Unusual Copper bearing massive sulfur collected from an off-axis caldera, east of the Northeast Lau Spreading Center

Abstract

Gray, copper- and other metal-bearing massive sulfur was collected from the central volcanic cone of an active off-axis caldera in the northeastern Lau Basin during the hydrothermal expedition undertaken by the Korea Deep Ocean Study (KODOS) program in 2006. The massive sulfur, composed of yellow and gray mixture, enveloped the frame and chain bag of the dredge sampler and showed plasticity 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 by same dredge. Low sulfur isotope composition of the gray sulfur (δ34S = -7.5 to -8.2 per mil) indicates that the sulfur was formed by magmatic degassing of SO2 and its disproportionation. In microscopic view, various sized and shaped covellite (CuS) inclusions occur within sulfur matrix ubiquitously. We propose that the covellite inclusions were possibly precipitated directly from magmatic vapor phase, which is supported by enrichment of other elements commonly associated with magmatic input (such as Mo, Sn, As, Sb, and Bi) and the low sulfur isotope composition in gray sulfur. The formation of metal-bearing gray sulfur may provide a directevidence for magmatic fl uids as major source of ore metals for the hydrothermal systems. However, an alternative model (hydrothermal origin of covellite) also should be examined.