** Shares of Horizon Minerals fall as much as 22.2% to A$0.049, set for their worst day since Sept. 20, 2011, if losses hold

** Gold miner says it has entered into an agreement to acquire nickel explorer Poseidon Nickel

** Says under the proposed deal Poseidon shareholders will receive 0.1156 shares of co for every Poseidon share held

** Co says it will raise A$14 mln($9.29 mln) via share placement at A$0.045 per share, representing a discount of 28.6% as of last close

** Stock hits its lowest level since Oct. 4

** Around 7.8 mln shares change hands, 7x the 30-day avg

** Stock down 28.6% YTD, including current session's moves

($1 = 1.5076 Australian dollars)

(Reporting by Kumar Tanishk in Bengaluru)

((Tanishk.Kumar@thomsonreuters.com;))