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Broken Hill Cobalt Project - Railway, Big Hill, Pyrite Hill |
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New South Wales, NSW, Australia |
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Co Ni
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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The Broken Hill Cobalt Project cobalt-nickel deposits Big Hill, Railway and Pyrite Hill are located on Thackaringa pastoral station, ~25 km WSW of Broken Hill, in far western New South Wales, and ~20 km east of the South Australian border (#Location: Pyrite Hill - 32° 5' 28"S, 141° 11' 37"E).
The deposits lie within the deformed and metamorphosed Palaeoproterozoic 1720 to 1640 Ma Willyama Supergroup that was deposited in the epi-continental Curnamona rift basin of the Willyama Block (or Broken Hill Domain), which forms the southeastern segment of the 200 x 400 km Curnamona Province.
For details of the tectonic, structural, geological and stratigraphic setting of the Broken Hill Domain see the separate Broken Hill record.
In the Broken Hill Cobalt Project area, the oldest rocks belong to the Statherian Thorndale Composite Gneiss which has a maximum depositional age of 1770 to 1700 Ma, based on detrital zircons (Page et al., 2005). It is largely composed of pelitic to psammitic gneiss, but also includes the Clevedale Migmatitic Member which comprises migmatite to migmatitic composite gneiss that is sporadically leucocratic. It also contains minor mafic gneiss and rare, thin, plagioclase-quartz rock and medium-grained biotite-rich quartzo-feldspathic gneiss. The Thorndale Composite Gneiss passes up into the Thackaringa Group which is host to the cobalt mineralisation of the project. This group comprises a sequence of variably albitic, psammite-dominated, metasedimentary rocks. The lower unit of this group is the Lady Brassey Formation, which is up to 600 m thick and comprises leucocratic sodic plagioclase-quartz rocks that occur as either discrete, massive, poorly 'bedded' units, or as thin to thick interbeds within psammitic to pelitic metasedimentary composite gneisses with a substantial conformable mafic gneiss component. These are overlain by the Cues Formation, which is up to 1000 m in thickness and is a composite gneiss that is interpreted to principally be a deformed sill-like granite which includes Potosi-type gneiss (garnet-biotite-rich quartzofeldspathic gneiss interpreted to represent a felsic volcaniclastic protolith), but also includes cordierite bearing pelitic paragneiss (metasedimentary rocks), and intercalated bodies of mafic gneiss. The sill is interpreted to have been a shallow intrusion emplaced into sandy marine shelf sedimentary rocks soon after sedimentation of the latter. It includes leuco-gneisses and 'Potosi-type' gneisses. Mafic gneisses form a substantial continuous interval in the mid sections of the formation, underlain by thinner, less continuous bodies. These mafic rocks are moderately Fe-rich, as indicated by abundant orthopyroxene and/or garnet, and are finely layered, in places with pale feldspar-rich bands. They are associated with medium-grained quartz-feldspar-biotite-garnet gneiss. The Cues Formation has been dated at 1700 ±4 Ma (U/Pb-Pb/Pb ion probe; Geoscience Australia) and is characterised by stratabound lenses of granular garnet-quartz ±magnetite rocks, quartz-iron oxide/sulphide, and quartz-magnetite rocks (Geoscience Australia, 2019). It is host to the Pinnacles Ag-Pb-Zn massive sulphide deposit along with widespread stratabound Fe-rich horizons. These rocks are also intruded by the Alma Granite Gneiss, which is mainly composed of medium to coarse-grained quartzo-feldspathic gneiss, with abundant megacrysts. In places it is a mafic gneiss with thin beds or bodies of leucocratic Na plagioclase-quartz rocks. The granite gneiss has crystallisation and metamorphic ages of 1704 ±3 and 1610 ±Ma respectively (U/Pb SHRIMP).
