The Proterozoic Kayad Zn-Pb deposit in Ajmer, Rajasthan is located within the Aravalli-Delhi Fold Belt in western India. Mineralization of sphalerite and galena, commonly associated with chalcopyrite and pyrrhotite, is hosted primarily by graphitic quartz mica schist (QMS) and subordinately by calc-silicate, quartzite and pegmatite. In QMS, both massive ore, evidently formed by remobilization, and laminated ore are present. Calc-silicate and quartzite contain disseminated sulfides while pegmatites contain sulfide veins that cause massive mineralization. Mineral replacement textures such as replacement of albite and muscovite by K-feldspars and biotite by chlorite suggest that Fe-Cu sulfides, represented by chalcopyrite and pyrrhotite, formed during potassic and acidic alteration. Trace element characterization demonstrates that between co-existing chalcopyrite and pyrrhotite, Ag, Zn, Sn, In, Cd, Ga are strongly portioned in chalcopyrite whereas Co and Ni are partitioned in pyrrhotite. High concentration of Ag (up to ~9000 ppm) in chalcopyrite adds to economic potential of the deposit. Barring the exception of massive ore in QMS, the trace element compositions of chalcopyrite and pyrrhotite are host-rock-independent suggesting profound control of the parental hydrothermal fluid on their composition. In laminated QMS, pegmatite and quartzite, higher Co, Cd, Mn and In in sphalerite and higher Ag, Sb, Bi, Se in galena compared to chalcopyrite suggest that these phases co-crystallized during their formation. In contrast in remobilized massive ore in QMS, higher Co, Cd and Mn in sphalerite over chalcopyrite and higher Ga, Sn and In in chalcopyrite over sphalerite and galena possibly indicate co-crystallization followed by recrystallization. Furthermore, constant yet distinct Cd:Zn ratios of chalcopyrite and co-existing sphalerite may indicate involvement of two different fluids. The observed trace element characteristics can be best explained by co-crystallization of chalcopyrite and sphalerite by fluid-mixing and subsequent recrystallization leading to the formation of part of the massive ore.