Phosphorus depletion controls Cu and Zn biogeochemistry in canola and corn rhizosphere on a calcareous soil

Phosphorus (P) deficiency may trigger rhizodeposition, including protons and organic compounds, with possible effects on metal solubility and speciation. To explore the relevance of this process, we investigated biogeochemical changes in the rhizosphere of P-deficient canola (Brassica napus L.) and corn (Zea mays L.) cultivars grown in a pot experiment on calcareous soil. Depletion of total soluble (0.005 mol/L Ca(NO3)2-extractable) P in the rhizosphere varied with crop species and cultivar but was generally strong and negatively correlated with dissolved organic carbon (DOC) in canola (R2 = 0.868) and corn (R2 = 0.844) rhizospheres, indicating rhizodeposition in response to limited P availability. DOC was correlated with dissolved Cu, explaining 86% of its variation in the rhizosphere and bulk soil solution of canola and corn cultivars, respectively, suggesting Cu mobilisation via the formation of Cu-organic complexes. In line with lower Zn-organic complex stabilities, the effect of rhizodeposition was less pronounced for Zn mobilisation. We show that the P nutritional status of plants and the related variation of rhizodeposition among crops and cultivars represents a major control of metal solubility in soil, with possible effects on micronutrient supply and toxicity. Hence, targeted P availability control should be considered in the management of polluted and micronutrient-deficient soils.

Are Vapor-Like Fluids Viable Ore Fluids for Cu-Au-Mo Porphyry Ore Formation?

Ore formation in porphyry Cu-Au-(Mo) systems involves the exsolution of metal-bearing fluids from magmas and the transport of the metals in magmatic-hydrothermal plumes that are subject to pressure fluctuations. Deposition of ore minerals occurs as a result of cooling and decompression of the hydrothermal fluids in partly overlapping ore shells. In this study, we address the role of vapor-like fluids in porphyry ore formation through numerical simulations of metal transport using the Gibbs energy minimization software, GEM-Selektor. The thermodynamic properties of the hydrated gaseous metallic species necessary for modeling metal solubility in fluids of moderate density (100–300 kg/m3) were derived from the results of experiments that investigated the solubility of metals in aqueous HCl- and H2S-bearing vapors. Metal transport and precipitation were simulated numerically as a function of temperature, pressure, and fluid composition (S, Cl, and redox). The simulated metal concentrations and ratios are compared to those observed in vapor-like and intermediate-density fluid inclusions from porphyry ore deposits, as well as gas condensates from active volcanoes. The thermodynamically predicted solubility of Cu, Au, Ag, and Mo decreases during isothermal decompression. At elevated pressure, the simulated metal solubility is similar to the metal content measured in vapor-like and intermediate-density fluid inclusions from porphyry deposits (at ~200–1,800 bar). At ambient pressure, the metal solubility approaches the metal content measured in gas condensates from active volcanoes (at ~1 bar), which is several orders of magnitude lower than that in the high-pressure environment. During isochoric cooling, the simulated solubility of Cu, Ag, and Mo decreases, whereas that of Au reaches a maximum between 35 ppb and 2.6 ppm depending on fluid density and composition. Similar observations are made from a compilation of vapor-like and intermediate-density fluid inclusion data showing that Cu, Ag, and Mo contents decrease with decreasing P and T. Increasing the Cl concentration of the simulated fluid promotes the solubility of Cu, Ag, and Au chloride species. Molybdenum solubility is highest under oxidizing conditions and low S content, and gold solubility is elevated at intermediate redox conditions and elevated S content. The S content of the vapor-like fluid strongly affects metal ratios. Thus, there is a decrease in the Cu/Au ratio as the S content increases from 0.1 to 1 wt %, whereas the opposite is the case for the Mo/Ag ratio; at S contents of >1 wt %, the Mo/Ag ratio also decreases. In summary, thermodynamic calculations based on experiments involving gaseous metallic species predict that vapor-like fluids may transport and efficiently precipitate metals in concentrations sufficient to form porphyry ore deposits. Finally, the fluid composition and pressure-temperature evolution paths of vapor-like and intermediate-density fluids have a strong effect on metal solubility in porphyry systems and potentially exert an important control on their metal ratios and zoning.