The use of trace Elements in Fe-Oxides in deducing the fractionation history of a silicate magma : A LA-ICP-MS study

Knowledge of the trace element compositions of Fe-oxides will help to develop indicator minerals for exploration in addition to improving our understanding of the petrogenesis of the deposit. This study has characterized Fe-oxides, using laser ablation ICP-MS, from the upper part of Sept-Îles layered intrusion (Québec, Canada), which hosts an Fe-Ti-P deposit, rich in magnetite, ilmenite and apatite. The Fe-oxides record a sequence of fractional crystallization that evolves from the bottom to the top of the deposit. Trace elements in both magnetite and ilmenite vary as a function of their stratigraphic position (Fig. 1). Aluminium, Co, Cr, Mg and V decrease in the Fe-oxides up sequence whereas Ga, Ge, Hf, Mn, Mo, Nb, W, Sc, Sn, Ta, Zr and Zn increase. These geochemical variations appear to be controlled by the partition coefficients of compatible (e.g., V) and incompatible (e.g., Mo) elements into the cocrystallizing phases (olivine, plagioclase, magnetite, ilmenite and apatite) and in the magma during fractional crystallization of the silicate magma. Although magnetite and ilmenite show similar variations (Fig. 1), elements partition differently between the two. For example, Hf, Mg, Nb, Sc, Ta, Ti, W, and Zr are more abundant in ilmenite whereas Al, Co, Cr, Ga, Ge, Mo, Ni, Pb, Sn and V are more abundant in magnetite. Figure 1: Variation of V and Mo in magnetite (Mt) and ilmenite (Ilm) as a function of depth in the Fe-Ti-P deposit of Sept-Îles. The layered sequence comprises, from bottom to top: magnetitite (grey), nelsonite (yellow) and nelsonitic-gabbro (orange). This study shows that the chemical composition of Fe-oxides varies over a wide range within a single Fe-Ti-P deposit as a result of fractional crystallization. Thus it is necessary to understand the processes that control the chemical variability of the Fe-oxides in any given deposit in order to develop an effective exploration tool for the mining industry.\ud \ud Figure 1: Variation of V and Mo in magnetite (Mt) and ilmenite (Ilm) as a function of depth in the Fe-Ti-P deposit of Sept-Îles. The layered sequence comprises, from bottom to top: magnetitite (grey), nelsonite (yellow) and nelsonitic-gabbro (orange).\ud \ud This study shows that the chemical composition of Fe-oxides varies over a wide range within a single Fe-Ti-P deposit as a result of fractional crystallization. Thus it is necessary to understand the processes that control the chemical variability of the Fe-oxides in any given deposit in order to develop an effective exploration tool for the mining industry