Late Archean subaqueous volcanic eruption, hydrothermal alteration, and microbial colonization: Evidence from the Abitibi Greenstone belt

An outcrop in the Blake River Group of the Abitibi Greenstone belt, exhibiting minimal late-stage alteration, provides an optimal natural laboratory in which to investigate Late Archean seafloor processes using a variety of geochemical analytical methods including major and trace element geochemistry, and stable isotopes O and N. This outcrop, at the Hurd Property in eastern Ontario, contains alternating volcanic facies of massive to lobate flows and brecciated hyaloclastite units that erupted during a time of subaqueous, basal-shield volcanism that pre-dates the collapse of the Misema Caldera. The major and trace element geochemistry of these rocks indicates that the volcanic protolith was an anorogenic, tholeiitic basaltic andesite, neither produced nor influenced by subduction-related arc magmatism. The rocks were variably altered to a maximum grade of greenschist-facies, involving hydrothermal fluids, with the more permeable hyaloclastite units experiencing the greatest fluid-rock interaction and associated chemical alteration. Primary igneous minerals were largely replaced by a classic greenschist mineral-assemblage of albite, chlorite, actinolite and magnetite, with calcite and minor epidote. The alteration is reflected in the major oxide element concentrations, including the anti-correlation of CaO and Na2O possibly representing albitization, and in fluid-mobile trace element concentrations. In particular, the large-ion lithophile elements (LILE: K, Rb, Ba, Cs), B, and Li concentrations vary greatly in the hyaloclastite units and are most enriched in those zones thought to have experienced the greatest fluid-rock interaction. Silicate delta 18O values reflect the generations of hydrothermal fluids that affected the units of each volcanic eruption cycle. Nitrogen enrichments and shifts in delta15N values indicate zones of greater alteration involving the stabilization of K-bearing clay, illite, and interactions with fluids carrying sedimentary/organic-rich signatures. The presence of titanite-filled, micron-sized, tubular structures in the volcanic glasses suggests the possibility that the volcanic rocks were a habitat for Late Archean microbial life. The Hurd outcrop also provides a useful analogue for Martian hydrothermal systems that affected the basaltic crust in the past and those that might still be active.