Intensified and apace bauxitization over the paleo-karstic surface linked to volcanism

Abstract

Recent studies indicate that volcanism may be a potentially important factor in the formation of karstic bauxite, but more evidence is needed. Six billion tons of bauxite formed upon paleo-karstic terrain within a short time in the Late Carboniferous in the North China Basin (NCB) and in the Late Permian in the Youjiang Basin (YB) of China. The factors that trigger their apace formation remain unclear. Herein, we proposed that extensive volcanic eruptions have accounted for this large-scale bauxite formation based on the mineralogy, Hg isotopes, and Hg concentration enrichment proxies of the samples of bauxitic sequence in the NCB and YB. NCB bauxite generally has three layers, namely bottom Fe-bearing claystone, intermediate bauxite ore, and top claystone, while bottom Fe-bearing claystone is usually absent in YB bauxite, which directly covers carbonate rocks. The mineral assemblages of NCB and YB bauxite confirm that they were deposited in a superficial alkaline and reducing karstic environment. Strong Hg enrichment peaks with corresponding near-zero ∆199Hg, a signal of excess volcanogenic Hg, were discovered in the upper claystone of NCB bauxitic sequences, which overlaid the Ordovician carbonate substrate. It denotes volcanism occurred immediately subsequent to deposition of terrestrial weathered Al-rich remnants in paleo-karstic depressions during the Late Carboniferous. This volcanism, occurring in the northern margin of the NCB, is considered to have triggered the apace bauxitization in the NCB after its long exposure and weathering. In the YB, the remarkable Hg enrichment and near-zero ∆199Hg were observed in the entire Late Permian bauxitic sequences with Late Permian carbonate as the substrate. It denotes enhanced volcanogenic Hg inputs throughout the whole deposition process of bauxite during the Late Permian. This episode of volcanism associated with the Emeishan large igneous province and contemporaneous arc system resulted in the short term weathering of source materials and the apace bauxitization in YB. Volcanism is suggested to aggravate surface acidic and oxidizing conditions, in which rapid decomposition of source materials occurred to release Al3+ to precipitate diaspore above the physiochemical barrier of underlying carbonate.