Periodic Reporting for period 1 - SEA METAL FLUX (Metal fluxes in the oceanic crust and formation of Volcanogenic Massive Sulfides in the Samail Ophiolite, Oman)
Reporting period: 2020-09-01 to 2022-08-31
The Samail Ophiolite in Oman is the world’s largest and best-exposed section of oceanic lithosphere. Representing an important source of base metals and revenue for the country, it hosts more than 20 volcanogenic massive sulfide (VMS) deposits. The purpose of the SEA METAL FLUX project was to study oceanic crust samples from this ophiolite to provide a deeper understanding of the magmatic–hydrothermal processes forming VMS deposits. For this it was necessary to unravel how sulfur and metals are mobilized during hydrothermal circulation and transported to form VMS deposits close to the seafloor. Evidence shows that hydrothermal circulation in the oceanic crust reaches the depths of the crust–mantle transition, but how it influences the shallow VMS-forming processes remains an open question.
Hydrothermal circulation in the oceanic crust is a first order, global process that modifies the composition of the oceanic crust that \impacts on the composition of the oceans and the mantle once it is subducted. Understanding hydrothermal alteration in oceanic crust is a key piece of our understanding of biological and geochemical cycles in our planet. It is also a crucial part of ore-forming processes leading to the formation of VMS deposits. which are formed in the oceans where they can theoretically be exploited in the seabed. Tectonic processes are able to transport VMS deposits to land, where they can be conventionally exploited. SEA METAL FLUX overall objectives are to understand what contributes to the metal signature of VMS deposits and what is the source and amount of metals that are mobilized from oceanic crust rocks during hydrothermal alteration to form them.
Hydrothermal circulation in the oceanic crust is a first order, global process that modifies the composition of the oceanic crust that \impacts on the composition of the oceans and the mantle once it is subducted. Understanding hydrothermal alteration in oceanic crust is a key piece of our understanding of biological and geochemical cycles in our planet. It is also a crucial part of ore-forming processes leading to the formation of VMS deposits. which are formed in the oceans where they can theoretically be exploited in the seabed. Tectonic processes are able to transport VMS deposits to land, where they can be conventionally exploited. SEA METAL FLUX overall objectives are to understand what contributes to the metal signature of VMS deposits and what is the source and amount of metals that are mobilized from oceanic crust rocks during hydrothermal alteration to form them.
The work and main results of SEA METAL FLUX can be summarised into its four main objectives
1. What contributes to different metal signatures of VMS deposits?: The type, amount and distribution of metals within a VMS deposits is affected by different factors such as composition of the lavas, how fluids rise through the crust and are vented on the seafloor or even by the oxidation of the ores by seawater after their formation. We studied ores, lavas and oxidised ores using various mineralogical and geochemical isotopic techniques, (osmium-Os and lead-Pb, strontium-Sr and sulfur-S) to understand the relative influence of those factors. We studied two VMS deposits with different features and concluded that growth mainly above seafloor as large in large hydrothermal fields is facilitated by faulting (Mandoos). Deposits sealed by impermeable rock (jaspers) grow by hydraulic fracturing beneath the seafloor. Metals are mobilised from the volcanic footwall, and reflect their composition, which defies the paradigm of metal leaching from deep reaction zones. We also provided important constraints to the effects of secondary processes on metal redistribution and upgrading.
2. What is the primary metal budget of the oceanic crust? We quantified how much metal exists in the oceanic crust that can be mobilized during hydrothermal alteration. For this, we studied a crustal profile from the upper crust down to the mantle transition drilled by the Oman Drilling Project. We concluded that the upper crust mostly comprises secondary (hydrothermal) sulfides, the hydrothermal/magmatic sulfides ratio decreasing with depth and increasing towards areas of higher fluid flow.
3. What are the fluid pathways of metal cycling? We strived to understand how hydrothermal fluids travel through the crust from very hot, near magmatic conditions (~900C) down to “cooler” temperatures (<350), and how metals and sulfur are scavenged to ultimately form a VMS deposit. We assessed hydrothermal alteration in different rocks based on S and Sr isotopes and how much metal (Copper, Zinc) was scavenged or deposite. We concluded that Sulfur and Sr isotopic signatures are often decoupled during hydrothermal processes and have characteristic patterns along the oceanic crust. Sulfur and metal remobilization in the upper oceanic crust far exceeds that in the lower crust although seawater is sometimes introduced at depth along faults. The crust-mantle transition is the most complex domain and a sequence of gabbros, dunites and rodingites (metasomatized gabbro sills) were thoroughly investigated
4. A model for metal fluxes in the oceanic crust Our ultimate objective is to bring all information together to produce a numerical model for metal sulfur and cycling in the oceanic crust using appropriate modelling software. Preliminary data show that sulfur and metal remobilization in the upper oceanic crust is much more significant than what is recorded in in-situ modern crust, supporting the view that metal and sulfur fluxes are much more significant in ophiolites.
Exploitation
SEA METAL FLUX scientific outcomes are exploitable for scientists and mining and exploration stakeholders (companies and governments) working with modern day or ancient Volcanogenic Massive Sulfides. Also, to scientists working with subseafloor hydrothermal processes or with the biogeochemical sulfur cycles and their relation with deep-life processes. All data pertaining VMS of Oman are of special interest to local stakeholders from Public Authority for Mining and Mining companies in Oman. All results will be made available in published conference and manuscripts, with datasets being made available in the ZENODO open database as per FAIR principles.
