兰坪盆地区域地球化学异常特征及找矿方向

西南三江中段兰坪盆地是著名的金属成矿区,区内矿产资源丰富,以铜、铅、锌、银为主。2012—2016年云南省地质调查院完成了1∶25万丽江市幅水系沉积物测量工作,平均采样密度为1.02个/km²,采样介质为代表汇水域基岩成分的岩屑,样品粒级为-10~+60目。以兰坪盆地内的1∶25万水系沉积物测量成果为基础,首先分析了盆地内微量元素的地球化学特征,指出仅从元素富集系数的大小来判断研究区内有利矿种的方法欠妥;进而为消除元素的风化富集效应而采用变值七级异常划分方案绘制了该区地球化学异常图,并对区内的主要成矿元素进行异常圈定和分级评价。结果表明所圈定的异常区不仅与该区典型矿床在空间上相吻合,而且单元素异常级别或平均异常强度也与矿床的规模相一致,已知矿床均位于平均异常强度达3级以上的异常区。除典型已知矿区外,区域地球化学异常特征显示兰坪盆地东北部老君山地区和盆地中南部白洋厂南部地区是盆地内寻找多金属矿床的有利地段。

Anomalies and Prospecting Directions of Regional Geochemistry Survey in the Lanping Basin of Yunnan Province,China

The Lanping basin located in the middle section of the Sanjiang range in southwest China is a famous polymetallic mineralization area in which dominant economic metals are copper, lead, zinc, and silver. The regional geochemistry survey with the scale of 1∶250 000 has been completed by Yunnan Geological Survey Institute from 2012 to 2016 in this area. The sampling medium is stream sediment with the size of -10 to +60 meshes, and the averaged sampling density is ca. 1.02 per square kilometer. Based on the analytical results of the regional geochemistry survey, the enrichment coefficients of 29 trace elements on average concentration in the study area are calculated relative to their average background values in the Southwest Sanjiang ore zone and discussed focusing on the main economic metals firstly. The results indicate that it is not suitable to determine economic metals only depending on the enrichment coefficients. Then geochemical anomalies of 29 trace elements are determined and classified in the Lanping basin with an area of 8 800 km² on the method of seven levels’ classification to eliminate the elemental enrichment effects during weathering. The results indicate that the determined anomaly areas are consistent with the known metal deposits spatially and metal deposits with larger scales are commonly located in the anomaly areas with higher levels. The known metal deposits in this area are commonly located in the anomaly areas with levels higher than 3 on the seven levels’ classification method. According to this criterion, two favorable areas are determined for future prospecting for polymetallic deposits. One is the Laojunshan area in the northeast of the basin, and the other is the south section of Baiyangchang area in central-south of the Lanping basin of Yunnan province, China.