摘要
大石桥晶质菱镁矿产于含方柱石及微含沉积菱镁质的中元古代硅质白云岩中,受区域挤压带控制,与元古代混合岩有空间联系。受千枚岩遮挡层和层间断裂控制时,矿体成层状,否则成不规则状。围岩具脱硅化、菱镁矿化及滑石化等蚀变,具热晕。成矿温度介于240—370℃之间。因白云石变成菱镁矿时体积有收缩,故出现收缩构造,如张性角砾岩和张性断裂等。角砾岩胶结物中有菱镁矿和滑石。成矿热液为贫硅富镁碱性热液,晚期转为含硅酸性热液。矿床成因为(蒸发)沉积-热液富集。
Taking the Dashiqiao ore deposit as an example, this paper deals with some of the important geological features of dolomite-hosted sparry magnesite deposits including regional ore-controlling factors, conditions governing shapes of orebodies, wall rock alterations associated with thermal halos, and contraction structures.There are three critical parameters governing the localization of magnesite deposits, i. e., dolomite containing high sili(?)eous materials and a little sedimentary megnesitic substances, strong compressive belt and Proterozoic migmatite. Generally speaking, the CaO/MgO ratio of most of unaltered dolomites varies from 1.5 to 1.25, while the SiO_2 content from 2.98 to 21.69%. Magnesite deposits are developed usually along three main strong compressive strike fault zones in the area, and there also exists a field of low and gentle magnetic anomalies. In the northern part of this magnetic field lie outcrops of Proterozoic migmatite which contains unevenly scattered accessory magnetites, while in the southern part, exposed magnesite deposits are seen. It is supposed that there may exist concealed migmatite below the magnesite deposits. In the eastern part of the area, if merely one or two of these three parameters are present, no orebody or only small and very lean orebodies can be discovered. The magnesite deposits have two shapes of orebodies, i. e., stratiform or stratoid and irregular. The fomer is formed in connection with impermeable phyllite barriers and bedding fractures, while the latter only with compressive-shear zones with no phyllite barrier.Five sorts of wall rock alteration are discerned, i. e., magnesitization, desilication. talcization, decolorization and dolomitization. Magnesitization known as (?)nagnesite replacing dolomite is frequently accompanied with desilication which. reduces the SiO_2 content of magnesite to the extent of being lower than that of dolomite. Talcization finds expression in the replacement of silicate rocks or minerals by talc or the formation of different-colored talc veins of various generations. The rock alterations are all associated with thermal halos whose margins show decolorization. The dolomitization is manifested by the formation of dolomite veins. The homogenization temperatures of gas-liquid inclusions in bladed magnesite are in the range of 230-300℃,whereas those of rhombohedral magnesite are around 150℃.The contraction structures, which comprise collapse breccia, tensile brecciated zones, tensional fractures and tensional network fissures, may be formed as a result of volume contraction during the replacement of dolomite by magnesite. Collapse structures and brec ciated zones result from the cementation of breccias of altered dolomite, fine-grained magnesite, talcized rocks and talcs by dolomite crystals, medium-to coarse-grained magnesite or leucocratic talc. Tensional fractures and network fissures are usually filled with magnesite or dolomite, forming veins.From the above discussion, this paper advances a polygenic hypothesis of evaporation-sedimentation followed by hydrothermal enrichment.
出处
《矿床地质》
CAS
CSCD
北大核心
1989年第1期71-84,共14页
Mineral Deposits
