Vol 57, No 12 (2019)

In this paper, based on the analysis of 1572 small and above-scale gold deposits in China, the gold deposits are divided into four types for prediction purposes: sedimentary, volcanic rock, magmatic hydrothermal and shear zone types. The spatial and temporal distribution of gold deposits in China is non–uniform, as 60% of the deposits are distributed in the land block and Mesozoic deposits account for 71.5% of the national total. It is found that intrusive and sub-volcanic rocks are the main ore–forming geological bodies. Here, the method for accurately identifying the spatial positions of concealed rocks is summarized. Through the geological profile study of 450 representative gold deposits, this paper suggests that there are mainly two kinds of gold deposits in China. One is characterized as an association of Au and Ag, with formation temperatures of approximately 250°C; it has very low concentrations of As and Sb. The fineness of the gold is generally below 900. Unequal proportions of K–feldsparization and albitization formed in the early stage. The other is an association of Au, As and Sb with a formation temperature below 250°C; the fineness of the gold is generally higher than 900; it has low Ag and high As and Sb concentrations. Alterations were mainly in the form of silicon with some secondary quartz formed in the early mineralization stage. Taking into account the elemental geochemical characteristics, the mechanism of gold migration and precipitation in the transient transition process from fluid to solid minerals in the ore-forming structure is discussed. The discussion provides a reference for future gold prospecting.

Chemical equilibria of iron and manganese aqueous species were analyzed at various Eh–pH and aqueous CO₂ concentration. Thermodynamic and equilibrium-kinetic simulations were conducted for the oxidations of iron and manganese aqueous species with reference to groundwater demanganation and deironing, using the most probable rate constants of congruent dissolution reactions of iron and manganese carbonates, rate constants of homogeneous iron oxidation, and surface catalytic oxidation of manganese on suspension of iron hydroxide in aqueous solution. A kinetic–thermodynamic model is suggested for iron and manganese oxidation in groundwater under the effect of dissolved oxygen. The numerical simulations show that deironing is relatively effective, whereas the dissolved manganese(II) concentration increases because of slower manganese oxidation and the dissolution of manganese-bearing siderite. Hence, effective groundwater demanganation is possible if the host rocks contain no manganese-bearing siderite.

The Khorat Plateau is one of the largest potash deposits in the world. In recent years, exploitation of potash resources accompanied by leakage of underground brines (UB) in potassium-bearing strata has threatened mining safety. However, there is little research on origin of UB in Laos. In this study, twelve samples of UB, salt springs (SS) and river waters were collected in Khammouane mining area, central Laos and analyzed for ionic and boron isotopic compositions. The results show that both of UB, dominated by K⁺, Mg²⁺, Na⁺ and Cl^– ions, and SS, enriched in Na⁺ and Cl^–, are chloride-type brines, while river waters abundant in Ca²⁺ and HC\({\text{O}}_{3}^{ - }\) ions are carbonate-type waters. K⁺ (35.1–53.7 g/L) and Rb⁺ (12.9–16.6 mg/L) concentrations of UB are higher than the peak (25.9 g/L, 9.4 mg/L) of seawater evaporation trajectories (SET). UB is also characterized by high Mg²⁺ (32.7–68.0 g/L) and Br^– (1795.2–2801.3 mg/L) concentrations and its lg(Cl/Br) ratios (1.9–2.1) deviate from the halite dissolution line. These comparisons indicate that the water chemistries of UB are mainly influenced by dissolution of carnallite and sylvite. High Sr²⁺ concentrations (31.8–58.6 mg/L) in UB might be simultaneously associated with dissolution of potash salt and Ca-bearing minerals (carbonate and anhydrite). The negative saturation indices (SI) of different salt minerals in UB suggest that salt minerals are unsaturated in UB. Meanwhile, boron (B) concentrations (396.0–426.0 mg/L) and δ¹¹B values (+19.8–+21.0‰) of UB fall in between low B–δ¹¹B end member (0.02 mg/L, +2.31‰) of river water and high B-δ¹¹B end member (126.7–890.8 μg/g, +25.94–+32.94‰) of evaporitic minerals. The comparison implies that UB was influenced and mixed by two end members. Therefore, combining with major and trace ionic concentrations, isotopic compositions and lithologies of salt-bearing strata revealed by drillings in mining area, we conclude that the major solute source in UB is derived from dissolution of carnallite and sylvite. These geochemical evidences provide insights into dealing with leakage of underground brines occurred in the underground mining of potash deposits in Laos.

New data were obtained during Cruise 42 of the R/V Akademik Boris Petrov (January–March 2017) and Cruise 68 of the R/V Akademik Mstislav Keldysh (June–August 2017) on the concentration of the anthropogenic radionuclide cesium-137 in the surface waters of the East China Sea, South China Sea, equatorial Indian Ocean, Red Sea, Mediterranean Sea, Bay of Biscay, English Channel, Baltic Sea, North Atlantic, Norway Sea, and Barents Sea. Low cesium-137 concentrations were observed in the surface waters of the Eurasian seas. An exception is the Baltic Sea, where the cesium-137 content is much higher than the values obtained before the Chernobyl accident. Compared with previous estimates, higher cesium-137 concentration was observed in the surface water of the East China Sea, which could be related to the extensive discharge of high-level liquid radioactive waste after the Fukushima Daiichi nuclear accident of 2011.