SUKHOI LOG, Russia—Beneath this plot of land in southeastern Siberia lie vast stores of gold, according to Russia’s biggest gold producer, Polyus PJSC, and tapping them could provide the Russian central bank with a huge and nearly sanction-proof backstop for its currency.

Tests commissioned by the company last year and undertaken by Australia-based AMC Consultants, along with a scoping study conducted in 2018, determined that there are 63 million ounces of gold at Sukhoi Log, Polyus has told investors. While independent mining...