Formation of hydrothermal vents in the North Fiji Basin: Sulfur and lead isotope constraints

Hydrothermal sulfides were recovered from the 16°50′S triple junction area in the North Fiji Basin, at a water depth of ca. 1900 m. The chimney samples can be divided into three groups based on their major metal contents: 1) type 1 (Fe–Cu-rich), 2) type 2 (Fe–Cu-rich with minor Zn), and 3) type 3 (Zn-rich) chimneys. Type 1 chimneys are mainly composed of chalcopyrite and pyrite, and are enriched in elements commonly precipitated under high temperature conditions (> 300 °C), such as Cu, Co, Mo, and Se. Type 3 chimneys consist dominantly of sphalerite and marcasite with traces of pyrite and chalcopyrite, and are enriched in elements commonly associated with relatively low temperatures (150 to 250 °C), such as Zn, Cd, Pb, As, and Ga. Type 2 chimneys have a mineralogy similar to that of type 1 chimneys although their trace metal contents range between those of type 1 and type 3 samples. Type 2 chimneys appear to be the products of low-temperature replacement, which might be related to formation of type 3 chimneys, of type 1 chimneys.The trace element composition of basaltic rocks indicates that magma generation in the triple junction area was influenced by two different sources: N-MORB and E-MORB. Sulfur and lead isotope patterns of the hydrothermal chimneys show distinct differences between the type 1 and type 3 chimneys. The type 1 sulfides (δ34S = 2.8 ± 1.2‰; 206Pb/204Pb = 18.082 to 18.132; 207Pb/204Pb = 15.440 to 15.481; 208Pb/204Pb = 37.764 to 37.916) are depleted in 34S and have lower Pb isotope ratios compared to types 2 and 3 chimneys (δ34S = 4.1 ± 0.9‰; 206Pb/204Pb = 18.122 to 18.193; 207Pb/204Pb = 15.475 to 15.554; 208Pb/204Pb = 37.882 to 38.150). The higher δ34S values for types 2 and 3 sulfides than type 1 sulfides can be explained by mixing of hydrothermal fluids with ambient seawater. The more radiogenic Pb isotope composition of types 2 and 3 chimneys compared to type 1 chimneys suggest that the formation of types 2 and 3 chimneys were possibly related to E-MORB volcanism that shows higher Pb isotope composition than N-MORB rocks. However, ferromanganese crust samples have radiogenic Pb isotope compositions that are identical to those of E-MORB. Therefore, input of hydrogenous Pb is also a possible source for radiogenic Pb isotopes in the types 2 and 3 chimneys.