From my studies, it seems that the gold plates did not contain the same information as what was covered in the Brass Plates (i.e. genealogy of the jews, first five books of Moses, etc.) If this was so important, what happened to the Brass plates. Did Moroni take them with him with the gold plates or did he bury them with the rest of the plates Mormon acquired?
What Moroni hid up in the Hill Cumorah and delivered to Joseph Smith was one volume of gold plates containing three separate records. The first third of the set contained two records. These were 1) Mormon’s abridgment of the Large Plates of Nephi, Moroni’s abridgment of the plates of the Jaredites, Mormon’s and Moroni’s personal accounts, and 2) the entire original record called the Small Plates of Nephi. The first part of our Book of Mormon, from First Nephi through Omni is a translation of the Small Plates which were written primarily by Nephi and Jacob, with short entries from the remaining leaders of the Nephites down to Omni, who lived somewhere between 279 and 130 B.C. The remaining two thirds of the record was sealed. Moroni tell us what is in the sealed portion in these words:
Behold, I have written upon these plates the very things which the brother of Jared saw; and there never were greater things made manifest than those which were made manifest unto the brother of Jared.
Wherefore the Lord hath commanded me to write them; and I have written them. And he commanded me that I should seal them up; and he also hath commanded that I should seal up the interpretation thereof; wherefore I have sealed up the interpreters, according to the commandment of the Lord.
For the Lord said unto me: They shall not go forth unto the Gentiles until the day that they shall repent of their iniquity, and become clean before the Lord.
And in that day that they shall exercise faith in me, saith the Lord, even as the brother of Jared did, that they may become sanctified in me, then will I manifest unto them the things which the brother of Jared saw, even to the unfolding unto them all my revelations, saith Jesus Christ, the Son of God, the Father of the heavens and of the earth, and all things that in them are (Ether 4:4-7).
So the Brass Plates of Laban were never in the hands of Joseph Smith. However, Nephi and Jacob copied 17 chapters from Isaiah that were taken from the Brass Plates; another chapter was quoted by Abinadi in Mosiah 14, and part of Isaiah 52 and Isaiah 54, as well as parts of Malachi were quoted by the Savior in Third Nephi. Shorter excerpts from the Brass Plates, such as the Ten Commandments, were also quoted by other writers.
It is interesting to note that the 20 chapters of the Book of Mormon Isaiah, hand copied from the Brass Plates by the Nephite prophets, and translated into English by Joseph Smith, “by the gift and power of God,” represent the most correct and the most ancient record of the Book of Isaiah in existence.