DescriptionAlluvial gold is widespread in southern Scotland, but the fundamental causes of mineralisation remain elusive. The gold is hosted by the Southern Uplands–Down-Longford Terrane (SUDLT), a Lower Palaeozoic accretionary prism and foreland fold and thrust belt dominated by turbidites and black shales. Structurally hosted lode-gold associated with As and Sb mineralisation is known from a small number of localities and the region has been classified as an orogenic gold province . However, late Caledonian calc-alkaline plutons of the Trans-Suture Suite (TSS)  together with minor intrusions of lamprophyre, granodiorite and appinite support a possible magmatic role in mineralisation. Alteration mineral assemblages are compatible with epithermal and intrusion related systems. Gold is associated with magmatic PGE-rich Ni-Cu sulphide within a diorite sill at Talnotry . Fluid inclusion data indicate that remote from known intrusions gold was deposited from a mixed magmatic-metamorphic fluid [3,4]. The As-Sb-Au deposits exhibit a spatial association with minor dioritic, monzonitic and granodioritic intrusions  but not clearly with the major plutons [3, 6]. Porphyry Cu, associated with late Caledonian granodiorites at Black Stockarton Moor also supports a magmatic source of sulphide and metals. Isotopic characteristics of the TSS plutons indicate a source in Avalonian crust that was underthrust beneath the Iapetus Suture, including a component derived from pelites of the Skiddaw Group . The lamprophyres and appinites were probably sourced in sub-continental mantle metasomatised by subduction. The granitoid magmas were probably hydrous due to a hydrated mantle source . At least some of the more primitive (appinitic) magmas have transported gold, PGE’s, Cu and Ni from the lower crust or mantle and have become sulphide-saturated at higher crustal levels. Recent U-Pb zircon ages show that TSS magmatism occurred between 425-380 Ma, coeval with that in the Grampian Highlands. Magmatism in both regions can be explained by slab break-off and partial delamination of the Avalonian lithosphere following the collision of Avalonia with Laurentia [6,9]. Extrusion of asthenosphere through the slab window caused melting of metasomatised lower lithospheric mantle, generating voluminous lamprophyric magmas that underplated the crust, triggering partial melting and generation of the intermediate and granitoid magmas that formed the major TSS plutons and minor intrusions. We extend this scenario and suggest, following [10,11], that sulphides in the metasomatised mantle or underplated lower crust were enriched in PGE’s, As, Cu and Au and that these elements could have been remobilised during partial melting and carried to higher crustal levels. Sulphur saturation, perhaps due to interaction with Skiddaw slate and/or Moffat Shale, could have triggered exsolution of an auriferous sulphide melt which, during crystalisation at shallow crustal levels (~5 km) would have released gold into aqueous fluids that could have migrated along active faults to form Au-As, Sb and Cu lode deposits of an epithermal and intrusion-related character. In addition, metamorphic reactions due to the heat flow associated with the magmatism could have generated sufficient metamorphic aqueous fluids capable of mobilising and concentrating dispersed gold from the metasedimentary country rocks of the SUDLT. The well-constrained geology of the SULDT provides excellent opportunities to investigate further the relationships between magmatism, tectonism, metamorphism and gold mineralisation during the transition from subduction to collision. References:  Goldfarb R. J. et al. (2001) Ore Geology Reviews 13, 7–27;  Brown M.J. et al. (2008) Geological Magazine 145, 235-256;  Steed, G.M. & Morris, J.H. (1997) Transactions of the Institute of Mining and Metallurgy section B: Applied Earth Science, 106, B109-B117;  Naden, J. & Caulfield, J.B.D. (1989) Transactions of the Institution of Mining and Metallurgy. Section B, Applied Earth Science, 98, 46–48;  Leake, R.C. et al. (1981) Mineral Reconnaissance Programme Report No. 46, Institution of Geological Sciences;  Brown, M.J., et al. (1979) Mineral Reconnaissance Programme Report No. 30, Institute of Geological Sciences;  Power et al. (2004) Mineralogical Magazine 68, 395-411;  Miles et al. (2014) Journal of Petrology 55, 181-207;  Miles et al. (2016) Gondwana Research 39, 250-260;  Graham et al. (2017) Ore Geology Reviews 80, 961-984;  Richards, J.P. (2009) Geology, 37(3), 247–250.
|Period||26 Sep 2017|
|Event title||Fermor Meeting 2017: Factory Earth|
|Location||London, United Kingdom|
|Degree of Recognition||National|