Johannesburg - ANC secretary general Gwede Mantashe warned against Cosatu becoming a "yellow federation" which is dominated by public sector unions."Once you have a federation dominated by the public sector you are in trouble," he told a National Union of Mineworkers (NUM) congress in Boksburg, east of Johannesburg on Wednesday."We should have industrial unions building themselves, as well as putting on our shoulders to help this union the NUM to be strong enough to deal with Amcu."NUM had seen a big drop in membership numbers since the formation of the Association of Mineworkers and Construction Union (Amcu).Mantashe warned NUM of people who wanted to be in leadership positions for financial gain."You have a duty comrades... to save your organisation from opportunism and greed," he said.He said the focus on leadership positions and financial gain had weakened the union, which created a space for Amcu to poach its members.This was because of policies decided on by NUM, such as leaders having a car and not working underground.'Shop stewards fight until there is no tomorrow'NUM shop stewards have access to offices, laptops and other benefits which makes the positions highly contested."You are victims of your own victories," Mantashe said."The next person wants to be entitled to those privileges. Shop stewards fight until there is no tomorrow. "He said NUM members could save the union from self-destruction.Mantashe said people who led the breakaway unions from NUM were former shop stewards of the union who had not been elected during branch conferences.The decision to form breakaway unions was informed by "greed for power".Joseph Mathunjwa, the president of Amcu, was a former NUM shop steward.Mantashe urged NUM members to come out of its congress united and to have candid and open discussions about these issues.NUM is holding its elective conference, where new leadership will be chosen.The union's general secretary, Frans Baleni, who has been in his position for nine years, is reportedly expected to be up against the Free State region's secretary, David Sipunzi.NUM's largest region, PWV, and its third largest region, North East, are said to be supporting Sipunzi to take over the position of general secretary.Acting president Piet Matosa, is expected to stand uncontested, while the position of deputy president is reportedly being contested by Mbuyiseni Hibana and Joseph Montisetsi.According to the Mail & Guardian, NUM has lost almost 100 000 members over the past three years.Earlier on Wednesday, at the start of the congress, there was contestation over credentials and how delegates were allocated for each union.