Oxidised gold deposits: Relationships between oxidation and relative position of the water‐table

Abstract

Many oxidised gold deposits exhibit features which strongly suggest an interplay between geo‐chemical processes and movement of the water‐table. Geochemical processes involving the dissolution of gold‐silver and gold‐tellurium alloys, and precipitation of high‐fineness gold (low Ag) in transition zones, neck‐oxide and lateral oxide zones can take place both above and below the water‐table. In general the lower availability of oxygen below water‐tables ensures that these processes occur at a slower rate in the saturated zone than in the overlying water‐undersaturated weathering profile. Mass‐balance calculations on the Golden Web deposit, Coolgardie, indicate that dissolution and reprecipitation occur above the water‐table. Transfer of gold downwards from the leached zone created to the transition zone located just above the water‐table, is quantitative. Variations in the rate of downward movement of the water‐table(s), relative to the rate of oxidation of the primary gold and sulfides contained in the system, can result in up to eight different types of oxidised gold deposits. Three of these occur in relict profiles (R1‐R3), three are related to erosional profiles (E1‐E3), and two are representative of depositional profiles (D1, D2). Type R1, relict profile with one (high) water‐table, is representative of new systems in which oxidation and downward leaching are incipient. Type R2, relict profile with recently receded water‐table, is representative of a more mature system in which there has been late‐stage stream capture or climate change resulting in water‐table recession, whereas type R3, relict profiles with multiple stillstands, occurs in yet more mature profiles with a succession of such events. When such relict profiles are eroded, three related profiles are developed with very different surface expression: type E1, eroded profile with recently receded water‐table; type E2, eroded profile with receding water‐table and very active weathering; and type E3, eroded profile with stillstands deep in the profile. In depositional profiles two distinct types can be recognised: type D1, depositional profiles with lateral flow; and type D2, depositional profiles with no lateral flow (for which salt‐lake environments provide a classic example). Examples for all but the first type are known in Western Australia. Gradual, rather than episodic reductions in water‐table can result in secondary gold in oxide remnants in the upper parts of weathered profiles. These styles of oxidation have considerable impact on exploration in two ways: they affect the methods of exploration, and the economics of mining the deposit. Deeply leached profiles clearly have large amounts of overburden; enriched transition zones can have a positive impact on profitability. Knowledge of the oxidation processes and styles of gold oxidation can be used to direct exploration to the optimum strategy.