The Middle Valley sulfide deposits, northern Juan de Fuca Ridge; radiogenic isotope systematics

The Bent Hill and the Ore Drilling Program massive sulphide deposits in Middle Valley, the sediment-filled rift valley at the N Juan de Fuca Ridge, are among the largest and richest sulphide deposits on the modern sea-floor. Drilling during ODP Leg 169 allowed a complete hydrothermal system to be sampled for the first time. Representative samples from this leg were analysed for radiogenic isotopes (Sr, Nd and Pb) with the objectives of determining the sources of metals in the deposits and the nature of fluid circulation and evolution of the hydrothermal systems. Samples were chosen both from the sulphide deposits and host units, the latter including sediments, basaltic flows and sills. Sr, Nd and Pb isotope compositions of the basalts and sills below the Bent Hill massive sulphide deposit confirm that both were highly altered by hydrothermal solutions. The data also show that the sills are cogenetic petrologically with the underlying basaltic basement. A silicified zone immediately above the Deep Copper zone may have played a major role in the formation of the deposits by serving as an efficient seal, preventing the deep-seated fluids escaping to the sea-floor. When the seal formed, the fluids were forced to circulate in the lower part of the sedimentary sequence and in the basalts. The seal would have insulated this hydrothermal system, leading to higher T in the fluids and subsequently to more efficient leaching of metals. This model offers an explanation for differences in the Pb isotope compositions of the massive sulphides and the underlying Deep Copper zone and feeder zones.