def lookup_key_from_chunk_key(chunk_key):
    """
    breaks out the lookup_key from the chunk_key.

    Args:
        chunk_key (str): volumetric chunk key = hash&num_items&col_id&exp_id&ch_idres&x&y&z"

    Returns (str): lookup_key col_id&exp_id&ch_id

    """
    parts = chunk_key.split('&')
    lookup_parts = parts[2:5]
    return "&".join(lookup_parts)