Microprobe element maps showing the distribution of arsenic, mercury, barium and iron in altered greywacke from deep within the hot spring system. The large crystals are marcasite (iron sulphide, FeS2). Mercury and arsenic are present at low levels (about 0.5%) in some parts of marcasite crystals, particularly on some edges. Similar amounts of mercury and arsenic are in some barium phosphate minerals. The mercury and arsenic are readily released into streams when the marcasite breaks down. Marcasite is very unstable in atmospheric oxygen and breaks down to iron oxide and sulphuric acid. The acid in turn breaks down the barium phosphate minerals, releasing more arsenic and mercury.