Continental-scale spatial distribution of chromium (Cr) in China and its relationship with ultramafic-mafic rocks and ophiolitic chromite deposit

摘要

With the increasing demands for Cr following China's new infrastructure projects, ascertaining the concentration and distribution of Cr is urgent and meaningful for prospecting Cr deposit and relieving Cr shortage in China. The China Geochemical Baselines (CGB) project provides continental-scale Cr data from 3382 top and 3380 deep catchment sediment/alluvial soil samples and 11605 rock samples. Statistical analysis shows that the median Cr value (baseline) of China is 50 mu g/g. The ultramafic and mafic rocks respectively have median Cr values of 1609 and 129 mu g/g based on 52 and 551 samples, much higher than those in intermediate rocks (47 mu g/g on 991 samples) and felsic rocks (7.0 mu g/g on 2558 samples). Parent rock types, especially ultramafic-mafic rocks, predominantly governed the distribution of Cr enrichments in the catchment sediment/alluvial soil of China. High Cr concentrations (>75th percentile) are closely associated with lithology and Cr mineralization, presented in regions where ultramafic rocks, basalts and black shales widely outcropped, such as Changbaishan basalts in Northeast China, Emeishan basalts in South China and black shales along the middle-lower Yangtze River. After eliminating the disturbances of regional background values, the chromium geochemical anomalous centers (>97.5th percentile) are well consistent with the distribution of two-types Cr deposits, especially with the widely-distributed ophiolitic podiform-type Cr metallogenic belts in China. The nation-wide Cr concentration and distribution would provide giant basic data for further exploration prospecting, environmental and hydrogeological studies.