Benchmark spot gold prices have moved since last Friday's close, with the spread between its highs and lows the narrowest of any week since August 2007.
Sibanye has faced strong criticism from unions, local media and the government after the earthquake at its Driefontein gold mine that killed the miners.
The takeover offer, which expires next week, offers great exploration potential because Avanco owns a vast landholding in the copper and iron ore-rich Carajás region, second only to giant Vale.