UNUSUAL METAL-BEARING MOLTEN SULFUR COLLECTED FROM AN ACTIVE ARC CALDERA IN THE NORTHEASTERN LAU BASIN: EVIDENCE OF MAGMATIC CONTRIBUTION TO ORE FORMING PROCESS?

Gray, carbon- and metal-bearing molten sulfur material was collected from the central volcanic cone of an active caldera in the northeastern Lau Basin during the KODOS H-06 hydrothermal expedition. The molten sulfur material, composed of yellow and gray sulfur mixture, enveloped the frame and chain bag of the dredge sampler and showed sticky nature after recovery of the dredge from the depth of ~1700 m. Fresh dacitic rock and volcanic breccia composed of native sulfur, black lapilli and ash, and alunite and silica were collected from same site. Although only elemental sulfur is identified using X-ray diffraction, the gray sulfur is rich in Cu, Mo, Sn, As, Sb, and Bi compared to dacitic rock. In microscopic view, the gray sulfur contains several silicates-, sulfides-, and carbon- grains as inclusions in the sulfur matrix and most of them are thought to be trapped during the formation of sulfur. However, Cu sulfide (covellite?), which is ubiquitous within sulfur matrix, is thought to be precipitated from magmatic vapor phase, which is supported by significant Cu contents in the inclusion-free matrix (up to ~10% using microprobe analysis). Sulfur isotope composition of the gray sulfur, δ³⁴S = -7.5 to -8.2 pet mil, indicates that this metal-bearing sulfur deposit might be formed by disproportionation of magmatic SO₂. Our finding may provide a direct evidence for partitioning of Cu into a magmatic vapor phase in submarine arc volcano, which can be incorporated into the formation of hydrothermal ore deposits.