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Bynoe Pegmatite Field - Finniss Lithium (Grants, Carlton, Sandras, Hang Gong SW, BP33), Mount Finniss Tin-Tantalum

Northern Territory, NT, Australia

Main commodities: Li Sn Ta
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The Finniss project comprises five lithium pegmatite deposits namely Grants, Carlton, Sandras, Hang Gong SW and BP33 within the 60 x 10 km, NNE-SSW trending Bynoe Pegmatite Field south of Darwin in the Northern Territory, Australia. These deposits are from 25 to 50 km SSW of Darwin. The same belt of pegmatites also includes the historic Mount Finniss and other smaller tin-tantalum deposits.

Tin-tantalum pegmatites were first discovered in the area near Mount Shoobridge in 1882, followed soon after by similar occurrences in the Bynoe Field in 1886. The lithium-bearing fluorophosphate mineral amblygonite was reported from the Pickets Sn-Ta pegmatite in the Bynoe area, with an analysed sample containing 47% P2O5, 35% Al2O3 and 7.9% Li (Crohn 1968). However, it was not until mid 2016 that the potential for significant lithium deposits was recognised in the Bynoe Pegmatite Field (Rawlings, 2017).

These pegmatites are hosted within the middle to upper greenschist facies metamorphosed Palaeoproterozoic Finniss River Group which is composed of siltstone, shale, phyllite, arenite and schist and comprises more than 90 mineralised pegmatite bodies (Ahmad and Munson, 2013). The Finniss River Group, with the underlying South Alligator Group comprise the Cosmo Supergroup in the Central Domain of the Pine Creek Orogen of northern Australia. These succeed the Woodcutters Supergroup which, in turn, overlies sparsely outcropping Archaean basement. The greenschist-facies Central Domain is bounded by the amphibolite- to granulite-facies Litchfield and amphibolite-facies Nimbuwah domains to the west and east respectively (Ahmad and Hollis, 2013).

Tin-tungsten pegmatites in the field have been dated at 1723.5±5.5 Ma and 1733±13 Ma at the Angers and Saffums pegmatites respectively (U-Pb SHRIMP; Frater 2005) indicating that mineralisation probably occurred between 1740 Ma and 1720 Ma. The
40Ar/39Ar muscovite ages have a much wider range, of 1735 to 1680 Ma (Ahmad and Hollis, 2013).

The Sn-Ta pegmatite bodies are linear or lensoidal, and up to 10 m wide and 250 m long. Most are poorly exposed and extensively weathered and altered, and are covered by a thin laterite layer. Feldspars are usually completely decomposed to white kaolinitic clay and has been removed by erosion, leaving behind rubbly or bouldery remnants of white massive quartz. Historic Sn-Ta mine workings are confined to a depth of less than 25 m in heavily altered material (Ahmad and Hollis, 2013).

The Sn-Ta pegmatites are composed of quartz, muscovite and feldspar that are crudely zoned, with a border zone of quartz-muscovite followed inward by an intermediate zone of feldspar-muscovite-quartz and a massive quartz core in some pegmatites. Ore minerals include cassiterite, tantalite and columbite, occurring as coarse crystals or aggregates of crystals. Minor minerals include amblygonite, montebrasite, wordite, augelite, rutile, ilmenite, magnetite, zircon and tourmaline. Beryl is present in significant proportions at the Labelle pegmatite (Ahmad and Hollis, 2013).

At Mount Finnis, the pegmatite bodies occur as three irregular interconnected, generally NNE trending lenses extending over a length of about 200 m. The dip varies from sub-vertical in the south, to ~30°E in the centre, to sub-horizontal in the north, pinching and swelling, with maximum true thickness is about 30 m. Zonation within the pegmatites comprises an outer up to 5 cm thick border of fine grained quartz-muscovite with up to 5 mm laths of tourmaline along the pegmatite-wallrock contact; a 30 mm wall zone of coarse quartz-mica; an intermediate zone of feldspar-mica-quartz; and an inner core of massive quartz. Mineralisation consists of coarse cassiterite and tantalite-columbite crystals, mainly in the intermediate zone. Some very large crystals are also present in the border zone. Kaolinisation of feldspar is ubiquitous, rendering the ore amenable to economic extraction and processing at grades far lower than those required to support a hard-rock operation (Ahmad and Hollis, 2013).

