The mechanisms of forming iron-base metal gossan minerals

Abstract

Abstract Laboratory studies of the weathering of sulphide ores have centred around using samples of ore as electrodes and accelerating the weathering processes by passing an electric current. The results of reacting 19 different ore types under varying conditions are compared with data from co-precipitating, Fe and Cu, Fe and Ni, Fe and Zn, Fe and Co, and Fe and Pb, over a pH range from 2 to 11. An electrochemical cell specially designed to fit onto an optical microscope has allowed direct observation of the changes in sulphide mineral grains as they are anodically weathered. These experiments are used to demonstrate that the pH of the environment during the weathering of sulphides to sulphates is the most important parameter in determining the initial gossan minerals that form. Factors that will cause the pH to be high are buffering from gangue and wallrock minerals, low iron content in the sulphide and a high metal to sulphur ratio in the sulphide. A low pH is favoured by the converse, namely a sulphide sufficiently massive to override the buffering effects of the wallrocks and any gangue minerals present, a high iron content in the sulphide and a low metal to sulphur ratio in the sulphide. Two mechanisms of iron hydrolysis dominate the weathering processes where iron is a major metal being released from a sulphide. 1. (1) The high pH process. Where there is sufficient buffering for the pH to remain at or above 7, most of the base metals including ferrous iron will be hydrolysed and pyroaurite type of minerals form for Ni, Zn and Co, while mixed Fe-Cu hydroxycarbonates and hydroxysulphates form for Cu, and mixed iron lead hydrocarbonates form for Pb. The iron is located in these initial compounds as a green rust where it is effectively bound as ferrous hydroxide. Subsequent oxidation of this hydroxide produces no further acid. 4Fe(OH)2 + O2 + 2H20 → 4Fe(OH)3 2. (2) The low pH process. Where the buffering is insufficient and the pH is below 7, even though some of the ferrous iron will have precipitated as an equivalent to Fe(OH)2, the solubility is such that sufficient Fe2+ will remain in solution so that further oxidation will produce acid. 4Fe2+ + O2 + 1OH2O → Fe(OH)3 + 8H+ This acid will bring more of the Fe2+ into solution to create more acid and the pH will gradually fall even further, so that the gossan forming environment will be at a pH less than 5 and may be as low as 3. At these low pH values, the base metals are soluble and not prone to co-precipitation or adsorption with the gossan minerals. Only elements present in solution as anions, such as Se, As, Mo and Sb, are likely to be bound into gossans forming at low pH. The results from weathering tests carried out on gossan minerals formed at higher pH show that these minerals are reasonably stable if treated with solutions that have a pH above 7, but they can break down if treated with a solution of pH 5. Thus they could be expected to be leached by rain water saturated with CO2. When investigating a likely gossan, all aspects — the iron oxides, the silicates, the carbonates and penetrations into the footwall and hanging wall — should be examined carefully, being ever mindful of the effect that pH would have had during the formation and reworking of the minerals. The composition of gossan minerals, their adsorption properties, the solubilities of metal ions, the mechanisms of Fe precipitation, the co-precipitation of other metals with Fe, the stability of carbonates and the binding of humic materials are all pH dependent in the same way. At high pH, metals are immobilized; at low pH, they tend to be in solution.