Isocubanite, a New Definition of the Cubic Polymorph of Cubanite CuFe<sub>2</sub>S<sub>3</sub>

The Cheoeum vent field (CVF) is the first example of an inactive ultramafic-hosted seafloor massive sulphide (SMS) deposit identified in the middle part of the Central Indian Ridge. Here, we report on the detailed mineralogy and geochemistry of ultramafic-hosted sulphide sample atop a chimney, together with a few small fragments. Hydrothermal chimneys are characterised by high concentrations of Au (up to 17.8 ppm) and Sn (up to 1720 ppm). The sulphide mineralisation in the CVF shows (1) early precipitation of anhedral sphalerite and pyrite–marcasite aggregates under relatively low-temperature (< 250 °C) fluid conditions; (2) intensive deposition of subhedral pyrrhotite, isocubanite, chalcopyrite, Fe-rich sphalerite (Sp-III), and electrum from high-temperature (250–365 °C) and reduced fluids in the main mineralisation stage; and (3) a seawater alteration stage distinguished by the mineral assemblage of marcasite pseudomorphs, altered isocubanite phase, covellite, amorphous silica, and Fe-oxyhydroxides. Electrum (< 2 μm in size) is the principal form of Au mineralisation and is mainly associated with the main mineralisation stage. The consistently high fineness of electrum (801 to 909‰) is indicative of the selective saturation of Au over Ag in the fluid during high-temperature mineralisation, which differs from the Au mineralisation associated with typical basaltic-hosted hydrothermal systems on mid-ocean ridges. Tin is mainly substituted in structures of sphalerite, isocubanite, and chalcopyrite as a solid solution, and not as mineral inclusions. The continuously ascending hydrothermal fluids enable the early formed Sn-bearing sulphide to be dissolved and reprecipitated, producing significantly Sn-enriched replacement boundaries between isocubanite and Sp-III. This study suggests that Au–Sn mineralisation could be facilitated by the low redox potential of ultramafic-hosted hydrothermal systems such as in the CVF, which may be a common occurrence along slow-spreading mid-ocean ridges.

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Low-temperature magnetic behavior of isocubanite from seafloor hydrothermal deposits in the Okinawa Trough

Kato,

Ohno,

Hatakeyama

et al. 2024

Phys Chem Minerals

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The characteristic behavior of magnetic remanence correlated with mineralogical textures and composition was observed using low-temperature magnetometry, microscopy, and chemical analysis of three isocubanite samples collected from hydrothermal deposits in the Okinawa Trough and a sample transformed from natural cubanite via heating. Both zero-field remanence acquired at 5 K and field cooling remanence acquired at 300–5 K of all samples sharply decreased with increasing temperature at approximately 100 K. In addition, low-temperature cycling of isothermal remanence at 300 K exhibited a transition at approximately 100 K; remanence increased with decreasing temperature and vice versa. The intensity of remanence at low temperature and sharpness of the transition varied across samples with different compositions and microscopic textures, that is, the presence or absence of chalcopyrite lamellae and their widths. The sample obtained from a hydrothermal chimney, in which the magnetic transition was most clearly observed, was also subjected to X-ray diffraction, Mössbauer spectroscopy, electrical resistivity, and magnetic hysteresis measurements. The obtained results were generally consistent with those reported previously for unnamed mineral CuFe3S4 with an ordered cation arrangement. The low-temperature magnetic behavior of isocubanite possibly depends on the degree of cation ordering and can be regarded as an indicator of chemical composition and cooling history. Therefore, low-temperature magnetometry is useful for the detection of isocubanite and a potentially powerful technique for the prompt estimation of its composition and texture, contributing to our understanding of the formation process of hydrothermal deposits.

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Isocubanite, a New Definition of the Cubic Polymorph of Cubanite CuFe₂S₃

Cited by 44 publications

References 22 publications

Mössbauer study on microwave synthesized (Cu,Fe) sulfide composites and correlation with natural mineral—cubanite

Mössbauer study on microwave synthesized (Cu,Fe) sulfide composites and correlation with natural mineral—cubanite

Gold and tin mineralisation in the ultramafic-hosted Cheoeum vent field, Central Indian Ridge

Low-temperature magnetic behavior of isocubanite from seafloor hydrothermal deposits in the Okinawa Trough

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Isocubanite, a New Definition of the Cubic Polymorph of Cubanite CuFe2S3

Cited by 44 publications

References 22 publications

Mössbauer study on microwave synthesized (Cu,Fe) sulfide composites and correlation with natural mineral—cubanite

Mössbauer study on microwave synthesized (Cu,Fe) sulfide composites and correlation with natural mineral—cubanite

Gold and tin mineralisation in the ultramafic-hosted Cheoeum vent field, Central Indian Ridge

Low-temperature magnetic behavior of isocubanite from seafloor hydrothermal deposits in the Okinawa Trough

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Isocubanite, a New Definition of the Cubic Polymorph of Cubanite CuFe₂S₃