According to Catholicism, how was Mary born without original sin?

I am slowly converting to Catholicism and am confused on the doctrine of Immaculate Conception. I do understand that it is Mary's conception that she was free from original sin. According to St. Augustine, original sin was passed down via sexual intercourse. Augustine also said that because Jesus did not have a human biological father and was not conceived with sperm. From what I understand of the story of St. Joachim and Anne, they had intercourse at the gates of Jerusalem. I may be wrong on these things as I am still learning. But taking these things into account, how can Mary be born without original sin if her parents had intercourse.