* Gold Fields forms Ghana JV with Canada's Asanko * $202.6 million deal sends shares down 6 pct * Bullion miner to fund transaction through cash and debt (Adds CEO quote, analyst comment on debt levels) By Tanisha Heiberg and Nqobile Dludla JOHANNESBURG, March 29 (Reuters) - South Africa's Gold Fields will buy a near 50 percent share of Asanko Gold Inc's Ghana subsidiary and take a stake in the Canadian miner in a $202.6 million deal, it said on Thursday. The deal fits with Gold Fields' strategy of buying cash generative, working mines rather than embarking on costly greenfield explorations but some investors were not convinced, sending the company's shares tumbling as much as six percent.