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Temporal, lithospheric and magmatic process Controls on ni, cu and platinum-group element (PGE) mineralisation: A case study from Scotland

Hughes, Hannah S. R. 2015. Temporal, lithospheric and magmatic process Controls on ni, cu and platinum-group element (PGE) mineralisation: A case study from Scotland. PhD Thesis, Cardiff University.
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A temporal and spatial relationship between plume magmatism, cratonic lithosphere and the occurrences of orthomagmatic Ni-Cu and platinum-group element (PGE) sulphide mineralisation has been documented in the literature. However the underlying causes for this correlation have yet to be resolved – is there an inherent feature of the cratonic lithosphere and its mantle ‘keel’ that controls mineralisation? Or is this correlation purely a preservational bias in the geological record? Scotland has experienced multiple tectono-magmatic events and provides an ideal testing ground, or ‘framework’, in which to assess the role of lithospheric mantle on chalcophile element (Ni and Cu) and precious metal (PGE and Au) abundances through time. Given the well-documented geological history of the region (including several suites of mantle xenoliths), coupled with exploration campaigns in Greenland (with which Scotland has comparable geology), this thesis aims to assess the contributions and influences of lithospheric mantle vs. asthenospheric mantle during melting and mineralisation. It also evaluates the Ni-Cu-PGE mineralisation potential for Scotland, particularly in a Noril’sk-type conduit-hosted setting within the British Palaeogene Igneous Province (BPIP). The earliest major tectono-magmatic event following cratonisation of the North Atlantic Craton (NAC) occurred c. 2.4 Ga during Palaeoproterozoic extension, forming the maficultramafic Scourie Dyke Swarm. Despite evidence for lithospheric mantle melting at this time, the subcontinental lithospheric mantle (SCLM) below the Scottish portion of the NAC did not become severely depleted in sulphides or PGE. Instead, spinel lherzolite mantle xenoliths from this region (e.g., Loch Roag) record an influx of carbonatite-associated sulphides at this time, enriched in PGE, and providing a deeper indication of continental extension that may be correlated to carbonatitic intrusions in Greenland. Subsequent collision and orogenesis of the NAC in the late Palaeoproterozoic (c. 1.9 to 1.7 Ga) represents a second significant tectonomagmatic event, recorded in the Scottish SCLM as sulphide (re-)melting and formation of discrete Pt-sulphide minerals (cooperite). Hence the lithospheric mantle here became appreciably enriched in precious metals during the Palaeoproterozoic, but crucially this preserved multiple co-existing populations of sulphides, distinct in their petrographic setting and geochemistry. Cratonic basement and associated mantle lithosphere are absent in the southern terranes of Scotland. This provides a direct comparison between lithospheric mantle geochemistry for Archaean-Palaeoproterozoic terranes north of the Great Glen Fault vs. Palaeozoic terranes south of the Great Glen Fault. Rifting of Rodinia and opening of the Iapetus Ocean in the late Neoproterozoic thus marks a significant change in geodynamic setting. This is especially apparent in the concentration of cobalt in lithospheric mantle sulphides, which appears to be inherently linked with the formation and/or later destruction (subduction) of oceanic crust during the Grampian event of the Caledonian orogeny. The impingement of the proto-Icelandic mantle plume initiated in the Palaeogene at the base of the NAC lithospheric mantle keel of Scotland and Greenland. The earliest Palaeogene magmas are enriched in Pt (i.e., have a high Pt/Pd ratio), whilst subsequent magmas associated with the opening of the Atlantic Ocean have successively lower Pt/Pd ratios. High Pt/Pd ratios are therefore coincident with magmas that have intruded through cratonic lithosphere. The SCLM at the margin of this region is known to be Pt-enriched (with cooperite) and therefore the changing Pt/Pd ratio of North Atlantic Igneous Province magmas suggests a fundamental interaction between the mantle plume and pre-enriched SCLM. Thus, whilst the concentration of metals, particularly Ni and Cu, is largely based on the high degree of asthenospheric mantle melting associated with the plume itself, the ratio of precious metals, such as Pt/Pd, can be strongly influenced by SCLM geochemistry. Overall, the intricate subtleties of metasomatic signatures recorded by mantle xenolith sulphides (or populations of sulphides) could allow for metallogenic ‘mapping’ of the upper mantle. This may identify areas of geochemical and mineralogical ‘preconditioning’, and together with geophysical constraints such as major lithospheric lineaments, it may be possible to establish the craton-specific fertility of a region. Finally, in order for orthomagmatic sulphide mineralisation to occur, magmas must achieve sulphur saturation in the upper crust, forming an immiscible sulphide liquid and thereby collecting PGE and chalcophile elements, possibly to economic grades. Thus a crucial part of assessing the mineralisation potential of a region must entail an investigation into the causes and locations of S-saturation. Given that crustal sulphur contamination is a common trigger for magmatic S-saturation, this thesis establishes the first S-isotopic (δ34S) framework for western Scotland in order to identify areas of sulphur contamination in the BPIP. In Scotland, the most readily available and S-rich rocks occur in the Mesozoic Hebrides Basin. Sulphur contamination of BPIP rocks is widespread and both S-saturation and S-undersaturation can be observed, suggesting that the region may be extremely fertile for orthomagmatic Ni-Cu-PGE mineralisation. By reconstructing the Hebrides Basin stratigraphy we can assess locations of contamination, even if these are above the current level of exposure (and since removed by erosion), and in some situations sulphide liquid sinking may be demonstrated, suggesting further possibilities for mineralisation present ‘up-stream’ in magmatic conduits. In conclusion, the Scottish BPIP represents a new exploration frontier not yet identified by industry for orthomagmatic Ni-Cu-PGE mineralisation. The conclusions are based on approximately 500 rock samples from across Scotland, which have been analysed for major elements and over thirty trace elements (including PGE) and S-isotopes. All data are available on an accompanying CD.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Earth and Environmental Sciences
Subjects: Q Science > QE Geology
Date of First Compliant Deposit: 30 March 2016
Last Modified: 13 Sep 2023 09:33

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