煤田自燃区上磁异常的成因

GENESIS OF MAGNETIC ANOMALIES OVER THE SELF-COMBUSTION DISTRICT OF THE COAL FIELD

通过对内蒙古平庄西露天煤矿顶板砂岩的磁性研究，指出这些煤矿顶板的上侏罗统灰色砂岩在天然状况下是弱磁性的，天然剩磁在n×10^(-3)A/m级次，磁化率在10^(-4)（SI）级次。但经高温热处理（＞600℃）后，磁性迅速增高，剩磁增大五个级次，磁化率增大四个级次。通过多种岩矿分析方法进一步搞清楚，这些砂岩的胶结物成分是弱磁性的碳酸铁（FeCO_3），经高温处理后，成为强磁性的磁铁矿（Fe_3O_4）。这种变化是我国北方地区许多侏罗纪煤田自燃区上出现磁异常的主要原因。文中还给出了黄铁矿热处理前后的磁性变化情况。最后利用测得的岩石磁性，设计了多种理论模型，正演计算了所产生的磁异常值。结果表明，利用高精度磁法勘探来区分这类煤田的自燃区与非自燃区是完全可能的。

: Based on an investigation into magmatism of roof sandstone of the Pi- ngzhuangxi opencut coal mine in Inner Mongolia. this paper points out that the Upper Jurassic grey sandstone of the roof of the coal mine is weakly ma- gnetic under natural conditions. The natural remanence and the magnetic su- sceptibility are n × 10-3A/m and 10-4 (SI) in order of magnitude respectively. After high-temperature heat treatmellt (>600℃), however, magnetism incre- ases rapidly: remanent magnetism and magnetic susceptibility increase by five and four orders of magnitude respectively. It is found through rock and ore analysis of various means that the cement of the sandstone is weakly magne- tic iron carbonate (FeCO3), which. after high-temperature treatment, turns into strongly magnetic magnetite(Fe3O4). Such change seems to be the major factor causing the appearance of magnetic anomalies over many Jurassic coal fields in North China. The change in magnetism of pyrite through heat treatment is also described. Finally, using magnetism of rocks that was determined, we have designed quite a few theoretical models and made direct calculation for values of magnetic anomalies. The results show that it is quite possible to distinguish self-combustion districts from non-self-combustion districts by means of high- precision magnetic exploration.