Mafic-hosted seafloor sulfide mineralization at the margin of a non-transform discontinuity on the southern mid-Atlantic ridge

Abstract

Mineralogical, geochemical and S-isotopic studies were carried out on seafloor massive sulfides (SMS) and hydrothermally altered rocks from the mafic-hosted TaiJi field (including TaiJi-1 and TaiJi-2 sites), which is located on the margin of a nontransform discontinuity (NTD) on the Southern Mid-Atlantic Ridge (SMAR). The main hydrothermal precipitates of TaiJi-1 were massive sulfides, while TaiJi-2 produced a large amount of semi-massive sulfides. Significant rock alteration is an important feature of this field, and large amounts of clay minerals (chlorite) occurred in the semi-massive sulfides. Geochemically, notable negative correlations were identified between rock forming elements (Mg) and major hydrothermal metal elements such as Zn + Cu and Co in semisulfides. Such mineralogical and geochemical characteristics, together with low Co/Ni ratio (similar to the surrounding rocks) of the semi-massive sulfides and the unique REE features of some altered rocks, tell us that the prominent mineralization mechanism for these semi-massive sulfides was probably related to rock mineralization. Thus, TaiJi-2 appears to differ significantly from the TaiJi-1 in terms of sulfide mineralization mechanism. Moreover, the TaiJi sulfides are remarkable for large δ³⁴S variation (7.2–15.3%). We suggest that seawater corrosion after sulfides precipitation were responsible for ³⁴S enrichment in sulfides. Furthermore, our work sheds some light on the comparison between TaiJi and other NTD-related fields along the Mid-Atlantic Ridge. We argue that rock mineralization may represent an important mineralization type in NTDs.

Keywords:

• Southern mid-Atlantic ridge
• nontransform discontinuity
• TaiJi field
• rock mineralization
• sulfur isotopes

Acknowledgements

We thank all the crews and scientists of RV Dayang Yihao Cruise 22 (Leg 2) and 26 (Leg 3), and An Ning who analyzed sulfur isotope for us at the Guiyang Institute of Geochemistry, Chinese Academy of Sciences.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was financially supported by National Natural Science Foundation [41506076], China Ocean Mineral Resources R&D Association project [DY135-S2-2-06], Taishan Scholar Program of Shandong province and Aoshan Talents Program Supported by Qingdao National Laboratory for Marine Science and Technology [No. 2015ASTP-ES16].