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The Watershed tungsten (scheelite) deposit is located in the headwaters of the Palmer and Mitchell Rivers, ~160 km NW of Cairns and 35 km NW of the Mount Carbine tungsten deposit, on Cape York Peninsula in Far North Queensland, Australia (#Location: 16° 20' 21"S, 144° 52' 59"E).

Regional Setting

The deposit lies within the Hodgkinson Formation of the Hodgkinson Province (Basin) of Far North Queensland. The Hodgkinson Province is composed of early to middle Palaeozoic (447±4 to 323±20 Ma) turbiditic sedimentary rocks which include subordinate limestone, chert and basic volcanic rocks. It extends for ~500 km over a north-south interval, from Cape Melville in the north, to south of Innisfail in the south, and inland for ~150 km from the coast to the major Palmerville Fault. The latter fault, which trends north-south in the north, and SE-NW to the south, separates it from the Proterozoic basement of the Etheridge Province, composed of the Forsayth and Yambo Sub-provinces to the south and north respectively. These basement blocks comprise sedimentary rocks deposited between 1700 and at least 1650 Ma in an intracratonic rift, that underwent a major metamorphic and deformational event at ~1550 Ma, accompanied by S-type granite emplacement.

The dominant lithologies within the Hodgkinson Province are turbiditic siliciclastics, mostly quartzo-feldspathic arenite and mudstone, representing deep-water density current deposits, interlayered with subordinate conglomerate, chert, metabasalt and minor shallow-water limestone. These rocks are mostly unfossiliferous, except for the limestone and chert.

The rocks of the Hodgkinson Province form distinct, mainly fault bounded belts, parallel to the Palmerville Fault, each in turn, disrupted extensively by numerous thrust faults. Older, probable early Ordovician, siliciclastic rocks are preserved in fault bounded lenses along the western margin of the province, adjacent to the Palmerville Fault. The oldest of these, the Mulgrave Formation, which is interpreted as Early Ordovician in age (dated as 450 to 443 Ma on conodont data and a SHRIMP U-Pb age of 455±5 Ma from a dacitic clast in a conglomerate), consists of quartz-rich sandstone that is interbedded with lesser mudstone, siltstone and shale, with local mafic volcanic rocks. This unit is structurally intercalated with the Mountain Creek Conglomerate, a massive conglomerate with subordinate limestone and sandstone.

Silurian to Devonian rocks in the Hodgkinson Province comprise two main units, the older Chillagoe Formation (mainly limestone, chert, mafic volcanic rocks and sandstone), which is exposed as a linear belt adjacent or close the Palmerville Fault in the west of the province, and the Hodgkinson Formation (mainly sandstone, siltstone and mudstone, with subordinate chert, mafic volcanic rocks and conglomerate), that constitutes the bulk of the province. The ages of these two formations are poorly constrained. Conodonts from the Chillagoe Formation, indicate a Telychian-Emsian age (407 to 399 Ma). Early to Late Devonian faunal assemblages in the western and central parts of the exposed Hodgkinson Formation are indicative of a Late Devonian to Lower Carboniferous age.

The province has been subjected to generally sub-greenschist facies metamorphism with localised higher grade zones associated with contact aureoles around late Palaeozoic intrusive bodies. The Hodgkinson Province has been affected by several significant deformational events of both regional and local extent. Melange is common, and the rocks are multiply deformed and metamorphosed. The Queensland Geological Survey Office (e.g. Garrad & Bultitude, 1999), suggest the basin represents an extensional regime with a possible rifted continental margin or back-arc basin setting. Mafic volcanic rocks in the Hodgkinson Formation, which include basalt, basaltic andesite and andesite/dacite, were initially considered to have a MORB-like geochemical signature, but additional data indicate (e.g., Nb depletion) a possible relationship to subduction. The Palmerville Fault follows the Tasman Line, which marks the breakup of the Rodinia Supercontinent in the late Neoproterozoic in eastern Australia. To the west of this line, basement comprises Precambrian continental crust, while to the east, most Palaeozoic rocks, including the Hodgkinson Province rocks, rest on Cambro-Ordovician oceanic crust.

