An exhaustive study of mining and smelting practices was writen in the mid 16th century by Agricola and transilated by Herbert Hoover. The book De Re Metallica is easily found in many libraries and used copies can be acquired on internet book search sites.
Well illustrated it explains the techniques acquired to smell metals in the period of history that you are interested in.
I read over your initial question. A very small amount of silver would have been required to do silverpoint sketching since the paper abrades a minute amount to lay down a thin film of silver that eventually darkens to create the image. I suspect the coinage of the Da Vinci's day may have provided enough silver to refine and make into the stylus he may have used.