The accumulation of temporal goods contrary to justice is always a mortal sin. And so Hab. 2:6 says: "Woe to those who pile up things not their own." Likewise, the accumulation of temporal goods, even if not contrary to justice, is a mortal sin if one makes them one's end.
Thomas Aquinas, De Malo 13.2 ad s.c. 3.
[Thomas Aquinas, On Evil, Regan, tr., Oxford University Press (Oxford: 2003) p. 395.]