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Mallina Basin Gold - Hemi, Brolga, Aquila, Crow, Falcon Diucon, Eagle, Withnell, Wingina,
Western Australia, WA, Australia
Main commodities: Au

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The 200 x 90 km, east-northeast trending Mallina Basin is developed between the East Pilbara and West Pilbara granite greenstone terranes and contains the Mesoarchaean 3020 to 2950 Ma Whim Creek greenstone belt and the 2970 to 2940 Ma De Grey Group. Gold mineralisation is distributed over a length of >150 km within this basin. Significant gold deposits include the Withnell and Wingina resources developed within the volcano-sedimentary Mallina Formation of the De Grey Group and the more recently discovered, much larger Hemi group of deposits that are hosted within intrusive rocks. The Hemi deposits are located ~60 km SSW of Port Hedland in the northern Pilbara region of Western Australia. Hemi is ~25 km east of Withnell, and 15 km WSW of Wingina.

The description below discusses the gold mineralisation of basin as known prior to the discovery of Hemi, followed by a focus on the Hemi deposits in the second half.

The Whim Creek greenstone belt comprises the lower 3009 Ma felsic to mafic volcanic rcoks of the Whim Creek Group, overlain by the Bookingarra Group, a sequence of abundant clastic material derived from the lower volcanics, and followed by high Mg basalts.

The De Grey Group is an up to 8000 m thick sequence that comprises lower turbiditic sandstone unit overlain by shale rich facies. The sequence includes conglomerate, wacke, feldspathic sandstone, arkose, shale, banded iron formation, basalt, high-Mg basalt, siltstone and chert. These are unconformably overlain by basalt and minor felsic volcanic rocks that form the base of the 2770 to 2680 Ma Fortescue Group.

These rocks are intruded by 2946±6 Ma alkaline granites, 2954 to 2945 Ma high Mg diorite, 2941 to 2931 high-K monzogranite and a late syn-Fortescue 2765±5 Ma monzogranite.

As known in 2002, the basin hosted a number of small gold deposits that fall into three types, namely:
i). lode Au associated with sericite-carbonate-pyrite alteration, the largest of which is the Withnell deposit within the east-west trending Mallina shear zone which comprises two parallel zones of steep south dipping and moderatley west plunging shoots over strike lengths of several hundred metres within turbidite hosts.
ii). lode Au deposits associated with pyrophyllite-bearing alteration assemblages, the largest of which is the Camel deposit, some 3 to 5 km west of Withnell.   It comprises 5 mineralised zones with similar geometry and controls to Withnell.   Ore occurs in a number of moderately south dipping stacked quartz veins within strongly silicified and carbonate altered siltstone and greywacke with 4 to 7 g/t Au over widths of up to 20 m.
iii). lode Sb-Au prospects (e.g., the Mallina and Peawah prospects).

The Camel and Withnell deposits combined had a resource in 2001 of 4.96 Mt @ 2.08 g/t Au for 10.32 tonnes of contained gold (Huston et al., 2002).
The expanded Withnell deposit Mineral Resource in June 2021 (De Grey Mining Limited June 2021 ASX Release) was:
  Measured Resource - 1.6 Mt @ 1.8 g/t Au;
  Indicated Resource - 11.7 Mt @ 1.8 g/t Au;
  Inferred Resource - 12.2 Mt @ 2.2 g/t Au;
  TOTAL Resource - 25.6 Mt @ 2.0 g/t Au, for 50 tonnes of contained gold;
The similar Wingina deposit, 40 km to the east of Withnell had a Mineral Resource as follows (De Grey Mining Limited June 2021 ASX Release):
  Measured Resource - 3.1 Mt @ 1.7 g/t Au;
  Indicated Resource - 2.5 Mt @ 1.5 g/t Au;
  Inferred Resource - 6.3 Mt @ 1.2 g/t Au;
  TOTAL Resource - 11.9 Mt @ 1.4 g/t Au, for 16.5 tonnes of contained gold.
NOTE: These resources for Withnell and Wingina are for the two 'mining centres' which each include multiple relatively small deposits distributed over zones that are tens of kilometres across.

In November 2019, an aircore drilling program first discovered mineralisation at what was to become the much larger Hemi discovery. The discovery was confirmed by reverse circulation drilling in March 2020. Hemi comprises a cluster of six individual gold deposits: Brolga, Aquila, Crow, Falcon, Diucon and Eagle. Mineralisation in these deposits differs from those previously known in the basin, as detailed above, both in scale and host lithology. Instead, it is hosted within intermediate intrusions, which comprise a series of brecciated and altered quartz diorite bodies that intrude the surrounding Archaean Mallina Basin meta-sedimentary rocks. These mineralised intrusions are part of a large intrusive complex spanning a 20 x 10 km structurally prepared corridor. Gold mineralisation is intimately associated with sulphide stringers and disseminations that are dominantly pyrite and arsenopyrite accompanied by sericite and silica alteration. The deposits consist of broad zones of gold mineralisation within well defined intrusive lithologies. The boundaries of those lithologies commonly coincide with mineralisation boundaries, although some of the mineralised zones do have gradational boundaries, with the limit of mineralisation based on a gold cut-off grade. The Archaean basement has been eroded and covered by a 25 to 45 m thick layer of transported sediment that does not contain gold.

