Joe Foster, who manages active gold funds at investment firm VanEck, which is Yamana's biggest shareholder, said the merger would create a more complex company with not much synergy, and both Yamana and Gold Fields would do better on their own than as a combined group.
VanEck is also Gold Fields' fourth-biggest shareholder.
"I don't endorse the deal on either side," Foster told Reuters on Thursday. "It's a deal that just doesn't make fundamental sense to me."
Gold Fields has argued the acquisition, which would create the world's fourth-biggest gold miner, will provide long-term growth and deliver cost savings.
Shares in the South Africa-listed miner dropped sharply when it announced the proposed deal in May, and the CEOs of both companies have fought to convince investors of its merits in the months since.
For the all-share deal to go ahead it must win the approval of 66.67% of Yamana shareholders and 75% of Gold Fields shareholders at votes scheduled for Nov. 21 and Nov. 22 respectively.
Which way the votes will go is "up in the air" at the moment, Foster said.
Yamana shares are trading at a discount to the offer price, indicating uncertainty in the market.
(Reporting by Helen Reid; Editing by Susan Fenton)
