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Isotope constraints on the contribution of metal-rich magmatic fluids to back-arc seafloor hydrothermal systems

Final Report Summary - ISOBAB (Isotope constraints on the contribution of metal-rich magmatic fluids to back-arc seafloor hydrothermal systems)

Executive Summary:

1.1. Project objectives.

Geochemical cycling (the flow of elements through the Earth’s reservoirs) is an important control of the environmental processes (in general) and Earth’s climate system, and influences human activities. Chemical exchanges between the Earth’s surface reservoirs, lithosphere-ocean-atmosphere, are focused at the active plate boundaries and are important in the global geochemical balance. The engine that transfers chemicals between the lithosphere and ocean is the hydrothermal circulation at these boundaries. However, the structure of lithospheric contribution (hydrothermal leaching of basement rocks vs. direct mantle degassing) to the ocean chemistry is poorly understood and deserves particular scientific attention. On the other hand, the insight into this problem (sources of metals and location of their deposition at the seafloor) becomes increasingly important with the current interest in mining seafloor deposits and has high societal relevance.

Hydrothermal circulation in mid-ocean ridges (MOR) results in relatively uniform both hydrothermal fluids and mineral deposits. Therefore, the back-arc basins (BAB) in which the hydrothermalism produces a wide range of vent fluid chemistry and mineral deposits are the best setting for investigation of the structure of lithospheric contribution to the ocean chemistry. Additionally, the BAB are considered as critically important for the genesis of volcanogenic massive sulfide deposits. While it has been suggested that input of magmatic fluids plays an important role in the chemistry of the BAB hydrothermal systems, its contribution as source of both transition and precious metals, has been a matter of debate.

It is presently unclear whether the unique patterns of metal enrichments and/or depletion in BAB settings reflect: (1) composition of the source host rock ranging from mafic to felsic; (2) direct input (i.e. degassing) of metal-rich magmatic volatiles; or (3) remobilization at low pH of previously deposited sulphides in sub-seafloor. These processes have been difficult to distinguish and evaluate based solely on chemical compositions of hydrothermal fluids and deposits and new approaches are required. Therefore, the major objective of our project was to develop and test new stable isotope proxies to gain an insight into the relative role of metal-rich magmatic fluids and sub-seafloor metal precipitation/remobilization in back-arc seafloor ore deposits. Through the study of Cd-Zn-Sb isotope systematic coupled with S-isotopes in seafloor hydrothermal vent systems (fluids, deposits and substrate rocks) in the Manus and Lau BAB, we aimed at providing new constraints to define the relative role of these two contributions to back-arc mineralization.

1.2. Work carried out to achieve the project’s objectives.

The project was analytically based and relied on analytical feasibility and sample availability in the Host Institution. Large sample collection (hydrothermal fluids, sulfide deposits, host rocks) from the Manus back-arc basin, Lau back-arc basin and East Pacific Rise (EPR) 9°50´N was investigated. The experimental work was focused on the samples from the Manus back-arc basin since they presented hydrothermal settings with a wide range of chemical and physical parameters: varying hydrothermal systems from basalt-hosted to dacite-hosted. Analyzed vent fluids spanned a wide range of pH, temperature, and chemical composition suggesting very different sub-seafloor processes. Hydrothermal fluids (45; 19 paired with chimney sulfides), sulfide deposits (53) and host rocks (10) from 12 vent fields (Vienna Woods, Roman Ruins, Roger’s Ruins, Satanic Mills, Fenway, Snowcap, Tsukushi, Desmos, Suzette, North Su, South Su, North East Pual) in 4 areas (Manus Spreading Centre, Pual Ridge, Desmos Caldera and SuSu Knolls) of the Manus Basin were investigated for chemistry (elemental composition and isotopes) and mineralogy. Hydrothermal fluids (10), hydrothermal suspension (21) and sulfide deposits (13) from four vents (Tica, Bio9″, P and Ty-Io) at the EPR 9°50′N were studied for chemistry and mineralogy as well.

Experimental work started with bulk geochemical and mineralogical investigation of the entire sample suite in order to lay foundation of the further isotope studies. The bulk geochemical (major, minor and trace elements) and mineralogical (optical microscope investigations of polished sections, X-ray diffraction, electron microprobe) data for sulfide and host rock samples, coupled with the detailed major, minor and trace element data for the hydrothermal fluid samples were used for determining the most appropriate samples for isotope analyses.

