The role of sulphide surfaces on the deposition of precious metals from hydrothermal fluids

Ore minerals have been separated from silicate and carbonate phases in lode gold ore by means of HF ± HCl acid digetion. Ore was derived primarily from four Archaean lode gold deposits in Zimbabwe. All deposits were mesothermal (250-400^oC) but each had differing host rocks, vein styles, tectonic controls and ore assemblages. Textural studies on ore separates have revealed specific grains (termed cling-ons) attached to the surfaces of sulphide minerals. The most common association is that of gold on pyrite, although a range of other cling-on minerals have been identified including native silver and tellurium, metal tellurides, metal sulphides and rarely metal selenides. Surface-attached grains range in size from sub-1μm to over 500μm. Occurring in euhedral to anhedral forms, they are commonly domal, although hackly, spongiform, micro-spherulitic, plated and other forms also occur. Cling-ons were also recognised on chalcopyrite, arsenopyrite, pyrrhotite, gersdorffite and galena substrates. Surface-attached grains are notably absent on sphalerite and stibnite. There is no apparent textural correlation between the presence of cling-ons and substrate grain shape or substrate size. The surface-attached grains were nucleated at specific sites on the sulphide surfaces. These sites have been recognised to be `defects', which have been classified as having formed during sulphide grain growth (primary physical defects), formed after the sulphides finished growing (secondary physical defects), or are related to non-stoichiometry and/or impurities in the substrate composition (chemical defects). Compositional controls on cling-on nucleation and growth were principally studied by electron and proton microprobe techniques. Electron microprobe analyses revealed no consistent relationship between major element concentrations and the presence of cling-ons. In cling-on-bearing sulphides, no systematic or consistent variation in element concentration was found between the grain centre and those edges where cling-ons were nucleated, nor edges where cling-ons were absent.