphides and are composed of quartz with minor amounts of plagioclase (albite).
D2 - SAS ZONE
• Silica-altered sedimentary rocks - a highly discontinuous quartz-cemented fault breccia which occurs between the Eastern and QTS North ore systems. It is composed of <50% country rock and <50% quartz altered clasts, both set in a quartz cement with minor calcite, and represents a non-mineralising quartz-rich, sulphide poor pulses of fluid. While un-mineralised these breccias are developed within zones which also hosts ore lenses and is synchronous with that mineralisation. The quartz within the breccia has undergone brecciation and recrystallisation, and appears to have been emplaced by fluids in a zone of brecciation either while it was still active or before it was reactivated. These breccia zones are 0.6 to 5 m thick, with individual irregular country rock clasts that range from 1 to 3 cm in width. The quartz matrix is milky and never forms well developed crystals.
All of these veins are found as both a halo encompassing and cutting the CSA deposit, and apart from the VA and VK are sub-vertical, cutting through the deposit from bottom to top with changes limited to the horizontal plane only. With the exception of the same two vein types, all increase in density of occurrence as a mineralised lens is approached. VA and VK type vein types formed independently of mineralisation and occur throughout the deposit with no particular spatial distribution. Veins VB and VJ represent the temporal limit to the hydrothermal fluid system, and occur up to 800 m distal to mineralisation. VC-E and VI may occur up to 500 m distal to mineralisation while VF, VG and VH may be found up to 100 m from mineralisation. VF, VG and VH veins are usually associated with major mineralised lenses.
ORE ZONES - The mineralisation that constitutes the ore lenses at CSA occurs as five distinct varieties. The first of these is the siliceous Semi-massive Sulphide with Quartz ore type which predominates in the core of the deposit, in the Eastern System, with minor pods also found on the eastern margin of, and at depth in the Western System. Semi-massive Sulphide with Chlorite occurs in the east of the deposit making up the QTS North, Central and South systems. Massive Sulphide ore forms a core to both the Semi-massive Sulphide with Quartz and Chlorite lenses, developed in the zone of most intense fault deformation. Massive sulphide with galena and sphalerite is largely restricted to the west of the deposit, dominating the upper Western System and to a lesser extent the western margin of the Eastern System to a depth of ~1 km. Pyrrhotite-rich Ore occupies the lower extremities of the Western System. In 2014, the mine geologists recognised the Semi-massive Sulphide with Quartz, Semi-massive Sulphide with Chlorite and Massive Sulphide ore types, whilst Pyrrhotite-rich Ore and Massive Sulphide with Galena and Sphalerite were introduced by Kyne (2014). Each of these ore types may be summarised as follows after Kyne (2014):
• Semi-massive Sulphide with Quartz - mineralisation occurs in the core of the CSA deposit and extends from ~1 to >2 km below the surface. It usually occurs as the sole ore type within an ore lense, although a transition to 'Semi-massive Sulphide with Chlorite' (see below) may be seen towards the margins of lenses and between adjacent lenses. It is composed of >35% chalcopyrite ±cubanite and pyrrhotite ±sphalerite ±ancanthite with trace native bismuth and galena, >50% quartz and other gangue which may include feldspar and <15% carbonate. It has a 'net-like' texture with sulphides infilling between discrete, 0.5 to 5 cm diameter almost circular, blob-like quartz grains. It occurs as semi-massive breccia replacement or as veins ranging in width from 5 from 25 cm that together constitute an ore lens.
