Evolution of the Lucky Strike vent field, Mid-Atlantic Ridge

Sánchez Mora, Dennis (2022) Evolution of the Lucky Strike vent field, Mid-Atlantic Ridge. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Seafloor hydrothermal discharge results in accumulations of sulfide minerals that can be rich in base, precious, and critical metals. An increasing demand in these metals in order to transition to a low-carbon economy has resulted in an interest to evaluate the potential of seafloor resources. This dissertation focuses on fundamental geological processes that lead to the formation of these hydrothermal metal-rich deposits. To achieve this, the age, rate of accumulation, composition, and efficiency of metal deposition of the hydrothermal deposits was determined at the Lucky Strike vent field, which is located on the Mid-Atlantic Ridge, ~400 km southwest of the Azores archipelago. Using the ²²⁶Ra/Ba dating technique, hydrothermal barite is dated from these hydrothermal deposits to determine that the hydrothermal field at Lucky Strike has been active at least for ~6,600 years. The tonnage of hydrothermal material that accumulated above the seafloor at Lucky Strike was estimated to be ~1.3 Mt, this was estimated using ~1 m resolution bathymetry at this site. This means that the hydrothermal deposits are accumulation at a minimum rate of ~194 tons/year. The hydrothermal deposits are dominated by barite, anhydrite, marcasite, pyrite, sphalerite, and chalcopyrite, and supergene alteration that consists of goethite and minor atacamite, covellite, and bornite. The bulk composition of these deposits is similar to other Mid-Atlantic ridge hydrothermal deposits but show elevated concentrations of Ba, Sr, and Mo, which reflects the E-MORB substrate geochemistry associated with the nearby Azores hotspot. Furthermore, the average deposit Ba/Co is useful to discriminate between different substrate compositions, such as E-MORB, N-MORB, and ultramafic-hosted sites. The sulfur isotopic composition of marcasite and chalcopyrite ranges from -2.5 to 8.7‰. The Capelinhos vent site, located 1.4 km east of the main Lucky Strike field is enriched in ³⁴S, which reflects differences in sub-seafloor fluid/rock interactions and fluid pathways at these two sites. Sub-seafloor precipitation at the main Lucky Strike field results in <20% of the reduced sulfur of the upwelling hydrothermal fluid reaching the seafloor. Moreover, data on the rate of accumulation and geochemical composition of the hydrothermal deposits and compared to available fluid flux and fluid chemistry was used to determine how much of the metals were lost to seawater and how much of this was precipitated as hydrothermal deposits. This efficiency of metal precipitation is estimated for two of the sites at Lucky Strike, Sintra and tour Eiffel. The metal precipitation efficiency was determined to be 38-99% for Cu, 64-78% for Zn, 14-76% for Fe, and <1% for Mn and Si and seems to be element specific and temperature dependent. In addition, these efficiencies vary at the different scales when this study is compared to the recalculated depositional efficiencies at other hydrothermal sites such as TAG and Endeavour. Furthermore, the efficiencies of precipitation vary at the vent field scale, with the Sintra site showing overall higher efficiencies for Cu, Zn, and Fe, when compared to Tour Eiffel. These estimated metal depositional efficiencies have implications for metal fluxes to the oceans as well as ore forming processes. Finally, fault geometry analysis indicates that hydrothermal venting and associated hydrothermal deposits are largely controlled by enhanced permeability that results from local scale offsets in stress field orientations that form relay ramps and fault linkages.

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