Metallogenesis in Southeast Pacific Ocean: Nazca Plate Project: ABSTRACT

Abstract

The Nazca Plate Project is a detailed and systematic geological, geochemical, and geophysical investigation of a distinct and tectonically active lithospheric plate of the southeast Pacific Ocean basin. Major tectonic boundaries of the Nazca plate are defined by the East Pacific Rise on the west, along which new oceanic crust of basalt is being generated; the Peru-Chile Trench on the east, where older oceanic crust is being consumed by subduction beneath the overriding South American plate; and the Galapagos spreading center and the Chile Rise on the north and south, respectively. The motion of this plate relative to the Pacific plate is estimated to be eastward at more than 16 cm/yr, as documented by stratigraphic, structural, paleontologic, magnetic, and seismic evidenc . Normal sediments of the deep-sea floor cover basaltic basement rocks over much of the plate, but are locally dominated by a metal-rich component on and near the East Pacific Rise and within the Bauer Deep. These metalliferous sediments, which are composed of crystalline ferromanganese hydroxyoxides (goethite, psilomelane, etc) and Fe-montmorillonite, contain anomalously high concentrations of Fe, Mn, Cu, Ni, Zn, and other transition metals. Elemental abundances and ratios, extraordinary high rates of sedimentation, lead-isotope ratios, and statistical analysis of the data suggest that Fe, Mn, Cu, and possibly other metals were deposited by hydrothermal fluids emanating from sources of basaltic magmatism along the East Pacific Rise. However, isotopic abundances of O, Sr, S, and C and di tribution patterns of the rare-earth elements imply that deposition of the metalliferous sediments occurred at normally low temperatures of the ocean floor and that the nonmetallic components were derived from and/or equilibrated with seawater.