近付く and 近寄る are almost the same meaning. But 近寄り is 連用形(れんようけい) of 近寄る. 連用形 means that verb or adjective follows after that(in this case, 近寄り). For example 近寄りがたい means unapproachable(近寄り-approach, がたい-un able). So you cannot use only 近寄り besides you use 近寄り as noun.