Composition of iron oxides in Archean and Paleoproterozoic mafic-ultramafic hosted Ni-Cu-PGE deposits in northern Fennoscandia: application to mineral exploration

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 Crmagnetite 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 Crmagnetite 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.

Magnetite geochemistry as a proxy for metallogenic processes: A study on sulfide-mineralized mafic–ultramafic intrusions peripheral to the Kunene Complex in Angola and Namibia

“…Besides pH, the oxidation state and fluid salinity also impact the solubility of Pt and Pd (e.g., Barnes and Liu 2012). Experimental data show that Magnetite analyses with Ni up to 5000 ppm and Cu up to 3000 ppm are commonly reported in literature (e.g., Dare et al 2012;Boutroy et al 2014;Nadoll et al 2014;Jiao et al 2019;Duran et al 2020;La Cruz et al 2020;Moilanen et al 2020;Frank et al 2022), and suggest minor ablation of sulfide phases present as microinclusions. For Type 4, partial contamination by sulfides is compatible with the nature of this type of magnetite as it formed by the replacement of sulfide.…”

“…This means that in a sulfide-mineralized rock we can expect primary magnetite (Fe 2 O 3 ), sometimes variably Cr-enriched, which crystallized from the silicate and/or the sulfide melt. Moreover, post-magmatic hydrothermal alteration at different temperatures can affect the iron oxide content of a sulfide deposit, causing desulphurization and progressive replacement of sulfides by secondary magnetite (e.g., Konnunaho et al 2013;Yang et al 2018;Moilanen et al 2020), or introducing latestage, low-temperature magnetite in fractures and microveinlets (e.g., Dare et al 2014;Nadoll et al 2014;Milani et al 2017;Huang et al 2019;Duran et al 2020).…”

“…9a, b, where the high Cr, V and Ti contents of Type 1 magnetite indicate that it crystallized from a primitive MSS (Dare et al 2012;Boutroy et al 2014). This is not unusual: among some of the world-class Ni-Cu-PGE sulfide deposits (e.g., Sudbury, Voisey's Bay and Lac des Îles in Canada, Noril'sk-Talnakh in Russia, Munali in Zambia, the Fennoscandian deposits in Finland), magnetite preserves a record of crystallization from MSS with a lack of extensive fractional crystallization of the sulfide liquid towards ISS compositions (e.g., Duran et al 2020, and references therein;Moilanen et al 2020).…”

“…Cr-spinel (chromite) and Cr-rich magnetite (ferritchromite, Cr-magnetite) may crystallise from a silicate melt or as primary phases from a sulfide liquid (Frost and Groves 1989;Fonseca et al 2008;Moilanen et al 2020). Secondary Cr-rich oxides are also common and may result from metamorphic events spanning a wide range of conditions that combine solid-state and hydrothermal processes (e.g., Merlini et al 2009;Ahmed and Surour 2016;Hodel et al 2020, and references therein;Moilanen et al 2020). Chromite, here defined as chromium-spinel, has been documented in the cumulates of the Ombuku North and South intrusions, and occurs as xenomorphic crystals, as discrete grains or in association with sulfides (e.g., Fig.…”

“…It commonly contains trace elements that will partition differently according to parameters like temperature, oxygen fugacity, and magma/fluid composition (Dupuis and Beaudoin 2011;Dare et al 2012). The properties of magnetite make this mineral a successful indicator in early-stage metal exploration, capable to constrain petrogenetic environments (Dupuis and Beaudoin 2011;Nadoll et al 2014;Boutroy et al 2014;Dare et al 2014;Liu et al 2015;Zhao and Zhou 2015;Duran et al 2016a;Ward et al 2018;Jiao et al 2019;Moilanen et al 2020), deposit types (Dupuis and Beaudoin 2011), and to discriminate between mineralized and barren rocks (Pisiak et al 2017;Ward et al 2018).…”