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Kanmantoo |
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South Australia, SA, Australia |
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Super Porphyry Cu and Au
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The Kanmantoo copper-gold deposit lies within the Kanmantoo Trough of the Adelaide Fold Belt, ~55 kms southeast of Adelaide in the Mount Lofty Ranges of South Australia.
(#Location: 35° 5' 43"S, 139° 0' 0"E).
Copper mineralisation was first recognised at Kanmantoo by two Cornishmen commissioned by the South Australian Company to prospect the district in 1846. The South Australian Company worked several pipe-like ore bodies down to depths of 60 m with dressed ore grades of 25 to 30% Cu shipped to Swansea (Wales) for smelting. Local smelting began in 1848, enabling treatment of ore with 12% Cu, too poor for the Swansea market. In 1851 the South Australian Company, disappointed with the lack of profitability of the mine, decided to withdraw from mining. Sporadic mining of small high grade copper and copper-gold underground mines took place, mainly before 1872, with the total production prior to World War I of over 60 000 tonnes of ore mined from the district, of which the Kanmantoo group of mines contributed ~24 000 t @ 8.5% Cu. In 1938, the deposit was drilled by Austral Development Company, who intersected 75 m @ 0.63% Cu, which at the time was uneconomic. Regional exploration by Broken Hill South Ltd., through its subsidiary Mines Exploration Pty. Ltd., in the 1960s, resulted in a drill program at Kanmantoo, the first of which intersected 130 m @ 0.95% Cu in October 1962, having drilled down the core of the pipe-like deposit. After a program of diamond drilling and underground testing, a 750 000 tpa of ore open pit was established in 1970 and the first concentrates shipped in 1971 (Verwoerd and Cleghorn, 1975). The operation was profitable until late 1974, when falling Cu prices rendered it unprofitable. The mine and facilities were owned by Kanmantoo Mines Ltd., a joint venture between Broken Hill South 51%, North Broken Hill 19.5%, Electolytic Zinc Company 19.5% and P.G. Hallof of Canada 10%. In 1972, following the liquidation of Broken Hill South, their share, and the management of the JV was assumed by CRA Limited, through its subsidiary Australian Mining and Smelting. Plans to develop an underground mine below the open pit were abandoned and the mine closed in June 1976 due to continuing low copper prices. Approximately 4.1 Mt @ 0.87% Cu and 0.07 g/t Au was mined from the open pit between 1971 and 1976. Exploration at Kanmantoo resumed in 2004 when Hillgrove Resources defined a cluster of eight Cu-Au-Ag deposits around the abandoned Kanmantoo Mines pit, enabling the development in 2010 of several open pit mines that were still in production in 2016. Open-pit mining ceased in May 2019, although processing of stockpiled ore continued into 2020. Rehabilitation and revegetation of the site is expected to take up to seven years. The total endowment at Kanmantoo in 2016 was estimated at ~0.35 Mt of Cu, 3 t Au and >90 t Ag (Rolley and Wright, 2017). Investigations were progressing in 2021 towards reopening the mine as a high grade underground operation, with construction of a decline commencing in November 2021.
Regional Setting
The Kanmantoo Trough is a fault controlled basin that developed during the Early Cambrian along the eastern margin of the late Neoproterozoic to Early Cambrian Adelaide Rift Complex. It forms a southward expanding wedge shaped basin, which is now bounded to the west by a curved, west-vergent thrust zone. The trough was formed in an oceanic rift basin during the break-up of the Rodinia Supercontinent. Extension leading to this breakup began in the Adelaide Rift Complex at ~827 Ma (Preiss, 2000). This extension first involved NW-SE trending fractures, intruded by mafic dykes of the 827±6 Ma Gairdner Dyke Swarm, and lesser mafic flows. Extension rotated to NE-SW, and formed an overall north-south complex of overlapping rift basins filled with thick siliciclastic, carbonate and evaporitic sedimentary rocks. Subsequent north-south trending extensional faulting at ~800 to 750 Ma widened the rift zone, whilst between ~670 and 660 Ma, additional NW-SE trending rifts to the east defined the margins with the Curnamona Province. The rift sequence was disconformably overlain by 660 to 650 Ma sag phase deposition that transgressed onto the Gawler Craton to the west as the Rodinia break-up intensified (Preiss, 2021). At the same time, rifting, break-up and separation of the eastern margin of the Rift Complex progressed, with extreme extension taking place along cross-cutting growth faults. Subsequently, the remaining rift basin to the west became a passive margin with a shore line that curved from ENE-WSW south of Broken Hill, following the southern margin of the Curnamona Province; then, gradually rotating to north-south oriented in the Mount Lofty Ranges, east of and parallel to the Gawler Craton. This shallow curvilinear shelf, the Nackara Arc, was bounded to the east by a steep continental slope and an ocean floor of attenuated Proterozoic basement and Cambrian oceanic crust, overlain by a thick turbidite fan that defined the Kanmantoo Trough.
