Amplats shares slumped 3.36% to R359.99 after the firm said annual profit likely fell by at least 20% due to this year's five-month strike.
Falling commodity prices, following weak economic data from China, added to the pressure on Amplats' shares and also hit stocks in rivals such as Impala Platinum [JSE:IMP] and Lonmin [JSE:LON], which fell between 1% and 2%.
"The Chinese with very average numbers this morning - HSBC PMI barely above growth territory - have seen a broader commodities complex slide," Sasha Naryshkine, an analyst at Vestact, said in a note.
The benchmark JSE Top-40 index was down 2.19% to 43 236 and the broader All-share index dropped 2.11% to 48 858.
Trade was robust, with 250 million shares changing hands, compared with last year's daily average of 176 million shares.
Other decliners were mobile phone group MTN Group [JSE:MTN], down 4.38% to R208.62 as the Nigerian naira, the currency of its biggest market, slid to a record low on the back falling oil prices.
Also taking a hit from oil prices, Sasol [JSE:SOL] plunged 6.81% to R430.50. Sasol makes fuel from coal and gas but sells at it at the same price as the companies that refine imported crude oil.
Among the gainers, Winhold Ltd [JSE:] surged 11.9% to 47 cents after the investment holding firm said full-year profit rose by nearly a third.