| Abstract:This paper deals with four major features of the magnesite ore, namely, ore structures and textures, solid solutions in the system MgCO3-FeCO3, mobility of some important trace elements and carbon and oxygen isotope composition. In the ore both relict sedimentary structure and metasomatic texture are observed. The former comprises relict bedding, relict banding, relict cross-bed-ding, and relict fossil remains whereas the latter includes metasomatic relict, corroded and sutured textures. Dolomite is replaced by magnesite, and fine-gained magnesite is replaced by medium-grained one which, in turn, by coarse-grained one. Magnesite grains of different sizes are associated with different accessory minerals and have different iron contents. The accessory minerals may be classified into four groups: (1) relict minerals such as quartz, dolomite, calcite, and sericite; (2) hydrothermal minerals like talc, tremolite, and Mg-chlorite; (3)residual hydrothermal minerals, including Mg-bearing siderite, rhodochrosite, veined dolomite and light-colored talc; and (4) secondary minerals, e. g., hematite and limonite. The FeCO3 content of the solid solutions of MgCO3-FeCO3 system increases with the falling of the ore-forming temperature, while the inhomogenization of the constituents of the solutions depends upon the evolution of mineral generations. In backscattered electron image, the earliest microgranular magnesite from magnesitized dolomite shows uniform composition, whereas the fine-grained and earlier medium grained one from minerals of earlier generations displays both light grey area and dark grey area interlacing in the crystals. Moreover, in the late medium- and coarse-grained magnesite of later generations, rhythmic variations manifested by some 1-2% difference in FeCO3 content outline the pulsatory growth of crystals. During the mineralization, such trace elements in dolomite as Ti4+, Fe2+, and Mn2+ whose ionic radii are all similar to the size of Mg2+ were transported together with magnesium from the unaltered dolomite to the ore; on the contrary, Sr and Ba were disperse. The variation in carbon isotope composition of the ore suggests that the magnesite might have inherited carbon from the replaced dolomite, for in spite of the variation in gas phase of the ore fluids during the falling of the ore-froming temperature, the δ13C values of ores remained constant. The oxygen isotope composition of the ore was also constant and did not rise as the temperature of the ore fluids got lower. Hot brine with properδ18O values might have uninterruptedly flowed in to counterbalance the effect of falling temperature. During the formation fo dolomite veins, the ore-fluids were so greatly mixed with hot groundwater that they turned from reducing state to oxidizing, and that their light oxygen isotope content became much than that necessary to balance the effect of falling temperature. In conclusion, the authos firmly hold that the ore deposit if evaporation-sedimentation followed by hydrothermal enrichment in genesis. |