Between ~586 and 510 Ma, extensive alkaline, rift volcanic rocks were deposited immediately offshore of the Nackara Arc to the east of the margin of the preserved Neoproterozoic passive margin sequence. These included the fault juxtaposed 585 Ma Arrowsmith and ~515 Ma Mount Wright Volcanics in the Broken Hill area (Koonenberry Belt), east and south of the Curnamona Province. Magnetic data and sparse intersections of similar volcanic rocks below the Cenozoic Murray Basin have been interpreted to trace the latter SW to the eastern Mount Lofty Ranges (Fabris, 2003), where the ~526 to ~521 Truro, Marne River-Teal Flat Volcanics are exposed (see below), and can be traced beneath cover in magnetic data to far west and southwestern Victoria, occurring as semi‐continuous fault-bounded belts (Cayley, 2021). This belt of magmatic activity has been interpreted to represent within-plate volcanics (e.g., Gatehouse et al., 1993), related to mantle decompression during extension and rifting (Foden, Song, et al., 2002 as quoted in Fabris, 2003). Similar rocks are found ~400 km to the east, in western Victoria, where basement resurfaces east of the Cenozoic Murray Basin. These include the 350 x 50 km Dimboola Igneous Complex which is composed of ~600 Ma ultramafic and mafic tholeiites, bonninites, volcaniclastic rocks and cumulate gabbros. To the immediate west of these, there is a similar sized belt of ~524 Ma basalts and gabbros with 'within-plate' and MORB chemistry (Rankin et al., 1991; VandenBerg et al., 2000). These rocks are also interpreted to be related to extensional magmatism and to the development of oceanic crust upon which the Kanmantoo Trough was floored in the east. To the west, the Kanmantoo Trough sequence laps onto Precambrian continental basement, and overlies attenuated Proterozoic basement in between. As such, the trough was developed between the remaining Australian cratonic block to the west, represented by the remnant Adelaide Rift Complex sequence and the Gawler Craton, and the retreating continental block to the east, which is now believed to be part of North America (and/or South China). The trough is interpreted to have opened as the result of transtensional deformation in response to NE-SW extension (Haines and Flömann, 1998). The contact between the Adelaide Rift Complex and the Kanmantoo Trough represents the Tasman Line in this part of eastern Australia. The Tasman Line separates the current Australian continental block to the west, with exclusively Precambrian basement, and an eastern domain predominantly floored by Cambro-Ordovician oceanic crust. However, in southeastern Australia, across the Kanmantoo Trough, deep seismic data indicates this 'line' is transitional over a broad east-west interval occupied by attenuated oceanic and continental crust, with some enclosed older microcontinental rafts (e.g., Cayley et al., 2011; Cayley, 2021).
