Metal bass riffs are used over metal chords and can be made up of chords themselves. For example, power chord riffs are often used in metal. The bass picking method used for the lead playing may be alternate picking for scales, and sweep picking on arpeggios for example. While chords are picked in various directions, one method is repeated, rapid fire down strokes, in one direction without alterations that makes the sound heavy and characteristic of the thrash metal style. Metal chords are also alternate picked many times as well, depending on the style that you want to emulate and your own personal preference.