Kirk gave Rolling Stone this explanation:

Without getting too deep into it, it's more a recording project than a bona fide Metallica album.

via Exclusive: Kirk Hammett Says Secret Metallica Project In the Works | Rolling Stone Music.

Kirk went on to explain that the band will try to record an album's worth of tunes in two weeks.  He did not say if they were originals, covers or nursery rhymes.

Now keep in mind that Buckcherry's big comeback c.d. was recorded in 15 days.  It all comes down to how rehearsed you are before you get into the studio.  So will it be more  "Death Magnetic" or more St. Anger?  My bet is the band has learned their lesson, and will be able to kick out a killer c.d. in two weeks.  Let's not forget they just spent two years playing and getting tight every night.