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

Résumé

Magnetite forms under a wide variety of conditions, crystallizing at high temperature from silicate magmas or precipitating at low temperature from hydrothermal fluids or seawater. Trace element content of magnetite may reflect the differences in these conditions. Therefore as part of a larger project examining the trace element content of magnetite, by laser ablation ICP-MS, we have characterized magnetite in magmatic massive Fe-oxide deposits (magnetite-ilmenite, ± apatite) from layered intrusions (Bushveld and Sept Iles Complexes) and a massif anorthosite (Lac St. Jean) in order to study how magmatic processes affect the trace element compositions. We have also collected trace element data from the enigmatic “magnetite lava flows” from El Laco, Chile, in order to consider whether these magnetites are indeed of igneous origin. Magnetite from the layered intrusions record the evolution of the fractionating silicate liquid (Fig. 1a), with those found lower in the sequence (more primitive) being richer in Cr, Mg, V and Ni whereas those found higher (more evolved) being richer in Ti, Nb and Ta. Magnetite layers from the uppermost parts of the intrusions contain apatite and this magnetite shows the most evolved composition (Fig. 1a). Magnetite from the anorthosite shows similar compositions to those of the layered intrusions. However, magnetite from the El Laco lava flow are much richer in Si (0.4 wt.%), Ca and P and poorer in Ti (<0.1 wt%), Al (<0.2 wt%) and Ga than magnetite from any magmatic Fe-oxide deposit (Fig. 1b) which raises doubts about the El Laco “magnetite lava flow” having formed by igneous processses.