By ~570 Ma, following the final Rodinia break-up, the Adelaide Rift Complex was inverted, reflected by a pause in deposition on the continental block to the west. This marked the top of the Cryogenian to Ediacaran (Neoproterozoic) Heysen Supergroup, and an unconformity. This unconformity was overlain from ~540 Ma by the Cambrian Moralana Supergroup that commences with a carbonate rich shallow water, continental shelf sequence of Early Cambrian limestone, the Normanville Group, deposited on the up to 250 km wide Ardrossan Shelf. This carbonate unit passes eastward and disconformably upward into the Early to Middle Cambrian, turbiditic Kanmantoo Group, a thick, rapidly deposited, deeper water, continental slope to bathyal sequence, deposited from at least 526 to 514 Ma (Belperio et al., 1998; Jago et al., 2003). There was an abrupt change in the zircon ages and detrital chemistry at the disconformity between the Normanville and Kanmantoo groups, from a provenance similar to the rocks of the Gawler Craton, to a source consistent with the sequences in East Antarctica to the south (e.g., Estrada et al., 2015; Ireland et al., 1998; Flömannet al., 1998). The latter share the same 600 to 500 Ma zircons also deposited in Ordovician turbidites of the Lachlan Orogen further to the east. This pulse of turbiditic deposition that filled the Kanmantoo Group is attributed to renewed attenuation in eastern Australia forming the Kanmantoo Trough, coupled with early shortening, uplift and erosion in the Ross Orogen in East Antarctica from ~540 Ma, while the Gawler Craton was subdued, and covered by shallow seas (Cayley et al., 2011). This uplift in the Ross Orogen is interpreted to have been due to subduction of oceanic crust ahead of the NW oblique advance of the Vandieland Microcontinent (Cayley et al., 2011).
The Kanmantoo Trough was dominantly filled by a 7 to 8 km thick (Jago et al., 2003) high density clastic (sandstone-siltstone-mudstone) turbidite/flyschoid sequence (Haines et al., 2001) of rapidly deposited psammitic (now quartz-feldspar-mica schists) and lesser pelitic (now garnet-andalusite-biotite schists) sedimentary rocks, the Kanmantoo Group (Belperio et al., 1998). This thick pile of sediments was deposited over a period of no more than 3 m.y. (e.g., Preiss, 2021). The contact between the upper Normanville and basal Kanmantoo Group includes the bimodal, 526 ±4 Ma (U-Pb; Cooper et al., 1992) Truro Volcanics. These extrusives, which include dark green, fine grained basalt and andesite flows; cream quartz-phyric metarhyolite dykes and sills; dark green metadolerite dykes and sills; grey-green metadiorite dykes and sills; dark grey-green mafic volcaniclastic sandstones and granulestone. They have alkaline, intraplate affinities to subalkaline, tholeiitic compositions similar to MORB (Rankin et al., 1991; VandenBerg et al., 2000; Foden et al., 2002; Crawford et al., 2003; Gibson et al., 2015) and may indicate the onset of rift detachment and separation as detailed above. The similar 521 ±4 Ma Marne River Volcanics calc-alkaline dacite, rhyodacite to rhyolite volcanics and nearby calc-alkaline basalt to andesite of the Teal Flat Volcanics are also taken to indicate rift-extension tectonics (Burtt et al., 2000). The strongly attenuated and dislocated Proterozoic basement intruded by mantle derived mafic rocks, which form the basement to the Kanmantoo Group is interpreted from deep seismic data to extend as far east as the Moyston Fault in western Victoria, which marks the eastern margin of the Delamerian Orogen (Cayley et al., 2018).
This extensional deposition within the Kanmantoo Trough was abruptly terminated by the onset of NW directed compression at ~514 Ma (Foden et al., 2001), initially predominantly affecting the southeastern corner of the Gawler Craton, with Cambrian extensional faults being reactivated as thrusts (Preiss, 2021). Consequently, folds, thrusts and foliation are NW-vergent on Fleurieu Peninsula, which experienced the maximum shortening (Preiss, 2021). This compressional event resulted in deformation, metamorphism and granitoid intrusion during the Middle to Late Cambrian Delamerian Orogeny that persisted until ~485 Ma. The onset of compression is interpreted to be the result of the Middle Cambrian oblique collision between the NW migrating VanDieland Microcontinent and the Kanmantoo Trough. This microcontinent includes most of Tasmania, its surrounding continental shelf, and central Victoria below the Melbourne Trough (Cayley et al., 2011).
The approach of this microcontinent was accommodated by a NNW-SSE trending, west-dipping subduction zone, in what is now western Victoria, which consumed the Cambrian oceanic crust that preceded the microcontinent. This subduction resulted in the development of the Mid to Late Cambrian (>~511 to 495 Ma) Stavely Arc in western Victoria, a calc‐alkaline andesite‐dacite igneous succession that apparently conformably interfingers with‐ and overlies‐ the upper Kanmantoo Trough sequence to the west. Basement to the east of the subduction zone is occupied by Cambrian oceanic crust. The Vandieland Microcontinent had approached from the SE, in the wake of the cratonic block (North American and/or south China?) that had been rifted and separated from Australia, as detailed above, and was retreating to the NE.
