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Reaction rims of myrmekitically-intergrown bournonite and chalcopyrite occur selectively along the contact of galena and ferriferous antimonian fahlore grains but are absent along galena and arsenian fahlore grain contacts-sometimes even in the same specimen-from Rajpura-Dariba deposit. For amenability to thermodynamic treatment ferriferous tetrahedrite (for which no thermodynamic data exist) is treated as equivalent to pure tetrahedrite plus pyrrhotite and assuming the reaction tetrahedrite + pyrrhotite +galena + bournonite + chalcopyrite (with increasing a_S_₂) ' the univariant line is calculated from published data. The particular sulfidation curve, set against the stability fields of associated minerals in the log a_S_₂ - T space, suggests that the texture is due to low temperature reaction between pre-existing ferriferous tetrahedrite and galena crystallized from a late stage pulsation of ore fluid. The finding is consistent with the known, biphase mode of origin of the ore body.