Hydrothermal mobilisation of Au and other metals in supra-subduction oceanic crust: Insights from the Troodos ophiolite

• Published in: Ore geology reviews 2017-06, Vol.86, p.487-508
• Main Authors: Patten, C.G.C., Pitcairn, I.K., Teagle, D.A.H.
• Format: Article
• Language: English
• Subjects: geologi; Geology; Gold; Hydrothermal alteration; Magnetite crisis; Troodos ophiolite; VMS deposit
• ISSN: 0169-1368; 1872-7360; 1872-7360
• DOI: 10.1016/j.oregeorev.2017.02.019

[Display omitted] •Large quantities of Au, As, Sb, Se are mobilised within the Troodos ophiolite.•Most of the metals mobilised by hydrothermal alteration are not trapped in VMS.•Primary metal distribution in Troodos is similar to that of the Manus Basin.•The magnetite crisis controls supra-subduction oceanic crust metal fertility.•The primary metal content the crust partly controls the style of VMS deposits. The Troodos ophiolite is an ideal location to investigate the relationships between the mobilisation of metals by hydrothermal fluids from the lower oceanic crust and the formation of volcanogenic massive sulphide (VMS) deposits. The ophiolite hosts 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 been poorly investigated. Low detection limit whole rock 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, Sb and Pb most likely due to addition from a subducting slab during crustal formation. During early stages of magmatic differentiation (9–3.5wt% 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 ∼3.5wt% MgO leads to sulphide segregation and depletion of strongly chalcophile elements (Au, Cu and Se) during continued differentiation (