Geology & Mineralogy

Geology & Mineralogy

Tungsten is one of the rare metals in nature, although it is much more abundant than gold or silver. It is present in the continental earth’s crust at about 1 g/t, comparable to tin, molybdenum, arsenic or uranium.

In contrast to some other metals like gold or silver, tungsten does not occur in its pure metallic form in nature, but only as a constituent of a wide range of minerals. Of those, two are of economic importance as they can form economic viable deposits. Those two minerals are scheelite and wolframite. Wolframite itself is a solid solution of two other minerals, ferberite and hubnerite, which can occur in their pure forms in some deposits. The properties of both scheelite and wolframite are summarised below:

WO3 content (wt.%)80.576.3 - 76.6
Density (g/cm3)5.9 - 6.17.1 - 7.5
ColourColourless, white, pale yellowBrown-black
Mohs hardness4-54.5
FluorescenceBlue, yellow if Mo-bearingNon fluorescent
Brown-yellow scheelite crystals grow next to a cube of violet fluorite on white quartz (from Cinovec, Czech Republic)..
Black wolframite crystals grow within white vein quartz (from Aue, Germany).

The formation of tungsten deposits is usually related to the intrusion of granites during orogenic processes (the formation of mountain ranges as a result of the collision of two tectonic plates). During the crystallisation of a granitic melt, the dissolved tungsten cannot be incorporated as a trace element in the usual rock forming minerals like quartz and feldspar. Therefore it is getting enriched during the process of granite crystallisation and can form a pegmatitic tungsten deposit from the last bit of melt, or, which is more abundant, is released into a hot aqueous solution (a hydrothermal fluid).

In case the fluid contains enough tungsten, it can form various styles of tungsten deposits, which often occur together. The hot aggressive fluid can react with the newly formed granite destroying minerals like feldspar replacing it with new ones, including wolframite. This style of deposit is called a greisen. If the fluid reacts with a nearby carbonate-rich rock (e.g. a limestone), the carbonates are getting replaced by calc-silicates and scheelite. This kind of deposit is called a skarn.

If the fluid intrudes into cracks within the now solid granite or the rocks above it, it can fill up these cracks with minerals like quartz but also scheelite and wolframite. These filled up cracks are called veins, and depending on the number of veins and their relationship to each other they can form a vein, vein swarm or stockwork-type (vein network) deposit. There are also a few other types of tungsten deposits including placers, but they are less significant in Australia.

Depending of the exact type of deposit, tungsten can be associated with economically interesting minerals such as cassiterite (tin), magnetite (iron), molybdenite (molybdenum), base metal sulphides, fluorite or gold.

The size and grade of tungsten deposits can differ widely range from small high-grade deposits with less than 1 million tonnes of ore at grades higher than 1 wt% WO3 to large low-grade deposits of more than 100 million tonnes and grades below 0.1 wt% WO3.

A tin-tungsten vein containing wolframite and cassiterite cuts through mica-schist. The veins in this deposit in Germany originate from a granite at depth which also contains tin-greisen bodies.