ARUSHA, Tanzania (Reuters) - Tanzania should stick to existing agreements in the fast-growing mining sector or investors will lose confidence, the company which owns the country's biggest gold mine said on Saturday. Major international miners are still in talks with the government two years after it passed mining legislation that included a rise in royalties on gold exports to 4 percent of gross value from 3 percent of netback value. The law also required mining companies to pay the government 0.3 percent of their annual turnover, up from the previous requirement of a maximum $200,000 a year. ...