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Emmie Bluff |
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South Australia, SA, Australia |
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Cu Au
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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The sub-economic Emmie Bluff iron oxide copper-gold-uranium deposit and the overlying sediment hosted Cu-Co-Ag mineralisation are located ~75 km south of Olympic Dam, 45 km NW of Carrapateena, and ~500 km NNW of Adelaide in northern South Australia (#Location: 31° 12' 00"E, 137° 10' 00"E).
Emmie Bluff, along with Carrapateena, Olympic Dam, Prominent Hill, Moonta-Wallaroo and Hillside, and all of the other significant known IOCG mineralised systems of the Gawler craton are hosted within Palaeo- to Mesoproterozoic rocks and are distributed along the eastern edge of the currently preserved craton to define the Olympic IOCG Province.
See the Gawler Craton and Olympic IOCGU Province record for a summary of the regional, cratonic setting of the Olympic IOCG Province.
The Emmie Bluff mineralisation comprises two significant zones of iron oxide alteration ~3 km apart in an ENE-WSW direction, reflected by two NW-SE elongated coincident positive gravity and magnetic anomalies. The SW zone gravity anomaly is ~2.5 km long, while the NE zone is 5 km in length. The mineralised basement is overlain by 750 to 900 m of the flat lying cover sequence.
This overlying sequence in the Emmie Bluff area includes, from the base, the Mesoproterozoic (~1420 Ma) Pandurra Formation hematitic sandstone, shale and siltstone, an arenaceous red bed sequence (locally ~10 to 750 m thick); unconformably overlain by the Neoproterozoic Tapley Hill Formation, (~20 m of dark, carbonaceous, finely layered pyritic dolomitic siltstones, black shales and dolostones; Whyalla Sandstone (~120 m of mature, bimodal lithic quartz arenite, composed of quartzite and carbonate clasts and grains, variably gritty siltstones to sandstones of aeolian, reworked aeolian, and alluvial origin), Tregolana Formation shales (~180 m of maroon shales with thin interbeds of slightly dolomitic green siltstones), Emmie Bluff Beds (an up to 10 m thick lens of fine grained buff dolomite composed of predominantly finely crystalline dolomite, mica, quartz and rare pyrite, with pyrolusite joint coatings); Coorabarra Sandstone (~25 m of maroon and white fine grained wackes and fine grained maroon quartzite); and the Simmens Quartzite Member (~60 m of fine grained, white quartzite with white clay galls and rare heavy mineral laminae) (Binks, 1993).
The deposit area is cut by a NW-SE-trending normal fault between the two mineralised zones. The block to the SW of this fault, has been down-thrown and the Pandurra Formation is unconformably underlain by 200 to 300 m of only gently dipping 1590 to ~1575 Ma Gawler Range Volcanics (GRV), which in turn overlie, across an angular unconformity, metasedimentary rocks of the <1850 to ~1740 Ma Wallaroo Group (Gow et al., 1994; Bastrakov et al., 2007).
To the NE of the fault, the Pandurra Formation is directly underlain, across an angular unconformity, by a moderately NE-dipping sequence of Wallaroo Group metasedimentary rocks, that include two thick units of fine-grained, laminated siltstone sandwiching an ~200 m thick band of coarse-grained arkosic rocks that grade up into the upper siltstone which comprises interbedded cherts, lutites and carbonate-facies sediments. Further to the NE of the fault, the upper siltstone unit is structurally overlain by a wedge of ~1850 Ma Donington Suite granitoids, overthrust from the NE along a bedding parallel thrust fault. Intersections of interpreted GRV altered felsic volcanic rocks have been encountered between the Pandurra Formation and the Wallaroo Group to the east of the main fault in the south (Gow et al., 1994; Bastrakov et al., 2007; Gunson Resources, 2009, 2011).
The SW mineralised zone consists of a highly fractured zone of magnetite-rich alteration, discordantly developed within both the GRV and the underlying Wallaroo Group laminated siltstones.
Diamond drill hole SAE7 that passed through this SW zone of mineralisation, intersected altered (GRV) rhyolite porphyry from the unconformity at the base of the Pandurra Formation at 898.2 m to 1134.0 m, and then altered meta-sediments of the Wallaroo Group to the end of hole at 1221.7 m. Alteration comprised intense replacement of all rock-types by magnetite and amphibole, with little or no copper mineralisation evident (Binks, 1993).
