The Middle Valley sulfide deposits, northern Juan de Fuca Ridge: Radiogenic isotope systematics

The Bent Hill and the Ore Drilling Program massive sulfide deposits in Midd le Valley, the sediment-filled rift valley at the northern Juan de Fuca Rid ge, are among the largest and richest sulfide deposits on the modern sea fl oor. Drilling during Ocean Drilling Program Leg 169 allowed a complete hydr othermal system to be sampled for the first time. Representative samples fr om this leg were analyzed for radiogenic isotopes (Sr, Nd, and Pb), with th e objectives of determining the sources of metals in the deposits and the n ature of fluid circulation and evolution of the hydrothermal systems. Sampl es were chosen both from the sulfide deposits and host units, the latter in cluding sediments, basaltic flows, and sills. Sr, Nd, and Pb isotope compos itions of the basalts and sills below the Bent Hill massive sulfide deposit confirm that both were highly altered by hydrothermal solutions. The data also show that the sills are cogenetic petrologically with the underlying b asaltic basement. Pb isotope compositions of the massive sulfides in both sulfide deposits va ry within narrow ranges (i.e., Pb-206/Pb-204 ranges of 18.83-18.89 and 18.7 7-18.89 for Bent Hill and Ore Drilling Program, respectively), which could reflect remobilization, replacement, and recrystallization of sulfides duri ng formation of the sulfide mounds, but also mixing and homogenization duri ng hydrothermal circulation of fluids below the sea floor. Data for the mas sive sulfides form elongate clusters between the basalts and the sediment c lusters in Pb isotope diagrams. The most simple explanation for these patte rns involves the mixing of Pb from a basaltic source (Pb-206/Pb-204 range o f 18.64-18.74) with Pb from isotopically heterogeneous sediments comprising a hemipelagic component with relatively higher Pb-207/Pb-204 and Pb-208/Pb -204 values, and a clastic component (turbidite) with relatively higher Pb- 206/Pb-204 values. A third potential source is represented by hydrothermal sulfides present in both the sediments and basalts that have relatively unr adiogenic Pb isotope ratios (Pb-206/Pb-204 range of 17.83-18.36), implying an old crustal source, possibly older than 1.5 Ga. The overlapping Pb isotope compositions of the massive sulfides in the two deposits suggest that the same sources were involved and in similar proport ions to form the deposits. Ore Drilling Program massive sulfide is extremel y enriched in Zn compared to Bent Hill massive sulfide. If the metal source s are the same for the two deposits, other parameters must have produced th e differences between them, such as different fluid chemistry, temperature, or factors at the depositional site. Pb isotope data for sulfides of the B ent Hill massive sulfide feeder zone and the stratiform Deep Copper zone (P b-206/Pb-204 range of 18.59-18.82) are both much less radiogenic than the m assive sulfides and overlap with the field for the underlying basalts. The Pb isotope ratios, a high Cu/Zn ratio, and intense alteration of the basalt s strongly suggest that the basalts were the main source for the metals in the Deep Copper zone and related feeder zones, rather than a higher sedimen t influence in the overlying massive sulfides. This conclusion is also supp orted by positive correlations between the Pb isotope value and Zn/Cu ratio contents of Pb and Mo. 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 to escape to the sea floor, as observed even today. When the seal formed, the fluids were forced to circulate in t he lower part of the sedimentary sequence and in the basalts. The seal woul d have insulated this hydrothermal, system, leading to higher temperatures in the fluids and subsequently to more efficient leaching of metals. This m odel offers an explanation for differences in the Pb isotope compositions o f the massive sulfides and the underlying Deep Copper zone and feeder zones .