The price is just over £1mln, payable in Ortac shares.
Ortac has also agreed to convert the US$2 mln convertible loan note previously issued to the company by Casa at an amended reduced share conversion price of US$0.5586.
Following the acquisition of the new block of shares and the conversion of the loan note, Ortac will hold just over 7.8 mln shares of Casa, representing 70.09% of the issued share capital in Casa.
Casa has gold properties in the Eastern Congo.
"The proposed acquisition of Casa is a turning point in the Company's recent history,” said Ortac chairman Nick von Schirnding.
“We are delivering on our strategy to focus on our portfolio of valuable African exploration assets and Casa is a very exciting and valuable part of our future. With a new board and a clear strategy in delivering shareholder value we look forward to a new chapter for Ortac and its shareholders. "