|
Savannah, Savannah North, Sally Malay |
|
|
Western Australia, WA, Australia |
| Main commodities:
Ni Cu Co PGE PGM
|
|
 |
|
|
 |
Super Porphyry Cu and Au
|
IOCG Deposits - 70 papers
|
All papers now Open Access.
Available as Full Text for direct download or on request. |
|
|
The Savannah, formerly Sally Malay, Ni-Cu-Co deposit cluster is located ~110 km NNE of Halls Creek and ~240 km south of Kununurra, in the East Kimberley region of Western Australia. It is composed of a group of mineralised intrusions, including Savannah and Savannah North, and a number petrologically similar mafic-ultramafic intrusions at Sub-chamber D, Dave Hill and Wilson’s Creek without known mineralisation (#Location: 17° 21' 18"S, 128° 1' 50"E).
Mineralisation in the Sally Malay Suite was discovered by Anglo American in 1973. Drilling between 1974 and 1978 culminated in a published resource of 7.2 Mt @ 2.15% Ni to a vertical depth of 900 m. In 1989, Normandy Poseidon acquired the interests around Sally Malay and undertook further exploration and development studies during the 1990s. Panoramic Resources, formerly Sally Malay Mining Ltd, acquired the project in March 2001. Additional drilling, and a bankable feasibility study, led to construction of the Savannah mine and processing facility commencing in August 2003 based on a premining resource of 3.74 Mt @ 1.74% Ni, 0.72% Cu, 0.09% Co. Mining of nickel copper and cobalt commenced in 2004. An open pit was mined for the first 18 months before mining transitioned underground. Operations continued until 2015 when lower nickel prices forced the closure of the operation which was put on care and maintenance. A drilling campaign at depth and to the north had commenced during 2013, aimed at finding the continuation of the Savannah Intrusion which was offset below one of two large thrust reverse faults in the mine area (known as the 500 and 900 faults). This resulted in the discovery of the Savannah North deposit in 2015 with a maiden Mineral Resource established later that year. Following further resource upgrades and technical studies, a decision to restart the operation at Savannah and extend underground workings to Savannah North was made in 2018. First concentrate shipment occurred in early 2019. Operations were suspended in April 2020 amidst the ongoing impact of the global pandemic and the falling nickel prices, having produced 0.18 Mt of nickel, 60 000 t of copper and 5000 t of cobalt. After this operational suspension, an extensive technical and economic assessment was undertaken, including a revised Mineral Resource and Mine Plan. The operation was successfully re-started in 2021 with the first shipment of concentrate leaving the port of Wyndham in December 2021. However, after nickel prices again declined, the operator, Panoramic Resources, went into voluntary administation in December 2023 and the operation was suspended in January 2024.
Regional Setting
The Palaeoproterozoic rocks of the East Kimberley region are part of the Halls Creek Orogen, formed between the proto-Kimberley and Tanami Cratons during the Halls Creek Orogeny (Hoatson and Blake, 2000). This crustal accretion episode was characterised by widespread volcanism and the emplacement of pre-, syn- and post-tectonic granitic to ultramafic intrusive complexes over an interval of several hundred kilometres between ~1865 and 1805 Ma. The major orogenic peak of this event was marked by widespread granitic intrusion between ~1835 and 1805 Ma. The orogen is divided into the eastern, central and western tectonostratigraphic terranes by a series of NE trending strike-slip faults. These terranes represent three distinct structural blocks of differing metamorphic histories, juxtaposed by complex faulting. The Savannah nickel deposit falls within the central terrane , bounded by the Halls Creek, Angelo
and Osmond Faults to the east, and the Ramsay Range and Springvale Faults to the west. The dominant and oldest unit within this terrane is the Tickalara Metamorphics, a deformed succession of intercalated mafic meta-volcanic and meta-sedimentary rocks, the regional metamorphic grade of which increases from SW to NE, represented by granulite facies garnet-cordierite-sillimanite paragneisses at Savannah (Hicks et al., 2017).
The Tickalara Metamorphics have a maximum depositional age of 1870 to 1865 Ma (Page and Hoatson, 2000). Numerous felsic and mafic-ultramafic intrusions were emplaced in the central zone in multiple pulses between ~1860 and1830 Ma, straddling a series of metamorphic events. The latter include a peak granulite facies event at ~1852 Ma (Page and Sun, 1994); followed by the ~1835 Ma Hall Creek Orogeny associated with amphibolite facies metamorphism; and a regional event associated with the 100 x 8 km Mabel Downs Tonalite located 5 km to the east of Savannah (Hicks et al., 2017).
