Abstract
Morphotectonic analysis of the inside corner intersection (14.0°S) between the southern Mid-Atlantic Ridge and the Cardno fracture zone indicate a young rough massif emerging after the termination of a previous oceanic core complex. The massif, which hosts an off-axis hydrothermal field, is characterized by a magmatic inactive volcanic structure, based on geologic mapping and sample studies. Mineralogical analyses show that the prominent hydrothermal deposit was characterized by massive pyrite-marcasite breccias with silica-rich gangue minerals. Geochemical analyses of the sulfide breccias indicate two element groups: the Fe-rich ore mineral group and silica-rich gangue mineral group. Rare earth element distribution patterns showing coexistence of positive Eu anomalies and negative Ce anomalies suggest that sulfides were precipitated from diffused discharge resulted from mixing between seawater and vent fluids. Different from several low temperature hydrothermal systems occurring on other intersection dome-like massifs that are recognized as detachment fault surfaces associated with variably metamorphosed ultramafic rocks, the 14.0°S field, hosted in gabbroic-basaltic substrate, is inferred to be of a high temperature system and likely to be driven by deep high temperature gabbroic intrusions. Additionally, the subsurface fossil detachment fault is also likely to play an important role in focusing hydrothermal fluids.
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