• Semi-massive Sulphide with Chlorite - ores are found in the eastern part of the CSA deposit, mainly as the QTS North, Central and South systems. It occurs in association with the 'Semi-massive Sulphide with Quartz' and 'Massive Sulphide' ores. As with the former, it tends to constitute the sole component of a lens with a transition to thae quartz-bearing ore type on its margins. Apart fromthis transition, there are only traces of quartz present. It also occurs in conjunction with a 'Massive Sulphide' core to most lenses (see below), and is characterised by areas of intense chloritic alteration. It is composed of semi-massive (>65%) sulphide including chalcopyrite ±cubanite and pyrrhotite ±sphalerite, with ~25% chlorite and <10% other gangue minerals. Cubanite only occurs when no pyrite is present. Lesser magnetite and trace amounts of acanthite, native bismuth, quartz and galena are also present. This represents the first appearance of magnetite in the deposit. Stilpnomelane, both bladed and fibrous, is found on the contacts between the altered country rock and sulphides, growing perpendicular outward from the boundaries of the sulphide-rich veins, as well as in clusters within the chlorite altered country rock. The Semi-massive Sulphide with Chlorite mineralisation type occurs in areas of intense chloritic alteration. It is very similar in appearance to the 'Massive Sulphide' ore type (see below), mainly occurring as semi-massive breccia replacement or as veins ranging from 5 to 25 cm in thickness. Breccias contains 0.5 to 5 cm clasts of very dark Mg-chlorite and stilpnomelane which appears to align sub-parallel to the regional S1 cleavage.
• Massive Sulphide - ore occurs in the centres of some ore lenses, predominantly those in the eastern part of the deposit area composed predominantly of Semi-massive Sulphide with Quartz and Semi-massive Sulphide with Chlorite. The cores of these lenses represent the zones of most intense faulting deformation i.e., of greatest permeability. These massive sulphides are composed of >80% chalcopyrite, with <10% pyrrhotite, <5% cubanite and trace sphalerite, acanthite, galena, pyrite, native bismuth, magnetite and stannite, and <5% gangue. Pyrrhotite occurs as blebs, either aligned with S2 or as blebs disseminated throughout the chalcopyrite. Native bismuth tends to occur in close association with galena, but not always. Cubanite is primarily found as lamellae within the chalcopyrite, but may also occur as isolated blebs. Acanthite is most abundant in massive sulphides. The massive sulphides have a massive texture, with the only discernable fabric the alignment of the pyrrhotite blebs in some sections.
• Massive Sulphide with Galena and Sphalerite - which is dominated by sphalerite, galena, lesser pyrrhotite and trace of native bismuth, with chalcopyrite appearing in minor amounts with depth. Sphalerite and galena are >80% of the mineralisation, with <5% chalcopyrite, <10% chlorite and <10% other gangue minerals. Sphalerite is the dominant sulphide mineral enclosing disseminated galena, pyrite and minor chalcopyrite in a gangue of quartz, dark Mg-chlorite and commonly calcite. Acanthite and cubanite are absent. Pyrrhotite occurs in patches as a transition towards the 'Pyrrhotite-rich Ore' below and Cu rich lenses at depth and to the east. The sulphides are massive, and is typically appears to be both folded and has a 'swirling' suggesting deformation. Deformation is interpreted to be D2. This ore type occurs as multiple lenses averaging 45 m in length and 7 m in thickness with down plunge extents of ~200 m.
• Pyrrhotite-rich Ore - occurs in the lower section of the Western System, separating the 'Massive sulphide with galena and sphalerite' mineralisation from Cu-rich ores to the east and down dip. It is always found within a few metres of the former, but can be much more distal to the Cu-rich ores. It is predominantly composed of >70% pyrrhotite with <10% chalcopyrite, sphalerite and galena, accompanied by <30% gangue minerals including chlorite and quartz. This mineralisation style occurs as either 1 to 4 cm thick veins and/or 4 to 12 cm wide semi-massive breccia style replacement 'veins' defining zones that are 5 to 10 m across.
These ore zones are intensely deformed, concentrated along zones of high strain. There is ample macro- and microscopic evidence of deformation of sulphides, including twins, folds and brittle cracking. Whilst no discrete faults are recognisable within ore lenses in drill core, the occurrence of sulphide-rich replacement of breccias and folds suggest that ore lenses are hosted in fault zones. The degree of deformation in all the ore lenses is much higher than in the adjacent wall rocks, whilst textural evidence shows increasing cleavage development as ore lenses are approached, grading into the more intense
deformation within the mineralised material. The host sequence is most intensely deformed adjacent to the Cobar Fault, although very few high strain
features are evident distal to the mineralisation. No known mineralisation post-dates cleavage development. This is taken to suggests all of the
mineralisation was pre- or syn-D2.
