Review of colloids in ore genesis

Advances in the theory of colloidal chemistry over the past decade indicate that this state of matter may take part in ore forming processes to a far greater extent than is at present generally recognised. The stability of a colloidal sol system is subject to temperature, pressure, pH, ions in solution, and other colloidal species present. Most sulphide sols are stabilized in the presence of hydrogen sulphide, even to such an extent that NaCl and KCl may not cause coagulation. Silica sol probably has a very important stabilizing effect on most metal hydrosols. In many sol systems the pH of the solution determines the charge on individual particles end thus the degree of dispersion, The effect of temperature on sol stability is unknown but recent work tends to suggest that in some cases increased temperature results in decreased stability. Colloform (globular, framboidal, oolitic, reniform and botryoidal) textures have been successfully produced in the laboratory from sulphide gels. Such textures may also develop due to crystallization from true solutions, but structural criteria exist to differentiate the two origins. Ore textures exhibiting a crystalline form do not necessarily indicate precipitation from true solutions as crystalline textures may equally develop due to ageing of gels. There is no inter-dependence between the transport of metals as colloidal sols and their deposition as colloidal gels, Fluid inclusions present in minerals precipitated via the gel stage, may give little information on the composition of the ore fluid, or the temperature of precipitation.