Sandfire spent A$5.3 million buying 5.27 million shares on-market over the past week.
The buys have taken Sandfire's stake in Adriatic to 11.1%.
Sandfire first became a 7.7% shareholder by cornerstoning Adriatic's May 2018 initial public offering and spent $827,845 in late 2018 to maintain its investment.
The company is working on a maiden resource estimate for the Rupice deposit at Vares, which will feed into a scoping study.
Sandfire last week announced a $167 million takeover of Botswana copper developer MOD Resources.
Shares in Adriatic dropped 0.8% to $1.135, while Sandfire was down 1.7% to $6.79.