The lithogeochemical and mineralogical setting of turbidite hosted arsenic-gold deposits in the Lower Palaeozoic of Scotland
Detrital gold is widely dispersed in the Southern Uplands and attributed to minor Au-bearing vein mineralisation hosted by Ordovician and Silurian turbidites. Field, mineralogical and geochemical studies in the Glendinning area indicate the pervasive and laterally extensive nature of hydrothermal alteration and element zonation associated with As-Sb-Au mineralization. Geochemical anomalies are characterised by elevated chalcophile (As, Sb, S, Cu. Pb, Ti, Hg) and depleted siderophile (Fe, Mg, Zn) and alkali group (Na) elements. Broad areas of sodium depletion are indicative of primary hydrothermal activity and together with anomalous arsenic values locate 8 zones of As-Sb- Au-Hg mineralization within 10km of the Glendinning mine and some 60 locations elsewhere in the Southern Uplands of Scotland and the Longford Down, Ireland. Electron microprobe studies demonstrate that an initial phase of arsenopyrite mineralisation forms the principal locus of submicroscopic and lattice-hosted gold concentration in the Glendinning, Knipe, Cairngarroch Bay and Clontibret As-Sb-Au deposits. Gold deposition was initiated during wallrock alteration and hydraulic brecciation, possibly as a result of fluid chilling, while later stibnite vein deposition was accompanied and overprinted by minor chalcopyrite, sphalerite, galena and a variety of sulphosalts. Hydrothermal alteration and As-Au mineralisation in the Southern Uplands postdates arc-related volcanism, turbidite deposition and early deformation. The model envisaged invokes the discharge of highly reducing, sulphur-rich hydrothermal fluids, related to the emplacement of structurally constrained, late Silurian calc-alkaline minor intrusives at the close of the Caledonian orogeny. Different levels of crustal emplacement and subsequent exhumation are considered to explain the various styles of As-Sb-Au mineralization exposed in the Southern Uplands. The complex tectonic hlstory of the Southern Uplands and Longford Down is mirrored by significant variations in the chemical composition of strike parallel greywacke tracts admixed with sediments derived from ophiolites, calc-alkaline volcanic arcs, stable cratons and carbonate shelves. Non-parametric K-means cluster analysis applied to 840 petrographlcally defined samples, determined the chemical variation within each petrofacies and provides a satisfactory method of classifying individual members. Geochemical traverses through the Southern Uplands and Longford Down reveal lateral continuity over a strike length of 350km.