The chemical formula for sphalerite and marmatite is (Zn,Fe)S not (ZnFeS2).  The iron and solid solution are in a solid solution and marmatite (there is technically no mineral called marmatite) is referred to as the high iron variety of sphalerite http://en.wiktionary.org/wiki/marmatite

They have the same flotation properties and the high iron content makes the sphalerite almost black.   Their was a lot of this “black jack” at Sullivan Concentrator (Cominco).  Marmatite is known as a dark form of sphalerite that is rich in iron on which +50% Zn grade was possible after very fine regrinding and column cell cleaning.

see http://www.mindat.org/min-6294.html

Generally marmatite will only generate a 46-50% Zn conc.