Metal mobility during metamorphism and formation of orogenic gold deposits: Insights from the Dalradian of Scotland
- Publisher: Stockholms universitet, Institutionen för geologiska vetenskaper
Gold mobility | ultralow detection limit | metamorphic devolatilization | Dalradian | Glen Esk | Loch Lomond | Stonehaven | fluid inclusions | HG-AFS
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.