Publications

The Aurora hydrothermal vent field is located on Gakkel Ridge, an ultraslow-spreading mid-ocean ridge in the Arctic Ocean, in a region of perennial sea-ice cover. Previous expeditions to Aurora revealed high-temperature black smoker chimneys on pillow basalts near an axial volcano summit, but challenges operating in drifting sea ice prevented detailed exploration and sampling of the site. The hydrothermal vents were imaged in detail for the first time during the 2021 HACON expedition, and rock sampling of three active vents and adjacent hydrothermal talus by remotely operated vehicle marked the first successful geological exploration of a hydrothermal field under ice. This study provides a geological characterization of Aurora using video footage and mineralogical and geochemical analyses of hydrothermal rock samples. While situated on mafic substrate, the hydrothermal precipitates contain Ni-Co sulfide minerals and low Si, Pb, and Ba concentrations, characteristics of ultramafic rock-influenced vent fields on slow-spreading ridges. Chimney structures are unusually thin and friable, lacking many of the lowtemperature minerals commonly found in other deposits. The results are discussed in the context of the poorly explored Gakkel Ridge, with implications for future interdisciplinary exploration of hydrothermal systems and for seafloor massive sulfide mineral potential in the Arctic.

Submarine hydrothermal systems occur where seawater infiltrates permeable oceanic crust at locations proximal to volcanic activity. Fluid-rock interactions in submarine hydrothermal systems can vary considerably, resulting in different influences affecting the formation and composition of associated seafloor massive sulfide (SMS) deposits. This dissertation utilizes the stable isotopes of sulfur (S), in conjunction with other geochemical, oceanographic, geophysical, and geobiological methodologies to constrain fluid cycling processes affecting hydrothermal systems and their contributing endmembers. Sulfur isotopes are a useful tool for tracing fluid-rock interactions, reservoir mixing and fractionation processes affecting SMS deposit formation. New S isotope data from five hydrothermal fields spanning two mid-ocean ridge systems—Juan de Fuca Ridge and East Pacific Rise (EPR)—indicate a progressive microbial alteration of host rocks that correlates with hydrothermal system maturity, host rock age, and tectonic setting. Data from the Juan de Fuca Ridge and EPR were utilized to create models in δ³⁴S and Δ³³S space that incorporate potential alteration and fractionation of endmember S sources for a more comprehensive understanding of fluid circulation. While host rock alteration is an important process affecting modern hydrothermal systems, the effect of minor variances in seawater SO₄²⁻, generally considered to be a homogenous endmember with respect to sulfur isotopes, has not previously been investigated. Using samples collected during a trans-Pacific research cruise, minor regional and water-mass specific variations in seawater sulfate S isotope signatures were found that correlate to biological productivity, ice melt influence, as well as potential atmospheric effects and hydrothermal input. These findings further demonstrate the utility of quantifying variances in endmember S reservoirs. These new ideas regarding evolution of hydrothermal endmembers over time were further applied to investigations of the newly discovered inactive Nawaay`as hydrothermal vent field on the West Valley segment of the Juan de Fuca Ridge, which was once a site of high-temperature venting. Increasing scientific understanding of the formation and subsequent preservation of such inactive seafloor massive sulfide deposits are critical for determining both the economical viability of these deposits, as well as their ecological significance in order to make informed, risk-mitigating seafloor mining marine policy decisions in the future.

The overarching goal of this project was to study seafloor hydrothermal processes along the Galapagos Spreading Centre. The project was designed around a multidisciplinary approach, combining the fields of geology, geophysics, biology, and oceanography to address a range of inter-related scientific questions related to occurrence, evolution, and ecological significance of high-temperature hydrothermal systems along the Galapagos Rift. Specific project goals included: 1. Evaluate the utility of SAS surveys as an effective tool for documenting/characterizing the seafloor environment (including geological and biological features), especially in regions of significant seafloor topography. 2. Characterize the mineralogical, geochemical, and biological evolution of hydrothermal deposits at inactive vent fields and inactive deposits within active vent fields. 3. Survey, explore, document, and sample previously unexplored vent fields. 4. Document the benthic soundscape at hydrothermal vents and measure CTD and sound velocity (i.e. bulk compressibility) to inform the equation of state at relatively high temperatures and salinities. 5. Refine the methodologies for integrating multi-resolution datasets to classify ecological and geological features on the seafloor and develop approaches for the use of these techniques for marine spatial planning and conservation.