Production, Reserves and Resources
Production from the CSA and Tinto mines between 1905 and 1957 was 0.121 Mt @ 3.84% Cu, 0.38 g/t Au, 25 g/t Ag, 3.4% Pb
Production from the CSA mine from 1961 to 1995 was 20 Mt @ 2.11% Cu, 0.62% Pb, 1.98% Zn, 22 g/t Ag
- resources at 1996 were 27.8 Mt @ 3.75% Cu.
Proved reserves in 2002 were - 0.71 Mt @ 6.01% Cu, plus an inferred resource of 4.97 Mt @ 3.90% Cu.
Remaining resources and reserves at December 31, 2014 were (Glencore Reserves and Resources Report, 2015):
- Measured + indicated resources - 5.7 Mt @ 5.73% Cu, 23.5 g/t Ag;
- Inferred resources - 6.7 Mt @ 5.8% Cu, 22 g/t Ag;
- Proved + Probable reserves - 5.4 Mt @ 4.02% Cu, 16.5 g/t Ag (included in resources).
Remaining Mineral Resources and Ore Reserves at Cobar (CSA) at December 31, 2018 were (Glencore Reserves and Resources Report, 2019):
Measured + Indicated resource - 6.0 Mt @ 5.53% Cu, 23 g/t Ag,
Inferred resources - 5.4 Mt @ 5.37% Cu, 20 g/t Ag.
Proved + Probable reserves - 6.4 Mt @ 3.81% Cu, 15.7 g/t Ag (included in resources).
Annual production between 2008 and 2017 has been ~45 000 t Cu and 13 t Ag from workings that are 1500 to 1700 m below surface.
The Peak - see the separate The Peak record.
NOTE: When these two records (Cobar Mineral Field - CSA and The Peak) were generated in the early 2000's, the CSA Cu-Zn-Au-Pb-Ag mine was the major producer in the field, and The Peak Au deposit was an auxiliary 17 km to the SSE. Hence the separate record. Since then, what was the isolated The Peak mine is now a cluster of deposits spread over 8 km in the southern to central parts of the field, while CSA remains an important producer on the northern end of the Cobar line of deposits.
Historic production from the shallow deposit between 1896 and 1951 - 0.299 Mt @ 21.7 g/t Au, 498 g/t Ag, for 6.5 t Au,
The total production + reserves in 1995 was 4.6 Mt @ 7.6 g/t Au, 0.9% Cu, 1% Pb, 0.9% Zn, 8 g/t Ag.
Resource and reserve figures published by Peak Gold Ltd (2008), as at 31 December, 2006, were:
Measured + indicated resources - 3.52 Mt @ 4.05 g/t Au, 1.12% Cu
Inferred resources - 1.99 Mt @ 6.5 g/t Au, 0.53% Cu
Proved + probable reserves - 1.68 Mt @ 6.75 g/t Au, 0.74% Cu.
The resources are exclusive of the reserves.
Remaining Mineral Resource and Ore Reserves as at 17 July 2018 (Aurelia Metals Limited, 2018 Annual Report) were:
Measured + indicated + Inferred resources - 10.8 Mt @ 1.64 g/t Au, 1.48% Cu, 0.96% Pb, 1.04% Zn;
Proved + probable reserves (Cu-Pb-Zn ore) - 2.0 Mt @ 3.48 g/t Au, 1.39% Cu, 0.66% Pb;
Proved + probable reserves (Pb-Zn ore) - 0.5 Mt @ 0.4 g/t Au, 0.17% Cu, 5.72% Pb, 6.15% Zn.
Resources are inclusive of reserves and relate to the five main Cobar mines of Aurelia Metals, namely Perseverance (which contains the bulk of the gold resource and is connected by underground workings with The Peak); The Peak and New Occidental (which share a decline) and Chesney and New Cobar (which are linked underground; Chesney has the best copper grade, while New Cobar is an important copper/gold source). Aurelia Metals also controls the Great Cobar Cu-Pb-Zn-Au deposit to the north of New Cobar.
Cobar Field Production - Other deposits
Historic production from the larger of these deposits, prior to the modern development of the CSA and The Peak, include:
Great Cobar - 4.186 Mt @ 2.75% Cu, 2.2 g/t Au, for 9 t Au, between 1871 and 1947,
New Occidental - 2.098 Mt @ 9.8 g/t Au, for 20 t Au, between 1889 and 1952,
New Cobar - 0.985 Mt @ 8.7 g/t Au, for 8.6 t Au, between 1890 and 1948,
Chesney - 0.723 Mt @ 1.2 g/t Au, for 0.88 t Au, between 1887 and 1952,
Queen Bee - 0.429 Mt @ 7.55% Cu, between 1902 and 1957,
Gladstone - 0.344 Mt @ 6.3% Cu, 10 g/t Ag, between 1908 and 1920.
