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Thunderbird

Western Australia, WA, Australia

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The Thunderbird heavy mineral sand deposit lies ~70 km west of the port of Derby, within Western Australia’s Kimberley region.

It lies within the Canning Basin, an intracratonic depository covering approximately 506 000 km2, of which 430 000 km2 are onshore (Forman and Wales 1981 Forman, D. J. and Wales, D. W. 1981). It consists of two NW-elongated troughs, each containing up to 15 km of predominantly Palaeozoic rocks, separated by a mid-basinal arch, where less than 2 km of sediment are preserved (Shaw, Sexton and Zeilinger 1994). This sequence spanned the period from the Ordovician to mid Triassic.

Both troughs and the mid-basinal arch are blanketed by a thinner late Mesozoic sequence comprising mainly shallow-marine sediments which host the heavy mineral sand deposit. This sequence commenced with a Late Jurassic transgression which led to the deposition of sand and mud in a marine environment above a depositional break and unconformity, including the Middle Jurassic Wallal Sandstone, Alexander Formation sandstone, interbedded mudstone and the Jarlemai Siltstone, capped by another hiatus and unconformity. A subsequent Early Cretaceous regression resulted in the deposition of the Lower Cretaceous Broome Sandstone in shallow-marine environment. As the regression continued, the Melligo Sandstone and its equivalents were deposited throughout the basin. In the Cainozoic, laterisation occurred and a variety of thin deposits of shoreline, alluvial, lacustrine and aeolian material was deposited across the Canning Basin (Towner and Gibson 1983).

The stratigraphic units at Thunderbird comprise sand units of the Upper Jurassic to the Lower Cretaceous, including the Jarlemai Siltstone, the Broome Sandstone and the Melligo Sandstone, which regionally lie on the flank of the Baskerville Anticline, a broad, west to NW-trending anticline (Smith, 1992).

The Jarlemai Siltstone has been dated as Upper Jurassic but may extend up to the Early Cretaceous (Crowe, Towner and Gibson 1978). It was deposited at the height of the Jurassic-Cretaceous marine transgression, varying from siltstone to claystone and sandstone, and is glauconitic to ferruginous in part (Towner and Gibson 1983).

The Broome Sandstone includes a variety of sandstone lithologies and sedimentary structures, interpreted to indicate deposition in a shallow-marine (tidal) environment as the Early Cretaceous sea regressed (Towner and Gibson 1980). Lithology varies from a fine to very coarse sandstone to a mudstone with some minor conglomerate, and common sedimentary structures such as ripple-marks, cross-bedding and bioturbation. Prior to the discovery of Thunderbird, heavy minerals had been recognised in the uppermost part of the unit (Towner and Gibson 1983).

The bulk of mineralisation at Thunderbird is hosted by deeply weathered sands of the Broome Sandstone, although not over its entire thickness. The mineralised section of the unit is locally known as the Thunderbird Formation, and is exposed at surface along the northeast edge of the deposit as brown, fine-grained ferruginous sandstone float.

Sub-surface, the Thunderbird Formation is dominantly a well-sorted, rounded, fine to very-fine-grained sand that is >90 m thick and is very rich in heavy minerals (up to 40 wt.%). It extends over an area of at least 8.5 km along strike and >6.5 km wide. The upper part of the unit varies from dark brown-yellow to purple brown in colour to a medium to light brown, yellow or red at depth. This colour variation is commonly an indicator of heavy mineral and clay content. Dark colours typically indicate high heavy mineral concentration whilst light coloured sands are usually richer in clays and low in heavy minerals.

The Thunderbird Formation includes thin, 2 to 20 cm thick layers of sandstone and iron cemented sandstone (ironstone), interpreted to have been formed by post-depositional chemical processes of silicification and ferruginisation from ancient water table movements. Theses bands are patchy, but occur throughout the deposit, with most either near surface or around the current water table at depth of 30 to 40 m. The iron oxides are regarded to most likely have been sourced from ilmenite, being most common in the top 5 to 10 m of the deposit where high HM grade material occurs near the surface. Siltstone is less common within the Thunderbird Formation, and is friable and closely jointed. A 1.5 to 6 m thick silt unit occurs toward the base of the Thunderbird Formation, evident primarily through its high slimes content (>25% and <38 µm diameter), locally named the Fraser Beds, which can be traced throughout the deposit and acts as an important marker.

The Thunderbird Formation has a gradational base, accompanied by a drop in HM grade to background levels of <0.9 wt.%, and a decrease in average grain size with clay, clayey sand and sandy clay beds becoming more common. This transitional zone at the base of the mineralised Broome Sandstone (Thunderbird Formation) separating it from the Jarlemai Siltstone is interpreted as early/basal Broome Sandstone.

The overlying Melligo Sandstone conformably to disconformably overlies the Broome Sandstone (and Thunderbird Formation). Its outcrop the Melligo Sandstone is variably silicified, with caps on hills formed from silcrete on sandstone. It is characteristically white, cream and grey in colour, and is typically a well-sorted, well-rounded, fine to medium grained quartz sandstone with fine planar and low-angle cross-bedding.

The principal heavy minerals in the Thunderbird deposit are altered ilmenite, ilmenite, pseudorutile, haematite, goethite, leucoxene, zircon, rutile, anatase, and monazite. There are also minor (<2%) amounts of tourmaline, spinel, staurolite and andalusite. The valuable heavy minerals have a median diameter of 70 to 90 µm.

The deposit is a thick, broad, gently tilted sheet-like, NW striking stratiform body, folded from a flat dip along the north-eastern flank, to ~four° to the southwest along the south-western flank. It persists under cover to the southwest, extending from surface to a maximum drilled depth of 155 m, although ~30% of the total resource area occurs within 6 m of surface. The average depth to the top of the main body of mineralisation is 36 m, with an average mineralised thickness is 38 m. The areal extent, width, grade, geological continuity and grain-size of the Thunderbird mineralisation are interpreted to indicate an off-shore, sub-wave base depositional environment, similar to the interpreted depositional environment of the WIM 150 deposit of the Murray Basin (Williams, 1990).

Within the overall mineralised envelope, there is a continuous, very-high grade (>7.5% HM) zone known as the GT Zone, which is up to 43 m thick (average 15 m) over an area at least 7.5 x 4 km, striking ~north-south, following the dip of the Thunderbird Formation. The GT Zone extends from surface to a maximum modelled depth of 126 m, averaging 35 m depth to its top, with an average mineralised thickness is 16 m. The higher grades are not associated with unit thickening or a change in grainsize, and are therefore interpreted to be the result of deposition in higher wave energy shoals off-shore, influenced by inflow directions of heavy minerals source.

Published measured + indicated + inferred mineral resources of valuable HM include (Boyd and Teakle, 2016):
  3.24 Gt @ 2.86% HM, comprising 0.57% zircon, 0.18% high-Ti leucoxene, 0.21% leucoxene, 4.0% ilmenite (3.0% total HM cut-off) -or-
  1.09 Gt @ 4.74% HM, comprising 0.91% zircon, 0.28% high-Ti leucoxene, 0.25% leucoxene and 3.3% ilmenite (7.5% total HM cut-off).

The most recent source geological information used to prepare this summary was dated: 2016.    
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
 References to this deposit in the PGC Literature Collection:
Boyd, D.M. and Teakle, M.G.,  2016 - Thunderbird heavy mineral sand deposit, Western Australia: in    Trans. IMM (incorp. AusIMM Proc.), Section B, Appl. Earth Sc.   v.125, pp. 128-139


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