This discordant alteration zone comprises veins and replacement mineralisation within fractured, sericitised, trachytic-textured GRV rhyodacite, which typically has chloritised phenocrysts. The veins contain high-temperature mineral assemblages that include combinations of albite and Ca-Fe-rich clinopyroxene, actinolite, magnetite (<10 vol.%), quartz, calcite, K feldspar, pyrite and allanite. Sulphides are not abundant, with only very minor pyrite and traces of chalcopyrite. These veins are up to 6 cm thick, with open-space growth textures and commonly prominent disseminated magnetite selvedges. They typically occur as multistage networks, interpreted to have formed as a result of high fluid pressures associated with intrusion and volatile release. Within zones of intense veining, the trachytic texture of the host volcanic rock has been destroyed by recrystallisation and metasomatism (Gow et al., 1994; Bastrakov et al., 2007).
In contrast, the NE zone is a conformable, gently- to moderately-NE-dipping, slab-shaped, hematite-rich body within the upper fine-grained laminated Wallaroo Group siltstone, and in brecciated zones of the structurally overlying Donington granitoids found immediately above and within the thrust that separates the two. The hematite zone overlies and overprints an earlier magnetite-rich zone, as described below.
Diamond drill hole SAE6 that passed through this NE zone of mineralisation, cut the basal contact between the Pandurra Formation and the underlying basement and intersected 75 m of foliated Donington Suite granite, then drilled 15 m @ 1.23% Cu in strongly hematite altered rock, underlain across a gradational boundary, by 104 m @ 0.27% Cu in shale that was partially to wholly replaced by magnetite and associated pyrite. Other drill holes cutting the unconformity below the basal Pandurra Formation, and passing directly into the Wallaroo Group, west of the faulted lower contact of the Donington Suite, intersected massive hematite to heavily hematised shale and sandstone with copper mineralisation at the base, underlain by mineralised shales that have been partially or wholly replaced by both hematite and magnetite, with associated pyrite. An example is drill hole SAE4, which passed through the unconformity into 78 m of alternating quartz-hematite, red hematitic brecciated shale and sandstone, hematite breccia with shale and sandstone clasts, and massive steely-grey hematite, that only averages 108 ppm Cu of the whole interval, with no detectable Au. This passes abruptly across a 60 cm fault breccia into 18 m @ 0.7% Cu and 0.2 g/t Au, comprising an upper and lower steely hematite, sandwiching 3m of patchily hematised sandstone breccia, all of which are sulphidic, carrying mainly pyrite, some chalcopyrite and rare bornite. This interval is underlain by 58 m of rocks that contain both hematite and magnetite, again as massive grey to black hematite/magnetite bands, brecciated, hematitic (red earthy) sandstones and shales, with disseminated sulphides (mainly pyrite with some chalcopyrite). This interval averages 0.50% Cu, 0.13 g/t Au. After passing through a fault, the sequence becomes mainly quartzite, with some sandstone and conglomerate, containing far less iron oxides, and copper assays of generally <10 ppm, with no Au values above the 0.05 ppm detection limit (drill logs in Binks, 1993).
This NE mineralised zone is up to ~150 m thick and ~3 km in diameter, and clearly overprints relics of an earlier higher temperature magnetite alteration stage. The bulk of the early magnetite is fine grained and disseminated (although locally grading to massive), but may also occur as larger grains with a bladed habit. On a microscopic scale, magnetite exhibits evidence of strong preferential growth in the quartz-poor layers within the fine-grained laminated units of the Wallaroo Group, and forms discontinuous bands and laminae which appear to have been tectonically disrupted. Some discordant magnetite veins have iron oxide aureoles that preferentially extend outwards along phyllosilicate layers in the host sedimentary rocks. On a mesoscale, magnetite is abundant in the fine-grained, ferruginous, phyllosilicate-rich (quartz-K-feldspar-muscovite) layers, while quartzose-arkosic interbeds and the underlying arkosic unit have remained relatively unaltered, although rare magnetite and hematite veins are observed. Magnetite is typically accompanied by a lower temperature assemblage of chlorite-quartz-magnetite±pyrite, with localised zones rich in actinolite, apatite, tourmaline or K feldspar. In contrast to the SW mineralised zone, the magnetite phase of the NE zone is not generally accompanied by significant amounts of amphiboles and feldspar (Huntington et al., 2006).
