HYDROTHERMAL MINERALIZATION ALONG SUBMARINE RIFT ZONES, HAWAII

This is the first article to describe mineralization of midplate subma rine rift zones and hydrothermal manganese oxide mineralization of mid plate volcanic edifices. Hydrothermal Mn oxides were recovered from su bmarine extensions of two Hawaiian rift zones, along Haleakala and Pun a Ridges. These Mn oxides form two types of deposits, metallic stratif orm layers in volcaniclastic rocks and cement for elastic rocks; both deposit types are composed of todorokite and birnessite. Thin Fe-Mn cr usts that coat some rocks formed by a combination of hydrogenetic and hydrothermal processes and are composed of delta-MnO2. The stratiform layers have high Mn contents (mean 40%) and a large fractionation betw een Mn and Fe (Fe/Mn = 0.04). Unlike most other hydrothermal Mn oxide deposits, those from Hawaiian rift zones are enriched in the trace met als Zn, Co, Ba, Mo, Sr, V, and especially Ni (mean 0.16%). Metals are derived from three sources: mafic and ultramafic rocks leached by circ ulating hydrothermal fluids, clastic material (in Mn-cemented sandston e), and seawater that mixed with the hydrothermal fluids. Mineralizati on on Haleakala Ridge occurred sometime during the past 200 to 400 ka, when the summit was at a water depth of more than 1,000 m. Hydrotherm al circulation was probably driven by heat produced by intrusion of di kes, magma reservoirs, and flow of magma through axial and lateral con duits. The supply of seawater to ridge interiors must be extensive bec ause of their high porosity and permeability. Precipitation of Mn oxid e below the seafloor is indicated by its occurrence as cement, growth textures that show mineralizing fluids were introduced from below, and pervasive replacement of original matrix of clastic rocks.