Comparative Interpretation Of Multi-Element Regional Geochemistry In Gobi Desert, Eastern Tianshan, NW China

Gobi Desert has long been the difficulty for geochemical survey due to its unique natural landscape. With the heart-stirring discoveries of large and ultra-large scale mineral deposits in this region, geologists started to believe that it is one of the most potential areas for finding giant ores. Therefore, developing proper geochemical survey technique is of great importance. To avoid the influence of aeolian sand and soluble salts, former researchers have developed coarse rock debris geochemistry (minus4to plus20mesh) and fine regolith deep-penetrating geochemistry (minus120mesh). In this contribution, the author has conducted comparative studies of the aforementioned two survey techniques in order to understand the features of data inner structure and spatial distribution patterns through the establishment of data system (including statistical, multivariate and geostatistical analyses), geochemical system (including sampling medium, preparation, analysis and quality control) and geological system (including regional mineralization formation, remote sensing geomorphological model and ASTER global digital elevation model), on which geochemical survey technique for Gobi Desert, data processing and interpretation procedures are based.The following conclusions and understandings can be drawn:Ⅰ. The particle size of more than90percent aeolian sand ranges from120to830μm, and the particle size that lager than830μm and smaller than96μm is hardly seen. Moreover, the most active portion ranges from200to400μm. Consequently, both coarse rock debris and fine regolith are proper media for reflecting geological processes. Geochemical datasets from the two surveys are obtained by multi-element analytical system with high accuracy, precision and strict quality control standard, which is considered reliable and comparable.Ⅱ. Basic statistical parameters show that concentrations of most elements (Ag, Co, Cr, Cu, Mn, Mo, Ni, Pb, Sr, Th, W, Zn, Al, Ca, Fe, Mg, K and Na) are generally the same in coarse rock debris and fine regolith samples. Au, U, Li, As and Sb show moderate enrichment and Hg shows moderate depletion in fine regolith samples. The results of principal component analysis demonstrate that there are distinct differences in the components related to Au and U mineralization. The component related to Au mineralization in rock debris samples have the combination of Au-As-Sb, and however, fine regolith samples have that of Au-U-Li-As-Sr-Ca. The component related to U mineralization with a combination of U-Mo-Na can only be found in fine regolith samples. This indicates that both coarse rock debris and fine regolith samples can reflect the outcropping ductile-shear zone type and hydrothermal Au deposits. And the latter one can also indicate the deeply-buried Au-U mineralization. Spatial variation analysis of the principal components show that they all have great continuity in the east-west direction, but poorest continuity in the south-north direction, which coincide with regional geology.III. The spatial distribution patterns of the principal components related to mafic-ultramafic elements and weathering of acidic magmatic rocks (mainly granite) in the two datasets show great resemblance respectively. In addition, these distribution patterns perfectly match with regional mineralization formation, geomorphological and digital elevation models. This kind of similarity in the two datasets demonstrates good geochemical inheritage of mafic-ultramafic elements (including Co, Cr, Ni, Cu, Fe, Mn, Mg and Zn) and elements related to weathering of acidic magmatic rocks (including Th, Pb, W, K, U) in the two sampling media.IV. The principal component representing Au mineralization shows both similarity and difference in the two datasets. In the outcropped and semi-outcropped regions, the distribution patterns for ductile-shear zone type and hydrothermal Au deposits reflected by coarse rock debris and fine regolith samples are the same. However, fine regolith samples also capture the information related to deeply-buried gold mineralization which could be transported to the surface by gases and groundwater through fault systems. Two geochemical provinces are indentified at Yandun-Hami and Shanshan areas.V. The principal component related to sandstone-type U mineralization in Turpan-Hami Basin can only be found in fine regolith samples, and three U-Mo geochemical provinces are detected at Yandun-Hami, Shanshan and Turpan areas. Concealed U mineralization could be transported to the surface by gases and groundwater through fault systems and absorbed by fine regolith samples. Coarse rock debris cannot be collected in argillaceous plain.VI. The above analysis indicates that these two datasets own the characteristic of general similarity with minor but significant differences. Coarse rock debris inherit the geochemical information derived only from physical weathering of bed rock, however, fine regolith samples are believed to not only own the above-mentioned geochemical information, but also contain the information related to deeply-buried mineralization due to its strong capacity of absorption. Therefore, a conceptual information model of geochemical samples of Gobi Desert is constructed, on which geochemical survey system for Gobi Desert is based. Classic statistical, multivariate and geostatistical analyses can effectively accomplish geochemical date mining, and the construction of regional geological, geomorphological and digital elevation models can well aid in the interpretation.