Precious metal mobility during serpentinization and breakdown of base metal sulphide

Abstract

Serpentinization of ultramafic rocks drives geochemical exchange between the hydrosphere, biosphere and lithosphere in surficial environments and at depth. Precious metals are implicated in these hydration reactions because of the well-known association between peridotite-hosted Au and placer platinum-group mineral (PGM) deposits sourced from serpentinized ophiolite complexes at surface. However, the distribution of precious metals in mantle rocks and their mode of occurrence at the onset of serpentinization are not well understood because early-stage features are typically obliterated with progressive hydration. Herein we report electron probe microanalysis (EPMA) and laser ablation inductively coupled mass spectrometry (LA-ICPMS) spot and mapping results for a suite of base metal sulphide (pentlandite, pyrrhotite, chalcopyrite), native metal (Cu and Fe) and Ni-Fe alloy (awaruite) from variably serpentinized peridotite and pyroxenite (Late Permian to Early Triassic Nahlin ophiolite, Cache Creek terrane; Atlin, British Columbia, Canada). Pentlandite and pyrrhotite occur with magmatic clinopyroxene and Cr-spinel as ultrafine inclusions (few μm) and as coarser interstitial base metal sulphides (≤200 μm) that are enveloped by native metal (Cu and Fe) and Ni-Fe alloy. Because native Fe- and awaruite-bearing mineral assemblages require reduced and/or low-fS2 conditions, we suggest that these replacement textures document destabilization of base metal sulphide phases during the conversion of olivine to serpentine and magnetite. Desulphurization reactions decoupled precious metals from relict pentlandite at the microscale, with Ag, Pd, Pt and Au partitioning from sulphides into both awaruite and native metal (Cu and Fe) at concentrations up to 100s of ppm. New high-resolution LA-ICPMS maps also point to clusters of ultrafine PGM, tellurides, bismuthides and metal alloys that were either remobilized within the serpentinized mesh and/or represent the completely desulphidized product of ultrafine, intergranular base metal sulphide. Other PGE (Os and Ir) are mostly hosted within relict pentlandite and/or pyrrhotite that were preserved during incomplete desulphurization, along with lesser microscale remobilization of these elements into ultrafine veins. New whole-rock PGE (nickel-sulphide fire-assay; NiS-FA) results demonstrate that precious metal remobilization was limited to the microscale during the earliest stages of serpentinization, which, in the case of Au, is consistent with its low solubility within such reduced, low fS2 fluids. Precious metal mobility during the early stages of serpentinization may therefore depend on the stability of Ni-Fe alloy, native metal (Cu and Fe) and PGM. Progressive serpentinization, complete replacement of olivine and destabilization of the reduced, low-fS2 mineral assemblage likely represents an important process to liberate, transport and concentrate precious metals within more oxidized and/or sulphur-bearing fluids in surficial and deep subduction environments.