THE ORIGIN OF ALLUVIAL GOLD IN THE LEADHILLS AREA OF SCOTLAND - EVIDENCE FROM INTERPRETATION OF INTERNAL CHEMICAL CHARACTERISTICS

A detailed chemical study has been carried out of 1794 gold grains fro m 55 sites within an area approximately 7 km by 18 km around Leadhills in the turbidite-facies, wacke-dominant Southern Uplands terrane of S cotland, formed as an accretionary thrust complex at the Laurentian co ntinental margin during Ordovician and Silurian subduction of the Iape tus Ocean. The Leadhills area has had a long history of alluvial gold working, since at least 1510, and probably much earlier, but few bedro ck sources of mineralisation have been located. Quantitative electron microprobe point analyses were obtained of sectioned and polished gold grains and of the associated microscopic inclusions, which in total c omprised 35 types of opaque minerals. Microchemical x-ray maps were al so obtained using a fully automated electron microprobe to show intern al chemical compositional variation, to resolve the components of mult iple and composite inclusions and to help in the identification of the smallest examples. Silver is present within most grains but Cu, Hg an d Pd were only detected in about 3% of the grains. The shapes of cumul ative frequency plots of the Ag contents of grains from different site s allow recognition of populations from different sources and enables comparison and differentiation between sites. In one of the main river s, the Shortcleugh Water, the nature of the gold is generally similar, both in terms of Ag content and inclusion type, but a further type of gold, relatively rich in Ag and containing distinctive Ni and Sb-rich inclusions, is also present in 4 adjacent samples in one sector of th e watercourse. This component of the samples reflects downstream dispe rsion, of up to 400 m, of gold grains from a mineralised structure cut ting the river almost perpendicularly. Elsewhere abrupt changes in the nature of the alluvial gold within streams reflect a series of differ ent sources and indicate limited glacial and alluvial dispersion. Four main types of gold which are considered to be derived from different varieties of source mineralisation can be recognised on the basis of c omposition and inclusion assemblage. The commonest type (ca. 70%) cont ains 8.4-13.18 Ag and inclusions, mostly of arsenopyrite, pyrrhotite, pyrite, chalcopyrite, galena, sphalerite and cobaltite, characteristic s typical of mesothermal gold from shear-zones within a turbidite envi ronment, suggesting sources within the local Lower Palaeozoic sediment ary rocks. The second type is richer in Ag (15.8-31.3%) with a basic i gneous signature indicated from inclusions of Sb-rich gersdorffite, py rite, Ni arsenide and other Ni-rich minerals, and shows some spatial a ssociation with discordant Tertiary basaltic dykes which cut the sedim entary rocks. The third type, mostly found in an area mantled by glaci al drift, is poor in Ag (less-than 1.2%) but frequently Cu-bearing and with inclusions of platinoids and Cu oxide. Associated with this type of gold are grains of the intermetallic compound Au3Cu and platinoids such as isoferroplatinum and a complex Ir-rich phase. Its source is p roblematical and could partly reflect detrital material associated wit h ophiolitic debris, which is common in the northernmost turbidites, a nd partly a local source of lode gold. The fourth gold type is also Ag -poor (less-than 3.2%), but frequently contains Pd, up to 6.3%, while the inclusions comprise selenides of Pb, Hg and Cu. In nearby areas th is type of gold is associated with Permian red beds and associated bas altic volcanics and also structures in the Lower Palaeozoic into which oxidising solutions derived from the red bed sequence have penetrated . (C) 1998 Elsevier Science B.V. All rights reserved.