Haematite and magnetite are important host or accessory minerals in many mineral deposits, particularly the "Iron-oxide, copper, gold" type of deposits and many skarn deposits. In many of these deposits quartz is absent, rare or a late stage overprint and not related to the mineralisation event. To understand the fluid conditions during deposition of these deposits it is necessary to study opaque minerals and the normal fluid inclusion methods cannot be used. Although we cannot see fluid inclusions in haematite and magnetite, they most certainly do exist and the baro-acoustic decrepitation method can be used to study the fluids in these opaque minerals. In many provinces there are stratigraphic iron oxide units co-existing with other valuable mineral deposits, which has led to major controversy in deciding if the deposits are of sedimentary or hydrothermal origin. The information from the fluid inclusions in the opaque minerals is crucial to determine the precise origin of these deposits and to resolve the controversies.

Baro acoustic decrepitation of some haematite and magnetite minerals from typical iron oxide deposits show intense decrepitation. This occurs in both magnetite and haematite. If, as some researchers have claimed, haematite is merely a supergene oxidation of primary magnetite, then this would have destroyed any fluid inclusions present. The observation of decrepitation in haematite shows that it is often a primary mineral and not merely a supergene alteration product.



The Agrium mine in Ontario is a magnetite bearing carbonatite, being mined for phosphate. The Warrego mine in Tennant Creek, NT., was a major producer of Cu and Au from a magnetite and chlorite host rock which was quartz deficient. The Nobles Nob mine in Tennant Creek, NT., was a major Au producer from haematite host rocks, and is now closed. The Afton mine, near Kamloops in British Columbia, has produced  Cu and Au and is now closed. The Upper Beaver deposit in Ontario was mined for Au, hosted in massive magnetite. The Lyon skarn deposit in Nevada, USA, was prospected as a potential Fe and Cu source. In almost all of these deposits quartz is not available to study the mineralising fluid system and the use of haematite and magnetite is the only way to understand the fluid processes which caused the mineralisation.


Decrepitation in haematite and magnetite usually continues up to the instrumental maximum temperature of 800 C, in contrast to quartz in which decrepitation is negligible above the alpha-beta transition temperature of 573 C. The decrepitation patterns are often quite complex and the results are compared with other nearby samples to map out variations in the decrepitation behaviour across an exploration area. In quartz, low temperature decrepitation peaks are caused by  CO₂ rich fluids, as explained here. Based on the same thermodynamics, this effect should also occur in haematite and magnetite, but such low temperature peaks are rarely observed. Perhaps this is because hydrothermal haematite and magnetite usually deposit from CO₂ poor fluids.



The decrepitation events observed in haematite and magnetite are not caused by mechanical or crystallographic effects, and are the result of explosive decompression accompanying fluid release from inclusions. The instrument only detects pressure waves generated by such decompression and cannot detect the shear waves generated by crystallographic transitions as shear waves cannot travel to the sensor through the intervening fluid medium (air) in the apparatus, as explained here.

Additional confirmation is provided by the fact that re-analysing a sample that has already been analysed, gives no decrepitation counts. The cause of the decrepitation counts must be an irreversible process, such as inclusion fracture and decompression and cannot be due to any reversible process such as phase transitions in a crystalline mineral or mechanical effects in the apparatus. Two samples have been reanalysed in this manner and demonstrate the absence of decrepitation when the same sample aliquot is re-analysed.



Another test of reanalysing a previously analysed aliquot used steel mill concentrartes from Quebec.

Summary

Fluid inclusions in haematite and magnetite can be used to help understand the fluid conditions during mineral deposition. Just because the mineral is opaque does not mean we should ignore the valuable information which can be ascertained by studying fluid inclusions. Although it is more difficult to work with opaque minerals, it is not impossible. Additional studies have been done using decrepitation in pyrite. There are also other methods which can be used on fluid inclusions in opaques, such as gas analyses by mass spectrometer during thermal decrepitation in a vacuum and observation using infrared light.

Several studies of haematite / magnetite mineralised systems have been carried out including the Great Bear magmatic zone, Canada and at  Tennant Creek, NT, Australia. And there is an  overview of many other haematite and magnetite studies here.

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