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Carbonate alteration of ophiolitic rocks in the Arabian–Nubian Shield of Egypt:: sources and compositions of the carbonating fluid and implications for the formation of Au deposits

Carbonate alteration of ophiolitic rocks in the Arabian–Nubian Shield of Egypt:: sources and compositions of the carbonating fluid and implications for the formation of Au deposits
Carbonate alteration of ophiolitic rocks in the Arabian–Nubian Shield of Egypt:: sources and compositions of the carbonating fluid and implications for the formation of Au deposits
Ultramafic portions of ophiolitic fragments in the Arabian–Nubian Shield (ANS) show pervasive carbonate alteration forming various degrees of carbonated serpentinites and listvenitic rocks. Notwithstanding the extent of the alteration, little is known about the processes that caused it, the source of the CO2 or the conditions of alteration. This study investigates the mineralogy, stable (O, C) and radiogenic (Sr) isotope composition, and geochemistry of suites of variably carbonate altered ultramafics from the Meatiq area of the Central Eastern Desert (CED) of Egypt. The samples investigated include least-altered lizardite (Lz) serpentinites, antigorite (Atg) serpentinites and listvenitic rocks with associated carbonate and quartz veins. The C, O and Sr isotopes of the vein samples cluster between −8.1‰ and −6.8‰ for δ13C, +6.4‰ and +10.5‰ for δ18O, and 87Sr/86Sr of 0.7028–0.70344, and plot within the depleted mantle compositional field. The serpentinites isotopic compositions plot on a mixing trend between the depleted-mantle and sedimentary carbonate fields. The carbonate veins contain abundant carbonic (CO2±CH4±N2) and aqueous-carbonic (H2O-NaCl-CO2±CH4±N2) low salinity fluid, with trapping conditions of 270–300°C and 0.7–1.1 kbar. The serpentinites are enriched in Au, As, S and other fluid-mobile elements relative to primitive and depleted mantle. The extensively carbonated Atg-serpentinites contain significantly lower concentrations of these elements than the Lz-serpentinites suggesting that they were depleted during carbonate alteration. Fluid inclusion and stable isotope compositions of Au deposits in the CED are similar to those from the carbonate veins investigated in the study and we suggest that carbonation of ANS ophiolitic rocks due to influx of mantle-derived CO2-bearing fluids caused break down of Au-bearing minerals such as pentlandite, releasing Au and S to the hydrothermal fluids that later formed the Au-deposits. This is the first time that gold has been observed to be remobilized from rocks during the lizardite–antigorite transition.
0020-6814
391-419
Boskabadi, Arman
934a59ff-f764-465f-9b20-fe773474bcf9
Pitcairn, Iain K.
e6c79a72-1150-49fe-8b03-95db13e69c2e
Broman, Curt
f1308402-cff9-4342-a474-076054741642
Boyce, Adrian
a1e40a10-98ba-45a9-8582-6327a14d6072
Teagle, Damon A. H.
396539c5-acbe-4dfa-bb9b-94af878fe286
Cooper, Matthew J.
54f7bff0-1f8c-4835-8358-71eef8529e7a
Azer, Mokhles K.
8a6d5916-8872-42f6-a0ab-8623a30680ac
Stern, Robert J.
91394f41-0094-4259-968a-c8e7f960e6a2
Mohamed, Fathy H.
8b230e9c-8faf-443c-8194-1df8ffec186e
Majka, Jaroslaw
3b213d3c-82ef-48d1-9d71-70e526becf2d
Boskabadi, Arman
934a59ff-f764-465f-9b20-fe773474bcf9
Pitcairn, Iain K.
e6c79a72-1150-49fe-8b03-95db13e69c2e
Broman, Curt
f1308402-cff9-4342-a474-076054741642
Boyce, Adrian
a1e40a10-98ba-45a9-8582-6327a14d6072
Teagle, Damon A. H.
396539c5-acbe-4dfa-bb9b-94af878fe286
Cooper, Matthew J.
54f7bff0-1f8c-4835-8358-71eef8529e7a
Azer, Mokhles K.
8a6d5916-8872-42f6-a0ab-8623a30680ac
Stern, Robert J.
91394f41-0094-4259-968a-c8e7f960e6a2
Mohamed, Fathy H.
8b230e9c-8faf-443c-8194-1df8ffec186e
Majka, Jaroslaw
3b213d3c-82ef-48d1-9d71-70e526becf2d