The Oceans cover 70% of the Earth's surface, yet only 25% have been mapped at a resolution of at least ~100 m, a resolution that is coarser than the size of many important geological features on the seafloor, such as hydrothermal vents or volcanoes. On land, geological maps typically end at the shoreline due to the inaccessibility of the seafloor and challenges of working underwater (i.e., pressure, light attenuation). Seafloor surveys are carried out using acoustic techniques, such as sonars, to obtain bathymetric maps and acoustic imagery of the seafloor by measuring the travel time and intensity of an emitted acoustic signal reflected at the seafloor, respectively. Seafloor mapping, however, involves a trade-off between data resolution and coverage, with higher resolutions obtained at the cost of coverage area. Typically, the study of fine-scale (e.g., 10s of m or less) geological features relies on a multi-scale and multi-tool approach, combining large-scale acoustic mapping (meter to sub-meter resolution) with fine-scale optical surveying (millimetre resolution) and sampling of the substrate. This thesis presents an investigation of the application of different seafloor survey technologies and approaches for geological characterization of the seafloor. The research focuses specifically on the study of seafloor hydrothermal vents, which are hot springs on the seafloor that support life in the deep-oceans, and where metals, including societally relevant base and precious metals, accumulate as mineral deposits. As a case study, the classic approach to the geological characterization of a hydrothermal deposit is applied to the Fåvne vent field, located on the Mohns Ridge in the Norwegian Sea, using 1 m resolution bathymetry, optical imagery, rock samples, and sediment cores. This study improves our understanding of metal enrichment processes at the seafloor and extends our knowledge on the global diversity of hydrothermal deposits. With the development of interferometric synthetic aperture sonar (InSAS), a new high-resolution survey technology, larger areas of the seafloor can be surveyed at much higher resolutions than using conventional acoustic techniques. While InSAS has mostly been applied to survey flat terrain, in this study a comparison of using InSAS on a flat continental shelf to a rugged mid-ocean ridge terrain is presented. Identification of survey parameters necessary to optimize InSAS data quality for effective use for scientific applications, shows that InSAS is an appropriate tool for surveys even in morphologically complex terrain. Through the identification of bedrock structures and overburden classes from InSAS data, InSAS is evaluated for geological applications, such as bedrock stratigraphy and tectonics, glacial reconstructions, and marine litter distribution. Overall, this thesis shows that, compared to conventional techniques, InSAS is an effective tool for high-resolution characterization of seafloor geological features in various geomorphological environments, thus opening the door for a significant increase in potential applications, including for the survey of hydrothermal deposits.

Seafloor hydrothermal discharge results in accumulations of sulfide minerals that can be rich in base, precious, and critical metals. An increasing demand in these metals in order to transition to a low-carbon economy has resulted in an interest to evaluate the potential of seafloor resources. This dissertation focuses on fundamental geological processes that lead to the formation of these hydrothermal metal-rich deposits. To achieve this, the age, rate of accumulation, composition, and efficiency of metal deposition of the hydrothermal deposits was determined at the Lucky Strike vent field, which is located on the Mid-Atlantic Ridge, ~400 km southwest of the Azores archipelago. Using the ²²⁶Ra/Ba dating technique, hydrothermal barite is dated from these hydrothermal deposits to determine that the hydrothermal field at Lucky Strike has been active at least for ~6,600 years. The tonnage of hydrothermal material that accumulated above the seafloor at Lucky Strike was estimated to be ~1.3 Mt, this was estimated using ~1 m resolution bathymetry at this site. This means that the hydrothermal deposits are accumulation at a minimum rate of ~194 tons/year. The hydrothermal deposits are dominated by barite, anhydrite, marcasite, pyrite, sphalerite, and chalcopyrite, and supergene alteration that consists of goethite and minor atacamite, covellite, and bornite. The bulk composition of these deposits is similar to other Mid-Atlantic ridge hydrothermal deposits but show elevated concentrations of Ba, Sr, and Mo, which reflects the E-MORB substrate geochemistry associated with the nearby Azores hotspot. Furthermore, the average deposit Ba/Co is useful to discriminate between different substrate compositions, such as E-MORB, N-MORB, and ultramafic-hosted sites. The sulfur isotopic composition of marcasite and chalcopyrite ranges from -2.5 to 8.7‰. The Capelinhos vent site, located 1.4 km east of the main Lucky Strike field is enriched in ³⁴S, which reflects differences in sub-seafloor fluid/rock interactions and fluid pathways at these two sites. Sub-seafloor precipitation at the main Lucky Strike field results in <20% of the reduced sulfur of the upwelling hydrothermal fluid reaching the seafloor. Moreover, data on the rate of accumulation and geochemical composition of the hydrothermal deposits and compared to available fluid flux and fluid chemistry was used to determine how much of the metals were lost to seawater and how much of this was precipitated as hydrothermal deposits. This efficiency of metal precipitation is estimated for two of the sites at Lucky Strike, Sintra and tour Eiffel. The metal precipitation efficiency was determined to be 38-99% for Cu, 64-78% for Zn, 14-76% for Fe, and <1% for Mn and Si and seems to be element specific and temperature dependent. In addition, these efficiencies vary at the different scales when this study is compared to the recalculated depositional efficiencies at other hydrothermal sites such as TAG and Endeavour. Furthermore, the efficiencies of precipitation vary at the vent field scale, with the Sintra site showing overall higher efficiencies for Cu, Zn, and Fe, when compared to Tour Eiffel. These estimated metal depositional efficiencies have implications for metal fluxes to the oceans as well as ore forming processes. Finally, fault geometry analysis indicates that hydrothermal venting and associated hydrothermal deposits are largely controlled by enhanced permeability that results from local scale offsets in stress field orientations that form relay ramps and fault linkages.