Thin polished sections were prepared from each sulfide sample for mineral determination and texture interpretation and supplemented by both X-ray diffraction and electron microprobe analyses. Isotope analyses were planned for the matching faces to these thin sections and latter acquired on individual mineral phases separated either with careful hand-picking or using micro-drilling on polished blocks. After chemical dissolution of these samples, splits of all solutions were analyzed by high-resolution ICP-MS for major and trace metal concentration (e.g. Zn, Cu, Fe, Cd, As, Au, Ag, Pb, Tl, Mo, Sb, In, Se, Bi) for mineral purity check and comparison with isotope composition and potential influence of magmatic volatiles. Matrix-matched pure standard solutions were used for data calibration.

In addition to the planned Zn, Cd and Sb isotope analyses we also investigated Fe, Cu and S isotope ratios using multicollector ICP-MS. Isotope analyses of hydrothermal fluids and sulfide separates followed previously developed methods with some modifications introduced in the course of the study. The samples for Fe-, Cu-, Zn- and Cd-isotopes were purified by a three-stage anion exchange chromatography column. Instrumental mass bias for Zn isotope measurement was corrected using Cu-isotopes and for Cu isotope measurements using Zn-isotopes. Instrumental mass bias correction for Cd-isotope analysis involved both the standard-sample bracketing method and double spike, which provided increased precision and accuracy. Samples for S-isotopes were purified by a one-stage cation exchange chromatography column. Standard-sample bracketing was used to correct for the instrumental mass bias. Sb-isotope analysis involved chemical purification of Sb using Thiol Cotton Fibers followed by hydride-generation coupled to MC-ICP-MS. A data base with over 1000 isotope data points (including standards and blanks) was received, analyzed and interpreted.

1.3. Main results.

We found that temperature, magmatic degassing and boiling at sub- and super-critical conditions control the Cu, Zn and Cd concentrations of the seafloor hydrothermal fluids. Cu-isotope composition of the vent fluids (measured for the first time here) shows spatial and temporal variations and a tendency for positive fractionation relative to the bulk Earth reservoir. Several processes contribute to the Cu-isotope variability in the vent fluids: (1) magmatic volatile input; (2) sub-seafloor precipitation; (3) sub-seafloor re-dissolution; (4) boiling and phase separation. Heterogeneity of the source rocks and seawater ingress in the upflow zones of the hydrothermal systems are not factors influencing the Cu-isotope signature of the venting fluids. A redox-induced distillation fractionation model is used to explain δ65Cu variation across the chimney wall. It is based on the fact that the reduction of Cu2+ in the vent fluid results in precipitation of Cu+-sulfides which preferentially incorporate 63Cu. During the gradual Cu discharge through the porous chimney wall the hydrothermal fluid becomes progressively depleted in 63Cu and passively enriched in 65Cu. δ65Cusulfide gradually increases towards the chimney exterior. It decreases sharply in the external chimney oxidation layer as a result of the alteration of primary sulfides at ambient seawater conditions and preferential leaching of 65Cu. Two heavy Cu-isotope fluid fronts meet in this external alteration layer: a hydrothermal fluid depleted in 63Cu due to sulfide precipitation in the interior and seawater-based alteration fluid enriched in 65Cu due to sulfide dissolution at the exterior. A binary mixing model of the dissolved Cu flux to the ocean shows that in addition to the riverine and seafloor hydrothermal fluxes a heavy Cu-isotope input is required in order to balance the oceanic Cu-isotope composition. The oxidative dissolution of seafloor hydrothermal sulfides provides a heavy Cu-isotope flux to the seawater which may appear to be the missing 65Cu supply.