Boskabadi, Arman, Pitcairn, Iain K., Broman, Curt, Boyce, Adrian, Teagle, Damon A. H., Cooper, Matthew J., Azer, Mokhles K., Stern, Robert J., Mohamed, Fathy H. and Majka, Jaroslaw (2017) Carbonate alteration of ophiolitic rocks in the Arabian–Nubian Shield of Egypt:: sources and compositions of the carbonating fluid and implications for the formation of Au deposits. International Geology Review, 59 (4), 391-419. (doi:10.1080/00206814.2016.1227281).

Record type: Article

Abstract

Ultramafic portions of ophiolitic fragments in the Arabian–Nubian Shield (ANS) show pervasive carbonate alteration forming various degrees of carbonated serpentinites and listvenitic rocks. Notwithstanding the extent of the alteration, little is known about the processes that caused it, the source of the CO2 or the conditions of alteration. This study investigates the mineralogy, stable (O, C) and radiogenic (Sr) isotope composition, and geochemistry of suites of variably carbonate altered ultramafics from the Meatiq area of the Central Eastern Desert (CED) of Egypt. The samples investigated include least-altered lizardite (Lz) serpentinites, antigorite (Atg) serpentinites and listvenitic rocks with associated carbonate and quartz veins. The C, O and Sr isotopes of the vein samples cluster between −8.1‰ and −6.8‰ for δ13C, +6.4‰ and +10.5‰ for δ18O, and 87Sr/86Sr of 0.7028–0.70344, and plot within the depleted mantle compositional field. The serpentinites isotopic compositions plot on a mixing trend between the depleted-mantle and sedimentary carbonate fields. The carbonate veins contain abundant carbonic (CO2±CH4±N2) and aqueous-carbonic (H2O-NaCl-CO2±CH4±N2) low salinity fluid, with trapping conditions of 270–300°C and 0.7–1.1 kbar. The serpentinites are enriched in Au, As, S and other fluid-mobile elements relative to primitive and depleted mantle. The extensively carbonated Atg-serpentinites contain significantly lower concentrations of these elements than the Lz-serpentinites suggesting that they were depleted during carbonate alteration. Fluid inclusion and stable isotope compositions of Au deposits in the CED are similar to those from the carbonate veins investigated in the study and we suggest that carbonation of ANS ophiolitic rocks due to influx of mantle-derived CO2-bearing fluids caused break down of Au-bearing minerals such as pentlandite, releasing Au and S to the hydrothermal fluids that later formed the Au-deposits. This is the first time that gold has been observed to be remobilized from rocks during the lizardite–antigorite transition.

Full text not available from this repository.

More information

Accepted/In Press date: 17 August 2016
e-pub ahead of print date: 26 September 2016
Published date: 12 March 2017
Organisations: Geochemistry, Southampton Marine & Maritime Institute

Identifiers

Local EPrints ID: 406712
URI: https://eprints.soton.ac.uk/id/eprint/406712
ISSN: 0020-6814
PURE UUID: 6e057093-19d1-4b54-8149-01409aa77458
ORCID for Damon A. H. Teagle: ORCID iD orcid.org/0000-0002-4416-8409
ORCID for Matthew J. Cooper: ORCID iD orcid.org/0000-0002-2130-2759

Catalogue record

Date deposited: 18 Mar 2017 02:30
Last modified: 14 Mar 2019 01:47

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