Opaque mineral assemblages indicate highly reducing conditions in Oman Drilling Project Hole BA3A

The hydrothermal alteration of the Fe²⁺- rich mantle peridotites (serpentinization) tectonically exhumed at the Earth’s surface is associated to the oxidation of Fe²⁺ to Fe³⁺ that in turns creates reducing conditions characterized by the formation of hydrogen, native metals and uncommon minerals like Fe–Ni alloys and sulfides. Opaque mineral assemblages in serpentinized peridotites offer key insights into these redox reactions, capturing changes in hydrogen/oxygen and sulfur fugacity, thus allowing to unravel the mechanisms governing fluid-rock interactions and their broader impact on geochemical cycles.

Oman Drilling Project Hole BA3A (International Continental Drilling Program) recovered ~300 meters of harzburgite, with subordinate dunite in the upper 180 meters. BA3A peridotites are intersected by minor gabbroic and clinopyroxenitic dikes. Alteration is pervasive and extensive, primarily focused around dikes and veins, where it forms distinct alteration halos. Serpentine is the dominant alteration mineral within the peridotites. The opaque mineral assemblage in Hole BA3A includes pentlandite ((FeNi)₉S₈), magnetite, awaruite (Ni₂Fe), heazlewoodite (Ni₃S₂), native copper, and covellite (CuS). Sulfide minerals are primarily located within the serpentine groundmass or serpentine veins and are rarely found within pyroxene. This spatial distribution suggests that most sulfides formed as secondary phases. In highly serpentinized samples, sulfides are often finely dispersed as grains smaller than 2 μm within the serpentine groundmass. Pentlandite, the most abundant sulfide mineral, is commonly associated with magnetite and awaruite, with minor occurrences of heazlewoodite. Magnetite is frequently observed within the cleavage planes of pentlandite, as narrow rims along its edges, or in association with awaruite as thin veins. Pentlandite is also closely associated with native copper. Covellite forms banded rims around pentlandite, native copper, and magnetite. Magnetite is widespread, occurring as intergrowths with pentlandite, as fine grains (<2 μm) in the centers of serpentine veins, as fine-grained veins cutting through the mesh texture, and as overgrowths in fully serpentinized samples. Thermodynamic modeling indicates that, except for covellite—formed at low aH₂ and high aH₂S—sulfides like pentlandite, heazlewoodite, native copper, and magnetite are stable under high aH₂ conditions. We propose that the observed Cu-bearing assemblages has formed at temperatures below 200°C under highly reducing conditions which is consistent with serpentinization.