Target settings to secure sustainable access to raw materials include seafloor massive sulphide (SMS) resources. Gold-rich SMS deposits, are often the result of complex interplay of multiple Au enrichment events. Recent studies have shown that high-grade Au ores result from Au remobilization from preexisting mineralization, driven by fluid-induced coupled dissolution-reprecipitation (CDR) reactions; however investigations into this process in modern Au-rich SMS, are lacking. To tackle this issue, Au-rich [AuBULK≤32ppm; Au/(Cu+Zn+Pb)=1.9], polymetallic (Sb, Tl, Hg, Ag, Mo, Te) diffuser chimney samples from the active Kolumbo shallow-water SMS system, Hellenic Volcanic Arc, were geochemically and texturally examined using combined SEM-EDS imaging, and LA-ICP-MS spot analysis and trace element mapping. Recrystallized subhedral auriferous arsenian pyrite2 (≤65 ppm Au, ≤13290 ppm As) records textures, being porosity growth concurrent with the presence of native gold and accessory pore-filling Pb-Sb sulfosalts, indicating that recrystallization proceeded via fluid-mediated CDR reactions. The latter caused replacement of earlier, colloform-banded, Au-rich arsenian pyrite1 (≤130 ppm Au, ≤9057 ppm As) by pyrite2, and liberated invisible Au (nanoparticles and/or lattice-bound) and associated elements (Pb, Sb). Furthermore, textural evidence indicates that porous orpiment with Pb-Sb sulfosalt inclusions, showing extreme Au enrichment (≤861 ppm Au) compared to other SMS deposits worldwide, was formed by replacement of Au- and As-rich Pb-Sb sulfosalts (≤132 ppm Au, ≤6550 ppm As) via CDR reactions. This study provides significant evidence that in arc-related Au-rich polymetallic SMS deposits, native and invisible Au are closely associated to various sulfides/sulfosalts, and CDR reactions may contribute to upgrading Au grades during hydrothermal reworking.
Stephanos P. Kilias1, Evangelia Zygouri1, Nikolaos Zegkinoglou1, Manuel Keith2, Thomas Zack3, Daniel J. Smith4, Paraskevi Nomikou1, Paraskevi Polymenakou5
1National and Kapodistrian University of Athens, Faculty of Geology and Geoenvironment, 15784 Athens, Greece; 2University of Erlangen-Nuremberg, GeoZentrum Nordbayern, 91054 Erlangen, Germany; 3University of Gothenburg, Department of Earth Sciences, SE-405 30 Gothenburg, Sweden; 4University of Leicester, School of Geography, Geology and the Environment, University Road, Leicester LE1 7RH, UK; 5Hellenic Centre for Marine Research, Institute of Marine Biology, Biotechnology and Aquaculture, 71003, Heraklion, Crete, Greece