Abstract
Castromil and Serra da Quinta are two orogenic gold deposits located in northwest Portugal, along the eastern flank of the NW trending Valongo anticline (Noronha et al. 2000). The deposits are structurally controlled by a NW–SE trending fault, which lies adjacent to an intrusive margin between Silurian meta-sediments and the Castello de Paiva granite (Vallance et al. 2003). Collectively, the Castromil and Serra da Quinta deposits comprise the Lagares project, currently undergoing exploration drilling by Medgold Resources Corp. Although previous work on the prospect has produced models for gold deposition and concentration, the source of sulphur, gold and other base metals is undefined.
This research project will aim to test whether the composition of mineralisation is controlled by fluids sourced from meta-sedimentary rocks. The counter-hypothesis is that the mineral assemblage is controlled by other factors, such as variations in precipitation conditions, different pulses of fluid, or by fluids sourced from the granite host. In addition, other orogenic gold deposits within northwest Iberia; Limarinho and Leiranco, which have no spatial association to meta-sediments, will be compared and contrasted in terms of mineralogy and potential fluid sources. This will subsequently provide constraints for future exploration targets and drilling strategies along the granite/meta-sedimentary interface.
Comparisons between each deposit are being made analysing mineralised and unmineralised meta-sedimentary rocks and granites. Reflected and transmitted light microscopy has been utilised to observe ore mineralogy and identify the location of gold. The mineral assemblage is pyrite and arsenopyrite with lesser galena, sphalerite and pyrrhotite. Minor phases of bismuthides and antimonides have also been identified using scanning electron microscopy (SEM). Geochemical data of mineralised core show a clear association between Au and pathfinder elements As, Bi and Sb. This is significant as these gold proxies may be correlated to values in the meta-sedimentary geochemistry to define a potential source rock. Further work will characterise the fluid chemistry using fluid inclusion microthermometry. Sulphur isotopes will also be examined in primary biogenetic sulphides found within meta-sedimentary rocks and will be compared to sulphides in the mineralised ore zones.