Metal sources in the Middle Valley massive sulphide deposit, northern Juan de Fuca Ridge: Pb isotope constraints

Massive sulphides drilled from the Bent Hill deposit during Ocean Drilling Program Leg 139 at Middle Valley, northern Juan de Fuca Ridge, have 206Pb/204Pb=18.43–18.86; 207Pb/204Pb=15.43–15.63; 208Pb/204Pb=38.08–38.60. These span 35% of the total Pb isotope range measured in hydrothermal sulphides from the East Pacific Ocean. 206Pb/204Pb ratios overlap the range of local basalts but 208Pb/204Pb and 207Pb/204Pb require a contribution from a more radiogenic source that is similar to the overlying sedimentary sequence. Three samples have lower 207Pb/204Pb ratios than the mean basalt value which may reflect the preservation of heterogeneities in the local basalts. The Pb isotopic compositions of primary pyrrhotites are indistinguishable from the secondary pyrite-dominated assemblage which recrystallised from low temperature hydrothermal fluids. This may be explained by the importance of the sulphosalts as a host for Pb in the deposit. Clastic sulphides from sediments at Bent Hill are at the radiogenic end of the massive sulphide range suggesting that surficial sulphides may not provide an unbiased sample of the past hydrothermal circulation responsible for massive sulphide deposition. If the Pb content of sediment is 100 times greater than the basalts the sulphide Pb isotopes imply that the hydrothermal fluids have extracted Pb from 60 to 900 times more basalt than sediment. Lead in the massive sulphide deposit has been extracted from 2×108–3×108 m3 of oceanic crust demonstrating the local nature of hydrothermal circulation at mid-ocean ridges. The 3He/4He of hydrothermal fluids in the sulphides (6.2×10−6–7.8×10−6) show only half the variation between the sediment and basalt end-members displayed by Pb isotopes. This is consistent with volatile acquisition from magma degassing, while the Pb is leached during hydrothermal fluid interaction with crystallised oceanic crust.