The Cues Formation is overlain by the Himalaya Formation, which regionally varies from 20 to 1000 m in thickness, and is the principal host to cobalt mineralisation at the Big Hill, Railway and Pyrite Hill deposits. It consists of medium-grained saccharoidal leucocratic psammitic and albitic meta-sedimentary rocks dated at between ~1700 Ma at the base, to 1690 ±11 Ma at the top (Geoscience Australia). The unit contains variably interbedded albite-quartz rich rocks, composite gneiss, mafic gneiss and horizons of thinly bedded quartz-magnetite rock. Pyrite-rich rocks are widespread towards the base of the formation. The albite-quartz gneiss, which is host to mineralisation within this formation, has a well defined, thin, regular and continuous layering, with 20 to 30 mm thick 'beds', and is sometimes massive and recrystallised with an average grain size of ~1 mm. It is stratabound, continuous and conformable, and has textures interpreted to reflect graded and cross bedding with scour and fill structures, with nodular and pebbly lenses. The average of 10 albite-quartz gneiss samples had the following geochemical composition: 68.35% SiO2; 0.67% TiO2; 18.11% Al2O3; 1.22% Fe2O3; 0.01% MnO; 1.08% MgO (in albite and trace biotite); 0.35% CaO; 7.96% Na2O; 0.81% K2O; 0.08% P2O5; 0.83% H2O (Plimer, 1976). The protolith is considered to have been sandy marine shelf sedimentary rocks with a variable evaporitic and/or hypersaline altered component. The cobaltiferous pyrite, occurs as massive intervals up to 10 m thick, and disseminated to 50m thick.
The Himalaya Formation is intruded by the Rasp Ridge Gneiss, interpreted to be a granite sill, with a maximum and minimum age of ~1683 ±3 and 1609 ±7 Ma respectively, the same age as the overlying Broken Hill Group. The Himalaya Formation is conformably overlain by the metapelite-dominated Allendale Metasediments, which is up to 1000 m thick, and is the basal unit of the Broken Hill Group. This contact marks a change from the dominant psammites of the Thackaringa Group to pelites of the lower Broken Hill Group. This formation is composed of albitic pelitic to lesser psammitic metasedimentary composite gneiss, including garnet-bearing feldspathic gneiss, sporadic mafic gneiss and quartz-gahnite rock, with the 1693 ±4 Ma Ettlewood calc-silicates at the base. These are successively followed by the up to 500 m thick Parnell Formation quartzo-feldspathic gneiss with lesser metasediments and bounding amphibolites; the up to 500 m thick Freyers Metasediments which are psammitic to pelitic and psammo-pelitic, with minor tourmaline-quartz rocks, and rare mafic gneiss and quartz-gahnite rock; and the up to 200 m thick, 1685 ±3 Ma Hores Gneiss quartz-feldspar-biotite-garnet gneiss, interpreted to be a metadacitic volcaniclastic suite with minor metasedimentary rocks. This latter unit hosts the major Broken Hill Zn-Pb-Ag deposit. The Hores Gneiss is the uppermost unit of the Broken Hill Group, and is overlain by the 1680 to 1670 Ma Sundown Group interbedded pelite, psammopelitic and psammitic metasedimentary sequence (Stevens et al., 2008).
The Broken Hill Cobalt Project deposits comprise large tonnage, stratabound, cobaltiferous-pyrite mineralisation, hosted by a quartz-albite gneiss, within an areally extensive sequence of quartz-albite-plagioclase rock of the Himalaya Formation. The mineralisation is well exposed at surface and forms prominent topographic highs of weathered pyritic albite. The pyrite content varies from disseminated to semi-massive, with the gneiss that constitutes the Mineral Resource averaging ~15% pyrite overall, ranging from ~10% at Big Hill to ~20% at Pyrite Hill (based on the S content in Mineral Resources, assuming all S is from pyrite with 53% S). The Big Hill, Railway and Pyrite Hill deposits occur as lenses of pyritic quartz-albite gneiss within a unit that has been dislocated by a series of NW-SE trending faults.
The cobalt and nickel is located within the pyrite lattice, and not present as a discrete minerals. Mineralogical studies indicate the majority of the cobalt, ~85%, is found in solid solution with primary pyrite (Henley 1998). As such, there is strong correlation between pyrite content and cobalt grade, but also between Co, Fe, S and Ni. Two pyrite generations are recognised, i). an early, coarse euhedral pyrite containing 0.35 to 0.9%
Co, but no Ni, with a grain-size typically of 0.5 to 1 mm, occurring as both massive and disseminated sulphide and constituting the bulk of the sulphides; ii). a younger and less abundant generation of finer colloform, crusty and interstitial supergene pyrite that occurs with and partially replaces minor pyrrhotite. Both of these latter sulphides contain lower cobalt but higher nickel contents of ~0.1% Co, 0.1 to 0.2% Ni. Very rare galena, sphalerite and chalcopyrite occur as minute grains and inclusions in pyrite. There is no gold, platinoids or REEs. The associated albite is typically responsible for high Na2O compositions of 6 to 10% (Pringle 2012).
