Jade Umbsaar, PhD

“Serpentinization” is a blanket term representing a multitude of redox reactions that occurs in ultramafic rock types when exposed to seawater, collectively resulting in a highly reduced hydrothermal fluid of variable pH, temperature, and critical metal load, that is capable of contributing to overlying seafloor massive sulfide (SMS) deposits, particularly in off-axis oceanic core complex settings of ultraslow-spreading mid-ocean ridge segments. Recent exploration aboard the R/V Falkor (too) in March-April of 2023 discovered three new active hydrothermal vent fields in the North Atlantic, between the ranges of 20 – 25°N, with each deposit representing a varied exposure to ultramafic rock types. These deposits are Puy de Folles, EMARK, and Grappe Deux. The Puy de Folles deposit is hosted exclusively within mafic volcanics at the intersection of the Mid-Atlantic Ridge, and a non-transform offset. The EMARK deposit is hosted within mixed mafic-ultramafic rock types on the inside corner high of the Kane Fracture Zone, wherein multiple minor episodes of detachment faults exposed several serpentinite blocks intercalated with young, glassy, mafic volcanics. Finally, the Grappe Deux deposit is hosted almost exclusively within ultramafic rocks at the toe of an oceanic core complex in a non-transform offset. Trends in the bulk geochemistry of the collected 32 grab samples indicate enrichments of mantle-incompatible critical elements within the mafic-hosted site (Pb, Ag, Hg, As, Sb, Mo); whilst the ultramafic hosted site is enriched in mantle-compatible critical metals (Co, Bi). However, the mixed hosted site demonstrates strong enrichments of critical metals (Ag, Sn, Ga, Ge, Zn, Si), and most notably Au. The mineralogy of the serpentinites provides insight into the hydrothermal and volcanic histories of these SMS deposits, with the mixed-hosted deposit containing “early-stage” serpentinite minerals and geochemical signatures indicative of mafic melt impregnation (elevated Al2O3/SiO2), whereas the ultramafic-hosted deposit contains signatures indicative of a more evolved serpentinite (abundant talc, tremolite, and magnetite), as well as minimal carbonate veinlets. The strong correlations between Au and low temperature phases such as silica and sphalerite at EMARK demonstrates the importance of basalt as a redox buffer for the mobilization of gold. The variable critical metal enrichments at these sites demonstrate the geochemical and temporal variability of ultramafic-hosted SMS deposits as serpentinizing reactions evolve through subsequent buffering interactions with the host rock. These observations provide insight into both the influence of host lithology on critical metal endowment, and the potential effects of prolonged hydrothermal circulation in SMS deposits.