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Metabasalts as sources of metals in orogenic gold deposits

Metabasalts as sources of metals in orogenic gold deposits
Metabasalts as sources of metals in orogenic gold deposits


Although metabasaltic rocks have been suggested to be important source rocks for orogenic gold deposits, the mobility of Au and related elements (As, Sb, Se, and Hg) from these rocks during alteration and metamorphism is poorly constrained. We investigate the effects of increasing metamorphic grade on the concentrations of Au and related elements in a suite of metabasaltic rocks from the Otago and Alpine Schists, New Zealand. The metabasaltic rocks in the Otago and Alpine Schists are of MORB and WPB affinity and are interpreted to be fragments accreted from subducting oceanic crust. Gold concentrations are systematically lower in the higher metamorphic grade rocks. Average Au concentrations vary little between sub-greenschist (0.9?±?0.5 ppb) and upper greenschist facies (1.0?±?0.5 ppb), but decrease significantly in amphibolite facies samples (0.21?±?0.07 ppb). The amount of Au depleted from metabasaltic rocks during metamorphism is on a similar scale to that removed from metasedimentary rocks in Otago. Arsenic concentrations increase with metamorphic grade with the metabasaltic rocks acting as a sink rather than a source of this element. The concentrations of Sb and Hg decrease between sub-greenschist and amphibolite facies but concentration in amphibolite facies rocks are similar to those in unaltered MORB protoliths and therefore unaltered oceanic crust cannot be a net source of Sb and Hg in a metamorphic environment. The concentrations of Au, As, Sb, and Hg in oceanic basalts that have become integrated into the metamorphic environment may be heavily influenced by the degree of seafloor alteration that occurred prior to metamorphism. We suggest that metasedimentary rocks are much more suitable source rocks for fluids and metals in orogenic gold deposits than metabasaltic rocks as they show mobility during metamorphism of all elements commonly enriched in this style of deposit.
0026-4598
373-390
Pitcairn, Iain K.
e6c79a72-1150-49fe-8b03-95db13e69c2e
Craw, Dave
c5701ce3-cff8-41c0-916a-5479e18b9bfc
Teagle, Damon A.H.
396539c5-acbe-4dfa-bb9b-94af878fe286
Pitcairn, Iain K.
e6c79a72-1150-49fe-8b03-95db13e69c2e
Craw, Dave
c5701ce3-cff8-41c0-916a-5479e18b9bfc
Teagle, Damon A.H.
396539c5-acbe-4dfa-bb9b-94af878fe286

Pitcairn, Iain K., Craw, Dave and Teagle, Damon A.H. (2015) Metabasalts as sources of metals in orogenic gold deposits. Mineralium Deposita, 50 (3), 373-390. (doi:10.1007/s00126-014-0547-y).

Record type: Article

Abstract



Although metabasaltic rocks have been suggested to be important source rocks for orogenic gold deposits, the mobility of Au and related elements (As, Sb, Se, and Hg) from these rocks during alteration and metamorphism is poorly constrained. We investigate the effects of increasing metamorphic grade on the concentrations of Au and related elements in a suite of metabasaltic rocks from the Otago and Alpine Schists, New Zealand. The metabasaltic rocks in the Otago and Alpine Schists are of MORB and WPB affinity and are interpreted to be fragments accreted from subducting oceanic crust. Gold concentrations are systematically lower in the higher metamorphic grade rocks. Average Au concentrations vary little between sub-greenschist (0.9?±?0.5 ppb) and upper greenschist facies (1.0?±?0.5 ppb), but decrease significantly in amphibolite facies samples (0.21?±?0.07 ppb). The amount of Au depleted from metabasaltic rocks during metamorphism is on a similar scale to that removed from metasedimentary rocks in Otago. Arsenic concentrations increase with metamorphic grade with the metabasaltic rocks acting as a sink rather than a source of this element. The concentrations of Sb and Hg decrease between sub-greenschist and amphibolite facies but concentration in amphibolite facies rocks are similar to those in unaltered MORB protoliths and therefore unaltered oceanic crust cannot be a net source of Sb and Hg in a metamorphic environment. The concentrations of Au, As, Sb, and Hg in oceanic basalts that have become integrated into the metamorphic environment may be heavily influenced by the degree of seafloor alteration that occurred prior to metamorphism. We suggest that metasedimentary rocks are much more suitable source rocks for fluids and metals in orogenic gold deposits than metabasaltic rocks as they show mobility during metamorphism of all elements commonly enriched in this style of deposit.

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Published date: March 2015
Organisations: Geochemistry

Identifiers

Local EPrints ID: 376015
URI: http://eprints.soton.ac.uk/id/eprint/376015
ISSN: 0026-4598
PURE UUID: 940f4bac-3ab8-4ce6-bece-44510b12d4c0
ORCID for Damon A.H. Teagle: ORCID iD orcid.org/0000-0002-4416-8409

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Date deposited: 13 Apr 2015 09:04
Last modified: 10 Dec 2019 01:49

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