What happened to Lyddie and Charlie's father?

Very little detail is given to readers in regards to Lyddie's and Charlie's father.  What we do know is that he is presumed alive at the beginning of the novel, but he is far away.  Readers are told that he left his responsibilities on the farm in search of striking it rich by finding gold.  He has not been heard from since leaving the farm.  The big problem is that the father not only left his family to take care of the farm themselves, he left them with a mountain of debt.  Paying off that debt propels much of the story and is the initial reason why Lyddie and Charlie are sold off into indentured servitude in the first couple of chapters.