Impact of mining on sedimentation: The Camel and Gannet estuaries, Cornwall

The mineralogy and geochemistry of the inter-tidal sediments in the Camel and Gannet estuaries on the north Cornwall coast has been examined to test the importance of mining on sediment supply. In the Camel Estuary there is a clear stratigraphical geochemical anomaly for Sri, Wand Zr which corresponds with abundant cassiterite, wolframite and zircon. This sediment was supplied to the estuary as a result of the release of mine waste tailings from hard rock mining of main stage mineralisation, probably from mines such as Mulberry and Wheal Prosper in the area around Lanivet. In contrast the sediments in the Gannet Estuary contain very high concentrations of Pb and Zn. In one core, maximum Pb concentrations are in excess of 8500 ppm, along with over 1600 ppm Zn. This same stratigraphical interval also has very significant enrichment in Zr, Ce, La and Y along with high values for Ag. The geochemistry of the Gannet Estuary sediments is reflected by the mineralogy with abundant galena, sphalerite and plumbogummite (Pb-P-Al phase). In addition to these detrital grains there are abundant diagenetic phases precipitated within the sediments, including authigenic Pb, Zn and Cu-Fe minerals, Early diagenetic calcite-siderite-Fe monosulphide concretions are also present. The likely source for this Pb-Zn-Ag mine waste is from the area around Newlyn Downs. In both cases, the release of particulate mine waste, possibly following mine closure in the latter part of the 19th century or early 20th century, had a significant impact on down stream estuarine, sedimentation. However, in the Camel Estuary the presence of abundant cassiterite is unlikely to have had a significant impact on the biosphere. whilst in the Gannet Estuary the presence of significant Ph and Zn and the mineralogical evidence that there is diagenetic mobility of Ph, Zn and Cu, is indicative that these elements were, and may still be, bioavailable.