The Savannah North intrusion appears to have preserved its original subhorizontal layering, whilst the Savannah intrusion has a near vertical orientation of both the cumulus layering and of the ore deposit. As a consequence, Savannah North has been inferred to be younger and not structurally rotated, although U-Pb zircon geochronology (SHRIMP 11 sensitive high-resolution ion micro probe 2 facility at Curtin University quoted by Hicks et al., 2017) indicates both intrusions were emplaced within the error of the U-Pb age determination of ~1845 to 1840 Ma. This has been interpreted to imply syntectonic emplacement of both intrusions on a structure with a large component of rotation (Hicks et al., 2017). All of the magmatic phases at Savannah intrude granulite paragneisses and mafic granulite of the Tickalara Metamorphics. Around Savannah, the paragneiss is interpreted ot have been derived from sub-aluminous pelite and psammo-pelite, whilst the igneous protoliths of the orthopyroxene clinoproxene plagioclase rich mafic-ultramafic granulite at Savannah have compositions indicating original dolerite or gabbroic dykes and sills (Hicks et al., 2017).
Savannah Intrusion
The 1.75 x 0.35 km elongated Savannah mafic to ultramafic intrusive complex dips subvertically. It has well-preserved primary magmatic cumulus textures and a broad fractionation trend, progressing from a lower harzburgite-peridotite zone to the south, overlain by clinopyroxene-orthopyroxene troctolite and olivine norite, followed by a narrowing, norite-dominated tail zone in the north. All of the lithologies are very fresh with only minor alteration of pyroxene to amphibole and plagioclase to white mica (Hicks et al., 2017).
The southern ultramafic zone has 50 to 55% olivine with variable 10 to 25% plagioclase, ~15% orthopyroxene and 5 to 10% hornblende. Whilst this ultramafic zone locally contains troctolitic rocks, most appear to contain <10% plagioclase. Minor phases contain biotite and discrete grains of green spinel. Narrow orthopyroxene and symplectite rims composed of amphibole and green spinel are relatively common along olivine-plagioclase grain contacts. In the central section of the Savannah intrusion, between the ultramafic and norite dominant tail zone to the north, hornblende after clinopyroxene is more prevalent (Hicks et al., 2017).
The igneous layering is preserved in a sub-vertical orientation in outcrop, immediately to the north where it is cut by the Turkey Creek Gabbro. Supported by the vertical plunge of the orebody, the intrusion is interpreted to have been tilted by almost 90° along a ~NW-SE axis (Hicks et al., 2017). These observations led to the interpretation that the Savannah intrusion as a bladed dyke-type intrusion with mineralisation occupying the keel (Barnes et al., 2016).
Primary mineralisation within the Savannah intrusion typically occurs as magmatic sulphides, predominantly pyrrhotite-pentlandite-chalcopyrite, hosted by a thin, contaminated marginal noritic unit, located at the contact between the basal peridotite section of the intrusion and the Tickalara Metamorphics country rocks. This mineralisation occurs as either discrete lenses of massive sulphide, or as polymict magmatic sulphide-matrix breccias. The latter are composed of a network of massive to semi-massive sulphides enclosing angular to subrounded clasts of the host marginal norite and of diverse lithologies derived from the Tickalara paragneiss country rock. The marginal norite clasts may, in turn, contain coarse blebby to disseminated sulphides. The discrete massive sulphide rich lenses may also enclose noritic clasts, although these are typically much smaller and more rounded. Subordinate and generally sub-economic stringer, matrix and coarse blebby sulphide mineralisation is also frequently developed in the marginal norite unit surrounding the primary mineralisation. Sulphide mineralisation within the overlying peridotite is restricted to broad zones containing 2 to 3% fine-grained disseminated magmatic nickel sulphides and the occasional thin massive sulphide lens located near to the contact with the underlying marginal norite (Hicks et al., 2017).
The mineralised marginal norite is typically 3 to 40 m thick and is predominantly composed of a plagioclase-orthopyroxene rock containing <10% biotite hornblende, ilmenite and magnetite. Pre-mining, it was exposed as a persistent, subvertically south-dipping limonite-goethite rich gossan that was ~250 m long, with a base of oxidation that was typically at a depth of 10 and 30 m below the surface. Particularly near to the contact with the enclosing Tickalara Metamorphics, the marginal norite contains heterogeneous clasts of stoped, partially resorbed country rock xenoliths and thin veinlets of sodic plagioclase, orthopyroxene, phlogopite, garnet and quartz. These are interpreted to reflect buoyancy-driven, fracture-filled zones of melted Tickalara Metamorphics. The overlying peridotite unit can either have a sharp contact, or reflect a gradual increase in olivine content (Hicks et al., 2017).