The Kanmantoo Group has been affected by three Delamerian deformation events (Foden et al., 2006). D1 produced bedding-parallel schistosity and possibly resulted in NW vergent overthrusting of the Kanmantoo Group onto the Adelaide Rift Complex. D2 overprinted D1 schistosity with an axial plane crenulation cleavage. It was the most intense deformation and coincided with peak metamorphism and generally north-south striking, upright, tight to isoclinal folds in a steep reverse and sinistral strike-slip regime with shallow southerly plunges (Offler and Fleming 1968). The principle folds and the pervasive fabric at the Kanmantoo deposits is due to the D2 event. D2 was accompanied by the intrusion of the Kinchina Quarry Granite 506 ±1 Ma (Foden et al., 2006). D3 produced a series of west to WNW thrusts and overturned west vergent, tight to isoclinal, west to NW trending folds in zones of high strain, and in late shear zones. D3 is not evident at Kanmantoo (Foden et al., 2006). D3 was followed by the 485 to 470 Ma Mannum Granite (Turner, 1991).
However, Curtis et al. (2021) and Curtis (2021; Geological Survey of South Australia Discovery Day presentation) suggested that D1 may represent an extensional regime where crustal attenuation and associated decompression melting of underlying asthenosphere led to the emplacement of migmatites and bi-modal magmatism which affected the lower sections of wet sediments of a pile that was still being accumulated. This may have led to the widespread high-temperature, low pressure metamorphism that characterises the Kanmantoo Trough turbidite sequence, as described below. Curtis presented dates for these intrusions, which included the granitic to granodioritic orthogneiss of the Rathjen Gneiss, found at the core of the high temperature-low pressure metamorphism, and the Black Hill Norite, both dated at 509.67 ± 0.72 Ma. Other associated intrusions in the trough included the Palmer Granite at 509 ± 0.24 Ma and the Encounter Granite 506.58 ± 0.27 Ma. It was implied that if D1 is extensional, this means that deposition of the thick Kanmantoo Group, may have exceeded the unlikely short 3 m.y. previously estimated. It also noted that this means the NW vergent overthrusting of the Kanmantoo Group is later.
The ~510 to 500 Ma I- and S-type granitoids are extensively developed. In South Australia, west of the Cenozoic Murray Basin cover, these were accompanied by high temperature (550 to 600°C), low pressure (3 to 5 kb) metamorphism which comprises a migmatite core, flanked by NNW-SSE aligned belts of amphibolite facies (inner sillimanite and outer andalusite-staurolite assemblages) and peripheral biotite to greenschist facies zones to the east and west. This zonation is centred on granitic intrusions within the migmatitic core and affects both Neoproterozoic and Cambrian turbidites (Sandiford et al., 1995; Belperio et al., 1998). These metamorphic facies zones reappear ~300 km to the east from below Cenozoic cover in southwestern Victoria in the Glenelg Zone in rocks correlated with the Moralana Supergroup (Crawford et al., 2003). Following the cessation of D3 convergent deformation at ~490 Ma, there is an abrupt transition to post-tectonic relaxation and extension, and the emplacement of a voluminous, undeformed, post-tectonic, bimodal magmatic association of mafic intrusions and felsic granites and volcanic rocks of S- to predominantly A-type affinities until ~470 Ma (Curtis, 2021; Foden et al., 2002; Belperio et al., 1998; Sandiford et al., 1995). Some of the intrusions, previously assumed to be part of this event, have now been shown to be related to D1 (Curtis et al., 2021), e.g., the Black Hill Norite (see above) in South Australia, and possibly the extensive, largely concealed, Hummocks Serpentinite in western Victoria. The latter is composed of dark green massive, serpentinite with locally preserved cumulate texture. These intrusions were apparently emplaced at shallow levels and are accompanied by local bimodal felsic and mafic volcanism. Together, the syn-D2 and D3 related and post- or pre-orogenic intrusions occur as large masses that are mainly found in the east of the trough and under cover over a curved NE-SW → north-south → NW-SE, concave to the east, elongated belt of ~600 x 50 to 180 km.
