The podiform chromitites occur in a well-preserved mantle sequence consisting of lherzolite-harzburgite with abundant lenses of olivine dunite. The podiform chromitite deposits are common as small and irregularly shap...The podiform chromitites occur in a well-preserved mantle sequence consisting of lherzolite-harzburgite with abundant lenses of olivine dunite. The podiform chromitite deposits are common as small and irregularly shaped masses in the Southeastern Desert (SED) of Egypt. The podiform chromitites exhibit a wide range of compositions from high Cr to high Al varieties. The Cr of chrome spinel ranges from 0.67 to 0.88 in olivine-dunite, quite similar to that of the high-Cr chromitite, whereas it is around 0.62 in lherzolite-harzburgite. Primary hydrous mineral inclusions, amphibole and phlogopite, in chrome spinel have been reported for the first time from the Pan-African Proterozoic podiform chromitites. On the other hand, petrographic and geochemical evidence suggests that podiform chromitites in the SED of Egypt were formed as a result of crystallization of mafic melts, probably of boninitic composition, the boninitic parental magmas were probably produced by a second stage of melting above a subduction zone. Three types of chromite ores can be distinguished within the SED of Egypt: (a) sulphide-poor podiform ores; (b) brecciated ores; and (c) sulphide-rich ores. Two textural types of inclusions in chromite are distinguished: (1) primary silicate inclusions generally have high Mg-number (>96), Cr and Ni, and are dominated by pargasitic amphibole, forsterite, diopside, enstatite and Na-phlogopite. A diversity of primary and secondary platinum group minerals (PGM) is described from the chromitites, including alloys, sulphides, sulpharsenides and arsenides of Ru, Os, Ir, Rh, Ni, Cu, Fe and Co; (2) in addition to primary PGM and hydrous silicates, the fluids are of low to moderate salinity, sodium-dominated aqueous solutions with complex gas contents. Variable amounts of water, hydrogen, hydrocarbons, carbon dioxides and nitrogen have been determined in inclusion-rich samples. The chondrite-normalized PGE patterns of lherzolite-harzburgite and olivine-dunite have negative Ir and Pt, and positive Pd and Au anomalies. Chromitites are homo-geneous in composition but texturally zoned on a large scale. They carry elevated IPGE, manifested in numerous, primary and secondary PGM phases.展开更多
The formation of many hydrothermal gold deposits is closely related to iron-rich rocks. The host rocks of the Madiyi Formation of the Mid-to Late Neoproterozoic Banxi Group for the Woxi Au(-Sb-W) deposit, which is loc...The formation of many hydrothermal gold deposits is closely related to iron-rich rocks. The host rocks of the Madiyi Formation of the Mid-to Late Neoproterozoic Banxi Group for the Woxi Au(-Sb-W) deposit, which is located in western Hunan Province of the western Jiangnan Orogen, South China, is rich in hematite, which provides a good example for studying the relationship between the formation of gold deposit and iron-rich rocks. Field investigation and petrographic observation on the unaltered, weakly altered and strongly altered rocks demonstrate that the bleaching is caused by a combination of carbonatization, sulfidation and sericitization. Mass balance calculation suggests that, during decolourization there is no change in TFe_(2)O_(3), while FeO is gained and Fe_(2)O_(3)is lost. Geochemical modeling found that Au was mainly present as AuHS(aq) and Au(HS)-2, and that the water-rock interactions decreased the sulfur fugacity which destroyed the stability of such aqueous complexes. Combined with the locally occurred native gold in quartz veins, it is concluded that the major gold precipitation mechanisms are sulfidation and fluid boiling. Based on previous geochronological and geochemical research further gold mineralization is proposed to be generated by deep sourced magmatic or metamorphic fluid migrated upward along the Woxi fault, and the iron-rich Madiyi Formation is the idea chemical trap for gold deposition. The decrease of sulfur contents caused by fluid-rock interactions and fluid boiling are the major mechanisms for gold mineralization.展开更多
文摘The podiform chromitites occur in a well-preserved mantle sequence consisting of lherzolite-harzburgite with abundant lenses of olivine dunite. The podiform chromitite deposits are common as small and irregularly shaped masses in the Southeastern Desert (SED) of Egypt. The podiform chromitites exhibit a wide range of compositions from high Cr to high Al varieties. The Cr of chrome spinel ranges from 0.67 to 0.88 in olivine-dunite, quite similar to that of the high-Cr chromitite, whereas it is around 0.62 in lherzolite-harzburgite. Primary hydrous mineral inclusions, amphibole and phlogopite, in chrome spinel have been reported for the first time from the Pan-African Proterozoic podiform chromitites. On the other hand, petrographic and geochemical evidence suggests that podiform chromitites in the SED of Egypt were formed as a result of crystallization of mafic melts, probably of boninitic composition, the boninitic parental magmas were probably produced by a second stage of melting above a subduction zone. Three types of chromite ores can be distinguished within the SED of Egypt: (a) sulphide-poor podiform ores; (b) brecciated ores; and (c) sulphide-rich ores. Two textural types of inclusions in chromite are distinguished: (1) primary silicate inclusions generally have high Mg-number (>96), Cr and Ni, and are dominated by pargasitic amphibole, forsterite, diopside, enstatite and Na-phlogopite. A diversity of primary and secondary platinum group minerals (PGM) is described from the chromitites, including alloys, sulphides, sulpharsenides and arsenides of Ru, Os, Ir, Rh, Ni, Cu, Fe and Co; (2) in addition to primary PGM and hydrous silicates, the fluids are of low to moderate salinity, sodium-dominated aqueous solutions with complex gas contents. Variable amounts of water, hydrogen, hydrocarbons, carbon dioxides and nitrogen have been determined in inclusion-rich samples. The chondrite-normalized PGE patterns of lherzolite-harzburgite and olivine-dunite have negative Ir and Pt, and positive Pd and Au anomalies. Chromitites are homo-geneous in composition but texturally zoned on a large scale. They carry elevated IPGE, manifested in numerous, primary and secondary PGM phases.
基金supported by the National Natural Science Foundation of China (No.41930428)the scientific research start-up fund for doctors of East China University of Technology (No.DHBK2019066)the science and technology research project of Jiangxi Education Department (No.GJJ2200754)。
文摘The formation of many hydrothermal gold deposits is closely related to iron-rich rocks. The host rocks of the Madiyi Formation of the Mid-to Late Neoproterozoic Banxi Group for the Woxi Au(-Sb-W) deposit, which is located in western Hunan Province of the western Jiangnan Orogen, South China, is rich in hematite, which provides a good example for studying the relationship between the formation of gold deposit and iron-rich rocks. Field investigation and petrographic observation on the unaltered, weakly altered and strongly altered rocks demonstrate that the bleaching is caused by a combination of carbonatization, sulfidation and sericitization. Mass balance calculation suggests that, during decolourization there is no change in TFe_(2)O_(3), while FeO is gained and Fe_(2)O_(3)is lost. Geochemical modeling found that Au was mainly present as AuHS(aq) and Au(HS)-2, and that the water-rock interactions decreased the sulfur fugacity which destroyed the stability of such aqueous complexes. Combined with the locally occurred native gold in quartz veins, it is concluded that the major gold precipitation mechanisms are sulfidation and fluid boiling. Based on previous geochronological and geochemical research further gold mineralization is proposed to be generated by deep sourced magmatic or metamorphic fluid migrated upward along the Woxi fault, and the iron-rich Madiyi Formation is the idea chemical trap for gold deposition. The decrease of sulfur contents caused by fluid-rock interactions and fluid boiling are the major mechanisms for gold mineralization.
