摘要
大石桥菱镁矿矿石的残余沉积构造及交代结构均很发育。菱镁矿交代白云岩、粗、中、细粒菱镁矿依序交代。副矿物分残余沉积变质矿物、热液矿物、残余热液矿物及次生矿物四类。菱镁矿为一组MgCO_(?)-FeCO_3固溶体,其铁含量依温度降低而增加,组分均勺性则依世代演化而降低。凡离子半径与镁近似的痕量元素随镁自围岩至矿石迁移,反之,则随钙分散。矿石中碳、氧同位素组成均不因流体冷却、分馏系数增大而变,可能有含适度轻氧的卤水不断注入,碳则因阳离子交换自白云岩继承而来。确定了矿物生成顺序表。
This paper deals with four major features of the magnesite ore, namely, ore structures and textures, solid solutions in the system MgCO_3-FeCO_3, 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 omprises relict bedding,relict banding, relict cross-bedding, and relict fossil remains whereas the latter includes metasomatic relict, corroded and sutured textures. Dolomite is replaced by magnesite, and finegrained 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, remolite 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 FeCO_3 content of the solid solutions of MgCO_3-FeCO_3 system increases with the falling of the ore-forming temperature, while the nhomogenization 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 FeCO_3 content outline the pulsatory growth of crystals. During the mineralization, such trace elements in dolomite as Ti^(4+), Fe^(2+), and Mn^(2+) whose ionic radii are all similar to the size of Mg^(2+) were transported together with magnesium from the unaltered dolomite to the ore; on the contrary, Sr and Ba were dispersed.The variation in carbon isotope composition of the ore suggests that the magnesite might have inherited carbon from the replaced dolmite, for in spite of the variation in gas phase of the ore fluids during the falling of the oreforming temperature, the <δ^(13)C 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 proper δ^(18)O values might have uninterruptedly flowed in to counterbalance the effect of falling temperature. During the formation of 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 more than that necessary to balance the effect of falling temperture.In conclusion, the authors firmly hold that the ore deposit is evaporationsedimentation followed by hydrothermal enrichment in genesis.
出处
《矿床地质》
CAS
CSCD
北大核心
1990年第1期77-85,T002,共10页
Mineral Deposits
