Gold-Rich Porphyry Deposits

Abstract Gold-rich porphyry deposits worldwide conform well to a generalized descriptive model. This model incorporates six main facies of hydrothermal alteration and mineralization, which are zoned upward and outward with respect to composite porphyry stocks of cylindrical form atop much larger parent plutons. This intrusive environment and its overlying advanced argillic lithocap span roughly 4 km vertically, an interval over which profound changes in the style and mineralogy of gold and associated copper mineralization are observed. The model predicts a number of geologic attributes to be expected in association with superior gold-rich porphyry deposits. Most features of the descriptive model are adequately explained by a genetic model that has developed progressively over the last century. This model is dominated by the consequences of the release and focused ascent of metalliferous fluid resulting from crystallization of the parent pluton. Within the porphyry system, gold- and copper-bearing brine and acidic volatiles interact in a complex manner with the stock, its wall rocks, and ambient meteoric and connate fluids. Although several processes involved in the evolution of gold-rich porphyry deposits remain to be fully clarified, the fundamental issues have been resolved to the satisfaction of most investigators. Exploration for gold-rich porphyry deposits worldwide involves geologic, geochemical, and geophysical work but generally employs the descriptive model in an unsophisticated manner and the genetic model hardly at all. Discovery of gold-rich porphyry deposits during the last 30 yr has resulted mainly from basic geologic observations and conventional geochemical surveys, and has often resulted from programs designed to explore for other mineral deposit types. The tried-and-tested approach is thought likely to provide most new discoveries for the forseeable future, although more rigorous and innovative application of the descriptive and genetic models can only improve the chances of success.