The Early Carboniferous 357±6 Ma Mount Formartine Supersuite (dominantly the strongly foliated and sheared Mount Formartine muscovite-biotite granite) intrudes the Hodgkinson Formation, as does the 347±6 Ma Early Carboniferous Emerald Creek Microgranite, and Permian (288 to 262 Ma) Tinaroo Granite. The Emerald Creek Microgranite is a variably deformed, white to grey and pale brown, medium- to even-grained to slightly porphyritic, muscovite-biotite granite (S-type), with local traces of garnet. The Tinaroo Granite, which stopes into, and is part of the Tinaroo Supersuite, comprises a white to pale grey, medium-grained, (S-type) slightly to moderately porphyritic biotite granite with traces of garnet, and has a moderately well-developed foliation in marginal zones. A 20 to 25 km diameter batholith is located ~10 km to the NE of Watershed, while another similar sized body is 5 km to the SW, with dyke and sill like masses intruding the intervening Hodgkinson Formation sedimentary rocks.

Deposit Geology and Mineralisation

The Watershed deposit area is occupied by skarn-altered conglomerate, psammite and slate units of the Hodgkinson Formation that have undergone four deformation events evolving from ductile, isoclinal, colinear folding with transposition (D1 to D3) to brittle ductile shear zones (D4). A prominent ridge of these sedimentary rocks, including minor chert, hosts the known tungsten mineralisation. The Hodgkinson Formation sequence within the deposit area incudes the following:

Conglomerate is the principal host to mineralisation and occurs as isolated pod-like bodies that are boudins and disrupted segments of layer within psammite, generally proximal to the contact with slate-siltstone breccia. Individual layer segments are up to 15 m in thickness, can be tens of meters in length, and may be layer-like lenses, ovoid, irregular or wispy in outline. It is a clast-supported, polymict conglomerate composed of rounded, 3 to 30 cm diameter clasts, set in a matrix of coarse sand or grit. Clasts comprise fine- to medium-grained calcarenite with variable amounts of carbonate, quartz and feldspar that have been altered to assemblages of garnet, clinozoizite and amphibole. They vary in colour from pale-pink (garnet-rich) to pale-green (clinozoisite-rich) and pale-grey (siliceous). The matrix is green-grey and is composed of quartz, clinozoisite, garnet, feldspar, and muscovite with minor carbonate, biotite, titanite, scheelite and pyrrhotite (Poblete et al., 2021).

Psammite is common in the Watershed area, ranging from quartzo-feldspathic greywacke to arkose. It comprises fine- to coarse-grained sandstone with a silt to mud matrix, composed of 45 vol.% angular to subrounded quartz, 40 vol.% euhedral to subhedral plagioclase and <5 vol.% fine-grained biotite and muscovite. It occurs in monotonous sequences of massive to poorly graded beds with few internal structures and rare intercalations of shale (Poblete et al., 2021).

Quartzite is rare, occurring as isolated dark grey beds within the psammite units, and comprises >70 vol.%, typically 2 to 5 mm diameter quartz grains with minor biotite and feldspar (Poblete et al., 2021).

Slate-siltstone breccia is the principal lithologic unit in the Watershed area, occurring as a 2 to 3 km wide zone that strikes NNW over a strike length of >20 km. It is composed of fragments of siltstone and minor sandstone in a foliated, dark-grey matrix of mudstone/slate. Siltstone fragments comprise between 10 and 90 vol.% of the rock and vary from 0.5 to 5 cm in width. They may be planar to linear in shape, with some preserving hook-like forms reflecting fold hinges. This unit has a homogeneous composition and represents the highly deformed and transposed equivalent of interlayered sandstone-siltstone-mudstone beds. Elongated aggregates of pyrrhotite are common along foliation planes (Poblete et al., 2021).

Chert is only found within slate-siltstone breccia occurring as 0.5 to 30 cm thick pods or layer fragments of thinly banded, black to cream siliceous rock with shale partings. The chert fragments represent disrupted layers that formed within the slate-siltstone-sandstone sequence (Poblete et al., 2021).

Multiple felsic to intermediate dykes cut the metasedimentary rocks at Watershed including the following: i). Carboniferous, monzonite dykes dated at 350±7 Ma (zircon U-Pb) emplaced during D1-2; and ii). Permian granite plutons and dykes dated at 291±6, 277±6 and 274±6 Ma (zircon U-Pb) and 281±5 Ma diorite (zircon U-Pb) emplaced during D4 (Poblete et al., 2021).