Foden et al. (2002) suggest that over the ~30 m.y. of the Delamerian Orogeny, from 514 to 485 Ma, intrusive complexes were developed in three stages:
i). lower-crustal mafic magmas (possibly derived from relatively shallow, ponded mafic underplating below the thinned/attenuated crust developed during the extreme extensional phase; Foden et al., 1990) that melted and were contaminated by mingling with melts of the local metasediments during compression, producing I- and S-type magmas during D2 and D3; Preiss (2021) suggests the very thick sedimentary pile in the Kanmantoo Trough depocentre, deposited in <3 m.y., was still wet and hot from the extensional phase, leading to early growth of high-temperature metamorphic minerals and facilitating partial melting in the lowest units of the Kanmantoo Group;
ii). during post-orogenic rebound and extension, crustal S-type magma melts formed above the resultant upwelling mantle, or in proximity to mafic or I-type granite intrusions;
iii). upper-crustal mafic intrusions became a closed-system, in which fractionation dominated, but melted enclosing country rock to produce A-type granite.
The Kanmantoo Trough in South Australia and its eastern extension in western Victoria are separated by the NNW-SSE trending topographic high the Padthaway Ridge which is largely occupied by a core of Delamerian granitoid and mafic intrusions, with lesser syn-orogenic plutons that intrude rocks of the Normanville and Kanmantoo groups.
The Kanmantoo Group is limited to the east by the Stavely Arc, where an accretionary wedge of the Stawell Zone further to the east is overlain by the thick Palaeozoic sedimentary successions of the Lachlan Fold Belt Bendigo Zone.
Numerous copper, gold, lead, zinc, silver and pyrite deposits are hosted by the pelitic meta-sedimentary rocks of the Cambrian Kanmantoo Group over a 300 km strike length within the Kanmantoo Trough. The Kanmantoo deposit is the largest of these. The Kanmantoo Group is composed of three transgressive-regressive events. The Tapanappa Formation, which represents the middle event, is uniquely characterised by sulphide rich siltstone lenses and Fe-Mn rich horizons within basinal greywackes-mudstone sequence. The Kanmantoo copper deposit is the largest of a group hosted by Cambrian schists after a sequence of sandstone and mudstone units, the Paringa Andalusite Member, a high-Fe quartz rich (graphite poor) meta-pelite in the upper part of the Tapanappa Formation.
It is likely that the thick sedimentary pile in the Kanmantoo Trough depocentre that Preiss (2021) suggests was still wet and hot from the extensional phase, was heated by the postulated shallow ponded underplate, facilitating circulation of basinal fluid during D2 deformation. Such hot, pregnant fluids might then be deposited in structural dilations (e.g., Kanmantoo), or when coming into contact with sites conducive to deposition, such as the reduced/pyritic units within the sequence (e.g., Mount Torrens, see the Angas record). Metals may have been leached from the thick sequence, or contributed in part by the underplate mantle material.
Geology and Mineralisation at Kanmantoo
The mineralisation at Kanmantoo extends over a strike of at least 6 km, predominantly within the iron-rich Paringa Andalusite Member, now represented by a distinctive, Fe-Mg-rich suite of metamorphic minerals, including chlorite, biotite, almandine-rich garnet, staurolite and magnetite, a member of a regionally extensive garnet-biotite schist unit. The host generally occurs as a quartz-chlorite-garnet±andalusite unit, referred to as garnet-andalusite-biotite schist (GABS; Rolley and Wright, in press) which is characterised by large andalusite porphyroblasts in a schistose matrix of quartz, biotite, garnet and staurolite. Andalusite occurs in the lode schists in approximate inverse proportion to chlorite. This GABS unit is ~600 m wide at Kanmantoo and hosts all of the mineralisation. It is flanked to the east and west by quartz-mica schists predominantly composed of quartz and biotite, with lesser muscovite and plagioclase (SchiIler 2000; Both, 1990) and has a north to NNE strike and steep east dip.
