Insights into Extinct Seafloor Massive Sulfide Mounds at the TAG, Mid-Atlantic Ridge

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.

Inactive Sulfide Ecosystems in the Deep Sea: A Review

“…The TAG active hydrothermal system on the slow-spreading Mid-Atlantic Ridge is exemplary of the footprint of such a potential deposit, with a surface expression on the order of 200 m in diameter, a maximum elevation of ∼50-60 m above the surrounding seafloor (Rona et al, 1986;Humphris and Kleinrock, 1996). Numerous inactive sulfide mounds of similar and even somewhat larger dimensions (e.g., Mir Mound: 400-600 m in diameter) lie within the ∼5 km 2 TAG Hydrothermal Field (Rona et al, 1993;Lehrmann et al, 2018), but are geographically isolated from the active mounds (at least at the surface). Other large sulfide accumulations may be found on intermediate spreading ridges as well as in island arc environments and marginal basins (Hannington and Monecke, 2009;Cherkashov, 2017).…”

Marine Mineral Exploration With Controlled Source Electromagnetics at the TAG Hydrothermal Field, 26°N Mid‐Atlantic Ridge

“…(b) Conductivity models across TAG mound. (c) Porosity versus conductivity of selected samples from TAG (stars) and other sources (rectangles) for basalt (black), sulfide (red), and jasper (blue, a thin, <1‐m, silicate‐based layer above the sulfides; Lehrmann et al, ) samples; the Hashin Shtrikman upper bound (solid line) separates the main conduction mode via electrolytes (left) and metals/semiconductors (right), and the dashed line represents Archie's law approximation for electrolytic conduction in hard rock (Evans & Everett, ; Müller et al, ) with a =1 and m =1.8. TAG = Trans‐Atlantic Geotraverse.…”

“…Previous studies with a seafloor EM transmitter and receiver spacings of <100 m (Cairns et al, ) revealed higher conductivities (up to ∼16 S/m), similar to the conductivities for a similar penetration depth of the MARTEMIS system at the inactive deposits (Haroon et al, ) which suggests that the generally lower conductivities observed with the DASI‐Vulcan system at TAG mound are probably related to the deeper penetration depth due to the larger transmitter‐receiver spacing (350 and 505 m) and resulting averaging between highly conductive material just beneath the seafloor and less conductive stockwork material at greater depth. The smaller size of the TAG mound compared to the inactive deposits may in this case play a larger role than the possible effects of anhydrite (Lehrmann et al, ; Murton et al, ) and hot fluids (∼360 °C) which are only present in the active system.…”

Mineralogical and Geochemical Signatures of Metalliferous Sediments in Wocan-1 and Wocan-2 Hydrothermal Sites on the Carlsberg Ridge, Indian Ocean

“…This assumption is supported by the dominant and minor abundance of chalcopyrite and secondary Cu-mineral assemblages (Table 2), and higher Cu (5.19 wt.%) contents in bulk sediment (Table 3) at TVG-10. A previous study [34] has suggested Fe-dominated mineral assemblages in extinct sulfide mounds in the TAG hydrothermal field. Also, the calculated sphalerite precipitation temperature (~233 • C, n = 5) further suggests the occurrence of a low-intermediate temperature hydrothermal venting, rather than extinction at TVG-10.…”

“…The results of 10 representative bulk sulfur isotope analysis (δ 34 S) are shown in Table 6. The δ 34 S values of the Wocan-1 sediment varied spatially from 3.0-3.6‰, from station TVG-08 to TVG-07 (mean value; 3.45‰, n = 4). The Wocan-2 sediments were slightly broader and heavier and varied spatially from 4.1-4.3‰ and 6.4-8.7‰ at station TVG-10 to TVG-05 ( Figure 3), mean value; 6.28‰, n = 6).…”