The reason they stopped by this door was that Kentucky felt the presence of a Golden-level ore inside. Since he couldn't enter without destroying the whole thing, Roan's group went in to check it out while he dealt with the Yin Ghosts outside.
Rean then approached a sarcophagus at the center of that room and opened it. Sure enough, there was Golden-level ore there. It's just that it wasn't a Life's Nemesis Rock. "No, it's Haluquio Metal, ore at the Golden High-level. It's quite unfortunate that this one has already been refined, so it wouldn't be of much use to me." When Rean created his weapons, he had to do that from the start when they were still ores so that he could apply the Elemental Gathering Technique. "Well, I guess the Blacksmith Hall and Guild would still pay a good amount of Rank One Divine Stones for it."
