JOHANNESBURG (Reuters) - South Africa's biggest insurer, Sanlam, reported a 23 percent jump in earnings for the first four months of the year on Wednesday after booking increased new business volumes and higher returns from investment services. The company established nearly a century ago said new business increased 21 percent to 59 billion rand ($5.5 billion) while earnings from financial services improved by 40 percent. Sanlam said on Tuesday it had concluded an agreement to purchase a 63 percent stake in Soras Group, Rwanda's biggest life and non-life insurer. The $24.3 million purchase is its first foray into the country. Sanlam has businesses in 16 countries including 11 other African countries and has set aside $370 million for emerging market acquisitions this year. Its shares fell 0.28 percent to 60.20 rand as at 1219GMT, ($1 = 10.7368 South African Rand)