The Watershed project area is dominated by arenaceous and argillaceous metamorphosed sediments of the Devonian Hodgkinson Formation. The arenites form ridges with the softer argillcaceous units being exposed in the valleys. Minor chert and granitic dykes have been mapped within the project area. Several major Permian granitic plutons are present around Watershed, some of which are related to tungsten mineralisation in the wider project area.
The Watershed deposit itself occupies the hinge zone of a major, upright anticlinal fold. The axis of the anticline strikes NNW-SSE and dips to the north. The limbs of the fold are composed of coarse grained to conglomeratic arenites and are enveloped by tectonically brecciated shales containing arenite clasts (a so called “melange” zone).
The regional scale “Tullah Fault” runs parallel to the Watershed ridge on its eastern flank. This zone contains a persistent granitic dyke, the so called “Eastern Dyke”. Several smaller dykes with various directions are present in the area as well.
The presence of the dykes, abundant contact metamorphic rocks in the west and geophysical modelling led to the conclusion, that Watershed is underlain by a major granite intrusion, which is expected to be closely related to the genesis of the Watershed tungsten deposit.
The tungsten mineralisation at Watershed occurs over a strike length of approximately 3km and runs sub-parallel to the regional north-northwest trend. The only economic interesting mineral is scheelite, which contains more than 95% of the tungsten at Watershed. Wolframite or other tungsten minerals are present only as traces. Scheelite occurs solely as a blue-white fluorescing variety, demonstrating that it contains insignificant concentrations of molybdenum (a penalty element in scheelite concentrates).
The majority of the mineralisation occurs in altered arenites. Calc-silicate and albite alteration are the most important alteration types, although a number of other types are present. Within the altered rock units, mineralisation occurs in three styles:
- fine to coarse grained disseminated scheelite in altered arenites (low to high grade mineralisation)
- medium to coarse grained scheelite in quartz-albite, mostly E-W striking veins (medium to high grade mineralisation)
- medium grained scheelite in albitised granitic dykes (high grade mineralisation)
Disseminated and vein-style mineralisation represent the majority of the scheelite mineralisation at Watershed. Main gangue phases (non-ore minerals) are quartz, feldspar, clinozoisite, epidote, amphibole, biotite, and muscovite. Compared to similar tungsten mineralisation, Watershed contains very little sulphide and oxide phases.
The tungsten mineralisation at Watershed is open to depth and along strike to the south, where the three tungsten prospects Watershed South, Desailly and Desailly North are located, which were also discovered by Utah and currently explored by Vital.
Vital Metals’ accepted model for the genesis of the scheelite mineralisation describes a sequence of magmatic/hydrothermal events broadly outlined as follows:
- Formation of calc-silicate (iron-poor skarn-type) rock units by a local metamorphic event involving the selective alteration of calcareous sediments;
- An early hydrothermal event that introduced disseminated scheelite mineralisation, with some scheelite-bearing veining, to the calc-silicate altered rock units;
- A later hydrothermal event resulting in the major veining event, accompanied by a quartz-albite-biotite-pyrrhotite alteration event, adding extra and high-grade vein-type, scheelite mineralisation.
Assumed controlling factors of the mineralisation are the proposed underlying granite as a heat and metal source, the occurrence and distribution of calcareous sediments as a source of calcium, and the folded nature of the meta-sediments, whereby impermeable argelliceous units channeled and trapped the tungsten-bearing fluids inside the arenite units.