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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).
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 arenaceous and argillaceous metamorphosed sediments of the Hodgkinson Formation. A prominent ridge of these sedimentary rocks, including minor chert, hosts the known tungsten mineralisation. The closest exposure of granitoid lithologies to the Watershed property is a persistent, NW-trending dyke of quartz feldspar porphyry exposed ~2 km to the east of the project area.
The main structural fabric comprises an upright, NNW trending cleavage, parallel to the strike of the host sedimentary package, which broadly corresponds to the fabric developed during the fourth regional deformation.
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 scheelite is hosted within calc-silicate and over-printing albite-muscovite altered rock units, occurring as scheelite in quartz-feldspar-scheelite vein swarms invading both the altered units and the enclosing unaltered host rocks, and as disseminations in vein selvedges and nearby fracture planes within calc-silicates, albite-muscovite altered arenites, and albitised felsic dykes.
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% WO3 cutoff
Measured resources - 9.47 Mt @ 0.16% WO3;
Indicated resources - 28.36 Mt @ 0.14% WO3;
Inferred resources - 11.49 Mt @ 0.15% WO3;
TOTAL resource - 49.32 Mt @ 0.14% WO3 = 7.04 million tonne units (mtu)
0.15% WO3 cut-off.
Measured resources - 2.69 Mt @ 0.34% WO3;
Indicated resources - 6.66 Mt @ 0.32% WO3;
Inferred resources - 2.83 Mt @ 0.35% WO3;
TOTAL resource - 12.18 Mt @ 0.33% WO3.
0.3% WO3 cut-off
Measured resources - 1.09 Mt @ 0.53% WO3;
Indicated resources - 2.4 Mt @ 0.52% WO3;
Inferred resources - 1.17 Mt @ 0.54% WO3;
TOTAL resource - 4.66 Mt @ 0.53% WO3.
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."
The most recent source geological information used to prepare this summary was dated: 2014.
This description is a summary from published sources, the chief of which are listed below.
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