A third of the South African workforce of world No. 4 bullion producer Gold Fields have downed tools in an illegal strike, the company said on Monday, the latest labour stoppage to hit the mining sector in Africa's largest economy.The wildcat strike action came as South African police readied for possible violence over Monday's back-to-work deadline for striking workers at Lonmin's Marikana platinum mine, where 44 people died in labour strife last month, most of them shot by police.Gold Fields last week resolved an illegal strike by 12,000 workers at its KDC East mine which started because of rank and file discontent with the local branch of the National Union of Mineworkers (NUM). The labout unrest has now spread to the KDC West operation with 15,000 workers going off the job, shutting production. Gold Fields said the reasons for the latest strike were unclear.Gold Fields' share price shed 2.75 percent in early trade despite gold's spot price holding near 6-1/2 month highs.In the platinum sector, the labour conflict has had its origins in a turf war for members between the dominant NUM and the more militant Association of Mineworkers and Construction Union (AMCU), which is mounting a challenge.There is swelling discontent against NUM among poorly-paid mineworkers who see its leadership as out of touch and too interested in politics because of the union leadership's close links to the ruling African National Congress (ANC).