Weathering is variable across the deposit area, although a deep weathering profile is typical of the mineralised zones, with the base of complete oxidation typically extending to depths of between 60 and 100 m below surface. The base of complete oxidation is separated from completely fresh rock by a 30 to 60 m thick mixed oxide-sulphide transition zone. Parts of the weathered portion of the mineralised zones are kaolin rich which is typical of the oxide zone, although there is also substantial amounts of remnant recrystallised sulphides.

The individual deposits, which extend over an area 3.5 x up to 2 km are as follows (De Grey Mining Limited June 2021 ASX Release):
Brolga - the largest resource delineated, with mineralised NE-SW elongated dimensions of 1000 x 300 m near surface, extending to at least 500 m depth. The deposit appears to continue down dip to the south into the north-south aligned Brolga South.
Aquila, which is located ~100 m to the NW of Brolga and represents a northeast to southwest trending zone on the southern margin of the Crow intrusion. It has a strike length of 900 m and occurs as a subvertical lode that is up to 50 m wide. It has stronger sulphide development and more intense alteration towards the western end with consistently higher grade gold mineralisation that persists to a depth of at least 500 m.
Crow is located immediately adjacent to, and to the NW of Aquila, with mineralisation being associated with the Crow intrusion. Mineralisation appears to be more complex than at the other Hemi deposits. The strongest mineralisation at Crow occurs within the McLeod lode which strikes east-west and merges with the Aquila lode on the eastern side of the Crow intrusion. The McLeod lode contains high grade gold mineralisation associated with infrequent quartz veining. The northern portion of the Crow deposit is characterised by a series of shallower dipping stacked lodes of generally lower grade mineralisation.
Falcon strikes NNE-SSW and is ~500 m to the WSW of Brolga and lies within a steeply dipping, linear intrusion that is ~1000 m long by 50 m wide with its northern limits intersecting the south-western extremity of Aquila. The strong continuous gold mineralisation extends to a depth of up to 500 m and remains open at depth and along strike.
Diucon and Eagle, where mineralisation is hosted within multiple stacked lodes contained within a broader intrusion. The stacked lodes extend to depths of >300 m, and are mineralised over broad zones of >20 m.

Mineral Resource estimates at Hemi are (De Grey Mining Limited June 2021 ASX Release):
Brolga   Indicated Resource - 28.1 Mt @ 1.3 g/t Au;
  Inferred Resource - 34.7 Mt @ 0.9 g/t Au;
  TOTAL Resource - 62.8 Mt @ 1.1 g/t Au, for 69 tonnes of contained gold;
Aquila   Indicated Resource - 10.6 Mt @ 1.5 g/t Au;
  Inferred Resource - 7.4 Mt @ 1.3 g/t Au;
  TOTAL Resource - 18.1 Mt @ 1.4 g/t Au, for 25 tonnes of contained gold;
Crow   Indicated Resource - 9.8 Mt @ 1.1 g/t Au;
  Inferred Resource - 19.5 Mt @ 1.1 g/t Au;
  TOTAL Resource - 29.3 Mt @ 1.1 g/t Au, for 32 tonnes of contained gold;
Falcon   Indicated Resource - 17.0 Mt @ 1.3 g/t Au;
  Inferred Resource - 16.6 Mt @ 1.0 g/t Au;
  TOTAL Resource - 33.7 Mt @ 1.1 g/t Au, for 37 tonnes of contained gold;
Diucon/Eagle   Inferred Resource - 48.5 Mt @ 0.9 g/t Au;
  TOTAL Resource - 48.5 Mt @ 0.9 g/t Au, for 44 tonnes of contained gold;
Combined Hemi Resource
  Indicated Resource - 65.5 Mt @ 1.3 g/t Au;
  Inferred Resource - 126.9 Mt @ 1.0 g/t Au;
  TOTAL Resource - 192.4 Mt @ 1.1 g/t Au, for 211 tonnes of contained gold.

NOTE: 0.3 g/t Au cut-off above 370 m depth; 1.5 g/t Au cut-off below 370 m. Of these resources, that below 370 m depth totals 4.4 Mt @ 2.9 g/t Au.
  This is a maiden resource, with the deposits being open, and potential for expansion. The deposit has the potential to be mined by large scale, low strip ratio, low cost, open pit

The Hemi details are drawn from De Grey Mining Limited June 2021 ASX Release, whilst the section at the head of the summary relating to Withnell was from Huston et al. (2002).

The most recent source geological information used to prepare this decription was dated: 2021.     Record last updated: 17/6/2022
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:
Huston D L, Blewett R S, Keillor B, Standing J, Smithies R H, Marshall A, Mernagh T P, Kamprad J  2002 - Lode Gold and epithermal deposits of the Mallina Basin, North Pilbara terrain, Western Australia: in    Econ. Geol.   v97 pp 801-818

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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