The Big Hill and Railway deposits together occupy a strike length of ~3.5 km, and persist for at least 350 m down dip, and are between 20 and 300 m, averaging 70 m across strike. These deposits are largely steeply dipping and linear, although they include a complexly folded interval to the NE. The linear portion is distinguished by a distinct high grade Western Hangingwall zone. The Railway deposit to the NE, strikes NE-SW in the NE, before rotating to NNE-SSW, then across a NW-SE fault returns to a NE-SW strike as the Big Hill deposit. The Pyrite Hill deposit, which is ~2 km to the west of Big Hill, has an arcuate surface trace, from NW-SE in the north, to north-south in the south, with a strike length of ~1 km, 400 m down dip extent and between 10 and 100 m width across strike. The Pyrite Hill mineralisation dips at 60 to 70°E, whilst Railway and Big Hill vary between vertical to steeply SE. In detail, the Big Hill deposit is split into two lenses, Big Hill North which has a strike length of ~450 m, and Big Hill South which is a ~850 m long, separated and offset in a dextral sense across NW-SE faults by ~100 m. The larger Railway deposit is also separated by ~250 m, but not substantially offset from the trend of Big Hill North across similarly oriented faults.
The host is a quartz-albite-cobaltiferous pyrite gneiss, defined by the presence of disseminated pyrite that is concentrated parallel to the primary foliation in a fine grained, recrystallised quartz-albite groundmass. Where pyrite is present, there is an increase in the silica content and a virtually complete absence of biotite and sericite. There is a gradational boundary from the enclosing biotite-schist country rock to quartz-albite to pyrite-quartz-albite, taken to suggest the introduction of sulphide may have accompanied silica-sodic alteration of a micaceous schist protolith.
Two key structural controls of mineralisation have been proposed by Cobalt Blue Holdings Limited geologists, namely: i). the primary foliation (lithological banding) which acted as a fluid flow pathway and site of deposition of cobaltiferous pyrite, and ii). primary foliation parallel shear zones marking the quartz-albite gneiss margins. These latter shear zones also appear to be responsible for fold thickening of the quartz-albite gneiss. Across these shear zones, as mapped at surface, the transition from biotite schist to mineralised pyritic quartz-albite gneiss is rapid, whilst where there is no shearing at the contact, the transition is gradational. The continuity of mineralisation is strong parallel to lithological banding and where shear zones parallel the primary foliation, particularly close to the shear zones.
Heliborne electromagnetic (EM) surveys (VTEM-Max) have detected the pyritic gneiss mineralisation as conductivity anomalies and indicated lateral extensions.
Geochemically, exploration has determined that the cobalt in these deposits is depleted in the oxidised zone, which variably extends to depths of up to 35 m, with no underlying recognised supergene enrichment. Assays of gossan at Pyrite Hill versus pyritic quartz-albite gneiss 10 m below are as follows in ppm or where shown, as percent (Pringle, 2012):
Gossan - 1 Co; <0.5 Ag; 97 As; 50 Ba; 6 Bi; 23 Cr; 24 Cu; 41.3% Fe; 16 Mn; <1 Ni; 8 Pb; 0.29% S; 6 Sr; <2 Zn; 0.05% Ca; 0.01% Na; 0.02% K.
Sulphide - 867 Co; <0.5 Ag; 27 As; 40 Ba; 4 Bi; 8 Cr; 32 Cu; 12% Fe; 17 Mn; 253 Ni; 35 Pb; 12.8% S; 65 Sr; <2 Zn; 0.21% Ca; 3.05% Na; 0.12% K.
NOTE: Compare the fresh pyritic quartz-albite gneiss values with those shown earlier of Plimer (1976).
Drill core containing high grade semi-massive to massive, cobaltiferous pyrite in quartz-albite (modified image from Cobalt Blue Holdings Limited website 2022).
