Weathered ores or oxidized ores are the product of sulphide destruction by weathering under oxidizing conditions. Pyrite is the most significant source of oxidized sulphur and indirectly, hydrogen ions, in a sulphide
An oxidized ore is defined as one in which the ore material has been oxidized or weathered, possibly in a zone that is atypical of the primary sulphide deposit, and for which some especial processing may be required bearing rock volume undergoing weathering. As these components are interrelated, oxidation of pyrite is important to produce altered sulphides.
Oxidation of pyrite comprises a series of processes that creates oxidized iron minerals whose generation affect the oxidation process of other metallic sulphides. In general, there is ferrous, ferric and sulphate ions tend to handle the oxidation process. In this way, hydroxyl-sulphates are generated constantly and part of these compounds will be deposited in parts of the rock during the formation process creating conglomerates of particles with high permeability. Under this fact, some leaching process are favored due to the leaching agent can penetrate into the particles. This is the success of many heap leaching operations that form the heap without a previous crushing stage.
The mineralogy of the host rock plays a very significant geochemical role in the development of gangue and ore mineral assemblages occurring in the oxidation zone. In general, host rock such as silicates and oxides are a very important part in the oxidized environment due to they have exchangeable cations able to take hydrogen. If the quantity of reactive silicates and carbonates is high, the ability to neutralize sulphate bearing solutions is enormous.
Weathered ores can be formed by hydrothermal alternation. In this case, alteration and mineralization are produced in two stages. First, magmatically vapours ascending along fractures are absorbed into groundwater and form an acid fluid. Second, the acid fluid leaches the host rock producing an porous rock, sometimes called vughy silica. These acids assemblages can be associated with clayed zones. Early advanced argillic alteration in the area creates additional permeability in fracture zones. Intense acid leaching created pathways for metal bearing fluids and produced the open space vughs which host metal sulphides and Au.
Try to understand the environment of formation is a key aspect of the economical and metallurgical performance of these deposits. Many deposits are characterized for having a reddish color due to the presence of hematite. The generation of red hematite during oxidation of supergene sulphides is explained by the oxidation of pyrite.
Ore types of oxidized gold ores have gold contents as high as 10 g/t and an agitated leaching process is a good and attractive alternative due to the leaching is high and consequently the gold extraction is high, typically more than 90%. However, sometimes there is a potential problem, the presence of high amounts of oxidized copper minerals. The problem is related to the high cyanidation consumption and low gold recoveries.
The flotation process is other alternative, but the excessive floatability of fine material is an issue to be considered. The presence of clay is other potential problem due to this material consumes reagents and modifies the rheological properties of the slurry. When the presence of fine particles is not problematic, silver and gold will be recovered.