The western splay of the Savannah deposit comprises a persistent lens of predominantly massive sulphide mineralisation and marginal norite that extends westwards for ~200 m from the Savannah intrusion into the Tickalara Metamorphics. This splay progressively tapers towards the west until the marginal norite host disappears and the splay grades into a thinner, less well developed, but consistent, zone of semi-massive sulphide mineralisation within the Tickalara Metamorphics. Based on underground observations the western splay is interpreted to have been emplaced along a pre-existing zone of weakness, simultaneously with the emplacement of the Savannah intrusion marginal norite unit (Hicks et al., 2017).
The grade of the primary mineralisation within the Savannah intrusion varies, depending on the intensity of the primary breccia textured mineralisation, from 1.5 → 2.7% Ni, 0.5% → 1.5% Cu and 0.1 → 0.2% Co. The sulphide tenors are 3.7 → 4.8% Ni, 0.2 →7.2% Cu, 0.18 → 0.23% Co, 170 → 370 ppb Pd and 5 → 30 ppb Pt. NOTE: The sulphide tenor is, the metal content in 100% sulphides, with the ranges quoted being the tenth and nintieth percentile values. The very high ratio of Pd relative to the other platinum group elements is a distinctive and unusual feature of this intrusion (Hicks et al., 2017).
Savannah North Intrusion
The Savannah North intrusion occurs directly to the north and NW of the Savannah mine. It is exposed as a 1.25 km2 complex of interlayered norite, troctolite and olivine gabbronorite, and is interpreted to have been emplaced along an east-west corridor over a strike length of >2 km. It's base dips at 10 to 20°N, interpreted to suggest gentle post emplacement tilting. From a depth of ~900 m, just to the east of the Savannah mine, this basal contact plunges to the west to ~1300 m below surface over a distances of 1 km to the west (Hicks et al., 2017).
The Savannah North intrusion, as at Savannah, retains well-preserved primary cumulus textures, reflecting an upwardly fractionated cumulate sequence, with olivine increasing in prevalence with depth. These cumulate layers, which can have sharp, but often gradational contacts, typically have a gentle ~20°N dip, consistent with the apparent tilt of the intrusion. An up to 20 m thick contaminated chilled zone consistently occurs on the margins of the Savannah North intrusion at the contact with the enclosing para-migmatite and mafic granulites of the Tickalara Metamophics. This chilled zone is dominantly composed of equigranular, fine- to medium-grained gabbroic to gabbronoritic composition rocks. The contact between the Savannah North intrusion and enclosing Tickalara Metamorphics is also occupied by younger mafic to aplitic dykes, particularly evident where the base of the Savannah North intrusion is proximal to the 500 fault, one of the large reverse/thrust faults at the base of the Savannah Intrusion (Hicks et al., 2017).
The Savannah North intrusion is interpreted to represent a funnel-shaped mineralised mafic intrusion (Barnes et al., 2016) still in its original orientation, slightly tilted to the north, with massive sulphide mineralisation mainly concentrated at its base (Hicks et al., 2017). The magmatic Ni-Cu-Co sulphide mineralisation within the Savannah North intrusion is divided into an upper and a lower zone. The Upper Zone is more or less developed at the base of the intrusion, whilst the Lower Zone occurs within the Tickalara Metamorphics, below the intrusion. Both typically comprise magmatic breccia textured pyrrhotite-pentlandite-chalcopyrite rich massive sulphides, that are very similar, both compositionally and texturally to the primary mineralisation within the Savannah intrusion (Hicks et al., 2017).
• Upper Zone - This zone of sulphide mineralisation is typically found within the contaminated gabbronorite chilled margin at the base of the intrusion, where it is in contact with the underlying Tickalara Metamorphics. However, it can also form discrete lenses above the contact. Mineralisation is developed over a 300 to 350 m interval along the contact, attaining maximum thicknesses of 10 to 15 m. Where not contaminated by later dykes, mineralisation typically grades from 2.0 → 2.7% Ni, 0.5 → 1.5% Cu and 0.1 → 0.2% Co. Although very similar to the Savannah orebody, the upper zone has somewhat lower sulphide tenors of 2.8 → 3.3% Ni, 0.8 → 2.2% Cu, 0.21 → 0.23% Co, 80 → 180 ppb Pd and 5 → 125 ppb Pt. The marked difference in sulphide composition, particularly in Ni and Pd, supports the conclusion that the Savannah North orebody is distinct from the Savannah body and not a tectonically displaced slice of the same deposit. Westward from tne main mineralised interval, the Upper Zone appears to migrate to be 20 to 30 m above the basal contact of the intrusion, localised at the contact between the base of the peridotite unit and the underlying chilled gabbronorite zone. Here, mineralisation comprises a 0 to 20 m thick zone of 20 to 40% matrix mineralisation, with thin intervals of coarsely blebby to semi-massive to massive sulphides occurring within the matrix sulphides. The upper zone matrix mineralisation is typically of lower grade at 0.90 → 1.2% Ni, 0.40 → 0.60% Cu and O.05 → 0.06% Co (Hicks et al., 2017).