Copper mineralisation is dominantly associated with a low-temperature assemblage of hematite-quartz-chlorite-pyrite, that is interpreted to overprint the magnetite stage. This second, lower temperature phase, involved martite replacement of pre-existing magnetite, with much of the martite/hematite having magnetite cores. A detailed study of one drill hole suggests that in the upper part of the NE mineralised zone, where almost all of the magnetite has been oxidised, the martite has recrystallised to fine-grained anhedral hematite, with subordinate fine-grained platy, and red dusty hematite, sometimes with associated vugs (Gow et al., 1994; Huntington et al., 2006).
The Cu mineralisation occurs as chalcopyrite, bornite and minor covellite. Some pre-existing pyrite accompanies the early high-temperature magnetite, and some chalcopyrite also occurs in contact with magnetite, with no evidence for reaction, suggesting that some of the chalcopyrite may have initially precipitated as part of the early magnetite assemblage or was deposited with the later hematitic stage but in equilibrium with magnetite. Never the less, the bulk of the sulphides are spatially and temporally associated with the lower temperature hematite-rich assemblage, implying the latter fluids were richer in sulphur (Gow et al., 1994). Chalcopyrite replaces pyrite, accompanied by chlorite in laminated siltstones, and sericitisation of feldspars in the meta-arkose and granitoids, with lesser chlorite. Most of the chalcopyrite is intimately intergrown with pyrite and hematite, and is present as either veins or as a replacement of the host rock. The greatest abundance of chalcopyrite broadly coincides with the most intense hematite developments (although less intense in vuggy zones). Most bornite and covellite are replacements of chalcopyrite (Gow et al., 1994; Huntington et al., 2006).
The highest grade mineralised intersections contain up to 2.8 wt.% Cu and up to 0.6 g/t Au over intervals of as much as 10 m, often enclosing thinner zones of >4 wt.% Cu and >1 g/t Au (Gow et al., 1994; Gunson Resources, 2009, 2011). Gow et al. (1994) concluded that the petrogenetic relations between the oxide minerals, combined with a comparison of the silicate assemblages from the two major phases of iron oxide deposition (i.e., early high-temperature calc-silicate assemblages followed by a lower temperature quartz-chlorite assemblage), suggest the later mineralising fluids were at a lower temperature, and were more oxidising and Cu-rich, than the earlier fluids that deposited the magnetite.
A set of reverse/thrust faults that produce stratigraphic repetitions in the upper Wallaroo Group metasiltstones and the structural emplacement of the Donington Suite granitoid appear to have channelled the mineralising fluids in the NE mineralised zone. The relation between these structures and the regional-scale fault that has localised magnetite deposition is unclear. Within the reverse/thrust fault that separates the Donington Suite and Wallaroo Group, veins of the chlorite-hematite-Cu sulphide ore-bearing assemblage both crosscut and occur within breccia clasts of the silicified fault zone, implying that the fault was synchronous with mineralisation. Negligible Cu sulphides are found in the overlying metagranitoid reflecting a lack of magnetite as a source of iron for the mineralising reaction, while immediately below the fault zone, extensive deposition of Cu sulphides occurred in the magnetite-rich Wallaroo metasiltstone. Enhanced permeability caused by brittle fracturing associated with the fault was also a major influence on the localisation of mineralisation. In contrast to the metagranitoid above the fault, which has few brittle-fracture textures, the highly mineralised Wallaroo Group metasiltstones are extensively brecciated, with a correspondingly high permeability. This intense brecciation persists for ~60 m below the fault, with the gradual decrease in brecciation corresponding to the gradual decrease in hematite-rich alteration and Cu grades below the fault (Gow et al., 1994).
All of these factors have resulted in a crude downward alteration zonation, from the upper breccia and cataclasite zones associated with the faulted contact between the Donington Suite and Wallaroo Group, where intense hematite-chlorite-quartz (or hematite-sericite-quartz, depending on the protolith lithology) alteration is accompanied by chalcopyrite-pyrite-bornite-covellite, to less brecciated relic magnetite-chlorite-pyrite-altered zones lower in the mineralised zone (Gow et al., 1994; Bastrakov et al., 2007).
Quartz and calcite veining occurred throughout the mineralising phase, locally producing multistage veins with open-space growth textures. The Cu sulphides are typically associated with the quartz phases. Pervasive sericite alteration and late-stage fluorite veins represent the final phases of the hydrothermal activity (Gow et al., 1994).
In 2012, the IOCG 'deposit' was a prospect being drilled in search of a sufficiently large development of economic grade mineralisation.
