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Sorby Hills - Norton, Omega, Alpha, Beta |
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Western Australia, WA, Australia |
| Main commodities:
Pb Zn Ag
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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The Sorby Hills deposit comprises a zone of sub-economic pods of low grade Pb-Ag-Zn mineralisation hosted by lower Carboniferous carbonates in the southern onshore section of the Bonaparte Basin in far northern Western Australia (#Location: 15° 26' 26"S, 128° 58' 10"E).
Reports of minor Pb/Zn mineralisation, both at surface and in a petroleum well, in the Sorby Hills area were followed up and led to the location of the 'Discovery Hill' gossan by Aquitaine Australia Minerals Pty Ltd in 1971. Several other small gossans were also located in the same area. Drilling around the Discovery Hill gossan failed to locate any significant mineralisation. Systematic auger drilling to bedrock on a 400 x 200 m grid defined a strong geochemical anomaly along the strike trend of the Sorby Hills Dolomite Member east and north-east of Discovery Hill. Detailed IP, aeromagnetics and drilling was undertaken between 1974 and 1977 resulting in the delineation of thirteen pods of mineralisation over an 8 km north-south strike length. Close spaced drilling revealed that the mineralisation was confined to stratabound breccias with an irregular distribution. Underground testing was attempted in 1978 to 79 but had to be abandoned due to poor ground conditions, excess inflow of water, and consequent flooding. Further drilling between 1981 and 1984 did not substantially modify the resource estimate of,
16.42 Mt @ 5.25% Pb, 0.6% Zn, 56 g/t Ag,
at a cut-off of 2.5% Pb+Zn (Jorgensen, et al., 1990). As of 1990, the mineralised pods were known to be distributed in four areas along two major NNE trends, paralleling the main Halls Creek Mobile Belt faults and the eastern margin of the Bonaparte Basin. Individual pods, as outlined at that stage, averaged around 500 000 t, with the largest, Sandy Creek, totalling 3.3 Mt @ 4.4% Pb, 2.5% Zn, 15 g/t Ag using a 3% Pb+Zn cutoff.
Subsequent exploration and evaluation drilling to 2021 has outlined a resources, from north to south, in a 7.5 km long, concave to the west crescent shaped trend, comprising the Alpha, Beta, Norton, Omega/Omega South, B and A pods, with sub-economic mineralisation linking all seven. These have JORC Compliant Mineral Resources as follows, at a 1.0% Pb cut-off (Boab Metals Limited, ASX announcement, 17 December, 2021):
Alpha - Indicated + Inferred Mineral Resource - 1.5 Mt @ 3.1% Pb, 0.9% Zn, 64 g/t Ag;
Beta - Indicated + Inferred Mineral Resource - 4.2 Mt @ 3.6% Pb, 0.4% Zn, 43 g/t Ag;
Norton - Measured + Indicated + Inferred Mineral Resource - 21.1 Mt @ 2.8% Pb, 0.4% Zn, 64 g/t Ag;
Omega - Measured + Indicated + Inferred Mineral Resource - 17.2 Mt @ 3.3% Pb, 0.4% Zn, 34 g/t Ag;
A - Inferred Mineral Resource - 0.6 Mt @ 5.3% Pb, 0.1% Zn, 23 g/t Ag;
B - Measured + Indicated Mineral Resource - 2.7 Mt @ 3.6% Pb, 0.3% Zn, 20 g/t Ag;
TOTAL - Measured + Indicated + Inferred Mineral Resource - 47.3 Mt @ 3.1% Pb, 0.4% Zn, 35 g/t Ag.
JORC compliant Ore Reserves at a 1% Pb cut-off (Boab Metals Limited, ASX announcement, 19 January 2023) were:
Proved Ore Reserve - 10.4 Mt @ 3.5% Pb, 42 g/t Ag;
Probable Ore Reserve - 4.9 Mt @ 3.5% Pb, 322 g/t Ag;
TOTAL Proved + Probable Ore Reserves - 15.2 Mt @ 3.5% Pb, 39 g/t Ag.
The following is summarised from Jorgensen, et al. (1990), with more recent updates from Boab Metals Limited (2023) and the Geoscience Australia stratigraphic database.