Zn- and Cd-isotope signatures of vent fluids (Cd-isotopes measured for the first time here) and sulfides deposited at the seafloor are controlled by two major processes: subsurface Zn-sulfide precipitation/remobilization and metal-rich magmatic fluid input. Subsurface Zn-sulfide precipitation defines a particular domain in the Zn-Cd-isotope relations diagram characterized by heavier Zn-isotope and light Cd-isotope composition relative to typical end-member high-temperature hydrothermal fluids that have not lost Zn and Cd to subsurface precipitation. Sulfide precipitation causes significant and reverse Cd- and Zn-isotope fractionation and this was figured out through investigating fluid/mineral pairs from vents saturated in wurtzite/sphalerite (mostly those from the Vienna Woods). There is significant variability of Zn- and Cd-isotope composition of the vent fluids that reflects Zn-sulfide remobilization and/or precipitation in subsurface environments due to pH and/or T variations. Some seafloor vent fluids are characterized by more negative Cd-isotope values and higher Cd/Zn ratios, whereas Zn-isotopes in the same fluids show smaller deviation from igneous values. The same fluids are characterized by negative δ34S values. These fluids are interpreted as affected by magmatic degassing. Using fluid-mineral pairs with high F2 and CO2, lowest pH and negative δ34S values (due to SO2 disproportionation) we found that there were systematics between Cd- (and Zn- to a lesser extent) and S-isotopes, and precious metal enrichment (Au, Ag) in sulfide minerals and in fluids that are diagnostic of a metal-rich magmatic fluid component.

Isotopically light S-isotope compositions of some of the studied fluids and sulfides have been found. They are interpreted as a result of disproportionation of magmatic SO2.

Sb concentration analyses of pairs hydrothermal fluid/sulfide and host rocks (basaltic to felsic) are in accord with the previous studies that suggest the Sb enrichment in back-arc setting results from higher Sb concentration in felsic rocks vs. basaltic rocks and during water-rock interactions at low pH. Sb was found to be enriched in Zn-rich deposits. It was established that the hydrothermal sulfides from the BAB tend to have δ123Sb values systematically lower than MOR sulfides. It is suggested that Sb derives from leached felsic rocks vs. seawater or subsurface metal remobilization. Hydrothermal deposits in BAB contain a significant range of Sb isotope variations. Most of the sulfides have δ123Sb values systematically higher than the basaltic values. Sb isotope variations likely reflect not only contribution from different Sb sources (seawater and host rocks), but also kinetic fractionation occurring at low temperature in aqueous media through reduction of seawater-derived Sb5+ in more reducing environment.

Sb-isotope data still needs to be completely interpreted from point of view to evaluate the effect of temperature, pH and mineralogy on Sb-isotope fractionation during sulfide precipitation, and to estimate the magmatic volatile contribution to the vent fluid on the basis of comparison of S- and Sb-isotopes.

1.4. Expected final results and their potential impact and use.

The results achieved so far (see 1.3.) will be published in a series of peer-reviewed papers (see 2.) in internationally recognized journals with high Impact Factor.

The achievements of this research are a contribution to the fundamental understanding (scientific impact) of the sources of metals to the seafloor massive sulfides (SMS) and the ways of their concentration. The non-traditional isotope systems are substantially elucidating these questions. The increased economic interest in the BAB’s seafloor hydrothermal deposits during the last 5 years gives a socio-economic dimension of the performed research. Demonstrated commercial interest in extracting high-grade SMS deposits of Cu, Zn, Au, Ag is going to provide an opportunity to use the non-traditional isotopes as powerful tools for tracing the source of metals.

This project has reinforced the fellow’s professional experience with isotope geochemistry, adding a new dimension to his competences and skills: profound knowledge in non-traditional isotope systems. The new competences and gained experience will be an additional prerequisite for his future academic development, will increase his competitiveness and will provide him a long-term stability in this highly-competitive academic field.

Given the original and innovative nature of the project and its proven potential to shed light on the sources of metals to the seafloor hydrothermal systems, its successful completion is considered to contribute to the excellence of European science and to the potential of Europe to play a key role in both the exploration of the Ocean and the development of the non-traditional stable isotope geochemistry.

The completed project is an excellent prerequisite for transfer of knowledge of the new field of the non-traditional stable isotope geochemistry from one of the world-class oceanographic centres (Host organization, IFREMER) to the fellow’s home country. Thus, the mobility accomplished is beneficial to the European Research Area with spreading of knowledge, know-how and methodology from the most developed west European countries to the new EU members. Isotope geochemistry (including non-traditional isotope systems) will be a substantial part of the fellow’s lectures on Geochemistry. This is supposed to further result in training of young scientists (graduate students and postdocs) in Isotope Geochemistry, which will generate future collaborative work with other EU research bodies and thus transferring knowledge and leading to a more balanced EU scientific architecture.

1.5. Target groups for whom this research could be relevant.

The results of this project will potentially be of interest for scientists (geochemists, mineralogists, economic geologists, marine geologists), and exploration and mining companies.