Fred,
This may help you in understanding what is happening with targets that weaken or sometime disappear - Including GOLD nuggets! There is no such thing as pure gold in nature iron, silver and copper will all decay in the ground but gold is a noble metal and stays in tact. Depending on the level of other minerals that will decay associated with the nugget, you could be dealing with the Halo and be very careful not to leave a little gold nugget in the ground.

https://www.youtube.com/watch?v=55L2z0_P4Og