It's radial velocity and galactic position show that NGC 3201 is orbiting the Galactic centre in a unique retrograde direction to that of the rotation of the Milky Way Galaxy. This has resulted in a belief that it is a captured globular though its similar age to other standard Milky Way halo globulars tends to refute this thinking. The much younger globulars like Pal 12 etc which indeed may be captures have lower abundances than most halo globulars.
Summarised from The Astronomical Journal 1998 August: Elemental Abundances In Giants In NGC 3201, A Globular Cluster With A Retrograde Orbit - Guillermo Gonzalez & George Wallerstein.
Pdf file here:
http://www.iop.org/EJ/article/1538-3...9-e7693580a719