The equilibrium distribution of arsenic and antimony between coexisting tetrahedrite-tennantite and enargite or famatinite is a function of temperature. Preliminary calibration suggests that this distribution may provide a useful geothermometer.Experimental syntheses of coexisting tetrahedrite-tennantite and enargite or famatinite were carried out at 500 degrees C and 400 degrees C. At 500 degrees C the tie lines between tetrahedrite-tennantite and enargite or famatinite are very nearly colinear with sulfur in the system Cu 3 As-Cu 3 Sb-S. With decreasing temperature, a given enargite coexists with increasingly more antimonian tetrahedrite-tennantite. The composition of the tetrahedrite-tennantite in equilibrium with both enargite and famatinite migrates from approximately tn 70 at 500 degrees C to tn 56 at 400 degrees C. Attempts to synthesize coexisting phases below 400 degrees C are continuing.Enargite appears to coexist with tetrahedrite (tn 0 -tn 50 ) immediately below 400 degrees C suggesting that the common association of arsenian Cu 3 (As,Sb)S 4 and antimonian Cu 12 (As,Sb) 4 S 13 in the same deposits is a function of partitioning of arsenic and antimony under mesothermal conditions.

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