Time series studies of vent fluids from the TAG and MARK sites (1986, 1990) Mid-Atlantic Ridge: a new solution chemistry model and a mechanism for Cu/Zn zonation in massive sulphide orebodies

Abstract

Abstract The hot springs at TAG and MARK on the Mid-Atlantic Ridge have been resampled after an interval of four years (1986–1990). The fluid compositions show the same temporal stability as observed elsewhere, e.g. 21°N, East Pacific Rise (EPR) and the Guaymas Basin. Although the MARK fluids have no chemical characteristics that would distinguish them from those on the faster spreading ridges of the Pacific, TAG has pronounced differences. The depletion in B and the small shift in δ 11 B indicate a substantial degree of reaction at intermediate temperatures along the recharge path that is unique to TAG. In addition, the TAG mound contains a large cluster of sphalerite-rich ‘onion domes’ that have been formed from an approximately 5:1 mixture of the primary hydrothermal end-member fluid and seawater that is formed within the deposit. This has resulted in the extensive precipitation of FeS within the mound and a resulting decrease in pH to values below 3. The low pH causes the large-scale remobilization of Zn from the interior of the deposit and its reprecipitation on the surface as the domes. Such compositional zoning is a common feature of ophiolite-type massive sulphide ore bodies and probably results by the same mechanism. The end-member data from both hydrothermal areas fall on the mixing planes defined by the EPR and Juan de Fuca data in a new three-component mixing model, indicating the presence of a phase-separated brine pool at depth under these Mid-Atlantic Ridge systems which is very similar in composition to those on the Pacific ridges.