Ah ... the old wet chemistry tricks. Ok, I'll bite:
If the white solid is soluble in nitric acid, and after drying and redissolving in water, gives a white precipitate with HCl, it is lead, or silver or mercury. Lead gives a fine precipitate with this method, silver's is distinctly floculent, like a snow globe. IIRC, and I may be totally wrong here, mercury's white precipitated chloride gradually becomes gray over a period of days. Long story short, white precip with HCl that persists is lead.
Iron is pretty easy too, do a google for spot tests, or look in the chemistry library for an old yellowing text on analytical chemistry.
The zinc may be a problem, as I recall, many other things may interfere with a spot test.
'Course, if you really care what's in your rock, you chip off a crumb and send it to an analytical lab. For a couple hundred UD$, they submit it to ICP spec and tell you all the elements to 4 decimal places.