• Lower Zone - This mineralised interval is very similar to the Upper Zone, but comprises a series of pyrrhotite-pentlandite-chalcopyrite rich massive sulphide lenses developed within the Tickalara Metamorphics beneath the Savannah North intrusion. These lenses dip at ~50°NW and are typically more massive, and slightly higher grade, with fewer breccia clasts. Mafic and aplite dykes, similar to those within the 500 and 900 reverse fault structures, are often found proximal to the Lower Zone mineralisation and may suggest a potential link between sulphide development and faulting.
• Other Mineralisation - In addition to the mineralised zones described above, broad zones of 3 to 5% fine-grained disseminated magmatic nickel sulphides occur in the thick peridotite zone at the base of the intrusion. This mineralisation is typically pyrrhotite-rich and grades contains between 0.15 and 0.35% Ni. Small areas of disseminated and coarse blebby mineralisation, occasionally accompanied by thin widths of semi-massive to massive sulphides, are found in the contaminated chilled zone surrounding the upper zone. Though not as well developed as the primary Upper Zone mineralisation, these zones do locally form economical widths and grade. Distal to the Upper Zone the contaminated chilled zone is significantly less mineralised (Hicks et al., 2017).
Reserves and Resources
The resources published in 2001 totalled: 3.736 Mt @ 1.74% Ni, 0.72% Cu, 0.09% Co, 2.16 g/t PGE to the 500 Fault.
Hicks et al. (2017) quote Resources as follows:
Savannah North - 10.272 Mt @ 1.70% Ni, 0.72% Cu, 0.12% Co;
Savannah - 2.939 Mt @ 1.47% Ni, 0.84% Cu, 0.08% Co;
Savannah Production - 9.00 Mt @ 1.30% Ni, 0.66% Cu, 0.07% Co.
Panoramic Resources (ASX Announcement, 2020) quote JORC Compliant Mineral Resources as follows at a 0.50% Ni cut-off:
Savannah, above the 900 Fault - Measured + Indicated + Inferred Resource - 1.575 Mt @ 1.56% Ni, 1.03% Cu, 0.07% Co;
Savannah, below the 900 Fault - Measured + Indicated + Inferred Resource - 0.905 Mt @ 1.65% Ni, 0.76% Cu, 0.10% Co;
Savannah North, Upper Zone - Measured + Indicated + Inferred Resource - 6.434 Mt @ 1.40% Ni, 0.56% Cu, 0.09% Co;
Savannah North, Lower Zone - Measured + Indicated + Inferred Resource - 3.612 Mt @ 1.79% Ni, 0.85% Cu, 0.12% Co;
Savannah North, Other - Measured + Indicated + Inferred Resource - 0.93 Mt @ 1.66% Ni, 0.47% Cu, 0.11% Co;
TOTAL - Measured + Indicated + Inferred Resource - 13.456 Mt @ 1.56% Ni, 0.70% Cu, 0.1% Co.
Panoramic Resources (ASX Announcement, 2020) quote JORC Compliant Ore Reserves as follows at a 0.50% Ni cut-off:
Savannah, Proved + Probable Reserves - 1.233 Mt @ 0.95% Ni, 0.66% Cu, 0.05% Co;
Savannah North, Proved + Probable Reserves - 7.041 Mt @ 1.28% Ni, 0.57% Cu, 0.09% Co;
TOTAL - , Proved + Probable Reserves - 8.274 Mt @ 1.23% Ni, 0.59% Cu, 0.08% Co.
The information in this description is largely taken from Hicks et al. (2017), as cited below.
The most recent source geological information used to prepare this decription was dated: 2017.
Record last updated: 12/1/2024
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.
Savannah Operation
|
|
|
|
Hicks, J.D., Barnes, S.J., Le Vaillant, M. and Mole, D.R., 2017 - Savannah nickel sulfide deposits: in Phillips, G.N., (Ed.), 2017 Australian Ore Deposits, The Australasian Institute of Mining and Metallurgy, Mono 32, pp. 435-440.
|
|
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.
|
Top | Search Again | PGC Home | Terms & Conditions
|
|
