Clifford G. C. Patten

Time: Friday October 14 2016, 10h00
Place: William-Olssonsalen, Geovetenskapens hus, Svante Arrhenius väg 14, Stockholm
Thesis title: Mobility of gold and other metals duringalteration of the oceanic crust
Implications for the formation of VMS deposits

Docent Iain Pitcairn, Department of Geological Sciences, IGV, Stockholm University
Professor Damon Teagle, School of Ocean and Earth Sciences, University of Southampton, UK
Professor Mark Hannington, GEOMAR Helmholtz-Zentrum für Ozeanforschung Kiel, Germany

Professor Richard Herrington, Head of Earth Sciences, The Natural History Museum, UK
Professor Holger Paulick, Oulu Mining School, University of Oulu, Finland
Docent Abigail Barker, Department of Earth Sciences, Uppsala University

Professor Victoria Pease, Department of Geological Sciences, IGV, Stockholm University

Tremendous physical and chemical exchanges occur along oceanic ridges between the lithosphere, the hydrosphere and thebiosphere. During these exchanges important mobilisation of metals by hydrothermal fluid circulation takes place withinthe oceanic crust. Volcanogenic massive sulphide (VMS) deposits are hydrothermal ore deposits rich in Cu-Zn-Pb bearingsulphide minerals that form during submarine venting of these hydrothermal fluids near the seafloor. A proportion of themetals 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 suchas As, Sb, Se and Te. The processes that form Au-rich VMS are still debated, due in part to our lack of understanding ofthe 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 ofthe oceanic crust. Three localities are studied: the Ocean Drilling Program (ODP) Hole 1256D in the Cocos plate, theTroodos ophiolite in Cyprus and the ODP Hole 786B in the Izu-Bonin forearc. The investigation has been carried out usingcutting-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 andTe and base metals during the alteration of the oceanic crust; 2) to determine the mineral reactions which promote thismobilisation; 3) to investigate the variability in metal mobility in different tectonic settings in the oceanic crust and 4) toinvestigate the extent to which the composition of  “source area” oceanic crust controls the composition of VMS depositsin different tectonic settings.The main outcomes of this study are fourfold. 1) The distribution of Au and related elements in primary crust variesconsiderably between different tectonic settings. Sulphide minerals play an important role in the behaviour of Au, Se andCu during magmatic differentiation and hydrothermal alteration, but have a lesser influence on other metals. The oxidationstate of the primary crust controls whether sulphide minerals are present, and thus is an important control on the budget andmobility of strongly chalcophile metals during hydrothermal alteration. 2) Large masses of Au and related elements aremobilised from the sheeted dyke complex in mid-oceanic ridge (MOR) and ophiolite settings. Significantly more metalsare mobilised from the source areas than are trapped in the VMS deposits observed in these settings. Therefore, most of themetals 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 ofAu mobilised from the Troodos ophiolite could potentially lead to Au-rich VMS formation but additional processes suchas vapour separation by sub-seafloor boiling or magmatic volatile input would be required to increase the Au : base metalratio. The lack of evidence for these processes in Troodos implies that Au-rich VMS deposits are not likely to be abundantin 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.

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