Constellation, le dépôt institutionnel de l'Université du Québec à Chicoutimi

Résumé

Pyrrhotites (Fe (sub 7) S (sub 8) ) from three different komatiite hosted massive nickel sulphide deposits have been analysed with electron backscatter diffraction analysis (EBSD) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) in order to understand how trace elements behave during deformation. EBSD data reveals strain localisation microstructures in sample from greenschist facies whereas in sample from mid-amphibolite facies, pyrrhotite contains multiple parallel low angle boundaries and crystallographic preferred orientation. A sample from the upper amphibolite facies, is characterised by very large >2cm grains that contains numerous deformation twins. Laser ablation ICP-MS data reveal increased concentrations of Pb, Bi and Ag along low angle subgrain boundaries and twin boundaries. The increased concentrations of Pb, Bi and Ag are explained by diffusion of the trace elements along fast diffusion pathways (low angle and twin boundaries). Trace element variations are developed at the low-temperature stage of the tectonic history of the three deposits. Diffusion of Pb, Bi and Ag is triggered by their low solubility in sulphide phase, pyrrhotite.