Really, this is about the leftover obsession from medieval days with alchemy, or the attempt to transform other metals into gold. Scientists (or pseudo-scientists) made some of the most important chemical discoveries in their unending quest to perform wizardry that would make them rich. A lot of the poetry of the time (and a lot of Donne's poems) deal with working with precious metals.
Line 16: John Donne likes puns (probably because they rhyme with his name). Even though in this case "elemented" means 'started' or 'sparked,' the word itself prepares us for the upcoming metaphor about gold.
Line 17: Just to make sure we get it, the next line uses another metalworking analogy. Scientists worked endlessly to purify their metals, trying to burn off any impurity. Donne is saying that his love is pure, that it isn't tainted with mere physical affection.
Line 24: Like we said, these guys were just understanding how their world operated. That included the properties of metals. As it turns out, gold is a soft metal compared to others. It can be beaten into a fine foil so that just a little bit of pure gold will stretch a long way. Obviously, that's how Donne sees his love.