High-temperature hydrothermal precipitation of precious metals on the surface of pyrite

Abstract

A KNOWLEDGE of high-temperature surface chemistry processes is essential to a complete understanding of the genesis of hydrothermal gold deposits and is also of considerable importance to industrial mineral processing. Here we have studied individual pyrite grains separated from four lode-type gold deposits using optical and scanning electron microscopy and looked at the textural relationships of late gold on the surface of the pyrite. The gold occurs as sub-spherical to subhedral, rarely euhedral, grains. In three of the deposits studied, it is variously accompanied by native tellurium, native silver and altaite. Discrete crystals of gold and the other metals are located at specific sites on the pyrite substrate, showing a marked affinity for crystal edges and surface defects. These observations confirm the importance of the conducting properties of sulphides to metal precipitation. We believe that adsorption–chemisorption processes are particularly effective at point defects in the crystal surfaces and that these were responsible for the destabilization of aqueous Au-thio-suphide, Au-thio-telluride, and Ag-bearing complexes, resulting in the precipitation of the gold, tellurium, and silver.