TETRAHEDRITE AND TENNANTITE - EVALUATION OF THERMODYNAMIC DATA AND PHASE-EQUILIBRIA

Tetrahedrite and tennantite are extremely common in a variety of ore deposits, but the genetic information they record is difficult to decipher because of the limited understanding of their phase relations in multicomponent systems. Most natural compositions can be described by the general formula (Cu,Ag)10(Fe,Zn,Cu)2(Sb,As)4S13. Consideration of the compositional variations found in natural samples suggests that results of electron-microprobe analyses of tetrahedrite and tennantite are best normalized on the basis of four semimetal atoms (Sb, As, Bi, Te). Expressions for the Gibbs free energy of formation for Fe-tetrahedrite (Cu10Fe2Sb4S13; FeTd), Zn-tetrahedrite (Cu10Zn2Sb4S13; ZnTd), Fe-tennantite (Cu10Fe2As4S13; FeTn) and Zn-tennantite (Cu10Zn2As4S13; ZnTn) have been derived that best fit the available experimental and natural phase- equilibrium constraints. The provisionally recommended expressions [J/mole; T(K); relative to the elements and ideal S2 gas] and the temperature ranges for which they are valid are: DELTA-G-degrees-FeTn = -1,933,305 + 865.7 T (25 - 602-degrees-C) DELTA-G-degrees-ZnTn = -2,119,070 + 908.3 T (25 - 420-degrees-C) DELTA-G-degrees-ZnTn = -2,130,050 + 923.9 T (420 - 602-degrees-C) DELTA-G-degrees-FeTd = -1,871,251 + 860.8 T (25 - 630-degrees-C) DELTA-G-degrees-ZnTd = -2,058,151 + 903.2 T (25 - 420-degrees-C) DELTA-G-degrees-ZnTd = -2,069,130 + 918.9 T (420 - 630-degrees-C) Calculated phase-relations indicate that tetrahedrite and tennantite form under normal hydrothermal conditions in terms of temperature, and sulfur and oxygen fugacities. The stability of the rare sulfosalt skinnerite (Cu3SbS3) is limited to Zn- and Fe-deficient environments and is restricted by reactions involving Zn- and Fe-tetrahedrite.