Sediment hosted copper mineralisation
Copper mineralisation has also been intersected in drill holes in the cover sequence over an area of ~2 x 1 km,occurring as a laterally extensive, flat-lying, sediment-hosted deposit at ~400 m depth, elongated parallel to the basement NW-SE structural trends. Copper sulphides dominated by chalcocite were concentrated in porous strata and breccia clasts above and below a Marinoan disconformity of the Whyalla Sandstone, but principally in the underlying brecciated, black dolomitic and graphitic shale of the Tapley Hill Formation. Intersections include 6 m @ 1.49% Cu, 21 g/t Ag in drill hole SAE6; 1.7 m @ 1.7% Cu, 38 g/t Ag in drill hole SAE5. A higher grade zone than that indicated by these intersections is concentrated in the southeastern part of the defined area, with widths of 0.65 to 1.9 m @ 1.7 to 5.2% Cu (Binks, 1993). Mount Isa Mines Ltd outlined a resource of ~25 Mt @ 1.3% copper beneath 400 m of Neoproterozoic, and younger sedimentary cover. There were significant silver, cobalt and zinc credits. This stratabound mineralisation may be sourced from and reflect the basement mineralisation (DMITRE SARIG Database).
In 2007 Argo Exploration Ltd drilled 3 holes ~2.5 km to the NW of hole SAE6 to identify a similar style mineralisation at the same stratigraphic level, and over a distance of 2.6 km. The mineralisation was considered to represent a northern extension of the Emmie Bluff stratabound mineralisation.
The distribution of the Tapley Hill Formation appears to be controlled by a NW-SE trending fault bounded palaeo-ridge of Pandurra Formation which runs just to the west of drill hole SAE6 (see diagram above). Over this ridge, the Pandurra Formation is directly overlain by the Tregolana Formation shales, and the Tapley Hill Formation and Whyalla Sandstone are absent. The Tapley Hill Formation is apparently confined to a narrow belt just to the east of the ridge, possibly a separate basin, largely disconnected from the open sea. It represents a highly reduced, thin sequence separating two red bed arenite sequences, the Pandurra Formation and Whyalla Sandstone. However, to the west of the palaeo-ridge the same unit is much thicker with up to several hundred metres intersected in drilling (Binks, 1993).
Mineralisation in the eastern basin occurs as disseminated grains of chalcocite, bornite and chalcopyrite in carbonaceous pyritic shale which is interbedded with thin (l to 20 mm) bands of grey dolostone and sandy dolostone. The mineralisation if richer in two bands, at the top and bottom of the Tapley Hill Formation, adjacent to the red bed sandstones where copper mineralisation also occurs as disseminated grains and narrow veinlets of chalcocite, extending for several tens of cm into the arenites (Binks, 1993).
In drill hole SAE6, close to the margin of the restricted basin, the Tapley Hill Formation is 15.5 m thick upper and lower mineralised intervals of 6 m from 388 m below collar of 1.49% Cu, 21 g/t Ag, 0.0512% Co, and 2 m from 400 m below collar of 1. 45% Cu, 10 g/t Ag, 0.057% Co. The zone includes 1. 35 m of basal conglomerate of the Whyalla Sandstone containing clasts of reworked Tapley Hill Formation that have also been mineralised. Lead and zinc values are anomalous with the highest values of 0.34% Pb and 0.4% Zn (Binks, 1993).
The most recent source geological information used to prepare this decription was dated: 2012.
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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Bastrakov E N, Skirrow R G and Davidson G J, 2007 - Fluid Evolution and Origins of Iron Oxide Cu-Au Prospects in the Olympic Dam District, Gawler Craton, South Australia: in Econ. Geol. v102 pp 1415-1440
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Fabris, A., 2022 - Geochemical characteristics of IOCG deposits from the Olympic Copper-Gold Province, South Australia: in Corriveau, L., Potter, E.G. and Mumin, A.H., (Eds.), 2022 Mineral systems with iron oxide coppergold (IOCG) and affiliated deposits Geological Association of Canada, Special Paper 52, pp. 247-262.
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Gow P A, Wall V J, Oliver N H S and Valenta R K, 1994 - Proterozoic iron oxide (Cu-U-Au-REE) deposits: Further evidence of hydrothermal origins: in Geology v.22 pp. 633-636
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Tonkin, D., 2019 - Recognition and definition of the copper-hosting Whyalla Sandstone, South Australia: in Mesa Journal v.89, pp. 39-44.
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Uvarova, Y.A., Pearce, M.A., Liu, W., Cleverley, J.S. and Hough, R.M., 2018 - Geochemical signatures of copper redistribution in IOCG-type mineralisation, Gawler Craton, South Australia: in Mineralium Deposita v.53, pp. 477-492.
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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