Although first discovered in1886, little work was done at Mount Finniss until 1906, when 1.5 t of tantalite concentrate was produced (Summers 1957). Mining resumed in 1925 and continued uninterrupted until 1951, for a total production of ~20 t of cassiterite and 12.26 t of tantalite concentrate. The mine re-opened in 1980 and by 1989, ~150 000 t of pegmatite had been mined as well as ~100 000 t of alluvium, that together yielded 145.643 tonnes of Sn concentrate and 53.152 tonnes of Ta concentrate (Ahmad and Hollis, 2013).

Proved+probable reserves to a depth of 30 m at Mount Finniss were estimated to be 0.160 Mt @ 160 ppm Sn, 35 ppm Ta and 70 ppm Nb (Nicholson 1988).

The Lithium bearing pegmatites of the Bynoe Field are predominantly hosted within the Burrell Creek Formation of the Finniss Group. They are classified as Lithium-Caesium-Tantalum type and appear to be related to the parent 1806 Ma (U-Pb SHRIMP) Allia Creek Granite and 1862 Ma age (U-Pb SHRIMP) Two Sisters Granite (Worden et al, 2008). The pegmatites range from narrow 'veins' to broad lozenge-shaped bodies up to 500 m long and 60 m wide, generally trending north-northeast, parallel to regional fabric. They are expressed at surface as strongly weathered smectite-kaolinite-quartz saprolite, close to the partially exhumed Cenozoic weathering peneplain (Rawlings, 2017).

The lithium-bearing mineral spodumene has not been recognised at the near 100 historic Sn-Ta pegmatite workings in the field, but there are local occurrences of the Li-phosphate mineral amblygonite [(Li,Na)AlPO
4(F,OH)] and Li-bearing micas. This is because spodumene has a low resistance to weathering and alteration and as such has been decomposed and removed in the deep weathering profile (Rawlings, 2017). In drill core, fresh pegmatite comprises, in decreasing order of abundance, very coarse 20 to 30% spodumene [LiAl(SiO3)2], quartz, albite, microcline and muscovite, with accessory amblygonite, apatite, cassiterite, ilmenite, rutile and rare columbite, tantalite, tourmaline (elbaite), fluorite, topaz and beryl. Spodumene is intergrown with the other minerals but is overall, late in the paragenetic sequence. At some prospects it contains numerous spherical quartz inclusions, while at others it forms inclusion-free, >10 cm long optically-continuous crystals, and has a diagnostic red-pink UV fluorescence (Rawlings, 2017). The depth of weathering is quite extensive, although the transition from oxidised to fresh pegmatite is very sharp.

Bynoe lithium pegmatites are not strongly zoned, with the exception of a narrow 1 to 2 m thick quartz-mica-albite wall facies. Internally they display subtle textural and mineralogical changes towards the core of the pegmatite body, and overall, have a very consistent lithium. Very little lepidolite has been recognised (Rawlings, 2017).

Banded facies reflecting rapid late crystallisation is recognised in the pegmatites of the field. There is minimal late-stage alteration or deformation within the pegmatite and host rocks. Andalusite and garnet are found the very periphery of the larger bodies.

JORC 2012 compliant Mineral Resources at the Finniss Project are 9.63 Mt @ 1.4% Li20 (Core Lithium website visited May, 2020).

The most recent source geological information used to prepare this summary was dated: 2017.    
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:
Ahmad. M. and Hollis, J.A.,  2013 - Pine Creek Orogen: in Ahmad M and Munson TJ (compilers), 2013 Geology and mineral resources of the Northern Territory, Northern Territory Geological Survey,   Special Publication 5, Chapter 5, pp. 5:1-133.
Rawlings, D.,  2017 - Lithium-rich pegmatites of the Bynoe Field: in    AGES 2017 Conference, NT Geological Survey,   Proceedings, pp. 65-67.


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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