Geology and geochemistry of the Paleoproterozoic borate deposits in Liaoning-Jilin, northeastern China : evidence of metaevaporites

Abstract

The borate deposits in Liaoning, Jilin province, China, are hosted in the Paleoproterozoic meta-volcanic-sedimentary sequence which comprises magnetite-microcline rocks, K- and Na-rich leptynites and leptites, magnesium marbles and amphibolites. The borates are exclusively hosted in the magnesium carbonates and Mg-silicate rocks. The association of these rocks, their sedimentary structures and unusual chemical composition suggest that their protolithes were sediments deposited in evaporite basins: red beds, zeolitized tuffs and evaporites. Therefore, the borate deposits are metamorphosed evaporites. The initial precipitates were magnesium carbonates and hydrous magnesium borates, reflecting a MgSO_4 brine stage in a playa lake environment. During the subsequent metamorphism, these borates were dehydrated to form anhydrous borates, e.g. suanite and ludwigite. These deposits are analogous to many modern evaporite sequences in rift systems. Mineralogically, these borates are comparable to the borates in the saline lakes in the Qinghai-Xizang (Tibet) plateau.