Sulphide alteration and biomineralisation in metalliferous sediments

Abstract

The fate of sulphide minerals as they age on the seafloor and the process involved in early diagenesis of near field hydrothermal sediments are largely unknown.  This study investigates near field metalliferous sediments from two relict high temperature zones within the Trans Atlantic Geotraverse (TAG) hydrothermal field at 26^oN on the Mid-Atlantic Ridge.  The core were analyzed for bulk mineralogy and geochemistry in order to identify diagenetic alteration processes and their impact on bulk geochemical signatures and the evolution of these deposits in space and time.  Specific attention was paid to sulphide-rich layers and sulphur isotopes and SEM imaging were used to unravel the complex paragenetic sequence.  Microbial interactions were investigated by geochemical, textural and strontium isotope analysis of Fe oxide filaments and associated gypsum. 

The downcore mineralogy and geochemistry of cores CD102/58 & 60 indicate deposition from a complex mix of sources.  The sulphide layer of DC102/58 represents mass wasting of high temperature chimney and upper mound debris.  The sulphide layer of CD102/60 represents mass wasting of an inner mound pyrite-quartz breccia.  Downcore metal distributions reflect mineralogical variability and the relative proportions of sulphide and oxide as well as remobilisation due to oxidative dissolution of sulphide (Cu & Cr), secondary mineralisation (e.g. atacamite), redox cycling (e.g. Mn), precipitation from low temperature diffuse fluids (e.g. opaline silica) and scavenging from seawater (V & P).

CD102/58 has δ³⁴S values ranging from +5.9-17.5‰.  The heaviest values are associated with primary sulphide phases and reflect precipitation initially at the mound surface where the isotopic composition is controlled by Rayleigh fractionation during DSR; followed by high temperature composition is controlled by Rayleigh fractionation during DSR; followed by high temperature precipitation (>250^oC) in the subsurface where the isotopic composition is controlled by partial reduction of seawater sulphate.  CD102/60 has δ³⁴S values ranging from +6.2±15.89‰.  The δ³⁴S value of primary sulphides reflect reaction of hydrothermal fluid with pre-existing anhydrite sulphate in the stockwork zone.  The sulphur isotopic composition of secondary sulphides is controlled by mixing of hydrothermal fluid and reduced seawater sulphate and reaction with previously precipitated sulphides.  These new data have extended the range of δ³⁴S at the TAG hydrothermal site from +4.4±10.3‰ to +4.4±17.5‰.  The upper end of this range is substantially heavier than values of δ³⁴S from other sediment free mid ocean ridge hydrothermal sites and suggests that sulphur contributions from reduction of seawater sulphate are much more important than previously recognised.

This study has contributed to our understanding of sediment formation and diagenesis in hydrothermal metalliferous sediments, in particular the fate of sulphide minerals during early seafloor alteration.

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