Specimens of quartz sold as with Marcasite inclusions from Brazil have been analized and are actually resuted to be of Pyrite. Usually these "Pyrites" are characterized by a flatted morphology filling fractures of the quartz crystals. They are formed by secondary pyrite generation. The "real" Pyrite inclusion also from Brazil are usually three-dimensional primary cristals included in quartz.  The shape is usually round and occasionally can be squared with a radial structure. Oxidation of the pyrite can results in yellow to brown halos and a complete alteration of the pyrite sulfide in Iron oxides.
The location of this deposit is described as Sao Joao de Safira, Minas Gerais, that is an area very rich in famous tourmalinipherous pegmatites as Pederneira and Cruzeiro Mines.