PROCESSES OF FORMATION AND DISTRIBUTION OF PB-BEARING, ZN-BEARING, CD-BEARING, AND CU-BEARING MINERALS IN THE TYNE BASIN, NORTHEAST ENGLAND- IMPLICATIONS FOR METAL-CONTAMINATED RIVER SYSTEMS

Historic mining has produced widespread Pb, Zn, Cd, and Cu contaminati on in the fluvial deposits of the Tyne River Basin, northeast England. Detailed mineralogical analysis of contemporary overbank river sedime nt, mining-age alluvium, and mine-waste tips and of suspended solids i n river waters has defined a general weathering reaction paragenesis o f pb-, Zn-, Cd-, and Cu-bearing minerals: sulfides --> carbonate, sili cate, phosphate, and sulfate weathering products --> iron and manganes e oxyhydroxides. Textural and chemical evidence suggests that the sulf ides alter to carbonates in high pH/pCO(2), limestone-dominated source terrains. These minerals and other contaminant metal-rich minerals su ch as silicates and manganese oxyhydroxides decline and disappear down stream in lower pH shale/sandstone-dominated environments. The concomi tant decrease in total Pb, Zn, Cd, and Cu sediment contents in the Tyn e and possibly other metal contaminated rivers may be related to these essentially chemical weathering and dispersion processes. These are a ugmented by physical, hydrodynamic processes that to a large extent ef fect dilution by premining Quaternary sediment and by uncontaminated s ediment from tributaries.