Hydrothermal Alteration of Basalts beneath the Bent Hill Massive Sulfide Deposit, Middle Valley, Juan de Fuca Ridge

摘要

Burial of midocean ridges by clastic sediments, particularly at continental margins, profoundly affects the geometry and chemistry of hydrothermal circulation and mineralization in the upper crust. Middle Valley, the sediment-covered northern extension of the bare-rock Endeavour segment of the Juan de Fuca Ridge, is host to the base metal-rich (Cu-Zn) Bent Hill massive sulfide deposit. At a water depth of 2,400 m, the approx 9 Mt Bent Hill deposit is a steep-sided body approx 200 m across and approx 100-m-thick. Ocean Drilling Program (ODP) hole 856H penetrates through the massive sulfide and underlying feeder zone extending to a total depth of 500 m below sea floor through the base of the strongly recrystallized (quartz + chlorite) sediment pile and into the uppermost volcanic basement. The basaltic rocks beneath the Bent Hill deposit include narrow sills intruded into indurated sediments, a volcanic flow erupted on top of sediments, and pillow lavas below the lowermost sediments recovered.Similar styles of alteration are present in both the sills and flows, and alteration is dominated by the effects of large-scale hydrothermal upflow rather than hydrothermal activity associated with individual eruptions or intrusions. The basalts are slightly to completely altered to greenschist fades secondary minerals, principally quartz, chlorite, and titanite, with subsidiary epidote, Cu-Fe sulfides, and rare actinolite. There are steep min-eralogical, chemical, and isotopic alteration gradients from the highly altered basalt-sediment interfaces down to the less altered flow interiors, suggesting the channeling of hydrothermal fluids along the basalt-sediment boundaries. Alteration is reflected in intense metasomatic changes in the basalts. Assuming immobile TiO_2, the most intensely altered basalts have undergone about 20 percent mass loss during recrystallization to chlorite-quartz rocks, with depletions in silica, alumina, and alkali, alkali earth, and base metals. Chloritized pillow margins with strongly light rare earth element-enriched chondrite-normalized patterns ([La/Sm]_N = 1.5; cf. fresh basalts, approx 0.7), that mimic profiles for midocean ridge hydrothermal fluids, require fluid-rock exchange with large quantities of hydrothermal fluid (W/R approx 27,000). Oxygen isotope compositions of chlorite-quartz rocks (delta~(18)O = 1.8-2.4 percent deg) suggest that alteration occurred between approx 320 deg and 370 deg C.