Terms of the deal were undisclosed.
Snowden said that one of its shareholders, CSIR of South Africa, had divested its Snowden shareholding.
Snowden was formed by husband and wife geoscientists Phil and Viv Snowden. The company has offices in Perth, Brisbane, Vancouver, Johannesburg and London.
Downer is an Australian-listed engineering firm with annual revenues of around $3.5 billion. Other mining companies under its umbrella include Roche Mining and JR Engineering.