GEOLOGICAL CONTROLS ON GOLD GRADE DISTRIBUTION IN CHIDLAW LINK-ZONE, GWYNFYNYDD MINE, DOLGELLAU, NORTH WALES

Gwynfynydd is a small, high-grade black shale-hosted vein-gold deposit in the county of Gwynedd, North Wales. The veins cut the Cambrian Mae ntwrog, Clogau and Gamlan Formations, which were intruded by dykes and sills of intermediate to basic composition (greenstones). The Chidlaw Link Zone is currently the only area being mined and contains some 24 000 t ore at a historically realized grade of 15.2 g/t (0.49 oz/t) go ld. Within the Link Zone a complex vein structure is observed, which c onsists of a 3- to 5- m thick composite mother vein and narrow, <0.5 m wide, variably gold-bearing leader veins. The mother vein shows minor variations in dip, strike and thickness, whereas the leader veins sho w marked changes in all three and display low lateral and vertical per sistence. The leader veins are hosted within the footwall zone of the mother vein, which is defined as the 2-3 m either side of the true foo twall (mother vein). Both the mother and leader veins are composed of quartz with galena, sphalerite, chalcopyrite, pyrite and minor arsenop yrite. The veins occupy dominantly normal faults, which exhibit some o blique component. Throughout the Link Zone the vein system is transect ed by a series of cross-vein and intra-vein dip-slip faults, variably hosting carbonate-marcasite-hematite mineralization, that disrupt the veins, giving rise to structural complexities. Where the veins cut the Maentwrog and Clogau Formations they are potentially gold-bearing, bu t elsewhere they are notably barren. The gold distribution exhibits a very strong nugget effect, the gold occurring in highly restricted poc kets, but which may contain tens of kilograms. These pockets occur at vein intersections or branches and within the complex footwall zone an d are often related to variations in vein attitude and/or geometry. Wi thin the Link Zone gold pockets are generally associated with the lead er veins, but small background occurrences of up to 30 g gold occur in the footwall zone. The Chidlaw Link Zone displays a complex, erratic distribution of gold mineralization and a multi-stage history of struc tural activity, fluid flow and mineral deposition. A detailed geologic al model is needed to guide the mining operation.