The formation and material sources of the superlarge Hada Gol Ga-bearing coal deposit in Jungar Banner, Inner Mongolia

Abstract：The No. 6 coal seam lies in the upper part of Taiyuan Formation within the Hada Gol Ga-bearing coal deposit of northeastern Ordos Basin. This coal seam is a low sulfidation type of bituminous coal with low coalification. Due to the special geological background and paleosedimentary environment,the bauxitic clay stone and the coal seam are extremely enriched in boehmite,Ga and REE. The average content of Ga is 44.8μg/g, and the ∑REE values vary from 110.82μg/g to 467.21 μg/g in seven coal segments, suggesting that Al, Ga and REE have similar geochemical behaviors and coexistence features. It is considered that Al, Ga and REE were derived from the same sources：① the erosion area of paleobasement, where there existed igneous rocks and metamorphic rocks with high K and Al；② clay stone and bauxite in Upper Carboniferous Benxi Formation overlying Middle Ordovician might have been an important provenance of Ga；③ clayey glutenite in coal measure strata is characterized by volcanic crystal fragments and volcanic ash,implying that they partly provided dissolvable Al and Ga materials； ④ Lower Permian Taiyuan Formation comprises such continental-marine interactive deposits as offshore coastal deposit, lagoonal deposit, delta deposit, barrier island deposit and lake-marsh sedimentary system. Such lithofacies paleogeography served as an advantage sedimentary environment of coal seams and bauxite strata for the accumulation of Ga. It is thus known that the origin of Ga in coal is characterized by multi-sources, inheritance, multiple periods and certain steps, suggesting that the accumulation process of Ga was very complicated.