Arsenian pyrite-bearing altered volcanics dredged SE of Cheshire Seamount, western Woodlark Basin, Papua New Guinea

Abstract

In many subaerial hydrothermal ore deposits arsenian pyrite is an important host for Au, however, arsenian pyrite is rare on the modern seafloor. During a recent survey for submarine hydrothermal mineralization in the western Woodlark Basin volcanic breccias containing abundant arsenian pyrite were dredged from the flanks of a volcanic seamount in a water depth of 2000 m. This area is particularly interesting because it is located at the transition from continental splitting to oceanic spreading where enhanced heat flow and deep crustal faults may fertilize mineralization. The sulfidic breccia is essentially monomictic and matrix-supported containing altered dacitic clasts. Mineralogical investigation of the breccia reveals silicification and sulfidation as the main alteration types. Quartz occurs in fragments and also constitutes the breccia matrix attesting to silicification as a significant alteration process. Pyrite is the dominant ore mineral with only minor amounts of Fe-oxyhydroxide and goethite. Bulk geochemistry shows a slight enrichment of Au (0.12 ppm) in association with elements such as As-Ag-Hg-Zn-Pb-Sb, key elements indicative of a low sulfidation environment. Three generations of pyrite are recognized on the basis of morphology. Arsenic-free, early framboidal pyrite (py1) is overgrown by arsenian colloform (py2) or massive pyrite (py3) containing up to 3.93 wt% As. Arsenic speciation in the pyrite is in the form of As1- and As3+. The presence of arsenian pyrite in hydrothermal breccias at this seamount indicates the potential for Au mineralization in the area.