As the exploration and exploitation of seafloor polymetallic deposits appears to be the next frontier in mineral exploration, developing and optimizing remote sensing methods to locate and study these deposits is becoming increasingly important for understanding the resource potential and environmental implications of mining from the deep seafloor. One such deposit type is seafloor massive sulfide (SMS) deposits, which form on and below the seafloor at sites of high-temperature hydrothermal fluid venting at a variety of tectonic settings where seafloor extension and magmatism takes place. SMS deposits have promise to offer new sources of Cu, Zn, Pb, Au, and Ag, but the remote environment in which they are located creates difficulties for their discovery and resource estimates. In particular, the proportion of ore found below the seafloor, versus that found within the sulfide mound, has only been estimated from a limited number of collected drillcores. These cores are expensive, with respect to time and money, and unless collected in large numbers, and to sufficient depth, offer limited geometric information of the subseafloor components of the deposit. Alternatively, magnetic voxel-based inverse models can be used to locate SMS deposits as magnetic lows, due to the hydrothermal fluids stripping much of the magnetite from the alteration zone, and the data can be inverted with a surface geometry inverse (SGI) modelling method to resolve subseafloor structures. Additionally gravity inverse models can be used to model the SMS deposits' massive sulfide layer as a density high. The SGI method inverts for the position of nodes in a wireframe mesh rather than physical properties within a fixed mesh. This thesis demonstrates the SGI method to be an excellent tool for modelling the contact surfaces between the sulfide mound, the hydrothermally altered chloritized basalt, and least altered basaltic host rock. The volumes within these surfaces can then be used to calculate an estimated tonnage for the ore located below the seafloor, developing a better resource model for SMS deposits.

Inactive seafloor hydrothermal vents on the seafloor are continuously exposed to oxygen-rich seawater and subjected to oxidative processes. These processes result in mineral breakdown, structural instability, and eventual collapse of the vents. Rates and timing of these processes are largely unconstrained, despite laboratory experiments using combinations of oxidizing agents, environmental conditions, and reacting mineral species. Here, sixteen sulfide- and oxyhydr(oxide)-rich samples were collected from inactive hydrothermal vents along the Endeavour Segment of the Juan de Fuca Ridge to document natural oxidative processes. Petrographic observations indicate preferential oxidation of low rest potential minerals due to the formation of galvanic cells between polymetallic sulfide minerals. Under ambient seawater conditions, sulphide-rich samples demonstrate dual production of iron oxide and oxyhydroxide precipitates produced by abiotic oxidation of sulfide minerals or microbially-mediated precipitation of Fe-oxyhydr(oxide) on exterior surfaces. The results suggest that dissolution of minerals hosting valuable metals like Cu and Zn will occur more rapidly than Fe-bearing minerals. Radiometric ²²⁶Ra/Ba ages were determined from a subset of the collected samples. This subset increases in age with distance from the current spreading center, supporting previous results indicating progressive aging of hydrothermal vents from the axial spreading center.

Submarine hydrothermal vent systems hosted at sedimented mid-ocean ridges are often associated with larger seafloor massive sulfide deposits than typical sediment-free mid-ocean ridge systems. The Juan de Fuca spreading ridge contains both sediment-hosted and sediment-free hydrothermal systems (Middle Valley and Axial Volcano, respectively). The ridge also contains the Endeavour vent field, which occurs at the outer extent of turbiditic sediments, where the seafloor is currently sediment-free and consists of basaltic lava flows, but evidence from hydrothermal vent fluid composition suggests the presence of buried sediment. Multiple sulfur isotope ratios of hydrothermal precipitates from these three sites were analyzed to isotopically fingerprint differences in hydrothermal sulfur cycling associated with sedimented and sediment-free substrates. A three-component mixing model in Δ³³S and δ³⁴S space was developed that represents the differing contributions of sulfur derived from seawater, magmatic sources, and both sediment and crustal sulfur sources that have been influenced by microbial activity.