Abstract
Over the last decade there has been an increasing interest in deep-sea mineral resources that may contribute to future raw metal supply. However, before seafloor massive sulfides (SMS) can be considered as a resource, alteration and weathering processes that may affect their metal tenor have to be fully understood. This knowledge cannot be obtained by assessing the surface exposures alone. Seafloor drilling is required to gain information about the third dimension. In 2016, three extinct seafloor massive sulfide mounds, located in the Trans-Atlantic Geotraverse (TAG) hydrothermal area of the Mid-Atlantic Ridge were drilled. A mineralogical and textural comparison of drill core and surface-grab samples revealed that in recent ceased mounds high-temperature copper assemblages typical for black smoker chimneys are still present whereas in longer extinct mounds the mineralogy is pre-dominated by an iron mineral assemblage. Zinc becomes remobilized early in the mound evolution and forms either a layer in the upper part of the mound or has been totally leached from its interior. Precipitation temperatures of sphalerite calculated using the Fe/Zn ratio can help to identify these remobilization processes. While the Fe/Zn ratios of primary sphalerites yield temperatures that are in very good agreement with fluid temperatures measured in white smokers, calculated temperatures for sphalerites affected by remobilization are too high for SMS. Overall drilling of SMS provides valuable information on the internal structure and mineralogy of the shallow sub-surface, however, additional drilling of SMS, at a greater depth, is required to fully understand the processes affecting SMS and their economic potential.
Highlights
Since the discovery of the first hydrothermal venting at the Galápagos Rift in 1977 [1], the number of discovered vent sites globally has been steadily increasing with more than 600 sites known in the oceans [2]
The polymetallic, coarse-grained mineral assemblages observed within surface samples from Shinkai Mound and New M. #2 & #3 are similar to those observed in the black smoker complex at the active Trans-Atlantic Geotraverse (TAG) Mound (Figure 9)
Pyrrhotite and anhydrite, which are representative of the early stage of black smoker formation [4]
Summary
Since the discovery of the first hydrothermal venting at the Galápagos Rift in 1977 [1], the number of discovered vent sites globally has been steadily increasing with more than 600 sites known in the oceans [2]. The formation of modern black smoker deposits is related to processes that have occurred on Earth since Archean times [3] and are well understood [4]. It remains unknown how seafloor massive sulfides (SMS) are affected by alteration and weathering processes once hydrothermal activity ceases, and what impact this has on their economic potential, that is, whether metal tenors become enriched, depleted, or entirely lost. Current studies, using bulk geochemical data from 95 sites published in the literature [5,6], suggest a global resource potential for modern SMS deposits along the neovolcanic zones of the seafloor of 600 million tons, with a median grade of 3 wt % Cu, 9 wt % Zn, 2 g/t Au, and 100 g/t.
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