Magnetite composition in Ni-Cu-PGE deposits worldwide: application to mineral exploration

Abstract

The concentration of trace elements in magnetite is determined primarily by the geological environment at the time of its formation, which makes magnetite a useful indicator mineral in the exploration of ore deposits. Magnetite is a common accessory mineral in magmatic Ni–Cu–Platinum-Group Element (PGE) sulfide deposits forming primary grains in massive sulfide ore and/or secondary grains during alteration of sulfide ore and host rock. We report magnetite composition from massive and disseminated sulfide samples (n=94), representative of 13 major Ni–Cu–PGE deposits, worldwide, and from a range of geological environments and formation ages (Archean to Permo–Triassic). The samples are divided into 6 different types according to the composition of the parental host magma: (1) komatiite, (2) ferropicrite, (3) picrite–tholeiite, (4) anorthosite–troctolite, (5) flood basalt and (6) impact melt. The minor and trace element composition of magnetite was measured by electron probe micro-analysis and a subset of samples (n=61) by laser ablation-inductively coupled plasma-mass spectrometry.The composition of all primary magnetite, crystallized from an immiscible sulfide liquid, plots on a trend in Ti vs. Cr and V vs. Cr. However, primary magnetite trace element content is controlled by element partitioning during sulfide liquid differentiation, from early-forming Fe-rich monosulfide solid solution (MSS) to Cu-rich intermediate solid solution (ISS). The concentration of most lithophile elements (Cr, Ti, V, Al, Sc, Nb, Ga, Ta, Hf and Zr) is highest in the early-forming magnetite, which crystallized with Fe-rich MSS. The fractionated Cu-rich liquid was depleted in lithophile elements so that late-forming magnetite, which crystallized with the Cu-rich ISS, has a low concentration of these elements. Compatible chalcophile elements partitioning into magnetite depends largely on the co-crystallizing sulfide mineral. Elements such as Ni partition preferentially in Fe-rich sulfide minerals (pyrrhotite or pentlandite) such that coprecipitated magnetite is depleted in chalcophile elements compatible with the sulfides. In contrast, magnetite co-crystallized with Cu-rich ISS is enriched in Ni because Ni is not compatible with the co-crystallizing Cu sulfides. Magnetite composition is also dependent on parental magma composition. Magnetite in massive sulfides from ultramafic parental magmas is most similar in compositions to primitive magnetite from Fe-rich sulfides in fractionated intermediate to mafic hosted orebodies.Two types of secondary magnetite are distinguished from the komatiite-hosted Thompson Ni-deposit (Manitoba, Canada): (1) magnetite formed by replacement of pyrrhotite and (2) magnetite formed during the serpentinization of the ultramafic host rocks. Secondary magnetite is depleted in most trace elements (Ni, Mn, V, Ti, V, Al, Cr), with the exception of Si and Mg, which are enriched.To be useful as a tool for mineral exploration, primary and secondary magnetite should not have the same minor and trace element composition. Primary magnetite from all 13 major Ni–Cu deposits plot in the field for Ni–Cu sulfide deposits in the Ni+Cr vs. Si+Mg discrimination diagram. Secondary magnetite plot outside of the field for Ni–Cu deposits. Therefore using this diagram, we can discriminate between primary and secondary magnetite using the low Ni+Cr content of secondary magnetite. Magnetite is resistant to surficial weathering and destruction during mechanical transport, such that it is a useful indicator mineral in exploration that can be used to detect eroded Ni–Cu–PGE deposits in surficial sediments.