Early diagenetic sulphide minerals in the Hayle Estuary, Cornwall

The Hayle Estuary, Cornwall, acted as an effective sediment trap for mine waste tailings and smelt waste released into the river catchments draining into the estuary. The stratigraphy of two 3 m cores recovered from Copperhouse Pool, Hayle comprises interbedded muds (interpreted as mine waste slimes) and sands in the upper 50 cm, passing down into sands composed predominantly of carbonate shell debris. Vacuum resin-impregnated core plugs sampled from more organic-rich intervals in the upper 150 cm of both cores were examined using scanning electron microscopy. Detrital heavy and opaque minerals include abundant grains of cassiterite, chalcopyrite, Fe oxides, arsenopyrite, sphalerite, polymetallic slags, detrital angular pyrite, ilmenite, monazite, zircon, wolframite, ?loellingite (As-Fe), galena, chalcocite/bornite and pyromorphite. Abundant diagenetic sulphide minerals also occur in these samples, and include Cu-Fe-(As) sulphides (probably chalcopyrite), As sulphides and pyrite. The precipitation of chalcopyrite occurred under reducing conditions with the reaction buffered by the Fe system. Possible copper concentrations in equilibrium with the authigenic chalcopyrite are so low that it is likely that there was an influx of more oxidising pore waters carrying higher levels of copper, which was then precipitated on reaching more reducing conditions. The precipitation of the As sulphides occurred from pore fluids with a high arsenic concentration under less reducing conditions than the chalcopyrite.