Native metals and intermetallic compounds in subduction-related ultramafic rocks from the Stanovoy mobile belt (Russian Far East): Implications for redox heterogeneity in subduction zones

Abstract

Plutonic associations in orogenic belts are potent indicators of multi-stage fractionation of primitive arc magmas and growth of island-arc crust under evolving redox conditions in ancient subduction zones. Ildeus-Lucha ultramafic–mafic complex (ILC) was emplaced at 232–233 Ma within the Mesozoic Stanovoy convergent margin and subsequently underwent multi-stage hydrothermal alteration and greenschist to amphibolite facies metamorphism (~140 Ma), followed by adakite and K-lamprophyre magmatism (114–117 Ma). Dunite, wehrlite, pyroxenite and gabbro in the ILC are composed of olivine, clinopyroxene, orthopyroxene, plagioclase and late-magmatic amphibole. Mineral compositions in the ILC are typical of island-arc plutonic complexes characterized by early crystallization of orthopyroxene, plagioclase intercumulus and presence of magmatic amphibole. Ultramafic-mafic rocks display high-field strength (HFS) element depletions coupled with large-ion lithophile (LIL) element enrichments, indicating formation via crystal fractionation of mafic primary magma derived from a subduction-related mantle source. Some ultramafic rocks from the ILC contain native metals (W, Pt, Au, Ag, Zn, Bi) and intermetallic compounds (Cu-Au-Ag, Pt-Rh-Pd, Cu-Sn-Zn, etc.) included in silicate minerals, or observed as discrete phases in the finer-grained silicate-oxide-sulfide matrix. Textural evidence and association with either primary (magmatic) or secondary (metasomatic or metamorphic) mineral phases, suggest either magmatic (high-temperature) or metasomatic (low-temperature) origin of metals and metallic compounds. The following models of their formation can be proposed on the basis of the presented data: a) crystallization (W and, possibly, Pt) from metal-rich arc magmas under unusually reduced conditions in subduction-related lithosphere; b) formation from magmatic sulfides during pervasive serpentinization and release of abiogenic hydrocarbons and c) precipitation from chlorine-rich, saline aqueous fluids associated with collision-related metasomatism of the Stanovoy island arc crust.