Wide-spaced sampling for delineation of geochemical provinces in desert terrains, northwestern China

Desert terrains are widely distributed in northwestern and northern China, but remain largely unexplored or under-explored. In the past, lag materials (coarse fraction of regolith) have been sampled for regional geochemical surveys. This sampling method was successful in mountainous and hilly regions, but it failed in areas of lower relief, such as slopes, plains, alluvial valleys and basins. Studies conducted since 1999 were aimed at understanding the environment and the nature of geochemical dispersion in order to find a cost-effective, wide-spaced sampling method for delineation of geochemical provinces. The results show that many elements related to mineral deposits are concentrated in the fine fraction (<120 mesh, 125 μm) in the weakly cemented horizon in the upper part of the vertical profile. Wide-spaced sampling was carried out in an area of c. 150 000 km2 at a density of one sample per 100 km2. Samples were collected from the weakly cemented sandy clay-rich horizon at a depth of 15–30 cm and sieved to <120 mesh in the field. The sieved samples were ground to 200 mesh (75 μm). Thirty-four elements were determined by ICP-MS, GF-AAS and AFS. A geochemical atlas and interpreted geochemical maps were generated using a GIS. Geochemical provinces of Au, Cu and U have been recognized. Five Au provinces, with threshold values of more than 2.0 ppb Au, have been delineated. Three of these are consistent with known large Au deposits, and preliminary follow-up surveys indicate that the other two are prospective for undiscovered Au deposits. Seven Cu provinces have been delineated, of which three are endowed with hydrothermal Cu deposits, porphyry Cu deposits and Cu-Ni deposits, respectively. Two provinces of U associated with Mo were delineated in the basin. One of them contains a large sandstone-hosted U deposit and in the other, a large new U deposit was discovered by follow-up drilling.