I have found that Richard Aloysius Pitt was declared insolvent in Melbourne in May 1864. He was a clerk at the General Post Office in Melbourne.
The last record of him was a call on his shares in a number of speculative gold mining companies in 1868.
If he went to the US, then he probably went around this time.
By the way, I am related on the Robert George Pitt side of the family.