Northern Territory Government

Fossicking in the Northern Territory

Geology of Central Harts Range

The rocks of the Mount Palmer area of the Harts Range region are all part of the Harts Range Group (Cover Sequence) which are lower to middle proterozoic in age.

Geology of Central Harts Range

They have a complicated structural and metamorphic history typical of the whole Arunta Block of which they form a small part.

The Harts Range Group has been described as forming a cover-basement relationship with the Irindina Gneiss forming the lowest part of the cover sequence.

This is based on the observation that the cover units have not undergone the granulite facies metamorphism of the basement, but have been later metamorphosed to upper amphibolite facies, which also imprints on the basement as a retrograde metamorphic event.

The Harts Range over rocks is thought to be a volcano-sedimentary suite deposited in a possible continental rift or failed rift of the basement after 1780 million years ago. Basic and ultrabasic volcanism and/or plutonism followed the initial rifting and sandstones, mudstones and some limestones were also deposited in the resulting basin. These then underwent heating and deformation resulting from compression of the crust caused by the continuation of the Strangways event, ending at around 1730 million years ago.

This later amphibolite facies metamorphism (involving folding and faulting) virtually destroyed the original characteristics of the rock except for the gross lithological variations which are now recognisable as compositional layering.

Other later orogenic events recorded elsewhere in the Arunta Block played a minor role in this area. The numerous crosscutting pegmatites are much younger and represent the conclusion of a tectonothermal phenomenon that occurred about 520 million years ago (Cambrian Period). The interaction of the pegmatites and the surrounding meta-sediments and meta-igneous rocks provides the right conditions for the formation of large, high-quality mineral specimens.

The Alice Springs Orogeny (Devonian to Carboniferous, 300-400 million years ago) is responsible for the latest major uplift which has brought these rocks sufficiently close to the surface for their ultimate exposure through erosion.


Garnets are formed by metamorphism of rocks with high aluminium content, which are usually clay-rich sediments. They are also formed in zones of intense deformation where they coincide with biotite mica which weathers readily to expose the more resistant garnet. Different varieties may form depending on the composition of the original rock such as andradite (Ca, Fe variety) from calcareous sediments, almandine (Fe, Al) from clay-rich sediments and pyrope (Mg, Al) from magnesium-rich basic and ultrabasic igneous rocks.


Muscovite (white mica) is common in the pegmatites which are formed by the very last stages of crystallisation of a granitic melt (not exposed in the Harts Range region). The growth of large crystals is assisted by the slow crystallisation, and the presence of volatile components of the melt (H20, C02, F). This results in the formation of large, low temperature minerals (quartz, microcline feldspar, muscovite and some biotite) and because of the efficient scavenging effects of the volatile elements, minerals containing less common elements form, such as beryl, tourmaline, apatite and occasional cassiterite and magnetite.