The immediate hosts to ore at the main Kavanagh zone, mined in the 1970s, and the most studied, are chlorite rich, comprising an assemblage of quartz-chlorite-garnet±pyrrhotite±chalcopyrite, forming a mineralised core within the garnet-andalusite schist host. Like the mineralisation, the chlorite rich host envelope is pipe like in shape, but is complex in detail, with a steep northerly plunge, in contrast to the shallow south plunges of folds in bedding in the surrounding schists. Both the lode schists and the sulphide mineralisation that comprises the Kavanagh zone (or Main Lode), occur in a series of lenses within the gross pipe-like structure, which has maximum horizontal dimensions of 120 x 180 m and has been intersected to depths of at least 450 m below the surface. It strikes at 10° and dips at 75°E, plunging 80°N. Mineralisation is largely controlled by a set of north-south and NE striking structures and is best developed where these two structural trends intersect (SchiIler 2000; Both, 1990).
The main structure defined in the mine area by the generally well preserved relict bedding, is the Mine Synform, which plunges ~15°S. The mineralised pipe lies on the eastern limb of this structure. The dominant structural fabric is the S2 schistosity, accompanied by an erratically developed mineral lineation, which are both very regularly oriented. The schistosity, which has a north-south strike and dips at 73°E, is axial planar to the Mine Synform and all other mesoscopic and macroscopic folds. This schistosity is assigned to the regional D2 deformation. The mineral lineation has a steep SE plunge, whilst most fold axes plunge shallowly to the south. As a consequence, the pipe-like Main Lode is discordant to both the fold axes in bedding and the mineral lineation. Post-D2 structures are limited to brittle-style rare crenulations, kinks and joints (SchiIler 2000).
The main mineralised pipe in the Kavanagh zone is composed of multiple sulphide lenses each paralleling the NNE trending and steeply east dipping schistosity of the hosts rocks. Additional such lenses surround the main pipe and account for much of the known resource (Both, 1990). Veins in the mine are quartz, quartz-sulphide, sulphide or coarse-grained silicates and are either parallel to S2, or are slightly oblique, folded and boudinaged. Veins, which are oblique to S2 in the immediate mine surrounds, are mostly quartz, and are folded and boudinaged. These are older veins and are generally barren. Many minor shear zones occur in the mine, principally on the western limb of the Mine Synform where they are parallel or subparallel to S2. Some shear zones are important controls on the copper distribution (SchiIler 2000).
The host GABS unit is unaltered with well-developed S2 schistosity and porphyroblastic andalusites where distal to the mineralisation. As the mineralisation is approached, there is a transitions to a staurolite rich assemblage with the andalusites having corroded margins, accompanied by the initial appearance of chlorite and quartz-sulphide veining. Within the mineralised zone, the alteration is an intense biotite-garnet-chlorite schist in which the S2 fabric and andalusite has been obliterated, with sulphide, chlorite and quartz veining dominant. Quartz veining is associated with all mineralisation, but is generally more prevalent in the western mineralised areas.
In addition to the main Kavanagh zone, a network of surrounding mineralised zones or shoots have been outlined at Kanmantoo, controlled by a series of north-south striking shears that are parallel to S2 and dip at ~80°E, linked by a number of NNE to NE striking cross-shears (e.g., Mathew, Valentine and Kavanagh West which generally strike north-south, and the Nugent, Spitfire and Kavanagh deposits that strike NNE). These deposits occurs in the form of discordant stockworks within highly altered GABS. At a local scale, the higher grade copper shoots occur at the intersection of the north and NNE shears, and generally plunge at 65 to 75° with and azimuth of 40°. Most fall within a single halo of pyrite and pyrrhotite that extends over an area of 1.5 x 0.8 km, although the separate Emily Star zone is within a 250 m diameter area, ~250 m to the SW of the main zone. Chlorite also forms a significant alteration halo to the copper mineralisation, over an area of ~1 x 0.5 km, typically as replacement of biotite and andalusite, and as 2 to 20 mm selvages of intense chlorite (Rolley and Wright, in press).
The Spitfire zone, 250 m SSE of Kavanagh has been affected by chlorite-garnet-biotite-magnetite alteration, and comprises a number of mineralised orientations including north-south (S2), 15° and 25° strikes. The latter orientation is distinctly characterised by strong magnetite alteration associated with intense chlorite development. This zone also has the highest gold endowment which may be a result of the stronger development of the NNE and NE fabrics.
