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Sulphide mineralisation in forearc setting at ODP site 786B: evaluation of magmatic inputs into oceanic crust hydrothermal system
Stockholm University, Faculty of Science, Department of Geological Sciences.
Stockholm University, Faculty of Science, Department of Geological Sciences.
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Volcanogenic massive sulphide (VMS) deposits are variably enriched in metals that are mobilised by two main processes: hydrothermal alteration of the oceanic crust’s lower sheeted dyke section and exsolution of metal-rich fluids into the hydrothermal system from differentiating magmas. The extent to which each process contributes to metal enrichment in VMS deposits varies, however, between different tectonic settings. Oceanic Drilling Program (ODP) Hole 786B recovers the volcanic section and the transitional zone of a supra-subduction zone oceanic crust including a 30 m wide mineralised zone at the base of the hole. Previous work has indicated that significant input of magmatic fluid into the hydrothermal system isresponsible for the formation of mineralisation. This study uses in-situ trace element and S-isotope analyses in sulphide minerals and whole rock data to characterise the metal endowment of the mineralised zone, the sources of the trapped metals and the signature of magmatic fluid inputs in the hydrothermal system. The mineralised zone shows strong enrichment in S, As, Se, Sb and Au, and probably Mo, Te and Bi, but little enrichment in base metals. It is subdivided in two main alteration domains: the upper alteration domain, characterised by mixing of high temperature hydrothermal fluids with sea water at relatively low temperature (150-200 °C), under reduced and near neutral pH conditions, and the central and lower alteration domain, characterised by extensive mixing of magmatic fluids with sea water at relatively high temperature (~250 °C), under oxidised and acidic conditions. Strong metal zonation occurs in the transitional zone with preferential enrichment of Zn, Cu, As, Au and Pb in the upper alteration domain and preferential enrichment of S, Se, Mo, Sb, Te and Bi in the central and lower alteration domain. This zonation is controlled by variations in fluid composition, temperature, redox, pH, and zone refining during sulphide paragenesis. The oceanic crust at Hole 786B has high As, Sb and Pb concentrations relative to mid oceanic ridge setting but similar Cu, Zn and Au, and low Se concentrations. The oceanic crust metal fertility suggests that the Cu, Zn, As, Sb and Pb enriched in the transitional zone could have been mobilised by rock buffered hydrothermal fluids but that the S, Se and Au must have been mobilised by magmatic fluids. Major and trace elements behaviour during magmatic differentiation of the oceanic crust at Hole 786B show evidences for a magnetite crisis event which is interpreted to trigger exsolution of metal-rich magmatic fluids into the hydrothermal system and which can account for the observed metal endowment in the transitional zone. The metal content in the oceanic crust at Hole 786B and the specific endowment of the mineralised zone suggests that significant fractionation between Au and base metals occur during fluid migration in supra-subduction oceanic crust and can promote the formation of Au-rich VMS deposits on the sea floor. Such process is as highlighted by the Au : base metal ratio close to unity in the mineralised zone.

National Category
Geology
Research subject
Geology
Identifiers
URN: urn:nbn:se:su:diva-132903OAI: oai:DiVA.org:su-132903DiVA: diva2:955592
Available from: 2016-08-25 Created: 2016-08-25 Last updated: 2016-08-30Bibliographically approved
In thesis
1. Mobility of gold and other metals during alteration of the oceanic crust: Implications for the formation of VMS deposits
Open this publication in new window or tab >>Mobility of gold and other metals during alteration of the oceanic crust: Implications for the formation of VMS deposits
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Tremendous physical and chemical exchanges occur along oceanic ridges between the lithosphere, the hydrosphere and the biosphere. During these exchanges important mobilisation of metals by hydrothermal fluid circulation takes place within the oceanic crust. Volcanogenic massive sulphide (VMS) deposits are hydrothermal ore deposits rich in Cu-Zn-Pb bearing sulphide minerals that form during submarine venting of these hydrothermal fluids near the seafloor. A proportion of the metals enriched in these deposits are mobilised from deeper crustal levels during high-temperature hydrothermal alteration. Gold-rich VMS deposits represent an important sub-set of VMS deposits that are enriched in Au and related elements such as As, Sb, Se and Te. The processes that form Au-rich VMS are still debated, due in part to our lack of understanding of the behaviour of these elements during formation and alteration of the oceanic crust.

In this thesis we carry out a systematic investigation into the behaviour of Au, As, Sb, Se and Te during evolution of the oceanic crust. Three localities are studied: the Ocean Drilling Program (ODP) Hole 1256D in the Cocos plate, the Troodos ophiolite in Cyprus and the ODP Hole 786B in the Izu-Bonin forearc. The investigation has been carried out using cutting-edge analytical techniques including ultra-low detection limit analyses of Au and other metals in rock samples. The objectives of the thesis are 1) to quantify the mobilisation of metals including Au, related elements As, Sb, Se and Te and base metals during the alteration of the oceanic crust; 2) to determine the mineral reactions which promote this mobilisation; 3) to investigate the variability in metal mobility in different tectonic settings in the oceanic crust and 4) to investigate the extent to which the composition of  “source area” oceanic crust controls the composition of VMS deposits in different tectonic settings.

The main outcomes of this study are fourfold. 1) The distribution of Au and related elements in primary crust varies considerably between different tectonic settings. Sulphide minerals play an important role in the behaviour of Au, Se and Cu during magmatic differentiation and hydrothermal alteration, but have a lesser influence on other metals. The oxidation state of the primary crust controls whether sulphide minerals are present, and thus is an important control on the budget and mobility of strongly chalcophile metals during hydrothermal alteration. 2) Large masses of Au and related elements are mobilised from the sheeted dyke complex in mid-oceanic ridge (MOR) and ophiolite settings. Significantly more metals are mobilised from the source areas than are trapped in the VMS deposits observed in these settings. Therefore, most of the metals mobilised from the source areas are lost, either during transport, venting, sedimentation or late fluid mobilisation. 3) Insufficient Au is mobilised from MOR settings at ODP Hole 1256D to form Au-rich VMS deposits. The quantity of Au mobilised from the Troodos ophiolite could potentially lead to Au-rich VMS formation but additional processes such as vapour separation by sub-seafloor boiling or magmatic volatile input would be required to increase the Au : base metal ratio. The lack of evidence for these processes in Troodos implies that Au-rich VMS deposits are not likely to be abundant in this area. 4) Isotopic and trace element evidence supports magmatic input in the hydrothermal system at ODP Hole 786B, implying that magmatic fluid input into hydrothermal systems leaves a specific signature which can be tracked.

Place, publisher, year, edition, pages
Stockholm: Department of Geological Sciences, Stockholm University, 2016. 44 p.
Series
Meddelanden från Stockholms universitets institution för geologiska vetenskaper, 364
National Category
Geology
Research subject
Geology
Identifiers
urn:nbn:se:su:diva-132782 (URN)978-91-7649-478-3 (ISBN)
External cooperation:
Public defence
2016-10-14, William-Olssonsalen, Geovetenskapens hus, Svante Arrhenius väg 14, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Submitted. Paper 4: Manuscript.

Available from: 2016-09-21 Created: 2016-08-23 Last updated: 2016-09-12Bibliographically approved

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Patten, Clifford G. C.Pitcairn, Iain K.
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