Textures and mineralogy of residual supergene copper silicates in oxidised overburden

Henne, A; Craw, D; Paterson, DJ; Southam, G

Textures and mineralogy of residual supergene copper silicates in oxidised overburden

Date

2021-03-15

Authors

Publisher

Elsevier

Abstract

Copper-bearing silicate minerals are relatively resistant to hydrometallurgical leaching. More efficient extraction of this residual Cu requires detailed knowledge of the mineralogical and textural settings of the minerals, which we outline in this study. Supergene oxidation and alteration of Fe-rich rocks at the Salobo iron-oxide copper–gold (IOCG) mine, Brazil, has caused transformation of primary phyllosilicates, especially biotite, to Cu-bearing clay minerals. The Cu replaced Fe and Mg most likely in the octahedral sites of vermiculite (up to 10 wt% Cu) and the Cu became part of the silicate structure, rather than being merely adsorbed to surfaces. This alteration from biotite has involved negligible textural changes, so that much of the Cu-vermiculite is encapsulated in less-altered biotite at µm to mm scales. More advanced alteration yielded Cu-bearing kaolinite (typically ~ 1 wt% Cu). Mobilisation of Cu under circumneutral pH conditions also led to local formation of veins of Cu-bearing minerals including chrysocolla. The most intense supergene alteration produced mm- to sub-μm-scale veins and cement made up of Cu-bearing quartz, kaolinite and goethite. Titanium derived from alteration of Fe-silicates pervades the altered rocks, coating Cu-silicate material at the μm-scale and this coating may further hinder leachability. Experimental acid (pH 2.2) (bio)leaching columns demonstrated that the presence of the bacterium Acidithiobacillus ferrooxidans did not significantly enhance Cu leach efficiency. However, the experiments, combined with thermodynamic modelling suggest that residual Cu from Cu-silicates can be extracted with acid solutions, although long-term leaching (months-years) may be required. Interactions between acid solutions and clay-altered silicates caused partial neutralisation (to ~ pH 5), further hindering leaching of residual Cu. Our study on the differences between alteration and precipitation textures provides useful context for designing suitable extraction processes for residual Cu from Cu-bearing silicate minerals in weathered rocks. © 2021 Elsevier Ltd.

Keywords

Mineralogy, Oxidizers, Overburden, Clays, Minerals, Iron, Magnesium, Biotite, Copper silicates, Gold, Leachates, Weathering, Rocks

Citation

Henne, A., Craw, D., Paterson, D., & Southam, G. (2021). Textures and mineralogy of residual supergene copper silicates in oxidised overburden. Minerals Engineering, 163, 106775. doi:10.1016/j.mineng.2021.106775