Geologiska studier i Lövsvedsgruvan, Folkärna socken, Dalarna

Abstract

The mine investigated — Lövsvedsgruvan — is situated in the iron-ore formation of central Sweden. Figs 1 and 2 show the ore-body. The main field of investigation was the bottom 303 m level, which has been mapped. On this level there are several kinds of rocks: leptite, granitic leptite, iron-ore and skarn, all occurring as fragments in diorite. Granite and amphibolite are also present.

The leptite is mostly a soda-rich plagioclase-leptite. Some of the iron-ore occurs in fragments as a thickly banded quartz-magnetite-ore lying apart from the main skarn-ore body, which has no primary features. Skarn occurs partly as heavy masses of garnet and diopside, partly as characteristic lenticular “quills” of dark minerals in leptite (Fig. 7). The quills indicate a linear structure which is parallel to the fold-axes of the main ore-body. It seems evident that the ore was fixed in the chimney-formed body, at the same time as the quills were formed. There is a gradual transition of leptite into granite, indicated by a slight increase in the grain size, and the appearance of microline. The “leptites” with these characteristics show that they once have had a certain degree of mobility. It seems probable that the main iron-ore body received its iron content from a sedimentary quartz-banded iron-ore deposit of the type that is very frequent in Central Sweden. When the leptite-ore formation was folded, the ore collected in an axial fold, and at the same time the quartz-bands disappeared. Not far from Lövsveden there are similar iron-ore chimneys, where the ore is of undoubted sedimentary origin. The skarn-formation, and the mobilisation of certain leptites, must have occurred during the folding of the primarily sedimentary complex. Thus the main and existing features of the ore, skarn and leptite were already established before the diorite intruded.

The diorite brecciated and destroyed this complex formation. The various kinds of rocks reacted quite differently on coming into contact with the diorite. The felsic rocks became plastic (Fig. 16) and do not seem to have been assimilated to any great extent. The mafic rocks kept their rigidity but were more easily assimilated by the diorite. Thus the diorite near the small ore-bodies contains more iron than usual, but less magnesium. It is possible that the magnesium which was driven out from the diorite and evidently displaced by iron, can now be found deposited in the extremely magnesium-rich dyke-like zones transversing the leptite. These zones seem to be of the same age as the diorite.

Younger than the diorite is the granite proper, which occurs near the shaft on the bottom level. This granite contains xenoliths of diorite. The strongly foliated amphibolite dykes are younger still.