Geological, Mineralogical and Textural Impacts on the Distribution of Environmentally Toxic Trace Elements in Seafloor Massive Sulfide Occurrences

Abstract

With mining of seafloor massive sulfides (SMS) coming closer to reality, it is vital that we have a good understanding of the geochemistry of these occurrences and the potential toxicity impact associated with mining them. In this study, SMS samples from seven hydrothermal fields from various tectonic settings were investigated by in-situ microanalysis (electron microprobe (EMPA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS)) to highlight the distribution of potentially-toxic trace elements (Cu, Zn, Pb, Mn, Cd, As, Sb, Co, Ni, Bi, Ag and Hg) within the deposits, their minerals and textures. We demonstrate that a combination of mineralogy, trace element composition and texture characterisation of SMS from various geotectonic settings, when considered along with our current knowledge of oxidation rates and galvanic coupling, can be used to predict potential toxicity of deposit types and individual samples and highlight which may be of environmental concern. Although we cannot quantify toxicity, we observe that arc-related sulfide deposits have a high potential toxicity when compared with deposits from other tectonic settings based on their genetic association of a wide range of potentially toxic metals (As, Sb, Pb, Hg, Ag and Bi) that are incorporated into more reactive sulfosalts, galena and Fe-rich sphalerite. Thus, deposits such as these require special care when considered as mining targets. In contrast, the exclusive concern of ultra-mafic deposits is Cu, present in abundant, albeit less reactive chalcopyrite, but largely barren of other metals such as As, Pb, Sb, Cd and Hg. Whilst geological setting does dictate metal endowment, ultimately mineralogy is the largest control of trace element distribution and subsequent potential toxicity. Deposits containing abundant pyrrhotite (high-temperature deposits) and Fe-rich sphalerite (ubiquitous to all SMS deposits) as well as deposits with abundant colloform textures also pose a higher risk. This type of study can be combined with “bulk lethal toxicity” assessments and used throughout the stages of a mining project to help guide prospecting and legislation, focus exploitation and minimise environmental impact.

Silver-Rich Chalcopyrite from the Active Cerro Pabellón Geothermal System, Northern Chile

“…In modern hydrothermal settings, studies presenting trace element data of chalcopyrite are predominantly restricted to submarine hydrothermal environments, i.e., seafloor massive sulfides and active hydrothermal chimneys. For example, previous studies have reported LA-ICP-MS data showing that chalcopyrite from black smoker vents can host variable amounts of Zn, As, Sb, Ag, Pb, Bi, Se, Te, Co, and Ni, among other trace elements [12,[20][21][22][23][24][25][26]. Furthermore, [27] presented micro-analytical data of chalcopyrite retrieved from drill cuttings at the seawater-dominated Reykjanes geothermal system in Iceland.…”

Inactive Sulfide Ecosystems in the Deep Sea: A Review

“…Enhanced oxidation of copper sulfides and copper dissolution in galvanic cells (as well as zinc, lead, and cadmium) during mining operations (Fallon et al, 2017;Knight et al, 2018), together with known toxic effects of soluble copper (Flemming and Trevors, 1989;Stohs and Bagchi, 1995;Simpson and Spadaro, 2016), suggest the potential for geochemical modification/degradation of the environment. The toxicity of sulfides differ from one place to another, depending on their geological setting, mineralogy, and crystalline texture (Fallon et al, 2019). Based on limited laboratory experiments, metal toxicity has been considered to be minor relative to the physical impacts of mining (Simpson and Spadaro, 2016;Knight et al, 2018), but this perspective does not take into account ecotoxicological effects in the pelagic environment, and empirical evidence in the field supporting this view is lacking.…”

Mechanical Tests and Numerical Simulations for Mining Seafloor Massive Sulfides

“…where R is the UCS of SMS, MPa; P is the failure load, N; A is the cross-section area perpendicular to the loading direction, mm 2 . During the UCS test, the static deformation parameters can be measured synchronously to obtain the strain-variable and calculate the elastic modulus E and Poisson's ratio µ.…”

“…Seafloor Massive Sulfides (SMS), formed by the hydrothermal or chemical processes on the seafloor, are rich in precious and base metals such as gold, silver, manganese, iron, cobalt, etc. [1][2][3][4][5]. Currently, SMS deposits are primarily exploited by a spiral drum with cutting picks.…”