The Nugent zone, 200 m SE of Spitfire, typically comprises staurolite-quartz-biotite alteration surrounding a strong quartz-chlorite ore zone. The mineralisation occurs as a planar, 45° strike and 80°E dipping body, with the north-south Biotite Schist unit contact faulted to the east where it is intersected by the Nugent mineralised zone.
The Emily Star mineralisation is located close to the hinge zone of a regional syncline. Mineralisation cross-cuts bedding and is not limited to the hinge zone, localised within S2 oriented and NW to NNW trending chlorite-quartz zones. Alteration is characterised by garnet-quartz-biotite with a smaller sulphur and chlorite halo surrounding the mineralisation.
The majority of the sulphide minerals in the mine area occur as disseminated, microscopic grains and as mesoscopic veins, both of which parallel the north-south S2 schistosity, or are found as irregular veins and patches. In vertical section, the copper mineralisation is very regularly distributed, forming steep shoots, some parallel to the schistosity and others slightly oblique with a NNE trend. Two styles of mineralisation, on the respective limbs of the Mine Synform, are defined: i). East Limb, characterised by a NNE trend, and an association with host rocks that have fabrics inferred to represent metamorphic crystallisation during high fluid pressures. and ii). West Limb, which are north-south trending in general, and are not associated with the fabrics of the East limb style (SchiIler 2000).
SchiIler (2000) indicates that microstructural studies imply sulphide minerals were involved in all stages of the metamorphic and structural history. He also records S1 fabrics preserved in andalusite, garnet, staurolite and biotite porphyroblasts, with little evidence of crenulation of SI, suggesting S2 developed from SI, generally by rotation and recrystallisation of SI during flattening. Not all porphyroblasts were developed are coeval, with a series of porphyroblast forming reactions indicated in most rock types. The peak metamorphic pressure (3-4 kb) and temperature (480 to 565°C which are variable, depending on the geothermometers used) are similar for both lode schists and surrounding metasediments (SchiIler 2000).
The dominant minerals in the ore are chalcopyrite, pyrrhotite and magnetite in approximately equal proportions, with lesser pyrite, and minor or traces of ilmenite, cubanite, pentlandite, marcasite, mackinawite, sphalerite, bismuthinite, bismuth, cobaltite, galena, molybdenite, wolframite, gold and silver (Both, 1990).
Overall, chalcopyrite dominant mineralisation is weakly anomalous in gold, averaging around 0.1 g/t Au, with a Cu%:Au g/t ratio of ~100:1. However, to the east, e.g., in the Nugent, Spitfire and Schultze zones, there is a lower Cu%:Au g/t ratio of around 25:1, generally associated with quartz-pyrite and chlorite-pyrite veins, and shears striking NE on linking structures between north-south shear zones. Clusters of fine grained free gold over 10 mm areas have been observed in drill core with individual gold grains to 1 mm (Rolley and Wright, in press).
In October 2006 Hillgrove announced an Indicated and Inferred Resource of 28 Mt at 0.94% Cu and 0.2g/t Au.
In addition approximately 4 Mt @ 1% Cu were mined from 1970 to 1976.
Published ore reserves and mineral resources at the end of February, 2013 were (Hillgrove Resources, 2014):
In situ resource
Measured Resource - 2.63 Mt @ 0.88% Cu, 0.10 g/t Au, 1.95 g/t Ag;
Indicated Resource - 21.77 Mt @ 0.82% Cu, 0.23 g/t Au, 2.21 g/t Ag;
Inferred Resource - 5.0 Mt @ 0.67% Cu, 0.13 g/t Au, 1.79 g/t Ag;
Long term stockpiles
Measured Resource - 1.39 Mt @ 0.46% Cu;
Indicated Resource - 0.50 Mt @ 0.18% Cu;
TOTAL Resources - 31.30 Mt @ 0.78% Cu, 0.20 g/t Au, 2.11 g/t Ag.
In situ reserves
Proved Reserve - 2.5 Mt @ 0.77% Cu, 0.08 g/t Au, 1.7 g/t Ag;
Probable Reserve - 18.2 Mt @ 0.72% Cu, 0.20 g/t Au, 2.0 g/t Ag;
Long term stockpiles
Proved Reserve - 1.4 Mt @ 0.46% Cu;
TOTAL Ore Reserves - 23.1 Mt @ 0.71% Cu, 0.18 g/t Au, 1.9 g/t Ag.
