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The nature and age of Mesoproterozoic strike-slip faulting based on Re-Os geochronology of syn-tectonic copper mineralization, Assynt Terrane, NW Scotland

Holdsworth, R.E.; Selby, D.; Dempsey, E.; Scott, L.; Hardman, K.; Fallick, A.E.; Bullock, R.

The nature and age of Mesoproterozoic strike-slip faulting based on Re-Os geochronology of syn-tectonic copper mineralization, Assynt Terrane, NW Scotland Thumbnail


Authors

D. Selby

E. Dempsey

L. Scott

K. Hardman

A.E. Fallick

R. Bullock



Abstract

In ancient basement regions such as the Lewisian Complex, NW Scotland, the ages of brittle deformation events are commonly poorly constrained due to a lack of datable fills. An array of NW-SE sinistral and antithetic E-W dextral faults related to a regionally recognized episode of brittle shearing cut Neoarchaean gneisses and c. 2.25 Ga quartz-pyrite veins close to the trace of the unexposed, regional-scale NW-SE fault. Copper-iron mineralisation occurs at an intersection between an antithetic dextral fault and an older c. 2.25 Ga quartz vein. Optical microscopy, SEM and XRD analyses reveal an array of intergrown, co-genetic copper-iron sulphides, hematite and barite. Complex mm-thick zoned alteration rims rich in epidote occur at contacts between the sulphides and gneisses. Rhenium-Osmium copper-iron sulphide geochronology yields an age of c. 1.55 Ga for the hydrothermal mineralization event associated with faulting. Fault movements demonstrably overlap with mineralisation based on the asymmetric fibrous growth forms of these minerals within local dextral shears which acted as local channelways for mineralizing fluids during and after faulting. We tentatively propose that this regionally recognised strike slip faulting, previously termed the ‘Late Laxfordian’, should be referred to as the ‘Assyntian’ in order to distinguish it from kinematically distinct Laxfordian events.

Citation

Holdsworth, R., Selby, D., Dempsey, E., Scott, L., Hardman, K., Fallick, A., & Bullock, R. (2020). The nature and age of Mesoproterozoic strike-slip faulting based on Re-Os geochronology of syn-tectonic copper mineralization, Assynt Terrane, NW Scotland. Journal of the Geological Society, 177(4), 686-699. https://doi.org/10.1144/jgs2020-011

Journal Article Type Article
Acceptance Date Feb 13, 2020
Online Publication Date Mar 25, 2020
Publication Date Jul 1, 2020
Deposit Date Feb 21, 2020
Publicly Available Date Mar 29, 2024
Journal Journal of the Geological Society
Print ISSN 0016-7649
Electronic ISSN 2041-479X
Publisher The Geological Society
Peer Reviewed Peer Reviewed
Volume 177
Issue 4
Pages 686-699
DOI https://doi.org/10.1144/jgs2020-011

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Copyright Statement
Holdsworth, R. E., Selby, D., Dempsey, E., Scott, L., Hardman, K., Fallick, A.E. & Bullock, R. (2020). The nature and age of Mesoproterozoic strike-slip faulting based on Re-Os geochronology of syn-tectonic copper mineralization, Assynt Terrane, NW Scotland. Journal of the Geological Society 177(4): 686-699. https://doi.org/10.1144/jgs2020-011 © Geological Society of London 2020.




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