Geology
The onshore Bonaparte Basin represents the north-south trending southern extremity of the overall NW-SE elongated basin where it has been attenuated into the Halls Creek Mobile Belt direction. The onshore section of the basin contains up to 5000 m of Cambrian to Permian sedimentary rocks which unconformably overlie the Proterozoic Kimberley Block to the west and are faulted against the Sturt Block to the east across sinistral late Devonian to lower Carboniferous faults of the Halls Creek Mobile Zone. As such, the Late Palaeozoic Devonian to Carboniferous, 380 to 320 Ma rocks of the Bonaparte Basin were deposited on the margin of a rift basin, bounded by Precambrian basement.
The Precambrian Pincombe Inlier in the southern section of the basin represents a NNE-SSW trending basement high which was largely emergent during the late Devonian to early Carboniferous and separated the Burt Range Sub-basin in the east from the Carlton Sub-basin in the west. The Sorby Hills mineralisation is found on the margins of this inlier.
The stratigraphic succession in the Sorby Hills area is as follows from the base,
Early Cambrian
• Antrim Plateau Volcanics up to 150 m thick - tholeiitic basalts which outcrop on the western side of the Sorby Hills.
Unconformity
Late Devonian
• Cockatoo Group, subdivided into the,
- Kelly's Knob Sandstone, up to 300 m thick - medium to coarse grained sandstone with conglomerate at the base,
- Abney Sandstone, up to 100 m thick - green and pink siltstone and medium grained arkosic arenite with common pebbly bands and pyrite nodules.
• Ningbang Group, including the,
- Buttons Formation, up to 300 m thick - brown and light grey-green massive crystalline dolomite, dolomicrite and minor siltstone. This unit is usually sandy at the base and represents dolomitised sandy and silty limestone deposited in a shallow marine, lagoonal or back reef environment. Entire reef complexes are represented to the east in the Nigbing Ranges area, but not at Sorby Hills.
Disconformity
Lower Carboniferous
• Burt Range Formation, comprises a series of dolomitised limestone and clastic units and represents continued marine deposition from the late Devonian. It has been subdivided into,
- Sorby Dolomite Member, up to 100 m thick - moderately developed cycles each of 3 to 5 m thick consisting of un-fossiliferous vuggy dolostone and dolomitic siltstone,
- DII Unit, 15 to 20 m thick - a breccia zone at the base of the Knox Member which generally hosts the mineralisation especially in the central Sorby area. The breccia has both tectonic and sedimentary characteristics and is made up of reworked clasts from the underlying Sorby Dolomite Member and the overlying Knox Siltstone Member,
- Knox Siltstone Member, up to 80 m thick - a transgressive clastic unit of dolomitic siltstone and shale which disconformably overlies the Sorby Dolomite Member,
- DII Unit, up to 15 m thick - a thin basal bioclastic marker found below the Gooseneck Member,
- Gooseneck Member, up to 100 m thick - bioclastic limestone, sandy dolostone and siltstone, characterised by common nodular bedding, and is locally transgressive over the Cockatoo Group in the North Sorby area where it also hosts significant mineralisation.
• Septimus Limestone, up to 50 m thick - a largely unaltered limestone sequence with a high proportion of siltstone and shale near the base.
• Weaber Group, including the,
- Milligans Formation, up to 300 m thick - black shale, transgressively overlying the Septimus Limestone in the central and southern Sorby areas, the Burt Range Formation in the northern Sorby area and the Cockatoo Group to the north-west of Sorby Hills.
The palaeo-depositional setting of the host sequence is as follows, after Boab Metals Limited. The Devonian-Carboniferous sea is interpreted to have inundated an incised coastline around the pronounced Pincombe basement palaeo-high. The oldest sedimentary rocks comprised a basal clastic suite, including gravel beds onlapping onto the basement inlier to the east. A thin succession of intertidal, and shallow marine to lagoonal facies were initially deposited within the deposit area while eastwards, in the Burt Range Sub-basin, the basin floor subsided rapidly to accumulate >2 km of marine sedimentary rocks, as indicated by seismic data. The articulated paleao-coast line is best seen W of the Norton deposit where an embayment of Devonian strata extends several kilometres to the SW, over the basement rocks.
Prior to the onset of Carboniferous deposition, the palaeo-topography appears to have been removed and a marine platfrom established, with correlation between down-hole gamma profiles showing a gradual thickening of Carboniferous strata towards the east in the deposit area and surrounds.
