The company said yesterday Thornton had sold about 2.63 million shares, after the publication of its 2019 results, and he now held about 2.64 million shares.
Using yesterday's close of C$26.88, the transaction would have been worth about $70 million (US$50 million).
Barrick said Thornton sold the shares "due to personal portfolio considerations".
"Mr Thornton sold Barrick shares that he purchased using his personal funds which were not subject to holding restrictions," the company said.
It said Thornton confirmed he would continue "to retain and seek to build on what remains a meaningful equity stake in Barrick".
Barrick's share price has risen from a one-year low of $15.73 in May 2019 to touch $29.94 in February.
The chart (below) from Barrick's latest annual report illustrates the company's share price movements.
Thornton has been a director since February 2012.
The company is capitalised about $48 billion (US$34 billion).