Abstract
Using electron probe microanalyzer (EPMA) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), we analyzed major and trace element compositions of iron oxides from Ni-Cu-PGE sulfide deposits hosted by mafic-ultramafic rocks in northern Fennoscandia, mostly focusing on Finland. The main research targets were the Archean Ruossakero Ni-(Cu) deposit; Tulppio dunite and related Ni-PGE mineralization; Hietaharju, Vaara, and Tainiovaara Ni-(Cu-PGE) deposits; and Paleoproterozoic Lomalampi PGE-(Ni-Cu) deposit. In addition, some reference samples from the Pechenga (Russia), Jinchuan (China), and Kevitsa (Finland) Ni-Cu-PGE sulfide deposits, and a barren komatiite sequence in the Kovero area (Finland) were studied. Magnetite and Cr-magnetite show a wide range of trace element compositions as a result of the variation of silicate and sulfide melt compositions and their post-magmatic modification history. Most importantly, the Ni content in oxide shows a positive correlation with the Ni tenor of the sulfide phase in equilibrium with magnetite, regardless of whether the sulfide assemblage is magmatic or post-magmatic in origin. The massive sulfide samples contain an oxide phase varying in composition from Cr-magnetite to magnetite, indicating that Cr-magnetite can crystallize directly from sulfide liquid. The Mg concentration of magnetites in massive sulfide samples is lowest among the samples analyzed, and this can be regarded as a diagnostic feature of an oxide phase crystallized together with primitive Fe-rich MSS (monosulfide solid solution). Our results show that magnetite geochemistry, plotted in appropriate discrimination diagrams, together with petrographical observations could be used as an indicator of potential Ni-(Cu-PGE) mineralization.
Highlights
Orthomagmatic sulfide deposits are important sources for base- and precious metals, such as Ni, Cu, Co, and PGE.Editorial handling: P
The LA-ICP-MS method was used for more detailed trace element analysis, because compared to electron probe microanalyzer (EPMA), it is more accurate for trace elements, and has lower detection limits, and the larger spot size allows determination of a more representative composition
1) Our results indicate that the trace element concentrations in oxide phases vary from two to three orders of magnitude, reflecting both the silicate and sulfide melt compositions from which they have crystallized, and post-magmatic modification processes
Summary
Orthomagmatic sulfide deposits are important sources for base- and precious metals, such as Ni, Cu, Co, and PGE.Editorial handling: P. The sulfide mineral assemblage in these deposits typically consists of pyrrhotite, pentlandite, chalcopyrite, and pyrite (Naldrett 2004). These sulfides are commonly associated with iron oxides (e.g., magnetite and Cr-magnetite) because of the capability of sulfide liquid to dissolve significant amounts of oxygen, leading to the crystallization of magnetite together with early-forming Fe-rich monosulfide solid solution (MSS) and later, at lower temperatures, Cu-rich intermediate solid solution (ISS) (e.g., Naldrett 2004; Dare et al 2012). Experimental studies have demonstrated that the solubility of oxygen increases with
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