Mineral Resource estimates at a 275 ppm Co Equiv. cut-off (Cobalt Blue Holdings Limited ASX Release, 16 Sept 2022) were:
Pyrite Hill
Measured Resource - 18 Mt @ 1276 ppm Co Equiv., 1030 ppm Co, 191 ppm Ni, 10.9% S;
Indicated Resource - 7 Mt @ 931 ppm Co Equiv., 742 ppm Co, 191 ppm Ni, 8.5% S;
Inferred Resource - 6 Mt @ 1171 ppm Co Equiv., 943 ppm Co, 191 ppm Ni, 10.1% S;
TOTAL Resource - 31 Mt @ 1176 ppm Co Equiv., 946 ppm Co, 191 ppm Ni, 10.2% S;
Big Hill
Indicated Resource - 11 Mt @ 742 ppm Co Equiv., 604 ppm Co, 129 ppm Ni, 5.8% S;
Inferred Resource - 7 Mt @ 655 ppm Co Equiv., 529 ppm Co, 105 ppm Ni, 5.5% S;
TOTAL Resource - 19 Mt @ 707 ppm Co Equiv., 574 ppm Co, 119 ppm Ni, 5.6% S;
Railway
Indicated Resource - 41 Mt @ 775 ppm Co Equiv., 619 ppm Co, 118 ppm Ni, 6.9% S;
Inferred Resource - 28 Mt @ 727 ppm Co Equiv., 571 ppm Co, 116 ppm Ni, 7.0% S;
TOTAL Resource - 68 Mt @ 755 ppm Co Equiv., 599 ppm Co, 118 ppm Ni, 6.9% S;
COMBINED
Measured Resource - 18 Mt @ 1276 ppm Co Equiv., 1030 ppm Co, 191 ppm Ni, 10.9% S;
Indicated Resource - 59 Mt @ 788 ppm Co Equiv., 631 ppm Co, 123 ppm Ni, 6.9% S;
Inferred Resource - 41 Mt @ 781 ppm Co Equiv., 619 ppm Co, 123 ppm Ni, 7.2% S;
TOTAL Resource - 118 Mt @ 859 ppm Co Equiv., 687 ppm Co, 133 ppm Ni, 7.6% S, containing 81 100 tonnes of Cobalt.
Metallurgical testwork has successfully produced a Mixed Hydroxide Product (MHP) containing 7% Ni and 38% Co with an 85 to 90% recovery of Co within the ore (Cobalt Blue Holdings Limited ASX Release, 16 Sept 2022). In 2022, the project was operated by the Broken Hill Cobalt Project Pty Ltd which is a wholly owned subsidiary of Cobalt Blue Holdings Limited.
The geological overview in the summary above is drawn from a range of sources, augmented by the Geoscience Australia, Australian Stratigraphic Units Database, while the deposit description and resource figures are from Cobalt Blue Holdings Limited ASX Releases, 16 Sept 2022 and earlier, except as shown otherwise.
History - The Pyrite Hill and Big Hill prospects have been intermittently explored since their discovery in 1885, with a major hiatus from 1889 to 1950 when there was no significant activity. Exploration commenced in earnest from the late 1960s and continued through a number of joint ventures and options
until ~2000 when Broken Hill Prospecting Limited gained control of titles over the area. There have been a combined total of 35 holes drilled at the Pyrite Hill and Big Hill prospects in that period by: a Metals Exploration-Central Austin joint venture at Pyrite Hill between 1967 and 1970; Broken Hill South at Big Hill in 1976; CRA Exploration at Pyrite Hill and Big Hill in 1980-81; Macedon Gold Mines at Pyrite Hill in 1993; and Hunter Exploration at Big Hill in 1998. Better intersections over this period ranged from true thicknesses of 5.5 m @ 0.28% Co to 64.4 m @ 0.12% Co at Pyrite Hill and 6.1 m @ 0.24% Co to 34.15 m @ 0.22% Co at Big Hill (after Simon, 1981). The same and other companies undertook regional and proximal exploration for extensions. Much of this exploration, including that of Broken Hill Prospecting, was primarily looking for Broken Hill type Zn-Pb-Au mineralisation that may have been related to the sulphide accumulations at these prospects or in the overlying Broken Hill Group. Never-the-less, exploration on these prospects has primarily been for large tonnage cobaltiferous pyrite deposits. As of 2010, the known resources were: Big Hill - 10.6 Mt @ 980 ppm Co at 0.5 ppm Co cut off; and at Pyrite Hill - 4.4 Mt @ 893 ppm Co at the same cut off. In 2016-17, after drilling had better defined and expanded the cobalt resource, Broken Hill Prospecting 'spun-off' Cobalt Blue Holdings Limited to earn an interest in its cobalt projects, which was fully realised by December 2019 when the latter acquired 100% of the project. A positive optimised Pre-Feasibility Study was received in 2020 and a Bankable Feasibility Study scheduled for completion by early 2023. History to 2010 from Hill, Randell and Border of Geos Mining (2010).
The most recent source geological information used to prepare this decription was dated: 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.
Pyrite Hill
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Selected References:
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Stevens, B.P.J., Page, R.W. and Crooks, A., 2008 - Geochronology of Willyama Supergroup metavolcanics, metasediments and contemporaneous intrusions, Broken Hill, Australia: in Australian J. of Earth Sciences v.55, pp. 301-330.
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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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