Analytical formulation of the phase equilibrium relationship between two sets of representative mineral assemblages (Fe-Zn tetrahedrite ss + sphalerite + pyrite + chalcopyrite and tennantite ss + bournonite ss + galena + boulangerite) from two distinct ore facies of different generations within the same ore deposit at Rajpura-Dariba, India, has been attempted through the Gibbs method. Textural studies and element-partitioning data between coexisting mineral pairs point to a close approach to chemical equilibrium in both the assemblages. There is a marked nonideality in both the mineral systems and the element distribution patterns follow a modified Nernst law.The algebraic approach, comprising a solution of a system of linear equations in matrix form and the relating of mineral compositions to various intensive thermodynamic variables, generates only semischematic mu (sub S 2 ) -T-X i diagrams. This is due to constraints imposed by some simplified assumptions and inadequacies of some input data. Nevertheless, several significant conclusions emerge:1. The Fe-Zn tetrahedrite ss + sphalerite (+pyrite + chalcopyrite) assemblage belonging to the system Cu 2 S-FeS-ZnS-Sb 2 S 3 -S 2 provides a reliable mu (sub S 2 ) and/or T sensor due to steep intersections of compositional isopleths of tetrahedrite ss and sphalerite in mu (sub S 2 ) -T space, provided the Ag concentration in tetrahedrite ss is ignored. In contrast, the assemblage tennantite ss + bournonite ss (+galena + boulangerite) is of little use as an environmental sensor, due to the very low angle intersection of As isopleths of tennantite ss and bournonite ss .2. Near-vertical disposition of analytical data points on the mu (sub S 2 ) -T diagram for the tetrahedrite ss + sphalerite system indicates a virtually constant temperature and variable mu (sub S 2 ) condition for equilibration--a situation expected in metamorphosed sulfide deposits.3. The As partition ratio between tennantite ss and bournonite ss was found to be a linear function of the ratio (dmu (sub S 2 ) /dmu (sub A s ) ) (sub P.T) . This linear dependence is interpreted to be the manifestation of an interactive control of chemical potentials of the two ambient species (S 2 and As) over mineral composition under isothermal-isobaric conditions. The alternative possibility, of the chemical potentials of the ambient species being controlled by the mineral composition on an extensive scale, can be ruled out due to the low concentration of the mineral species in the orebody as a whole--although such a situation may be possible on a microenvironmental scale.