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Sulphide mineral evolution and metal mobility during alteration of the oceanic crust: Insights from ODP Hole 1256D

Sulphide mineral evolution and metal mobility during alteration of the oceanic crust: Insights from ODP Hole 1256D
Sulphide mineral evolution and metal mobility during alteration of the oceanic crust: Insights from ODP Hole 1256D
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 Ocean Drilling Program (ODP) Hole 1256D. The ODP 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.
magmatic sulphide, sulphide leaching, metal mobilisation, hydrothermal alteration, gold
0016-7037
132-159
Patten, C.G.C.
37453afd-d13c-435a-9665-73923173ebfd
Pitcairn, I.K.
fc7fc820-5cd9-4070-9623-23325563bc9f
Teagle, D.A.H.
396539c5-acbe-4dfa-bb9b-94af878fe286
Harris, M.
b0e2d8cb-44f8-47a5-8e1b-6ca9c04fe776
Patten, C.G.C.
37453afd-d13c-435a-9665-73923173ebfd
Pitcairn, I.K.
fc7fc820-5cd9-4070-9623-23325563bc9f
Teagle, D.A.H.
396539c5-acbe-4dfa-bb9b-94af878fe286
Harris, M.
b0e2d8cb-44f8-47a5-8e1b-6ca9c04fe776

Patten, C.G.C., Pitcairn, I.K., Teagle, D.A.H. and Harris, M. (2016) Sulphide mineral evolution and metal mobility during alteration of the oceanic crust: Insights from ODP Hole 1256D. Geochimica et Cosmochimica Acta, 193, 132-159. (doi:10.1016/j.gca.2016.08.009).

Record type: Article

Abstract

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 Ocean Drilling Program (ODP) Hole 1256D. The ODP 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.

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Patten et al_16-GCA-ODP 1256D sulphide paragenesis-final submission.pdf - Accepted Manuscript
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More information

Accepted/In Press date: 4 August 2016
e-pub ahead of print date: 10 August 2016
Published date: 15 November 2016
Keywords: magmatic sulphide, sulphide leaching, metal mobilisation, hydrothermal alteration, gold
Organisations: Geochemistry

Identifiers

Local EPrints ID: 402610
URI: https://eprints.soton.ac.uk/id/eprint/402610
ISSN: 0016-7037
PURE UUID: f00b4a7e-0151-49ae-a3e5-110fc65c0e0a
ORCID for D.A.H. Teagle: ORCID iD orcid.org/0000-0002-4416-8409

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Date deposited: 10 Nov 2016 15:46
Last modified: 10 Dec 2019 06:20

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Contributors

Author: C.G.C. Patten
Author: I.K. Pitcairn
Author: D.A.H. Teagle ORCID iD
Author: M. Harris

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