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Panton Mine, Panton Sill, Alice Downs Station, Halls Creek Shire, Western Australia, Australiai

Regional Level Types
Panton Mine	Sill
Panton Sill	- not defined -
Alice Downs Station	- not defined -
Halls Creek Shire	Shire
Western Australia	State
Australia	Country

A sill is a tabular thin sheet of magma which intrudes older layers of rock along weakened sections such as fractures, paralleling the existing bedding planes. The Panton Sill is 50 kilometres north of Halls Creek, 10 kilometres long by 3 kilometres wide, and 1000 metres thick. In this zone, an exploration company planned to open (2013) a mine for platinum and palladium, which is found within strataform reefs of chromite. Hamlyn refers to the general area of the sill, focusing on chromite.

Hamlyn describes normal alteration of chromite, along with an antipathic form previously unknown to that point. Along the way Hamlyn describes the species found in the sill. In summary the alteration is related to the formation of secondary spinel in the groundmass during the hornblendization of the primary silicates, under metosomatic conditions accompanied by low oxygen activity.

Panton Sill is a layered mafic-ultramafic lopolith as part of the Lamboo Complex, one of several in the area called the Alice Downs Ultrabasics. At the base is peridotites and dunites,then above this alternating melancratic and leucocratic gabbros, then finally anorthosites. The lopolith has intruded Archaean sediments and associated volcanic rocks of the Halls Creek Group, then later tectonic processes folded it into a south plunging syncline.

Chromite is confined in the periodotites and dunites as disseminations and massive veins 0.5 to 60cm thick. There is cumulus olivine and chromite with some bronzite, secondary chlorite and amphibole filling between.

The olivine forms nearly equant crystals 2.0-4.0mm in diameter as subhedral and anhedral forms. Serpentine veining and secondary magnetite is common, with olivine grains frequently surrounded by these minerals. Chromite is found between the large olivine grains as octahedral 0.025-1.0mm in diameter, with small chromite euhedra often as inclusion in the olivine.

Tremolite is commonly found in the matrix but only makes 5% of the volume. It forms bladed crystals associated with chromite or penetrating olivine. Carbonate forms veinlets replacing silicates.

Chlorite often penetrates the chromite as secondary inclusions within the grain margins where chromite reflectivity is high. Where low, amphibole is dominant with minor chlorite. Where alteration has progressed so that there is less olivine, the mass is dissected by serpentine and secondary magnetite veinlets, replaced at the margins by a zone of enveloping hornblende. There is some bronzite but this is usually replaced by a hornblende-spinel-chlorite assemblage. Chromite is sparse, usually altered to spinel nearly identical to the groundmass. Where major alteration of olivine has occurred, it is replaced by glomeroblastic aggregates of hornblende. Here tremolite is rare and relict, pseudomorphing to a bluish hornblende, with spinel and minor chlorite. In this is occasionally found tourmaline (schorl?) to 4mm in length.

The secondary spinel is xenoblastic and granular fine homogenous grains, associated with hornblende, with the spinel having a composition of ceylonite or hercynite. Orientated ilmenite exsolution lamellae frequently occur in the chromite, displaying spinel type, or 'ferritchromit' alteration.

The chromite grains acted as sites for the nucleation of the developing spinel. In the absence of the grains, spinel developed independently through variable recrystallization of the primary silicate phases. There were two episodes of metamorphism where tremolite and chlorite formed at the expense of silicates, then these reacted to produce hornblende and spinel by metasomatism from a consolidating granite pegmatite body.

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References

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