In an annotated bibliography by John Sorenson of sources relating to metals and metallurgy mentioned in the Book of Mormon, he notes that according to Warwick Bray, "estimates are given for the Spanish 'take' of 350 kg. of silver and 4,000 of gold from Mexico."
Sorenson notes that there was a special metal in ancient America called tumbaga, in which the silver content--up to 25%--was not intentional but was a natural impurity of the gold derived from Mexico to the Northern Andes. [John L. Sorenson, "Metals and Metallurgy Relating to the Book of Mormon", F.A.R.M.S., pp. 14-15]
“Gold”
According to John Sorenson, gold and silver specimens are well known in ancient America. Some show the "lost-wax" method of casting, known in Mesoamerica, Peru, and also the Near East. However, the only form specified in the scriptures is the flat "plate" on which historical and religious records were kept. It would not be feasible to manufacture those plates other than by hammering. Thin hammered metal we know well, but metal sheets for record keeping are not yet attested archaeologically in the New World. (A nineteenth-century historian in Oaxaca, Mexico said that the ancestors of the Mixtecs made very thin gold plates on which were engravings of ancient hieroglyphs, but we do not know the source of his information.) [John Sorenson, An Ancient American Setting for the Book of Mormon, F.A.R.M.S., p. 282]