Exsolution in the systems bornite-chalcopyrite and pyrrhotite-pentlandite: an approach to textural interpretation

The development of early textural forms occurs through nucleation and growth and is marginally affected by coarsening of the exsolution products. Therefore, the initial degree of supersaturation and the values of the diffusivities, both of which are functions of the temperature, determine the early textural features of the experimental charges. The final textures derive from further growth of the early forms but may also involve substantial coarsening and spheroidization above certain temperatures. Therefore, textural features may serve as geothermometers.

Coarsening experiments on cpy lamellar forms and cooling experiments on homogeneous mss of various compositions were also performed. The experimental results of this study indicate that bn-cpy mutual boundary textures form above about 250° C and, therefore, are compatible with

1. Simultaneous precipitation of bn and cpy;

2. Exsolution during slow cooling from temperatures above the solvus;

3. Metamorphism to temperatures in excess of about 250° C.

Widmanstätten textures involving bn-cpy are not easily explained by slow cooling, but indicate:

1. Low-temperature replacement; or

2. Exsolution of cpy lamellae from anomalous bornites during mild metamorphism (i.e., heating to temperatures not exceeding 200°-250° C).

Massive pn in po develops by exsolution during slow cooling from temperatures above the solvus, in the 250°-610° C temperature range. Pentlandite bodies oriented along one direction in the matrix are compatible with exsolution at temperatures between about 150° and 250° C. Pentlandite flames form by exsolution at or below 150° C. Widmanstätten textures consisting of pn lamellae in mss-obtained experimentally under high degrees of supersaturation-cannot form by exsolution by slow cooling.