The total global endowment of the field at the end of 2016 was 140 t of Au, 1120 t of Ag and 2 Mt of Cu.
Sections of this record were summarised from:
Kyne, R., 2014 - Genesis and Structural Architecture of the CSA Cu-Ag (P-Zn) Mine, Cobar, New South Wales; Submitted as a Doctorate of Philosophy Thesis, CODES, School of Earth Science, University of Tasmania, Hobart Australia, 503p. and
Stegman, C., 2007 - Structural and Geochemical Controls on Ore Formation at the New Occidental Gold Deposit, Cobar, New South Wales, Australia; Submitted as a Doctorate of Philosophy Thesis, CODES, School of Earth Science, University of Tasmania, Hobart Australia, 499p.
The most recent source geological information used to prepare this decription was dated: 2017.
Record last updated: 3/6/2019
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
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Selected References:
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Apaza, E. and Neumayr, R., 2017 - The discovery of the QTS Central ore system, CSA mine, Cobar, NSW, Australia: in Tenth International Mining Geology Conference 2017, Hobart, Tasmania, 20-22 September, 2017 The AusIMM, Melbourne, Proceedings, pp. 151-156.
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Brooke W J L 1975 - Cobar mining field: in Knight C L, (Ed.), 1975 Economic Geology of Australia & Papua New Guinea The AusIMM, Melbourne Mono 5 pp 683-694
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Brown, R.E., Coffey, J., Hosken, J., Johnston, C. and Lenard, M., 2017 - Cobar district mineral field: in Phillips, G.N., (Ed.), 2017 Australian Ore Deposits, The AusIMM, Melbourne Mono 32, pp. 739-746.
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Cook W G, Pocock J A, Stegman C L 1998 - Peak gold-copper-lead-zinc-silver deposit, Cobar: in Berkman D A, Mackenzie D H (Ed.s), 1998 Geology of Australian & Papua New Guinean Mineral Deposits The AusIMM, Melbourne Mono 22 pp 609-614
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Giles, A.D. and Marshall, B., 2004 - Genetic significance of fluid inclusions in the CSA Cu-Pb-Zn deposit, Cobar, Australia: in Ore Geology Reviews v.24, pp. 241-266.
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Glen R A 1987 - Copper- and Gold-rich deposits in deformed Turbidites at Cobar, Australia: their structural control and hydrothermal origin: in Econ. Geol. v82 pp 124-140
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Glen, R.A., 1991 - Inverted transtensional basin setting for gold and copper and base metal deposits at Cobar, New South Wales: in BMR J. of Aust. Geol. & Geophys. v.12, pp. 13-24.
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Hinman M C, Scott A K 1990 - The Peak Gold deposit, Cobar: in Hughes F E (Ed.), 1990 Geology of the Mineral Deposits of Australia & Papua New Guinea The AusIMM, Melbourne Mono 14, v2 pp 1345-1351
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Russell, R.T. and Lewis, B.R., 1965 - Gold and copper deposits of the Cobar District: in McAndrew J and Madigan R T (Eds.) 1965 Geology of Australian Ore Deposits Eighth Commonwealth Mining and Metallurgical Congress, Australia and New Zealand, The AusIMM, Melbourne, v.1, pp. 411-419.
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Scott A K, Phillips K G 1990 - C.S.A. Copper-Lead-Zinc deposit, Cobar: in Hughes F E (Ed.), 1990 Geology of the Mineral Deposits of Australia & Papua New Guinea The AusIMM, Melbourne Mono 14, v2 pp 1337-1343
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Shi B L, Reed G C 1998 - CSA copper-lead-zinc deposit, Cobar: in Berkman D A, Mackenzie D H (Ed.s), 1998 Geology of Australian & Papua New Guinean Mineral Deposits The AusIMM, Melbourne Mono 22 pp 601-608
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