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Sorby Hills

Western Australia, WA, Australia

Main commodities: Pb Zn Ag
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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.

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 an 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 failed due to bad ground conditions and flooding. Further drilling between1981 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. The mineralised pods are 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 average 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.

The following is summarised from Jorgensen, etal., (1990).

Geology

The onshore Bonaparte Basin represents a north-south trending extremity of the general NW-SE basin attenuated into the Halls Creek Mobile Belt direction. The onshore section of the basin contains up to 5000 m of Cambrian to Permian sediments 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.

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 two sub-basins. 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, - Kellyıs 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.

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 dolomite 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,

- DIII 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 dolomite 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.

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.

Mineralisation

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 are sub-divided into four groups based on their structure and styles of mineralisation, as follows,

Pods A to G inclusive - which comprise half the strike 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 - which 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 is the largest.

The Beta Pods - where mineralisation is largely found in association with tensional collapse breccias within the DIII 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 DIII 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 dolomite. 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 dolomite. 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 are 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 summary was dated: 1996.    
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.


  References & Additional Information
   Selected References:
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


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 takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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