1. What contributes to different metal signatures of VMS deposits?: The type, amount and distribution of metals within a VMS deposits is affected by different factors such as composition of the lavas, how fluids rise through the crust and are vented on the seafloor or even by the oxidation of the ores by seawater after their formation. We studied ores, lavas and oxidised ores using various mineralogical and geochemical isotopic techniques, (osmium-Os and lead-Pb, strontium-Sr and sulfur-S) to understand the relative influence of those factors. We studied two VMS deposits with different features and concluded that growth mainly above seafloor as large in large hydrothermal fields is facilitated by faulting (Mandoos). Deposits sealed by impermeable rock (jaspers) grow by hydraulic fracturing beneath the seafloor. Metals are mobilised from the volcanic footwall, and reflect their composition, which defies the paradigm of metal leaching from deep reaction zones. We also provided important constraints to the effects of secondary processes on metal redistribution and upgrading.
2. What is the primary metal budget of the oceanic crust? We quantified how much metal exists in the oceanic crust that can be mobilized during hydrothermal alteration. For this, we studied a crustal profile from the upper crust down to the mantle transition drilled by the Oman Drilling Project. We concluded that the upper crust mostly comprises secondary (hydrothermal) sulfides, the hydrothermal/magmatic sulfides ratio decreasing with depth and increasing towards areas of higher fluid flow.
3. What are the fluid pathways of metal cycling? We strived to understand how hydrothermal fluids travel through the crust from very hot, near magmatic conditions (~900C) down to “cooler” temperatures (<350), and how metals and sulfur are scavenged to ultimately form a VMS deposit. We assessed hydrothermal alteration in different rocks based on S and Sr isotopes and how much metal (Copper, Zinc) was scavenged or deposite. We concluded that Sulfur and Sr isotopic signatures are often decoupled during hydrothermal processes and have characteristic patterns along the oceanic crust. Sulfur and metal remobilization in the upper oceanic crust far exceeds that in the lower crust although seawater is sometimes introduced at depth along faults. The crust-mantle transition is the most complex domain and a sequence of gabbros, dunites and rodingites (metasomatized gabbro sills) were thoroughly investigated
4. A model for metal fluxes in the oceanic crust Our ultimate objective is to bring all information together to produce a numerical model for metal sulfur and cycling in the oceanic crust using appropriate modelling software. Preliminary data show that sulfur and metal remobilization in the upper oceanic crust is much more significant than what is recorded in in-situ modern crust, supporting the view that metal and sulfur fluxes are much more significant in ophiolites.
Exploitation
SEA METAL FLUX scientific outcomes are exploitable for scientists and mining and exploration stakeholders (companies and governments) working with modern day or ancient Volcanogenic Massive Sulfides. Also, to scientists working with subseafloor hydrothermal processes or with the biogeochemical sulfur cycles and their relation with deep-life processes. All data pertaining VMS of Oman are of special interest to local stakeholders from Public Authority for Mining and Mining companies in Oman. All results will be made available in published conference and manuscripts, with datasets being made available in the ZENODO open database as per FAIR principles.
SEA METAL FLUX contributed to a deeper understanding of hydrothermal processes in the oceanic crust. For the first time, it is possible to characterise and quantify the extent to which sulfur and metals are scavenged from the seafloor in VMS discharge zones to the crust-mantle Moho Transition Zone (MTZ). The project revealed a large decoupling in hydrothermal circulation above and below the small magma chamber beneath the ridge. The lower crust is overall less altered but channelized fluid flow areas record unequivocal introduction of seawater deep into the crust. The crust-mantle transition has a complex history of sulfur loss during serpentinization and late re-fertilization during gabbro sill intrusion and rodingite formation, leading to extreme isotopic shifts.
The project’s findings support that ophiolitic crust is several orders of magnitude more altered than in-situ oceanic crust and that the upper crust records the most pervasive hydrothermal fluxes. There is compelling and independent evidence that metals forming VMS deposits are scavenged not just from the sheeted dike gabbro transition (“reaction zone”), but also from the volcanic section hosting the deposits. Many of these outcomes challenge the current views on VMS deposit formation thus fulfilling the ambition that SEA METAL FLUX could provide transformative knowledge on oceanic crust hydrothermal and ore-forming processes.
The clearer understanding of hydrothermal circulation in the oceanic crust provides support for future exploitation of geological and biogeochemical processes by a range of scientific areas. These finding are relevant to the quest for understanding deep life reservoirs whereas hydrothermal vents and serpentinization are considered fundamental ingredients to have supported early life on earth. Additionally, societal impacts are tightly connected to socio-economic factors improving ore models for VMS deposits which will benefit their exploration and mining processes.
The project’s findings support that ophiolitic crust is several orders of magnitude more altered than in-situ oceanic crust and that the upper crust records the most pervasive hydrothermal fluxes. There is compelling and independent evidence that metals forming VMS deposits are scavenged not just from the sheeted dike gabbro transition (“reaction zone”), but also from the volcanic section hosting the deposits. Many of these outcomes challenge the current views on VMS deposit formation thus fulfilling the ambition that SEA METAL FLUX could provide transformative knowledge on oceanic crust hydrothermal and ore-forming processes.
The clearer understanding of hydrothermal circulation in the oceanic crust provides support for future exploitation of geological and biogeochemical processes by a range of scientific areas. These finding are relevant to the quest for understanding deep life reservoirs whereas hydrothermal vents and serpentinization are considered fundamental ingredients to have supported early life on earth. Additionally, societal impacts are tightly connected to socio-economic factors improving ore models for VMS deposits which will benefit their exploration and mining processes.