Malachi was an Old Testament prophet who lived around 430 BC, several generations after Lehi left Jerusalem, so his writings were not on the brass plates the Nephites carried. When the resurrected Christ visited the Nephites, he commanded them to write the words the Father had given Malachi and then expounded them (3 Nephi 24:1). The two chapters he quoted are Malachi 3 and 4: the command to bring tithes into the storehouse (3 Nephi 24:10) and the promise that Elijah would be sent before the great and dreadful day of the Lord to turn the hearts of the fathers to the children and the children to the fathers (3 Nephi 25:5-6; Malachi 3-4).
Christ told the Nephites that the Father had commanded these scriptures be given to them because they were to be preserved for future generations (3 Nephi 26:2).
Moroni repeated Malachi’s prophecy of Elijah’s coming when he appeared to Joseph Smith. The prophecy was fulfilled when Elijah appeared to Joseph Smith and Oliver Cowdery and committed the keys of the dispensation, turning the hearts of the fathers to the children and the children to the fathers (D&C 110:13-16).