Oxidative dissolution of hydrothermal mixed-sulphide ore: An assessment of current knowledge in relation to seafloor massive sulphide mining

Abstract

The weathering process of seafloor massive sulphide (SMS) deposits can be considered analogous to weathering of terrestrial volcanogenic massive sulphides (VMS) deposits. However, in the context of SMS deposits, the process occurs in chemically buffered waters of near neutral pH, resulting in the formation of insoluble Fe oxy-hydroxide minerals including goethite and hematite as well as sulphates such as jarosite. As a result of this precipitation, it is commonly assumed that any SMS deposit is unlikely to exhibit a significant loss of metals (dissolution and release of heavy metals) into the water column. However, galvanic interactions have never been considered in this seafloor context, whilst they have already been shown to have the ability to increase dissolution significantly in terrestrial deposits. If heavy metal release is not temporally balanced by precipitation of oxide phases, there is the potential that these metal occurrences lose economic value. This is specifically significant if there is an SMS deposits Oxidation Galvanic Cells Metal release Economic potential Mining impact

Characterisation of Mineralised Material from the Loki’s Castle Hydrothermal Vent on the Mohn’s Ridge

“…Au content of high temperature chimney material is consistent with that of massive sulphides from the interior of the mounds and the underlying stockwork zones [7] which suggests the Au grades measured in this study could be continuous throughout the site. Similarly, Zn grades are good, Ag comparable, and Cu low when compared to other SMS deposits [7,12,16,52]; for example, the TAG has a central volume grading under 0.01% Zn, but over 2% Cu [36]. Land-based mafic-hosted VMS deposits have typical Cu, Zn, Au, and Ag grades of 2%, 1.9%, 2.5 ppm and 25 ppm, respectively [6], which are comparable with the values found in this study.…”

Identifying Toxic Impacts of Metals Potentially Released during Deep-Sea Mining—A Synthesis of the Challenges to Quantifying Risk

“…If the scientific community continues with this approach, progress in this field will be too slow to be incorporated usefully into recommendations for contractors (see also, the arguments in Jager et al, 2006). As an alternative, we propose that it will be necessary to assess the "bulk toxicity" of each mineral deposit to identify a priori the potential toxic risk of each mineral resource to be mined within a license area (e.g., Harris et al, 2014;Fallon et al, 2017;Simpson et al, 2017; see also discussion of the "Weight Of Evidence" approach below). In practice, it actually may not be necessary to quantify the individual toxicity of each metal within each resource (although assessment of mineral composition is undoubtedly integral to resource classification; e.g., International Council on Mining Metals, 2013).…”

Sulfide Breccias from the Semenov-3 Hydrothermal Field, Mid-Atlantic Ridge: Authigenic Mineral Formation and Trace Element Pattern

“…Over the recent decades, great progress has been achieved in understanding trace element behavior in sulfide minerals from oceanic hydrothermal fields [1][2][3][4][5][6][7][8]. One of the reasons has been the development of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and its application in mineral geochemistry [1][2][3][4][5][6][7][8][9][10]. Most studies of oceanic hydrothermal fields concern the geochemistry of sulfides from smoker chimneys, diffusers, and massive or colloform ores, which are the result of medium-to high-temperature hydrothermal processes.…”

“…Another key point of seafloor hydrothermal sulfide fields is their possible future mining. Concern about environmental hazards (even for exploration of inactive hydrothermal fields) and the relatively small sizes, tonnages and grades of seafloor sulfide bodies compared to many on-land VHMS deposits [9,[20][21][22][23] have generated skepticism about the real mining potential of marine resources, although first steps have already been made to approach their possible exploitation [24,25]. In seafloor hydrothermal sites, the valuable base and precious metals are mostly contained in Cu-(Fe) (chalcopyrite, isocubanite, covellite and related minerals) and Zn (sphalerite, wurtzite) sulfides, but Fe sulfides make up a significant part of both seafloor and subseafloor ores [4,10,[26][27][28][29][30][31][32][33][34][35][36][37].…”

“…This circumstance should be taken into account during seafloor mining and extraction of valuable components, since pyrite can host up to a few percents of Cu and Zn, as well as noticeable contents of As, Co, Ni, Se, Hg, etc. (see review in [9]).…”