I would say it is a color variety of Port Cichlid (Cichlasoma portalegrensis).
The genera Cichlasoma and Aequidens were split up by the Ichthyologist Sven Kullander in the late 80's and early 90's. He is still working on the group actually.
He restricted the genus Cichlasoma to a group of roughly 14 described species. Most of the species were in the old genus Aequidens.
The Black Acara: Cichlasoma bimaculatum, The Port Cichlid: Cichlasoma portalegrensis the the Amazonian species Cichlasoma amazonarum are the most common species you will find in the hobby.
Cichlasoma portalegrensis is one of the only species in the genus with a reflective sheen like your fish has. Some populations of C.boliviense have a simular color pattern, but have strong black markings on their flanks. They are also uncommon in the hobby.
There have been allot of exports coming out of Argentina,Uruguay and Southern Barzil these days. This happens to be the home range of the true Cichlasoma portalegrensis. They are also finding new species in this area so it may be a possibile that you have an undescribed species.
Many of the other species of Cichlasoma were imported and sold as Port Cichlids even though thats not whet they were.
In either case, you scored yourself a very nice fish. I try to buy uncommon fish like this in groups for breeding. Pick up some more if you can. The whole genus is easy to breed.