An instrument for fluid inclusion decrepitometry and examples of its application

A decrepitation instrument has been constructed which provides rapid, reliable and fully automated analyses and it has been used to analyse some 3 000 samples to evaluate the application of decrepitation data in mineral exploration. Monomineralic samples of 0.5 g of crushed, sieved grains are analysed and extensive studies have been done using magnetite, pyrite, galena, carbonates and quartz. Quartz vein samples often show three or more distinct decrepitation peaks. A peak at 570 °C is related to the weakening of quartz during the alpha to beta phase transition, which facilitates the decrepitation of inclusions. A low temperature peak (below 300 °C) is due to the presence of gas-rich inclusions (the most common gas being CO2), while the intermediate temperature peak is due to primary inclusions. Pyrite and quartz of hydrothermal origin usually give strong decrepitation responses whereas sedimentary pyrite or low temperature cherts give negligible decrepitation. Carbonate samples give very intense decrepitation up to their thermal decomposition temperatures, at which point the decrepitation suddenly ceases. Because of the speed and low cost of the analyses, the technique is useful for exploration projects and for scanning and selecting samples prior to conventional microthermometric studies.