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Metal fluxes during magmatic degassing in the oceanic crust: sulfide mineralisation at ODP site 786B, Izu-Bonin forearc

Metal fluxes during magmatic degassing in the oceanic crust: sulfide mineralisation at ODP site 786B, Izu-Bonin forearc
Metal fluxes during magmatic degassing in the oceanic crust: sulfide mineralisation at ODP site 786B, Izu-Bonin forearc

Volcanogenic massive sulfide deposits are enriched in metals that are either derived from hydrothermal alteration of the basement rocks or supplied by exsolution of metal-rich volatiles during magmatic differentiation. The extent to which each process contributes to metal enrichment in these deposits varies between different tectonic settings. Ocean Drilling Program Hole 786B recovered > 800 m of upper oceanic crust from a supra-subduction zone setting and includes a 30-m-thick mineralised zone. In situ S isotopic compositions of pyrite decrease from 5.9 ± 2.9‰ in the upper mineralised zone down to − 3.3 ± 2.1‰ in the extensively altered central mineralisation zone, potentially indicating strong magmatic fluid input in this area. Whole rock data and in situ trace element analyses in sulfide minerals show enrichment of Ag, As, Au, Bi, Mo, S, Se, Sb and Te in the mineralised zone. Evaluation of metal behaviour during magmatic differentiation and primary metal fertility of basement rocks suggests that degassing melt is the main source for the high Au, Se and S enrichment observed in the mineralised zone. Magmatic volatile exsolution occurred late during the magmatic differentiation (~ 2 wt.% MgO), concomitant with oxide crystallisation and metal depletion in the melt. Comparison of Ocean Drilling Program Hole 786B with volcanogenic massive sulfide deposits hosted by boninitic volcanic successions, such as in the Semail ophiolite, the Newfoundland Appalachians and the Flin Flon Belt, suggests that magmatic fluid exsolution could be a common mechanism for Au enrichment in bimodal mafic volcanogenic massive sulfide deposits.

0026-4598
1-21
Patten, C. G.C.
37453afd-d13c-435a-9665-73923173ebfd
Pitcairn, I. K.
e6c79a72-1150-49fe-8b03-95db13e69c2e
Alt, J. C.
d2e22a46-a2e0-4d56-abbb-37199de80dbc
Zack, T.
42982af3-1d9b-46c4-8cf9-9baa967c12fb
Lahaye, Y.
98f32285-bd83-4338-8cc1-72ddbb808d0b
Teagle, D. A.H.
396539c5-acbe-4dfa-bb9b-94af878fe286
Markdahl, K.
bfa6f385-c383-4368-be17-8a33240dc369
Patten, C. G.C.
37453afd-d13c-435a-9665-73923173ebfd
Pitcairn, I. K.
e6c79a72-1150-49fe-8b03-95db13e69c2e
Alt, J. C.
d2e22a46-a2e0-4d56-abbb-37199de80dbc
Zack, T.
42982af3-1d9b-46c4-8cf9-9baa967c12fb
Lahaye, Y.
98f32285-bd83-4338-8cc1-72ddbb808d0b
Teagle, D. A.H.
396539c5-acbe-4dfa-bb9b-94af878fe286
Markdahl, K.
bfa6f385-c383-4368-be17-8a33240dc369

Patten, C. G.C., Pitcairn, I. K., Alt, J. C., Zack, T., Lahaye, Y., Teagle, D. A.H. and Markdahl, K. (2019) Metal fluxes during magmatic degassing in the oceanic crust: sulfide mineralisation at ODP site 786B, Izu-Bonin forearc. Mineralium Deposita, 1-21. (doi:10.1007/s00126-019-00900-9).

Record type: Article

Abstract

Volcanogenic massive sulfide deposits are enriched in metals that are either derived from hydrothermal alteration of the basement rocks or supplied by exsolution of metal-rich volatiles during magmatic differentiation. The extent to which each process contributes to metal enrichment in these deposits varies between different tectonic settings. Ocean Drilling Program Hole 786B recovered > 800 m of upper oceanic crust from a supra-subduction zone setting and includes a 30-m-thick mineralised zone. In situ S isotopic compositions of pyrite decrease from 5.9 ± 2.9‰ in the upper mineralised zone down to − 3.3 ± 2.1‰ in the extensively altered central mineralisation zone, potentially indicating strong magmatic fluid input in this area. Whole rock data and in situ trace element analyses in sulfide minerals show enrichment of Ag, As, Au, Bi, Mo, S, Se, Sb and Te in the mineralised zone. Evaluation of metal behaviour during magmatic differentiation and primary metal fertility of basement rocks suggests that degassing melt is the main source for the high Au, Se and S enrichment observed in the mineralised zone. Magmatic volatile exsolution occurred late during the magmatic differentiation (~ 2 wt.% MgO), concomitant with oxide crystallisation and metal depletion in the melt. Comparison of Ocean Drilling Program Hole 786B with volcanogenic massive sulfide deposits hosted by boninitic volcanic successions, such as in the Semail ophiolite, the Newfoundland Appalachians and the Flin Flon Belt, suggests that magmatic fluid exsolution could be a common mechanism for Au enrichment in bimodal mafic volcanogenic massive sulfide deposits.

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Patten_etal_2019_MinDep_ODP_Hole_786B_accepted - Accepted Manuscript
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Accepted/In Press date: 13 May 2019
e-pub ahead of print date: 13 June 2019

Identifiers

Local EPrints ID: 432711
URI: http://eprints.soton.ac.uk/id/eprint/432711
ISSN: 0026-4598
PURE UUID: 13087396-b472-4599-8e6f-caecb05801e9
ORCID for D. A.H. Teagle: ORCID iD orcid.org/0000-0002-4416-8409

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Date deposited: 24 Jul 2019 16:30
Last modified: 26 Nov 2021 06:00

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Contributors

Author: C. G.C. Patten
Author: I. K. Pitcairn
Author: J. C. Alt
Author: T. Zack
Author: Y. Lahaye
Author: D. A.H. Teagle ORCID iD
Author: K. Markdahl

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