If at all it has a 8 Pin and a 6 Pin power connector 
Normally the 8 Pin should work alone. e.g. the Powercolor Red Devil works without the 6 pin connector.
According to Sapphire product page:

1 x 8-pin + 1 x 6-pin (optional) AUX Power Connector
http://sapphirenitro.sapphiretech.com/en/580-LE.html