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

Résumé

In order to use trace element composition of magnetite as an indication of its origin it is necessary to understand the processes that control the trace element concentrations in magnetite. We have characterised trace element distribution in magnetite, using laser ablation ICP-MS, from magmatic ore deposits (Fe-Ti-V-P and Ni-Cu-PGE) where the paragenetic sequences are well constrained. Changes in composition of the liquid, driven by crystal fractionation, are recorded by magnetite in both silicate and sulfide melts. The composition of magnetite is sensitive to co-crystallizing phases, with marked depletion in Ti when ilmenite crystallizes before magnetite and in Cu when a sulfide liquid segregates. Multi-element variation diagrams show that magmatic magnetites have chemical signatures distinct from hydrothermal magnetite due to differences in fluid composition and different conditions of formation (e.g., competing phases, redox and temperature). Chemical fingerprinting of magnetite from the magnetite ‘lava flows’ of El Laco, northern Chile, provides new evidence to support the hydrothermal alteration model rather than a magmatic origin.