Twenty samples of interstitial waters and pressed saprolite were collected within undisturbed saprolite blocks to determine the actual gold transporting mode in the auriferous laterites of Misséni, Mali. The results of the analyses indicate a very high solubility of gold in the interstitial waters of the Misséni laterites (from <2 to 16.7 ppb Au; Eh, 0.356–0.419 V; pH, 6.7–7.7). The calculated theoretical concentrations of auric chloride ion, [Formula: see text], (0.3 × 10−14 – 32.7 × 10−4 ppm) and thiosulphate ion, [Formula: see text], (2.5 × 10−212 – 6.7 × 10−180 ppm), which are generally inferred to be the ions transporting the gold in a supergene environment, are insignificant in the interstitial waters of the Misséni saprolite. Given the absence of significant concentrations of chloride or thiosulphate ions as measured in the interstitial waters of Misséni (Cl−, 1.5 × 10−4 – 1.2 × 10−2 mol L−1; [Formula: see text], 1.1 × 10−111 – 4.2 × 10−95 mol L−1), the gold monohydroxide, AuOH(H2O)0, could be responsible for the gold mobilization in the studied saprolite. However, its transport could be limited by the conspicuous (Fe, Mn) oxides in the Misséni saprolite, which can act as adsorbents. The calculated concentrtion of humic acid (0.004–0.03%), which can solubilize the gold contained in the analyzed interstitial waters, is 2 to 4 times lower than that of the measured organic matter in the samples of pressed saprolite (from <0.016 to 0.07% in organic carbon). These latter results could indicate that a part of the gold solubility in the Misséni superficial saprolite is linked to the humic acids. [Journal Translation]
Soil age and soil organic carbon content shape biochemical responses to multiple freeze–thaw events in soils along a postmining agricultural chronosequence
