What's the field near? The approximate age of the location in terms of activity?
Coke is used, as you probably know, in the iron and steel making industry.
Those granular size pieces you have there....is about the size I see everyday where I work and it's that small so they can shoot it up pipes to the top of the mill, then added through additional pipes to the molten steel.
Coke is made from coal and coal will ring up as hot rocks sometimes, depending on what type of coal it is.
Now the lead? Don't know why that would be in the same place.
Where I work, they add all kinds of stuff to the steel...copper, aluminum, silicon, magnesium, nickle.....ect. Haven't seen lead anywhere in the processes.
Maybe there was an old ironworks or lead works in your approximate area?