"The mining sector requires people with a very strong character to be able to transform, rather than a mere opinion-maker who shouts from the sidelines with no concomitant action," the National Union of Mineworkers (NUM) said on Wednesday.
"The NUM requires people with a certain level of pragmatism, rather than theorists, to occupy these strategic positions in order to influence decisions that would favour the majority."
Gold Fields announced Ramphele's resignation earlier on Wednesday.
"Dr Ramphele has decided to retire as chair to further her socio-economic and political work," spokesperson Sven Lunsche said in statement.
"She has resigned from most of her private sector directorships, and will relinquish all other remaining private sector directorships and positions to focus on these activities."
Ramphele is 'uncaring'
He said non-executive director Cheryl Carolus would succeed Ramphele.
NUM accused Ramphele of not caring about the workers.
"Gold Fields – now Sibanye [Gold] – has been one of the worst performing [mines] in recent years in terms of health and safety of mineworkers, under the watchful eye and leadership of chairperson Mamphela Ramphele," it said.
"It is common cause from what she said at last week's mining indaba that she sees no problem with African workers losing their jobs in favour of some future one."
During the 2012 Mining Indaba in Cape Town Ramphele said sustainable mining is the only future for South Africa.
"We have to accept that the traditional way of mining in SA, with its reliance on cheap and low-skilled labour, is over. You [had] better get used to it. It is not sustainable," she said.
NUM said it had not yet been consulted by Gold Fields on Carolus' appointment. – Sapa