(Bloomberg) -- Even after the world's biggest gold-mining companies posted their best profit margins in four years, investors can't seem to head for the exits fast enough. They're betting the industry's returns have reached a high-water mark, at least for a while.

The biggest ETF linked to the producers, VanEck Vectors Gold Miners ETF, has seen net withdrawals this month of $617 million following a record outflow in April of almost $1.1 billion, data compiled by Bloomberg show. The $10 billion fund tracks the value of global producers including Barrick Gold and Newmont Mining.

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