"Red, Semidouble, St. Damasus, Pope, Martyr --
"Pope St. Melchiades ruled the Church at the close of the era of persecution. St. Augustine styled him "a true son of peace and a true father of Christians." He died January 10, 314, having sat as Pope two years, six months, and eight days - SM"
This also would have been the 3rd day within the Octave of the Immaculate Conception.
The Baronius Press edition of the '62 missal also says that he "died peacefully, after undergoing great sufferings in the persecution of Maximian."
I thought the information was a bit scant about the details of his death -- so I further looked to an article about him in the Catholic Encyclopedia. It's odd that he's called a martyr, I think. Seems that his reign was towards the end of the persections, indeed, the civil authorities in his time had started to give back church properties which had been confiscated. This pope also had to deal with a lot of heretics and lapsi.