<?php

$arr = array(
    array("0000000009999999","20181209","000000","REM BASICA","0.00","C"),
    array("0000000009999999","20181209","000000","CRED JUROS","0.02","C")
);


$chaves = array('Conta', 'Data', 'Cod', 'Historico', 'Valor', 'Op');

$novo = array();

foreach($arr as $item){
	$novo[] = array_combine($chaves, $item);
}

print_r($novo);