Tungsten mineralisation is predominantly restricted to skarn-altered conglomerate, where the peak metamorphic mineralogy formed during ductile deformation is preserved. This comprises garnet (Garnet 40-87: Almandine 0-35, Spessartine 1-25, Adradite 0-16), actinolite, quartz, clinopyroxene (Diopside 36-59, Hedenbergite 39-61, johannsenite 1-5) and titanite. The first mineralisation event comprised crystallisation of disseminated scheelite in monzonite dykes (pre-D3) and adjacent units, with scheelite grains aligned in the S1-2 fabric and affected by D3 folding, resulting in the Hodgkinson Formation becoming enriched in tungsten (Poblete et al., 2021).

However, the bulk of the scheelite mineralisation was emplaced during a second event and is concentrated in multistage, shear-related, quartz-oligoclase-bearing veins with muscovite selvages that yield a weighted average age of 276 ±6 Ma (
40Ar-39Ar), which formed during D4. These multistage veins were preferentially developed in competent, skarn-altered conglomerate units and were emplaced synchronously with four retrograde alteration stages. During retrograde stages 1 and 2, the retrograde skarn assemblage developed comprised clinozoisite after garnet with quartz, plagioclase, scheelite and phlogopite and minor sodium-rich amphibole. Stage 3 involved by later muscovite, calcite and chlorite, whilst stage 4 was a late-tectonic, non-economic sulphide stage (Poblete et al., 2021).

Poblete et al. (2021) interpret the main controls on scheelite mineralisation at Watershed to have been:
• Early monzonite dykes enriched in scheelite;
• D4 shear zones that acted as fluid conduits transporting tungsten from source areas to traps;
• skarn-altered conglomerate lenses that provided a competent host to facilitate vein formation and a source for calcium to form scheelite; and
• an extensional depositional environment characterised by vein formation and normal faulting, which provided trapping structures for tungsten-bearing fluids, with decompression being a likely control on scheelite deposition.

Tungsten mineralisation occurs over a strike length of ~3000 m, sub-parallel to the regional NNW trend, and is almost exclusively present as scheelite. The observed mineralisation is predominantly within quartz-scheelite vein swarms, which are usually oriented east-west, with some locally trending NNW (parallel to the dominant foliation) although observation from closely spaced drilling indicates that some shallow dipping mineralised structures may also be present. Vein widths observed in drill core range from 0.5 to 100 cm, although the vein width and abundance generally increase downwards. Minor pyrrhotite, pyrite and arsenopyrite may sometimes also be present.

The highest grade tungsten is found within veins where biotite is present in addition to the calc-silicate alteration of the enclosing rocks. The mineralised vein swarms are best developed in the arenaceous units and are relatively attenuated in argillaceous interbeds. Quartz-scheelite veins are most abundant in the arenite where it occurs in the hinge zone of the anticline which forms the Watershed Ridge.

JORC compliant mineral resources are as follows, at varying cut-off grades (Vitalmetals website, viewed August, 2014):
  0.05% WO
3 cutoff
      Measured resources - 9.47 Mt @ 0.16% WO
      Indicated resources - 28.36 Mt @ 0.14% WO
      Inferred resources - 11.49 Mt @ 0.15% WO
    TOTAL resource - 49.32 Mt @ 0.14% WO
3 = 7.04 million tonne units (mtu)
  0.15% WO
3 cut-off.
      Measured resources - 2.69 Mt @ 0.34% WO
      Indicated resources - 6.66 Mt @ 0.32% WO
      Inferred resources - 2.83 Mt @ 0.35% WO
    TOTAL resource - 12.18 Mt @ 0.33% WO
  0.3% WO
3 cut-off
      Measured resources - 1.09 Mt @ 0.53% WO
      Indicated resources - 2.4 Mt @ 0.52% WO
      Inferred resources - 1.17 Mt @ 0.54% WO
    TOTAL resource - 4.66 Mt @ 0.53% WO

Note: The regional setting is derived from the "Queensland Minerals Geological Framework" from the Queensland Department of Natural Resources website, and the GeoScience Australia dforms stratigraphic database (both viewed August, 2014). The deposit geology and mineralisation, and the resources are from the Vitalmetals website, visited on the same date, subsequently updated from Poblete et al. (2021)."

The most recent source geological information used to prepare this decription was dated: 2021.     Record last updated: 15/3/2021
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:
Poblete, J.A., Dirks, P.H.G.M., Chang, Z., Huizenga, J.M., Griessmann, M. and Hall, C.,  2021 - The Watershed Tungsten Deposit, Northeast Queensland, Australia: Permian Metamorphic Tungsten Mineralization Overprinting Carboniferous Magmatic Tungsten: in    Econ. Geol.   v.116, pp. 427-451.

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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