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Mobility of gold and other metals during alteration of the oceanic crust: Implications for the formation of VMS deposits
Stockholm University, Faculty of Science, Department of Geological Sciences.
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: urn:nbn:se:su:diva-132782ISBN: 978-91-7649-478-3OAI: oai:DiVA.org:su-132782DiVA: diva2:955241
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
List of papers
1. Mobility of Au and related elements during the hydrothermal alteration of the oceanic crust: implications for the sources of metals in VMS deposits
Open this publication in new window or tab >>Mobility of Au and related elements during the hydrothermal alteration of the oceanic crust: implications for the sources of metals in VMS deposits
2016 (English)In: Mineralium Deposita, ISSN 0026-4598, E-ISSN 1432-1866, Vol. 51, no 2, 179-200 p.Article in journal (Refereed) Published
Abstract [en]

Volcanogenic massive sulphide (VMS) deposits are commonly enriched in Cu, Zn and Pb and can also be variably enriched in Au, As, Sb, Se and Te. The behaviour of these elements during hydrothermal alteration of the oceanic crust is not well known. Ocean Drilling Program (ODP) Hole 1256D penetrates a complete in situ section of the upper oceanic crust, providing a unique sample suite to investigate the behaviour of metals during hydrothermal alteration. A representative suite of samples was analysed for Au, As, Sb, Se and Te using low detection limit methods, and a mass balance of metal mobility has been carried out through comparison with a fresh Mid-Oceanic Ridge Basalt (MORB) glass database. The mass balance shows that Au, As, Se, Sb, S, Cu, Zn and Pb are depleted in the sheeted dyke and plutonic complexes by -46 +/- 12, -27 +/- 5, -2.5 +/- 0.5, -27 +/- 6, -8.4 +/- 0.7, -9.6 +/- 1.6, -7.9 +/- 0.5 and -44 +/- 6 %, respectively. Arsenic and Sb are enriched in the volcanic section due to seawater-derived fluid circulation. Calculations suggest that large quantities of metal are mobilised from the oceanic crust but only a small proportion is eventually trapped as VMS mineralisation. The quantity of Au mobilised and the ratio of Au to base metals are similar to those of mafic VMS, and ten times enrichment of Au would be needed to form a Au-rich VMS. The Cu-rich affinity of mafic VMS deposits could be explained by base metal fractionation both in the upper sheeted dykes and during VMS deposit formation.

Keyword
Volcanic massive sulphide, Au-rich VMS, ODP Hole 1256D, Hydrothermal alteration, Oceanic crust
National Category
Earth and Related Environmental Sciences
Research subject
Geology
Identifiers
urn:nbn:se:su:diva-127363 (URN)10.1007/s00126-015-0598-8 (DOI)000369006600001 ()
Available from: 2016-03-10 Created: 2016-03-02 Last updated: 2016-08-30Bibliographically approved
2. Sulphide mineral evolution and metal mobility during alteration of the oceanic crust: Insights from IODP site 1256D
Open this publication in new window or tab >>Sulphide mineral evolution and metal mobility during alteration of the oceanic crust: Insights from IODP site 1256D
2016 (English)In: Geochimica et Cosmochimica Acta, ISSN 0016-7037, E-ISSN 1872-9533, Vol. 193, 132-159 p.Article in journal (Refereed) Published
Abstract [en]

Fluxes of metals during the hydrothermal alteration of the oceanic crust have far reaching effects including buffering of the compositions of the ocean and lithosphere, supporting microbial life and the formation of sulphide ore deposits. The mechanisms responsible for metal mobilisation during the evolution of the oceanic crust are complex and are neither fully constrained nor quantified. Investigations into the mineral reactions that release metals, such as sulphide leaching, would generate better understanding of the controls on metal mobility in the oceanic crust. We investigate the sulphide and oxide mineral paragenesis and the extent to which these minerals control the metal budget in samples from International Oceanic Discovery Program (IODP) Hole 1256D. The IODP Hole 1256D drill core provides a unique sample suite representative of a complete section of a fast-spreading oceanic crust from the volcanic section down to the plutonic complex. The sulphide population at Hole 1256D is divided into five groups based on mineralogical assemblage, lithological location and texture: the magmatic, metasomatised, high temperature hydrothermal, low temperature and patchy sulphides. The initiation of hydrothermal alteration by downward flow of moderate temperature (250-350 °C) hydrothermal fluids under oxidising conditions leads to metasomatism of the magmatic sulphides in the sheeted dyke and plutonic complexes. Subsequent increase in the degree of hydrothermal alteration at temperatures >350 °C under reducing conditions then leads to the leaching of the metasomatised sulphides by rising hydrothermal fluids. Mass balance calculations show that the mobility of Cu, Se and Au occurs through sulphide leaching during high temperature hydrothermal alteration and that the mobility of Zn, As, Sb and Pb is controlled by silicate rather than sulphide alteration. Sulphide leaching is not complete at Hole 1256D and more advanced alteration would mobilise greater masses of metals. Alteration of oxide minerals does not release significant quantities of metal into the hydrothermal fluid at Hole 1256D. Mixing of rising high temperature fluids with low temperature fluids, either in the upper sheeted dyke section or in the transitional zone, triggers local high temperature hydrothermal sulphide precipitation and trapping of Co, Ni, Cu, Zn, As, Ag, Sb, Se, Te, Au, Hg and Pb. In the volcanic section, low temperature fluid circulation (<150 °C) leads to low temperature sulphide precipitation in the form of pyrite fronts that have high As concentrations due to uptake from the circulating fluids. Deep late low temperature circulation in the sheeted dyke and the plutonic complexes results in local precipitation of patchy sulphides and local metal remobilisation. Control of sulphides over Au, Se and Cu throughout fast-spreading mid-oceanic crust history implies that the generation of hydrothermal fluids enriched in these metals, which can eventually form VMS deposits, is strongly controlled by sulphide leaching.

Keyword
magmatic sulphide, sulphide leaching, metal mobilisation, hydrothermal alteration, gold
National Category
Geology
Research subject
Geology
Identifiers
urn:nbn:se:su:diva-132899 (URN)10.1016/j.gca.2016.08.009 (DOI)
External cooperation:
Available from: 2016-08-25 Created: 2016-08-25 Last updated: 2016-09-14Bibliographically approved
3. Hydrothermal mobilisation of Au and other metals in supra-subduction oceanic crust: insight from the Troodos ophiolite
Open this publication in new window or tab >>Hydrothermal mobilisation of Au and other metals in supra-subduction oceanic crust: insight from the Troodos ophiolite
(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:nbn:se:su:diva-132901 (URN)
External cooperation:
Available from: 2016-08-25 Created: 2016-08-25 Last updated: 2016-08-30Bibliographically approved
4. Sulphide mineralisation in forearc setting at ODP site 786B: evaluation of magmatic inputs into oceanic crust hydrothermal system
Open this publication in new window or tab >>Sulphide mineralisation in forearc setting at ODP site 786B: evaluation of magmatic inputs into oceanic crust hydrothermal system
(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:nbn:se:su:diva-132903 (URN)
External cooperation:
Available from: 2016-08-25 Created: 2016-08-25 Last updated: 2016-08-30Bibliographically approved

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