A fractional crystallization model for the deposition of argentian tetrahedrite

Electron microprobe analyses of natural hydrothermal tetrahedrites reveal that most spec-imens are heterogenous in composition. A simplified formula for natural tetrahedrite, showing the three common solid solutions, is (Cu, Ag)•o(Fe, Zn)2(Sb, As)4S•.•. Variations in Ag and Sb are positively correlated within each individual sample. However, the mathematical slopes and intercepts of the linear correlations are different for different samples, and the bulk composition of the samples varies with position within the ore deposit. Nearly 5,000 new point analyses are presented for tetrahedrites from Coeur d'Alene, Idaho, and Orcopampa and Julcani, Peru, to document hese observations. Similar heterogeneity has been observed in tetrahedrites from other hydrothermal deposits. A fractional crystallization model is presented to explain the compositional patterns observed in hydrothermal tetrahedrites. Heterogeneity within a single sample could arise when a particular depositional site receives more and less evolved fluids at different times. Modeling with various partition coe•cients and starting fluid compositions defines conditions under which fractional crystallization could have produced the observed tetrahedrite compositions. Calculations how that the correlations between Ag and Sb in tetrahedrite are positive, and that samples from the central or deeper parts of a deposit are lower in Ag and Sb, as observed, only for certain values of the fluid-crystal partition coe•cients.