Structural transposition is ubiquitous in Precambrian craton. Yet, stratigraphic division and structural study are still based on transposition foliation in many Precambian craton researches. The present work discusses the transposition features and their effects on banded iron formation in Banshigou iron deposit in north-eastern part of North China Craton (NCC). Banshigou iron deposit is a typical Precambrian banded iron formation which is composed mainly of magnetite quartzite, plagioclase amphibolites and granites, and considered to be part of an Archaean green stone belt (Qu et al, 1993). Rigorous structural analysis has proved that the Archaean rock group was subjected to intense plastic deformation (D1) during Late Archaean, which produced a set of penetrative S+F+L structures and replaced most strata of the mining area. S1 is axial planar structure to F1 folds. It was the most obvious planar structure trending E—W with nearly upright dip in the mining area and transposed the preexisting S0 to various degrees. F1 folds commonly exhibit tight to isoclinal form. Competent iron commonly became hook-shaped. L1 is the associated linear structure, such as minor fold axes, boudin lines and mullions. These all belong to B-lineation, showing maximun fabric pattern and representing the fold axis plunge 60° to 266°. In many cases, S1 is the surface form of F1 which usualy exhibit asymmetric "Z" type indicating a structural flow deformation mechanism (Милл EP, 1983) and the transposition pattern is much similar to the pattern proposed by Turner and Weiss (1963). Strain analysis shows very small k-values, indicating a dominated flatten deformation regime. Both X (movement direction) and Y (fold axis direction) have been elongate and X diretion’s elongation is larger than that along Y direction. Detailed large-scale mapping shows that contrast to the original juxtaposed lenses, the ore bodies in each of 19 ore groups are multiple fold hinges. There are only 2 or 3 layers of iron ore originally. They are tightly folded by F1, thickened in hinge areas and finally became hook-shaped folds which can be mechanically mined. Ore bodies in the limb areas between ore groups are usually broken down or attenuated. The deeper extension of the hook-shaped folds is mainly controlled by L1 generally plunging moderately to the west. The strong deformation not only broke the ore bodies in X diretion, it also attenuated or even broke the hook-shaped folds in Y direction leading to the formation of the hook-shaped folds in horizontal sections and boudinaged folds in vertical profiles. Two prospecting directions for each ore group have been proposed based on the structural controlling effects on ore bodies. The first one is enveloping line direction, expected to find the neighbouring hook-shaped folds broken off in X direction. The other is fold axis direction, expected to find the deeper boudinaged folds broken off in Y direction. Exploitation schemes for 1, 3 ore groups have been designed in this papers. Now the exploitation schemes have been conducted and millions of tons of iron ore have been found.
Qu Fenxiong,Zhang Baohua,Liu Ruqi. STRUCTURAL TRANSPOSITION AND ITS CONTROLLING EFFECT ON IRON DEPOSITS: AN EXAMPLE FROM BANSHIGOU IRON DEPOSIT, JILIN PROVINCE, CHINA[J]. Chinese Journal of Geology, 1997, (1): 103-109.