17:15 〜 18:45
[SGC33-P05] Platinum group minerals in the Sartohay chromitite: implications for recycling of PGE
キーワード：Platinum group minerals, chromitite, Sartohay ophiolitic belt, PGE
Chromitite is generally considered to be a magmatic rock, or the product of melt/rock reaction at high temperature (Arai, 2021; Arai et al. 2022). Inclusions of metallic alloys and sulfides occur in many podiform chromitites, indicating a mantle source and a relationship between sulfur and platinum group elements (PGE). The chromite and PGE-bearing minerals (PGM) generally are not cogenetic in ophiolite. Both primary PGE-bearing arsenides-sulfarsenides (Kapsiotis et al. 2011; Eslami et al. 2023) and hydrothermal sulfide-arsenides have been found in ophiolitic chromitites (Brough et al. 2015; Zhu and Zhu, 2020; Evans et al. 2023; Liu et al. 2023).
The Sartohay chromitite deposit, hosted in the Sartohay ophiolite mélange in western Junggar of China, contains irarsite and breithauptite. Petrographic observations indicate that the chrome-spinel was firstly altered to Fe-rich chromite at low-temperature with Fe3+/Fe2+ ratios of 0.14-0.34, and such Fe-rich chromite was transformed to magnetite at relatively oxidizing environment. Afterwards, irarsite and breithauptite formed during hydrothermal processes. This suggests complex evolution path in the P-T-fS2-fO2 space. Enrichments of Os, Ir, Co, As, Sb, and Te in disseminated irarsite, and breithauptite reflect element mobility during low-temperature hydrothermal process overprinting the chromitites in the Sartohay ophiolite mélange (Zhu et al. 2016). Sulfides, arsenides, and PGM were observed mostly in the Fe-rich chromite, magnetite, and chloritite surrounding the chrome-spinel porphyroblasts in the Sartohay ophiolitic mélange. Sparse, minute grains of PGM, generally associated with heazlewoodite, millerite, and godlevskite, fill fractures or occur on edges of the Fe-rich chromite in the Sartohay chromitites. All the studied PGM grains are sulfides or sulfarsenides. Most PGM grains are less than 2μm across. Some PGM grains contain detectable amounts of Fe, Ni, As, and Sb. All sulfides in the Sartohay chromitites are secondary that cannot control the distribution of PGE, and IPGE probably were fractionated by irarsite as this mineral is the most common PGM found in the Sartohay chromitites. PGE are chalcophile elements and should fractionate into a sulfide phase owing to their large partition coefficient into the sulfide phase. The Al-rich chromitite in the Sartohay deposit is thought to have formed by reaction between depleted harzburgite and upwelling MORB-type magma, and that the system reached S-saturation before the formation of the chromitite. This indicates that the parent magma of the Sartohay deposit had low concentrations of PGE and was especially depleted in Pd and Pt.
It is concluded that chrome-spinel was transformed to the Fe-rich chromite at first during serpentinization at reduced condition. The Fe-rich chromite was altered to magnetite at relatively oxidizing condition, which probably happened during the formation of chloritite envelope surrounding chromitite. The following hydrothermal alteration introduced S, Sb, and Ni, which replaced magnetite and formed pentlandite, breithauptite, manucherite, and other sulfides. Millerite, heazlewoodite, and godlevskite generally replaced pentlandite. The irarsite formed at the final stage of hydrothermal process with introduction of As, Ir, and Os.
This work is financially supported by the National Natural Science Foundation of China (Grand No. 42072077).
References: Arai S et al (2022) OGR 140, 104422; Arai S (2021) Minerals 11, 209; Eslami A et al. (2023) Lithos 442-443, 107093; Evans KA et al (2023) Lithos 446-447, 107132; Liu X et al (2023) Lithos 436-437, 106957; Zhu QM and Zhu YF (2020) OGR 119, 103401; Zhu YF et al (2016) OGR 72, 299–312.
The Sartohay chromitite deposit, hosted in the Sartohay ophiolite mélange in western Junggar of China, contains irarsite and breithauptite. Petrographic observations indicate that the chrome-spinel was firstly altered to Fe-rich chromite at low-temperature with Fe3+/Fe2+ ratios of 0.14-0.34, and such Fe-rich chromite was transformed to magnetite at relatively oxidizing environment. Afterwards, irarsite and breithauptite formed during hydrothermal processes. This suggests complex evolution path in the P-T-fS2-fO2 space. Enrichments of Os, Ir, Co, As, Sb, and Te in disseminated irarsite, and breithauptite reflect element mobility during low-temperature hydrothermal process overprinting the chromitites in the Sartohay ophiolite mélange (Zhu et al. 2016). Sulfides, arsenides, and PGM were observed mostly in the Fe-rich chromite, magnetite, and chloritite surrounding the chrome-spinel porphyroblasts in the Sartohay ophiolitic mélange. Sparse, minute grains of PGM, generally associated with heazlewoodite, millerite, and godlevskite, fill fractures or occur on edges of the Fe-rich chromite in the Sartohay chromitites. All the studied PGM grains are sulfides or sulfarsenides. Most PGM grains are less than 2μm across. Some PGM grains contain detectable amounts of Fe, Ni, As, and Sb. All sulfides in the Sartohay chromitites are secondary that cannot control the distribution of PGE, and IPGE probably were fractionated by irarsite as this mineral is the most common PGM found in the Sartohay chromitites. PGE are chalcophile elements and should fractionate into a sulfide phase owing to their large partition coefficient into the sulfide phase. The Al-rich chromitite in the Sartohay deposit is thought to have formed by reaction between depleted harzburgite and upwelling MORB-type magma, and that the system reached S-saturation before the formation of the chromitite. This indicates that the parent magma of the Sartohay deposit had low concentrations of PGE and was especially depleted in Pd and Pt.
It is concluded that chrome-spinel was transformed to the Fe-rich chromite at first during serpentinization at reduced condition. The Fe-rich chromite was altered to magnetite at relatively oxidizing condition, which probably happened during the formation of chloritite envelope surrounding chromitite. The following hydrothermal alteration introduced S, Sb, and Ni, which replaced magnetite and formed pentlandite, breithauptite, manucherite, and other sulfides. Millerite, heazlewoodite, and godlevskite generally replaced pentlandite. The irarsite formed at the final stage of hydrothermal process with introduction of As, Ir, and Os.
This work is financially supported by the National Natural Science Foundation of China (Grand No. 42072077).
References: Arai S et al (2022) OGR 140, 104422; Arai S (2021) Minerals 11, 209; Eslami A et al. (2023) Lithos 442-443, 107093; Evans KA et al (2023) Lithos 446-447, 107132; Liu X et al (2023) Lithos 436-437, 106957; Zhu QM and Zhu YF (2020) OGR 119, 103401; Zhu YF et al (2016) OGR 72, 299–312.