The 1907 LDS vest-pocket edition and the 1911 LDS edition introduced the repeated preposition for before the last phrase of this coordinate structure. Subsequent LDS editions have followed this emendation. The RLDS text has retained the earlier text without the for (since it is not found in the printer’s manuscript or in any of the early editions).
It is quite possible that the original manuscript read for here and that the word was accidentally dropped when Oliver Cowdery copied the text into the printer’s manuscript. We have fragments of 𝓞 for part of this verse, and there is definitely room for the word for between the extant fragments. Nonetheless, the lacuna is quite long; thus this evaluation cannot be conclusive (see the transcript for lines 29–30 on page 98 of 𝓞). But we can determine that if the for was in 𝓞, it would have occurred at the end of a manuscript line, a place where Oliver frequently made mistakes (by either misreading or deleting a line-final word) when he copied from 𝓞 into 𝓟.
Internal evidence from parallel coordinate structures elsewhere in the text supports the repetition of the preposition for at least the immediately following conjunct whenever the two preceding conjuncts are gold and silver:
The only exceptional example is one where the preposition is repeated for the third conjunct but not for the second one:
There is a possibility that the original text for this passage read “with gold and with silver and with precious things”. For discussion, see under Mosiah 11:9. Yet even this example repeats the preposition with for the third conjunct, thus providing indirect support for the reading with the repeated preposition in Jacob 2:12: “for gold and for silver and for all manner of precious ores”.
Finally, note that for over half of these additional examples (8 out of 15) the third conjunct involves the adjective precious ( just like in Jacob 2:12). These examples with precious give further support to the emendation of Jacob 2:12 to read with a repeated for before “all manner of precious ores”.
Summary: In accord with the spacing between extant fragments of 𝓞, follow the emended reading for Jacob 2:12 that supplies the repeated preposition for before the third conjunct in “to search for gold and for silver and for all manner of precious ores”; the systematic nature of usage elsewhere in the text supports this emendation.