Trace Element Assemblages of Pseudomorphic Iron Oxyhydroxides of the Pobeda-1 Hydrothermal Field, 17°08.7′ N, Mid-Atlantic Ridge: The Development of a Halmyrolysis Model from LA-ICP-MS Data

Abstract

Halmyrolysis, as one of the global processes of alteration of seafloor hydrothermal sediments, needs to be recognized in terms of mineral and trace element evolution to elaborate new criteria for metallogenic and geoecological forecasts with respect to ocean exploration. The purpose of this paper is to explain trace elements’ behavior during the halmyrolysis of sulfide deposits. This task is resolved using an LA-ICP-MS analysis of iron oxyhydroxides (IOHs) on examples of oxidized pyrrhotite-rich diffusers of the ultramafic-hosted Pobeda-1 hydrothermal field (Mid-Atlantic Ridge). The IOHs formed after the sulfides were enriched in seawater-derived trace elements (Na, K, Mg, Ca, Sr, P, U, Mo, V, REE, Cr). Six trace element assemblages (TEAs) are statistically recognized for the IOHs. TEA-I (Cu, In, Sn, Bi, Se, Te) is inherited from chalcopyrite, isocubanine and bornite microinclusions. TEA-II is typical of Zn sulfides (Zn, Cd, Sb, Tl, Ag) interacted with seawater (Mg, U, Mo, Ni, Na, K) and hydrothermal fluid (Eu). TEA-III (Ca, Sr, Cu, Si, Se, P, As) reflects the inclusions of aragonite, opal, atacamite and possibly native selenium, while P and As occur as absorbed oxyanion groups on IOHs or Ca–Fe hydroxyphosphates. TEA-IV (Al, Ga, Ge, Tl, W, Ti ± Mn, Co, Ba) indicates the presence of minor clays, Co-rich Mn oxyhydroxides and barite. TEA-V with Pb and V is closely related to TEA-VI with REEs except for Eu. The halmyrolysis of sulfides includes two stages: (i) oxidation of S(II) of primary sulfides and the formation of supergene sulfides, which scavenge the redox-sensitive elements (e.g., U, Mo, Ni, Eu), and (ii) oxidation of Fe (II) to Fe (III) and absorption of most elements of TEAs III, IV, V and VI by IOHs.

Fluid Inclusion Studies of Barite Disseminated in Hydrothermal Sediments of the Mohns Ridge

“…A basic explanation of the mineralogical and geochemical diversity of deposits in the studied systems is the degree of maturity of the ore-generating hydrothermal systems, which is determined by the stages of mineral depletion of source rocks, beginning with the depletion of olivines, pyroxenes, and other dark-colored minerals and finishing with feldspars [76]. The formation of the opal, barite, Fe-and Zn-rich edifices of the Jan Mayen and the anhydrite, talc, Fe-and Cu-rich edifices of Loki's Castle is confined to this one of the last stages of depletion.…”

“…According to [38], Loki's Castle may be a more mature vent field than Jan Mayen. The mineralogy of pyrrhotite-rich surface sediments of the Loki's Castle vent field suggests the first stage of alteration of seafloor sulfide sediments, halmyrolysis, according to [76], where two stages of hydrothermal sulfide halmyrolysis have been identified. Pyrite-and marcasite-rich surface sediments of the Troll Wall vent field are attributed to the transition between the first and second stages of halmyrolysis.…”