ABSTRACT: Metalliferous Sediments Adjacent to Hydrothermal Fields: Distribution and Geochemistry

Georgiy A. Cherkashev

The study of metalliferous sediments located at small distances from their sources (10-15 km), such as modern or ancient hydrothermal fields, indicate essential differences in geochemistry compared with metalliferous sediments that occur at greater distances from geothermal fields. Thus, within areas of well-known metalliferous sediment, such as the TAG hydrothermal field, Galapagos Ridge, northern East Pacific Rise (near 13°N), and triple junction zone in the Indian Ocean, are areas of sediment showing the following compositional features: (1) anomalously high concentrations of metals building up sulfide edifices in the central parts of hydrothermal fields (Fe, Cu, Zn, Pb); (2) high noble metal concentrations; (3) rare-earth element patterns characterized by a europ um anomaly (Eu/Eu* >1) and with no negative cerium anomaly (Ce/Ce* >=1), the latter being a peculiar feature of rare-earth composition of "normal" metalliferous sediments. The first two features may not always be distinguished based on examination of the bulk rock. Metalliferous sediments of this type are commonly restricted to exposed igneous rocks that supply lithogenic material (volcanic glass, clastic basalt) to the sediments during subsea weathering. Most elemental concentrations (including nonferrous and noble metals) are diluted by this lithogenic material. The diluting effect can be eliminated by recalculation on a detrital-free basis using the concentration of titanium, the element enriched in the detrital component and depleted in hydrothermal sedimentary component. The g ochemical anomalies are caused by genetic features, namely that the metals are derived from the oxidation of sulfide minerals either previously building up the hydrothermal body (and transported by bottom currents after oxidation) or incorporated into "black smokers" (suspension in fluids). The rare-earth composition is inherited by the sediments from the hottest fluid that affected it. The small size of this type of metalliferous sediment makes it valuable as an indicator of the proximity to sulfide mineralization in the ocean.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990