Sulfide Mineralization from a Newly Discovered Hydrothermal Field on the Carlsberg Ridge: Evidence from Mineralogy and LA-ICP-MS study

The mafic-hosted and inactive Wocan hydrothermal field, located on an axial volcanic ridge at a depth of ~3000 m at 6°21′N along the slow-spreading Carlsberg Ridge, was discovered and sampled by TV-grab at five stations in 2013 on the DY28 cruise. Preliminary investigations show that the field consists of two mineralization mounds called Wocan-1 which located on the east, and Wocan-2 which ~1.7 km away from Wocan-1 on the west. The recovered   hydrothermal precipitates demonstrated a diverse range of mineralization, which can be classified into four group: (i) Cu-rich chimneys at Wocan-1, mainly consist of bornite and chalcopyrite, minor digenite, sphalerite and covellite, as well as trace pyrite, iron oxihydroxides and atacamite; (ii) Cu-rich massive sulfides at Wocan-1, composed of chalcopyrite, pyrite, sphalerite, marcasite, bornite, digenite and opal-CT; (iii) Fe-rich massive sulfides occur both at Wocan-1 and Wocan-2, mainly consisting of pyrite, with minor sphalerite, chalcopyrite, as well as trace barite and amorphous silica; (iv) Si-rich massive sulfides at Wocan-2, including abundant amorphous silica and opal, and sulfides (mainly pyrite and trace chalcopyrite). The observed sulfide mineral assemblages with their paragenetic sequence indicate: (i) the mineralization of Wocan-1 has progressed through a cycle of heating with Cu-rich chimney growth and subsequent cooling, with later seafloor weathering process; (ii) differ to Wocan-1, the mineralization of Wocan-2 consist of an earlier high-temperature circulation and later epithermal cycle with the silica cement on the mount. 

Laser-ablation ICP-MS studies indicate that there is a distinct systematic trace element distribution throughout the different sulfide minerals of the four group samples. In general, Pb, As, Mo, Ga, Ge, V, and Sb (middle- to low-temperature association) are enriched in chalcopyrite in the Cu-rich chimney, whereas trace elements are relative lower in chalcopyrite in other groups, except Se (a high-temperature element) is enriched in Cu-rich and Si-rich massive sulfides, sometimes, Ag and Sn are enriched in Si-rich massive sulfides. Low-temperature association (Pb, Mo, Mn, U, Mg, Ag, and Tl) is typical present in colloform/framboidal pyrite, whereas high-temperature association (Se, Co, and Bi) are enriched in the early euhedral pyrite. The   sphalerite in the Cu-rich chimney and Fe-rich sulfides at Wocan-1 are characterized by high contents of Ga, Ge, Pb, Cd, As, and Sb, and indicated they might precipitate under middle- to low-temperature. The primary bornite which mainly occurs in the middle zone of the Cu-rich chimney is typical enriched in Sn and In, and the second bornite mainly presents in the interior and external zone of the chimney and has significantly higher concentrations of Ag, Au, Mo and U. The digenite is poor in most trace element, except Ag and U. The notable Ag enrichment in the late stage mineral assemblages (e.g., colloform pyrite, secondary bornite, and digenite) both at Wocan-1 and Wocan-2, possibly due to the lower temperature and higher fluid pH. Differences in trace element distributions throughout the Wocan deposit at Carlsberg Ridge can be linked to the history of metal mobilization and mineralization within this inactive hydrothermal system.