GEOLOGICAL AND GEOCHEMICAL CONTROLS ON THE SILVER CONTENT (FINENESS) OF GOLD IN GOLD-SILVER DEPOSITS

Abstract

A compilation of microprobe fineness data and a re-evaluation of bullion production fineness data suggest that the gold fineness model of Fisher (1945) can be updated and applied to a wide range of gold-mineralised environments. The major deposit classes are characterised by the overall average or range of deposits averages and total range of gold fineness values as follows: View Within ArticleThe Archaean (including Witwatersrand), Slate Belt and Plutonic classes are characterised by a high and consistent average fineness and a narrow fineness range. There is no clear subdivision within these classes according to deposit style or inferred genesis. The Porphyry, Volcanogenic and Epithermal classes have a variable average deposit fineness and a wide total range of fineness values. Within these three classes, higher fineness is typical of deposits in andesitic rather than rhyolitic volcanic settings; deposits with CuAu rather than CuMo or PbZn element associations; and deposits with telluride or selenide minerals. Solubility calculations indicate that in all these environments, gold and silver form the same aqueous species with AgCl2-dominating for silver and Au(HS)2-dominating for gold. In all the environments, the fluids transport an equal amount of gold and silver in the form of their bisulphide complexes. Therefore, the observed gold-fineness values can not be explained by the transporting mechanism only. Low silver in the ores and higher gold-fineness values observed in the Archaean, Plutonic and Slate Belt environments can be attributed to sulphidation as the dominant mechanism of ore deposition. Sulphidation of wall rocks destabilises only the bisulphide-complexes of gold and silver. In contrast Epithermal, Volcanogenic and Porphyry environments are characterised by a diversity of ore-depositing mechanisms (cooling, boiling, mixing) which act alone or in combination and destabilise chloro- and bisulphide-complexes of gold and silver. The high silver concentration in the ores, low fineness values and large variation in the fineness values in these environments can be attributed to the complexities in the ore-depositing mechanisms.