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No description available.

The Great Glen Fault (GGF) is a major northeast to southwest trending structure and has been interpreted as a terrane boundary separating the Precambrian Moine terrane to the northwest from the Precambrian Dalradian terrane to the southeast (e. g. Bluck & Dempster 1991). If the GGF is a terrane boundary no `Moine' rocks could be found southeast of the GGF and no `Dalradian' rocks could be found to the northwest of the GGF and each crustal block would have distinct tectonometamorphic, provenance and igneous intrusive characteristics. To assess this, carefully selected orthoamphibolite and metasediment samples were collected from both the Northern and Central Highlands, and were analysed by a combination of petrography, geochemistry and geochronology. Geochemical analysis suggests that a Neoproterozoic metagabbro and metadolerite suite was emplaced during crustal extension across the Northern and Central Highlands at approximately the same time, and that this suite represents an earlier intrusive event to that represented by amphibolites in the Dalradian Appin Group. Geochemistry also suggests that the Upper Morar Psammite Formation of the Moine Supergroup in the Northern Highlands is unlikely to correlate with the pebbly psammite formations in the Central Highlands and shows that the Upper Shiaba Psammite Formation metasediments on the Isle of Mull are geochemically distinct from the Upper Morar Psammite Formation metasediments on the mainland. The Glen Urquhart Complex in the Northern Highlands cannot be correlated with the Ord Ban Subgroup or Grantown Formation in the Central Highlands. However, despite these differences U-Pb detrital geochronology shows that the Upper Morar Psammite and Central Highland pebbly psammite formations are dominantly derived from similar Mesoproterozoic and Palaeoproterozoic sources, with a small amount of material of Archaean derivation. This, together with the matching amphibolite suites, means that the Great Glen Fault is very unlikely to be a terrane boundary.

Orogenic gold deposits occur within metamorphic belts throughout the world and have through time represented the source for over 25% of the world’s gold production. Although orogenic gold deposits are of great economic importance, controversies exist on the subject of fluid and metal sources and there have been few studies of gold´s distribution and mobility outside of large economic deposits. Research made by Pitcairn et al. (2006), on the Mesozoic Otago and Alpine schists of New Zealand, observed systematic depletion of Au and a suite of 6 associated elements with increasing metamorphic grade. This depletion was identical to the suite of elements enriched in the Otago gold deposits and provided strong evidence that orogenic gold deposits form due to metamorphic processes. The mobilization of metals was attributed to the recrystallization of sulfide minerals during prograde metamorphism causing dehydration and release of metal-rich metamorphic fluids.  This thesis is part of a larger project aimed at testing the “Otago model” in a classic metamorphic terrain: The Dalradian metamorphic belt of Scotland. Rocks in the study are from the southern higlands group and the Appin and Argyll group which range in metamorphic grade from chlorite zone greenschist facies to sillimanite zone amphibolite facies. Three main aspects, which supplement earlier research, are addressed in this study: 1) Investigation of the sulfide paragenesis at Loch Lomond and Stonehaven was carried out to map the evolution of sulfides with metamorphic grade and the possible relations to the distribution of gold. Using SEM scanning to quantify the abundance of different sulfide minerals together with previous data on the Glen Esk region, a complex sulfide evolution pattern for the Dalradian Supergroup is identified. The sulfide evolution describes the same changes in texture and chemistry as observed in the Otago Schists but is made complex by the difference in geological evolution for the different regions. 2) Reinvestigation of the higher grade zones of Glen Esk (staurolite to sillimanite) was carried out as samples from the previous study were very weathered. Results from ultralow detection limit methods (HG-AFS and a gold detection method developed by Pitcairn et al. 2006) showed significant systematic depletion of Au and As with metamorphic grade. From chlorite to sillimanite zone average values of Au and As were showed to decrease by 65% and 88% respectively. Furthermore, a suite of 10 major and 12 trace elements were analyzed using ICP methods showing no trends of systematic depletion with increased metamorphic grade.  3) Investigation of Pb-Ag Veining and vein samples from each of the metamorphic index mineral zones in the Glen Esk area was carried out to identify fluid composition and ore mineralogy. Using microthermometry and Raman laser spectroscopy two distinct fluids were identified. The first type is a H2O-CO2-N2-salt fluid of low salinity (0-15 weight percent NaCl equivalent) and medium temperature (150 to 250 °C) locally containing minor amounts of CH4. It is found in the veins from the mineral index zones of Glen Esk and was formed in the ductile regime most likely related to late stage metamorphic devolatilization released during Caledonian uplift of the Dalradian. Pb-Ag veins from the locality of Hardhill host the second fluid type which was formed in the brittle regime  accompanied by brecciation as a high salinity (15 to 20 weight percent NaCl equivalent) low temperature (70-140°C) H2O-salt fluid with calcic composition was precipitated. This fluid bears much resemblance to Carboniferous calcic brines responsible for economic base-metal precipitation with widespread occurrence in southwest Scotland and Northern Ireland. Results of this thesis show many similarities with the Otago study, with a connection between metal mobility and metamorphic grade, providing support for the dehydration model as a viable mechanism for the generation of orogenic gold deposits.

Results from petrographic analyses, chemical analyses and mineral phase diagram calculations show that staurolite minerals grew in selvages adjacent to quartz veins. Previous studies show the same relationships between quartz veins and garnet, as well as kyanite growth. The selvages are formed as metamorphic fluids flow through cracks, altering the bulk composition by mass transfer and triggers the nucleation and growth of new minerals. A pseudosection for a staurolite absent sample has been calculated using THERMOCALC. No stability field correlates to visually observed mineralogy. This indicates that a reaction forming staurolite never was triggered since no fluids was present during metamorphism.