A Canadian firm that oversees $21 billion in investments has made an undisclosed investment in Schumacher Group, a Lafayette-based provider of emergency room and hospital staffing.

Onex Corp., based in Toronto, expects to complete the deal in the third quarter. Onex specializes in buying the controlling share of businesses and partnering with existing management. Since its founding in 1984, the company has built more than 80 operating businesses and completed more than 490 acquisitions with a value of about $58 billion.

Dr. William C. “Kip” Schumacher, who founded Schumacher Group in 1994, will remain as chairman of the board and continue to own “a significant interest” in the company, along with its current management.

Schumacher said the partnership with Onex is a natural next step.

Schumacher operates in 28 states and treats more than 4 million patients a year.

In October, the company refinanced its debt with $120 million through GE Capital’s Healthcare Financial Services.

Chief Executive Officer Rich D’Amaro said the Onex deal will allow Schumacher Group to take advantage of new opportunities.

“We believe that Onex’s resources, strategic guidance and experience in working with companies like ours will allow Schumacher to accelerate its growth,” D’Amaro said.

The equity investment will be made by Onex Partners IV, Onex’s $5.7 billion flagship fund. Onex has offices in Toronto, New York and London. The company acquires and builds businesses in partnership with management teams.