Mineralogy and geochemistry of contrasting hydrothermal systems on the Arctic Mid Ocean Ridge (AMOR) : the Jan Mayen and Loki’s Castle vent fields

Tese de doutoramento, Ciências do Mar, Universidade de Lisboa, Faculdade de Ciências, 2016

Underwater hydrothermal vent fields are known since the 1970s and occur in a variety of oceanic settings, each field with unique particularities. The potential importance of these hydrothermal deposits for the global economy is growing and each new discovery opens new ground for exploration for metal resources. The Arctic Ocean presents itself as an opportunity to study new interesting areas and numerous hydrothermal systems (active and inactive) were already discovered (or detected) along the Arctic Mid Ocean Ridge (AMOR). The AMOR is an ultra-slow spreading ridge, in which, according to modern views, there is a reduced but more focused magma supply, capable of sustaining long-lived hydrothermal circulation which increases the potential for the formation of large polymetallic hydrothermal deposits. Jan Mayen and Loki’s Castle are two hydrothermal vent systems discovered in 2005 and 2008 respectively in the Mohns Ridge, a segment of the AMOR. The work presented here originated from the need to understand these vent fields from a ore resource viewpoint, in order to evaluate the possible economic interest of each area. Sediments from each area were studied to establish the possible extent of hydrothermal influence in the surrounding terrains. Additionally, rocks of basaltic nature helped to constrain the geochemical influences on the hydrothermal fluid and understand the nature of the hydrothermal products generated on the seafloor. And finally, hydrothermal products composed of chimney structures and, in the case of Loki’s Castle, also sediments from the hydrothermal mound were studied to understand the polymetallic sulfide paragenesis of each system. There was a first macroscopic approach on these samples, followed by X-ray diffraction to better comprehend the expected mineralogy. Selected samples were polished and studied in reflected light and analyzed in the electron to understand the mineral chemistry in each vent field. A set of subsamples was powdered for whole rock geochemistry and for isotopic analyses of Sr and Nd. The results show two very distinct vent fields, with different mineralogy dominating each. Sediments from Jan Mayen show a moderate metallic enrichment towards the venting area. Sediments from the northern part of the Mohns Ridge were very far away from Loki’s Castle venting area and the occasional metal enriched layers detected are suggestive of other venting provenances. The rocks of basaltic nature from Jan Mayen have a characteristic E-MORB signature, superimposed by varied enrichment in elements as Ba, U and Pb resultant from different degrees of hydrothermal influence. The fresh basaltic sample from Loki’s Castle area show a D-MORB signature with a minor Ba and U enrichment and Pb depletion and no traces of hydrothermal influence. The differences in each system settings results in very distinct hydrothermal activity and products. Jan Mayen chimneys, located at 550-700 m water depth, are venting light colored fluids of up to 270ºC while Loki’s Castle, situated at almost 3000 m, shows a typical black smoker fluid with temperatures reaching 317ºC. These constrains coupled with the different host rocks sustaining the systems, contributes to the different paragenesis observed in each system. Jan Mayen is a Zn-rich, barite dominated system, with chimney structures composed of barite with amorphous silica towards the outer wall. Sulfides are present interlocked in the barite grid. Pyrite and marcasite are seen precipitating with colloform textures in an intermediate area of the structures, together with minor amounts of sphalerite, galena and chalcopyrite. Towards the inner walls there is an increase in the sulfide content composed of sphalerite and chalcopyrite either in a mass-like form or with a dendritic texture. Loki’s Castle is Fe- and Cu-rich, dominated by anhydrite and talc. The studied structures were composed of anhydrite, often already dissolved, and talc and talc-like material. Sulfides filling the inner walls are pyrrhotite and chalcopyrite dominated and less abundant pyrrhotite is seen towards the interior of the walls with only minor chalcopyrite and sphalerite coverings. The intermediate walls are dominated by Mg-phyllosilicates talc-derived and Fe-oxyhydroxides together with occasional blades of pyrrhotite with variable degrees of alteration. Sediments from the overlapping mounds area are Cu-rich and dominated by Mg-phyllosilicates. Pyrrhotite is ubiquitous and chalcopyrite is often altered to Cu-Fe sulfide phases as bornite, covelite and digenite, rendering a Cu enrichment in specific layers. Framboidal pyrite is occasional in these sediments. Isotopic information from Sr and Nd indicate ratios of close proximity to the host rocks from the Jan Mayen samples, while Loki’s Castle values reflect a higher seawater contribution/input. Considering an economic perspective, Jan Mayen products are much richer in base metal sulfides although the area of activity is smaller and does not have big hydrothermal mounds underneath. Loki’s Castle studied products show less abundant sulfides, probably due to alteration, but venting occurs in a wider area that is sustained by a sizable hydrothermal mound capable of hosting promising prospective mineralogy as suggested by the hydrothermal sediments addressed here. For a realistic consideration on the economic potential of each system, more work still has to be conducted with a systematic sampling approach of surface products but, more imperative, deep drilled cores to understand the system behavior in a tridimensional perspective.