On May 26 1863, gold was discovered in Alder Gulch, Montana Territory which reinforced the Federal treasury -- already bolstered by the California gold production.
Source Civil War Quiz and Factbook by Rod Gragg
Edit - Well, it's interesting to know about the Montana gold discovery, and it certainly could be considered a "financial bonanza." On the other hand, as demonstrated by our players' numerous and varied answers, a lot of other things could also be considered a "financial bonanza." There wasn't enough information in the question to make it clear that there should be one and only one correct answer and that the answer should be the Montana gold discovery.
I'm going to rule that all answers were correct on this one.
hoosier