LAUSR

Abstract Precambrian iron formations (IF) carry rich information on the interplay between the geosphere and the early biospheres. Here, we report mineralogical and petrologic characterizations of the Yunmengshan IF deposited at ~1.7 Ga, the transition between Paleoproterozoic and Mesoproterozoic eras. The IF contains granular hematite with evident oncoidal structures and lacks laminated structures, which are distinct to the typical banded iron formation (BIF) or granular iron formation (GIF) deposited early in the marine environments with high silica content. Based on observations of clastic sedimentary structures around the Yunmengshan IF, including parallel bedding, cross-bedding, asymmetric ripple mark, and mud crack, we deduce that the Yunmengshan IF was mainly deposited in a supratidal to intertidal zone. Electron microscopic observations show two types of hematite euhedral crystals: laminar hematite of 200–500 nm thick and 3-5 μm in diameter and granular hematite of 200–300 nm in diameter randomly distributed in the matrix. Stilpnomelane and minnesotaite, as Fe2+-silicates in the typical Archean- and the early Paleoproterozoic BIF, are identified but with extremely low abundance. The morphology and paragenetic association of these minerals imply that the Yunmengshan IF was probably deposited in an aquatic environment with low Si concentration. These results indicate that the Yunmengshan IF represents a transitional type of iron deposition between BIF/GIF and ironstone in the geological history of the iron cycle on the surface of Earth.