Underground Mineral Resources as at 31 December 2020 (Hillgrove Resources Annual Report, 2020):
Kavanagh Zone
Indicated Resource - 0.583 Mt @ 1.97% Cu, 0.24 g/t Au, 6.0 g/t Ag;
Inferred Resource - 0.560 Mt @ 1.7% Cu, 0.2 g/t Au, 5 g/t Ag;
West Kavanagh Zone
Measured Resource - 0.105 Mt @ 1.42% Cu, 0.06 g/t Au, 2.0 g/t Ag;
Indicated Resource - 0.300 Mt @ 1.1% Cu, 0.06 g/t Au, 2.0 g/t Ag;
Nugent Zone
Measured Resource - 0.202 Mt @ 1.40% Cu, 0.47 g/t Au, 3.2 g/t Ag;
Indicated Resource - 0.457 Mt @ 0.30% Cu, 0.70 g/t Au, 2.7 g/t Ag;
TOTAL Mineral Resource - 2.208 Mt @ 1.56% Cu, 0.32 g/t Au, 4.1 g/t Ag.
A JORC Compliant Exploration target as at 31 December 2020 (Hillgrove Resources Annual Report, 2020) was:
8 to 16 Mt @ 1.0 to 2.0% Cu, 0.2 to 0.4 g/t Au.
The Mineral Resource Estimate (Hillgrove Resources, Annual Report, 2022) as at 31 December 2021:
Kavanagh Underground at a 0.6% Cu cut-off - below the main Giant open pit,
Indicated Resource - 3.530 Mt @ 1.1% Cu, 0.11 g/t Au;
Inferred Resource - 1.480 Mt @ 1.01% Cu, 0.1 g/t Au, 5 g/t Ag;
Nugent Underground at a 0.8% Cu cut-off,
Measured Resource - 0.202 Mt @ 1.40% Cu, 0.47 g/t Au, 3.2 g/t Ag;
Indicated Resource - 0.457 Mt @ 0.30% Cu, 0.70 g/t Au, 2.7 g/t Ag;
TOTAL Mineral Resources - 5.669 Mt @ 1.10% Cu, 0.33 g/t Au, 4.1 g/t Ag.
The most recent source geological information used to prepare this decription was dated: 2021.
Record last updated: 10/11/2021
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.
Kanmantoo
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Selected References:
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Both R A, 1990 - Kanmantoo Trough - Geology and mineral deposits: in Hughes F E (Ed.), 1990 Geology of the Mineral Deposits of Australia & Papua New Guinea The AusIMM, Melbourne Mono 14, v2 pp 1195-1203
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Pollock, M.V., Spry, P.G., Tott, K.A., Koenig, A., Both, R.A. and Ogierman, J., 2018 - The origin of the sediment-hosted Kanmantoo Cu-Au deposit, South Australia: Mineralogical considerations: in Ore Geology Reviews v.95, pp. 94-117.
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Rolley, P. and Wright, M., 2017 - Kanmantoo copper deposit: in Phillips, G.N., 2017 Australian Ore Deposits, The Australasian Institute of Mining and Metallurgy, Mono 32, pp. 667-670.
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Seccombe P K, Spry P G, Both R A, Jones M T and Schiller J C, 1985 - Base metal mineralization in the Kanmantoo Group, South Australia; a regional sulfur isotope study : in Econ. Geol. v80 pp 1824-1841
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Tott, K.A., Spry, P.G., Pollock, M.V., Koenig, A., Both, R.A. and Ogierman, J., 2019 - Ferromagnesian silicates and oxides as vectors to metamorphosed sedimenthosted Pb-Zn-Ag-(Cu-Au) deposits in the Cambrian Kanmantoo Group, South Australia: in J. of Geochemical Exploration v.200, pp. 112-138.
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Verwoerd P J and Cleghorn J H, 1975 - Kanmantoo copper orebody: in Knight C L, (Ed.), 1975 Economic Geology of Australia & Papua New Guinea The AusIMM, Melbourne Mono 5 pp 560-565
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