The in-house gold mine, the wildly lucrative Medallion fund, was suddenly looking for capital.

The firm told staff that they would be allowed to increase their Medallion stakes by at least 50 percent. The catch: they had just weeks to come up with the money.

It was a remarkable about-face for RenTech, the secretive quant firm that has restricted Medallion’s size since the early 1990s to protect its ability to generate 40 percent annualized returns after fees. Only current RenTech employees are allowed in Medallion, and even they had faced strict limits designed to keep net assets from exceeding roughly $10 billion.

As a result, many RenTech staffers were caught off guard when told in early December that these limits would be significantly raised for the first time in years. At least six executives entered into borrowing arrangements, according to regulatory filings, including co-Chief Executive Robert Mercer, whose financial and strategic support is widely thought to have helped propel Donald Trump into the White House. […]