Impact of mining on the sediment geochemistry and mineralogy of the Helford River, Cornwall

The geochemistry and mineralogy of the intertidal sediments of the Helford River, Cornwall have been examined to assess the potential impact of mining activity on sediment supply. Cores from Polpenwith and Polwheveral creeks show a pulse in Sn (1000-1100 ppm), Cu (800-900 ppm) and Zn (500-600 ppm) at a depth of 30 cm below the present day sediment surface; As and Pb values are typically low and show little down-core variation (<130 ppm As and <78 ppm Pb). Two cores recovered near Gweek have generally low and invariant down-core geochemical signatures, except for a single sample from the base of Core 2 which shows a sudden increase in Sn to > 1800 ppm. In addition, two cores were collected from the mouth of Mawgan Creek. Core 4 shows a loxv but invariant geochemical signature but Core 3 shows a significant down-core increase in Sn (> 1900 ppm Sn), Cu (588 ppm) and Zn (1297 ppm). The heavy mineral assemblage is dominated by cassiterite, chalcopyrite and sphalerite, along with less abundant zircon, monazite, ilmenite, rutile/anatase, sphene, wolframite, barite and rare slag products. Diagenetic pyrite, bornite and Fe oxides also occur. The geochemistry and mineralogy are consistent with the historical release of mine waste tailings into the Helford River. Pb-210 dating of two cores suggests that the sediments are younger than 1880. Based on these data the most likely sources of the mine waste are from Wheal Caroline and Wheal Vyvyan to the north of the Helford River which are documented as being active between 1827 and 1864.