Mineralisation
The Sorby Hills mineralisation occurs in a transitional facies, the DII Unit, between the Knox Siltstone Member in the hanging wall and the Sorby Dolomite Member in the footwall. At Sorby Hills, this DII Unit Transition Facies, is ~20 to 25 m thick, consisting of 1 to 2 m thick cyclic bedded, beds of massive dolostone, silty dolomite and clay matrix breccias in the immediate footwall of the Knox Siltstone Member and the uppermost interval of the Sorby Dolomite Member. A massive micritic fossiliferous dolostone interval is located in the hanging wall. Strata generally dip shallowly, but variably to the east, southeast and northeast. The seven mineralised pods that form the 7 km long, curvi-linear north-south trending belt, as listed above, are sub-parallel to the eastern margin of the Precambrian Pincombe Inlier, and are linked by sub-economic mineralisation.
The extensive dolomitisation of the host is inferred to be genetically related to the mineralisation and has completely recrystallised the primary calcite-rich rocks, creating porosity estimated to be up to 15% in part of the Sorby Dolomite (Ferguson 1999). A secondary dolomitisation event is indicated by coarse dolostone in fracture linings and vughs.
The base metal mineralisation at Sorby Hills is incorporated in thirteen irregularly shaped pods located in four groups over a north-south strike length of 8 km. Several of these pods, which have an average thickness of 7 m, are connected by lower grade mineralisation. Some of the pods had not been closed off as at 1988 despite concentrated 100 m centres drilling.
The Sorby Hills mineralised pods, as known in 1990 were sub-divided into four groups based on their structure and styles of mineralisation, as follows,
• Pods A to G inclusive as identified in 1990, but by 2021 comprised the separate pods A and B, whilst C is Omega South, and D to G inclusive are the Omega orebody - together these comprise half the length of the Sorby Hills mineralised strike interval. These pods are all found within the DII Unit breccias which have been interpreted as occupying palaeo-channels cutting the underlying Sorby Dolomite Member.
• Pods H, I & J as known in 1990, were amalgamated by 2021 as the Norton orebody. These are also within the DII Unit, but are believed to be controlled by a series of broad Precambrian palaeo-highs reflected as relatively flat lying anticlines in the Burt Range Formation. The I Pod with 2.9 Mt (in 1990) is the largest.
• The Beta Pods - where mineralisation is largely found in association with tensional collapse breccias within the DII Unit and locally throughout the whole Burt Range Formation.
• The Alpha Pod - which is the northernmost of the main pods and lies adjacent to the north-eastern part of the Pincombe Inlier. Mineralisation is associated with both tectonic and sedimentary breccias found along the faulted margin of the inlier. The DII Unit is the most important host.
The ore mineralogy is simple with the dominant minerals being coarse grained galena and finer grained sphalerite in an average ratio of 10:1. Minor amounts of pyrargyrite (the main Ag mineral), tetrahedrite-tennantite and bournoniteare also present found mainly as inclusions in galena. Common gangue minerals include marcasite, pyrite, calcite and crystalline dolostone. The only zoning evident is in the preference of Zn for lower stratigraphic levels and the lateral margins of the pods.
There are apparently three main textural types of mineralisation representing three stages of paragenesis, as follows,
• The first and earliest type characterised by mineralised sedimentary breccia, usually within silty dolostone. Galena is the only base metal mineral, occurring as coarse grained cubes within the silty matrix of the breccia. This is the earliest and most widespread of the three styles.
• The second represents an intermediate phase occurring as coarse grained octahedral galena filling fractures and pores and as the matrix in the tectonic breccias. Minor colloform sphalerite may also fill the fractures. This style is the most abundant and can be massive over thicknesses of up to 1.5 m in places.
• The third and more minor type is represented by dominant sphalerite and some galena lining fractures and solution cavities which may cut earlier breccias and other structures.
Numerous oil and gas occurrences are known within the Palaeozoic sediments of the Bonaparte Basin. There is an extensive system of tensional faults that flank the Pincombe Inlier immediately to the west of the line of mineralised pods. In addition there is a well developed, diagenetically produced permeability in the carbonates of the host sequence due to the extensive dolomitisation of the limestones of the succession.
For detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 2021.
Record last updated: 7/8/2023
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.
Sorby Hills
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Selected References:
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Jorgensen G C, Dendle P K, Rowley M, Lee R J 1990 - Sorby Lead-Zinc-Silver deposit: in Hughes F E (Ed.), 1990 Geology of the Mineral Deposits of Australia & Papua New Guinea The AusIMM, Melbourne Mono 14, v2 pp 1097-1101
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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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