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Hydrothermal mobilisation of Au and other metals in supra-subduction oceanic crust: insight from the Troodos ophiolite
Stockholm University, Faculty of Science, Department of Geological Sciences.
Stockholm University, Faculty of Science, Department of Geological Sciences.
(English)In: Ore Geology Reviews, ISSN 0169-1368, E-ISSN 1872-7360Article in journal (Refereed) Submitted
Abstract [en]

The Troodos ophiolite is an ideal location to investigate the relationships between metal mobility from source areas and the formation of volcanogenic massive sulphide (VMS) deposits. The ophiolite is host to the classic “Cyprus-type” Cu-rich VMS deposits as well as abundant zones of epidosite alteration in the lower sheeted dyke section that are significantly depleted in base metals including Cu and Zn, and are considered to be the source of the metals enriched in the overlying deposits. Previous research indicates that the Troodos VMS deposits are irregularly enriched in Au and related elements As, Sb, and Se, but the behaviour of these elements during the hydrothermal alteration of the Troodos ophiolite hitherto has not been investigated. Analyses of fresh glass samples reveal that the Troodos primitive crust has a similar metal content and distribution to modern-day arc-related environments such as the Manus Basin. Compared to mid-oceanic ridge basalt (MORB), the Troodos primitive crust is enriched in As, Sband Pb most likely due to addition from a subducting slab during crustal formation. During early stages ofmagmatic differentiation (9-3.5 wt.% MgO) Au, As, Sb, Se, Cu, Zn and Pb behave as incompatible elements due to the sulphide-undersaturated nature of the melt. The onset of magnetite crystallisation, however, at ~3.5 wt.% MgO leads to sulphide segregation and depletion of strongly chalcophile elements (Au, Cu and Se) during continued differentiation (<3.5 wt.% MgO) whereas poorly chalcophile elements (As, Sb, Zn and Pb) remain incompatible. These differences in metal behaviour can account for the Curich, Zn-Pb-poor of the Cyprus-type VMS deposits as the source area rocks show high Cu fertility compared to Zn and Pb. Mobilisation of metals during hydrothermal alteration of the Troodos ophiolite is more extensive than observed in hydrothermally altered MORB. Mass balance calculations show that the epidosite zones are significantly depleted in Au (-88±16 %), As (-89±23 %), Sb (-60±12 %), Se (-91±20 %), Cu (-84±18 %), Zn (-63±9 %) and Pb (-60±8 %). Background altered diabase from outside epidosite zones shows similar metal depletions which suggests that the source areas of VMS are not restricted to epidosite zones but are extended to the lower sheeted dyke section. The masses of metals mobilised from a source area of 10.9 km3, (composed of a 5 km3 epidosite zone and 5.9 km3 of background altered diabase) in the Solea graben are 47 t Au, 21 kt As, 1220 t Sb, 3080 t Se, 2.4 Mt Cu, 1.8 Mt Zn and 27 kt Pb. Comparison of metal quantities mobilised from lower sheeted dike section in the Solea graben with those hosted in VMS deposits shows trapping efficiencies ranging from 4 to 37 % indicating that most of the metals is lost by other processes.

National Category
Geology
Research subject
Geology
Identifiers
URN: urn:nbn:se:su:diva-132901OAI: oai:DiVA.org:su-132901DiVA: diva2:955588
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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