Auriferous mineralization in some shear-zones: A three-stage model of metallogenesis

Studies of numerous shear zones hosting auriferous quartz veins suggest a three-stage model of formation for such veins. These stages are indicative of the progressive increase of the gold concentration with the evolution of the shear zone and comprise an early stage with “invisible” gold, an intermediate stage with finegrained gold, and a late stage with gold nuggets. The early stage, consisting of two consecutive episodes, comprises the development of the shear zone sensu stricto which results in the formation of a structure characterized by mylonite. This structure subsequently acts as a drain for hydrothermal fluids. Under the combined action of deformation and hydrothermal circulation, the rocks in this structure undergo mineralogical transformations that depend on the initial composition of the rock. These phenomena occur with increasing intensity towards the cores of the structures, the sites of substantial silicification and sulfidation. Gold is first fixed in the crystal lattice of pyrrhotite (up to 30 ppm of gold), disseminated throughout the structure as a whole. In the core, it is destabilized to pyrite-marcasite during the second episode of this early stage and the gold thus freed is trapped in the lattice of locally abundant ferriferous sulfides: pyrite, arsenopyrite (up to 1.6 wt% gold), and berthierite. At the intermediate stage, again consisting of two consecutive episodes, the shear zone can generate openings, allowing the emplacement of lenses and veins of milky white quartz. When these various veins are in turn crushed by continuing tectonic activity, microsaccharoidal quartz forms by cataclasis, and acts as a “receptacle” for the gold mineralization. Therefore textbook “auriferous quartz veins” are only one particular aspect of this stage, characterized by the appearance of visible native gold caused by destabilization of the auriferous sulfides of the early stage. This gold is generally very fine-grained (several micrometres) and relatively silverfree. The late stage, i.e. the fifth episode, in a brittle domain, can occur within a structure of the early or intermediate stage. It corresponds to a stage of opening, during which stockworks of quartz or of quartz and carbonate form, causing in situ remobilization of the auriferous mineralization from the earlier stages. This phenomenon is accompanied by the appearance of gold nuggets several millimetres in size, very often rich in silver (electrum).