Weathering of sulfide ore-bodies

The first step in recovering metals from their minerals is that of finding ore-bodies in which metal sulfides are present in a sufficient quantity, and at a sufficient concentration, for mining to be economic. One of the six basic geophysical exploration groups of methods commonly employed in the search for minerals is based on the sensing of mineral conductivity and of currents flowing as a result of oxidation of the sulfide minerals in natural weathering underground. The induced polarization exploration technique involves applying a series of current pulses to two electrodes inserted into the ground and analyzing the induced voltage at two different electrodes, also inserted into the ground. Electromagnetic measurements use alternating magnetic fields to induce measurable electrical currents in the earth and these depend on conductivity. This approach can be carried out from aircraft and it is then possible to screen large areas for later ground-based investigation. In the self-potential method, the potential between two electrodes inserted in the ground is measured and analyzed to detect electrochemical interaction between ore-bodies and ground water.

The electrochemical interaction between ore-bodies and ground water takes place by the same type of mechanism as occurs in the corrosion of metals. That is, the process involves anodic oxidation of the sulfide, a reaction that gives electrons to the mineral; these electrons are released again by the simultaneous cathodic reduction of oxygen. Since the sulfide ore-body is conducting, the anodic and cathodic processes can occur at different places. Thus, ore-bodies, when exposed to weathering processes, develop a zonal arrangement of different mineral associations, which reflect different degrees of oxidation. Oxygen dissolves in the ground water from the air and diffuses to metal sulfide surfaces where it can be reduced. This cathodic reaction takes up the electrons released by the anodic oxidation of sulfide minerals in the underlying ore-body and allows the process to proceed. Thus, electrons will flow through the ore-body to the surface and result in the ore at higher levels having a more positive potential than that at greater depth. The alteration of the composition of the ore-body will be greatest closer to the surface since the current path will be shorter. In this way, for example, alteration of ore-bodies containing the major economic copper mineral, a copper iron sulfide (chalcopyrite), often occur to form an overlying copper sulfide (chalcocite) zone and a cap containing oxidized copper minerals and native copper. It is the native copper that was the original source of this metal and was mined as early as 6,000 BC. Later copper mining involved leaching of copper oxides followed by cementation with iron. Cementacion is Spanish for precipitation, which better describes the process, and the use of the term cementation derives from the process being first carried out on an industrial scale in the copper mines in the Rio Tinto region of Spain in the mid 18th century.