Vemdalens järnmalm — en eokambrisk spillra bland de autoktona fjällrandssedimenten

Abstract

Peculiar autochthonous iron-bearing beds at the eastern border of the Caledonides — in Vemdalen, County of Jämtland — have been known since more than a century but have received but little attention from the geologists who have carried on investigations in these regions. The present writer, having studied them on two occasions, has therefore found it appropriate to publish his observations, with some inferences drawn from them.

The sedimentary series, resting on a smooth and partly striated basement of Precambrian granite and truncated by an overthrust nappe of Eocambrian (Varegian) quartzite, consists of alternating arenaceous and argillaceous beds with an aggregate preserved thickness of some 3 m. The matrix of the sandy beds consists of crystalline hematite and; as the percentage of iron amounts to about 25 %, the rock may be called a lean iron ore. The clastic material contains abundant fresh feldspar together with the quartz. The type of sedimentation preferably suggests deposition in a coastal lagune, apparently prior to the Varegian and Cambrian transgression during which sediments of a normal marine type were formed. And since the striations on the surface of thè, subjacent granite must have been engraved during the Varegian glaciation, the series is younger than the latter. Owing to the absence of chemical weathering the iron cannot have originated from decomposed ferruginous rocks; neither could the sharply defined multiple ore-beds have resulted from such processes. The only acceptable hypothesis seems to be that the iron was delivered by intermittent sub-aquatic volcanic exhalations, although, admittedly, there are no traces of volcanic activity in this region at the time.

A remarkable deposit of a similar nature, at Hsüan-Lung, NW of Peking, also evidently owes its origin to volcanic exhalations.