The graven images of their gods shall ye burn with fire: thou shalt not desire the silver or gold that is on them, nor take it unto thee, lest thou be snared therin: for it is an abomination to the Lord thy God. (Deuteronomy 7:25)
Metals used to overlay wooden idols and ornaments adorning them (see Isa. 30:22). These would only become a snare to Israel (Joshua 7:21, 22), and were not of permanent value anyway.