Fluid-mediated coupled dissolution-reprecipitation (CDR) reaction drives gold remobilization in shallow-water massive sulfides at the Kolumbo arc-volcano, Greece

Recent studies have shown Au-ores result from Au remobilization from preexisting mineralization, assisted by fluid-induced, coupled dissolution-reprecipitation (CDR) replacement reaction and scavenging by sulfosalt melts rich in low-melting point chalcophile elements (LMCE). However, these processes are poorly understood in modern, arc-related Au-rich polymetallic seafloor massive sulfides. To tackle this, Au-rich sulfide samples from diffuser chimneys emitting boiling-fluids, Kolumbo arc-volcano, Greece, were examined geochemically and texturally using FEG-SEM-EDS imaging, and, LA-ICP-MS spot analyses and trace element mapping. Gold concentrations were detected in pyrite, marcasite, galena, sphalerite, chalcopyrite, (Tl-As-Ag-Mo)Pb-Sb sulfosalts, stibnite, and (Tl-Zn-Sb-Pb-Cu-Ag)-rich amorphous, orpiment-like As-sulfides. Subhedral auriferous As-pyrite2 (<= 54 ppm Au) documents textures such as porosity that are concordant with the presence of native gold and accessory (Tl-As-Ag)Pb-Sb sulfosalts, indicating boiling-associated CDR-replacement of colloform LMCE-rich auriferous pyrite1 (<= 131 ppm Au), which liberated Au and associated elements. Arsenic-sulfides, on the chimney surface, with pores, (Tl-As-Ag)Pb-Sb sulfosalt inclusions, and extreme Au enrichment (<= 861 ppm Au), were formed by CDR-replacement of Au-rich (Tl-As-Ag) Pb-Sb sulfosalts (<= 132 ppm Au). We demonstrate that Au is released in the Kolumbo arc-related SMS, by fluid-assisted CDR-replacement reaction, and further suggest that Au re-precipitation and up-grading may be aided by polymetallic sulfosalt melt generated by LMCE-rich boiling-fluids at temperatures similar to 265 degrees C.