Terrane and basement discrimination in northern Britain using sulphur isotopes and mineralogy of ore deposits

Abstract

This study of four well characterized and adjacent terranes in Northern Britain outlines the sulphur isotope variations, assesses the overall importance of crustal and mantle sulphur, and presents a model that can be applied to terrane distinction throughout the North Atlantic Caledonides. The characteristics of metal components within the mineralization provide additional information that can be related to the nature of underlying basement and events from the onset of sedimentation to the cessation of mineralization within stratigraphically linked packages of rock.The ~345 data show that the dominant crustal units in each terrane, whether upper crustal sediments or cratonic basement, provide the main alternative sulphur source to the mantle and act also as the main contaminant of subcrustal melts. The 834S values of granitoidrelated mineralization are either within the subcrustal melt-range of -3%0 to +3%0 or deviate toward the values of major crustal units in the terrane, i.e. toward 345 depletion in the Southern Uplands and toward 345 enrichment in the Lakesman and Grampian terranes. More complex mineralization in the Northern Highland terrane is linked to the presence of thick North Atlantic craton beneath upper crustal metasediments. Across the region the vein systems beyond the influence of magmatic components represent homogenized sulphur, metals and fluids from local upper crustal units. The sulphur isotope data and style of mineralization for the British terranes are compared with terranes of similar age along strike in Eastern Canada revealing notable correlations. Anderson et al. 1989;Hall et al. 1991;Lowry et al. 1991a;Scott et al. 1991;Steed & Morris 1997), and pyrite in metasediments (e.g.Hall et aL 1987, 1988, 1994a, b). Our analysis of this dataset highlights distinctive isotopic variations, which not only provides further evidence for the delineation of the putative terrane boundaries, but also suggests deep crustal and possibly subcrustal variations across the region.

Contrasting mechanisms for crustal sulphur contamination of mafic magma: evidence from dyke and sill complexes from the British Palaeogene Igneous Province

“…There is a very weak and inconclusive correlation between La/Nb ratio and δ 34 S for sills (r 2 = 0.31). Lewisian gneisses have variable but low sulphur concentrations and δ 34 S ranging from −1.4‰ to +1.2‰ (this study) and from −1.4‰ to +5.5‰ on the NW Scottish mainland (Lowry et al 2005). Torridonian sediments measured in this study always have δ 34 S > 0‰.…”

“…Moine metapelites also have low or very low concentrations of sulphur, and where measurable have δ 34 S of +2.5‰ (this study) and +3.4‰ to +4.6‰ in some psammites (Lowry 1991). Accordingly, Lowry et al (2005) found that each crustal terrane across Scotland could be characterized by its dominant crustal units, reflected in the range of δ 34 S for that region. In the Northern Highland Terrane (which includes Skye) complexities in the terrane-scale δ 34 S were suggested as reflecting the thick North Atlantic Craton (i.e.…”

How the Neoproterozoic S-isotope record illuminates the genesis of vein gold systems: an example from the Dalradian Supergroup in Scotland

“…4; Lowry et al 1995) and, specifically, sulphides from the Etive granite (Fig. 1) have an average d 34 S of +2.1‰, suggesting that the granite may have derived some sulphur from crustal sources during emplacement (Lowry et al 2005). …”

Tectono-magmatic controls of post-subduction gold mineralisation during late Caledonian soft continental collision in the Southern Uplands-Down-Longford Terrane, Britain and Ireland: A review

“…The range of δ 34 S values for hydrothermal mineralisation falls between subcrustal magmatic sulphur and the strongly depleted sulphur of the SUDLT metasedimentary rocks (Moffat Shale) indicating hydrothermal equilibration between fluids and host rocks and/or mixing between magmatic-hydrothermal fluid and fluid derived by contact metamorphic dewatering of the country rocks (Lowry, 1991;Lowry et al, 1997;Lowry et al, 2005). There is no evidence for deep burial, melting, assimilation or contamination of magma by metasedimentary rocks of the Southern Uplands.…”