A Comparison of Sulfide Deposits from Modern Sediment-Covered Spreading Axes with Besshi-Type Deposits of Japan

Abstract

A comparison of the occurrence, associated lithologies, compositions, and tectonic settings of massive sulfide deposits from the sediment-covered spreading axes at Escanaba Trough (ET), Middle Valley (MV), and Guaymas Basin (GB) in the northeast Pacific with massive sulfide deposits of the Besshi type in the Sanbagawa metamorphic belt of southwestern Japan and at the Shimokawa mine in northern Japan reveals numerous similarities but also some important differences. Major differences are greater thickness of sediment and the absence of an igneous basement complex in the Sanbagawa terrane. At the Shimokawa deposit, an ophiolitic assemblage is interleaved with turbidites and massive sulfides in a tectonic melange. In addition, the mafic lavas associated with Besshi-type deposits have both alkalic and midocean ridge basalt (MORB) compositions whereas the basalts associated with ET, MV, and GB are restricted to MORB compositions. The principal compositional differences are the lower Cu/Zn ratios; higher Pb and Ba contents; more abundant sulfate and carbonate gangue mineralization in the sediment-hosted seafloor deposits; and the greater pyrite content and Co enrichment in the Besshi-type massive sulfides. The massive polymetallic (Zn-Cu-Pb-Ag-As-Sb) sulfide present at ET is apparently absent in the Besshi deposits. Pb isotope ratios for Besshi deposits are somewhat similar to those for GB sulfides and hydrothermal fluids; they indicate a contribution of Pb from both basalt and sediment. The similarities in deposits formed on sediment-covered ridge crests in different tectonic settings (openocean spreading axis and rifted continental margin) indicate that deposits with Besshi-type characteristics, both ancient and modern, are not unique to one tectonic environment. Deposits with similar characteristics may be forming in other modern oceanic rift environments (e.g., narrow back-arc or interarc basins, small ocean basins surrounded by continental highlands, and subduction zones in which an oceanic ridge is underthrusting the continental margin) where the sedimentation rate exceeds the spreading rate.