Cobar_style_deposits_early_thoughts_MichaelOstrowski
- 1. Cobar Style Deposits Orogenic or VHMS Michael Ostrowski 2011
- 2. 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
- 3. 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
- 4. 2Cobar Style Deposits….Orogenic or VHMS Figure 1 Tectono-Stratigraphic units of the Cobar Super Basin and mineral deposits (Adapted from David 2005)
- 5. 3Cobar Style Deposits….Orogenic or VHMS 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)
- 6. 4Cobar Style Deposits….Orogenic or VHMS 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.
- 7. 5Cobar Style Deposits….Orogenic or VHMS 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)
- 8. 6Cobar Style Deposits….Orogenic or VHMS 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)
- 9. 7Cobar Style Deposits….Orogenic or VHMS 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.
- 10. 8Cobar Style Deposits….Orogenic or VHMS 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)
- 11. 9Cobar Style Deposits….Orogenic or VHMS 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.
- 12. 10Cobar Style Deposits….Orogenic or VHMS 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.
- 13. 11Cobar Style Deposits….Orogenic or VHMS 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
- 14. 12Cobar Style Deposits….Orogenic or VHMS 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.
- 15. 13Cobar Style Deposits….Orogenic or VHMS 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.
- 16. 14Cobar Style Deposits….Orogenic or VHMS 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
- 17. 15Cobar Style Deposits….Orogenic or VHMS 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)
- 18. 16Cobar Style Deposits….Orogenic or VHMS 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.
- 19. 17Cobar Style Deposits….Orogenic or VHMS 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.
- 20. 18Cobar Style Deposits….Orogenic or VHMS 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.