560 REPORT—1904. 
TUESDAY, AUGUST 23. 
The following Papers and Reports were read :— 
1. On the Origin of the Great Iron-ore Deposits of Lapland. 
By Dr. Heice BackstR6m. 
The great ore sheet of Kirunavara-Luossavara occurs between old lava streams 
and volcanic conglomerates, in other words between rocks formed by volcanic action 
at the surface. It seems to mark an interval between two separate eruptions or 
eruptive periods, as the rocks on both sides are different, although showing a 
distinct consanguinity. The ore bed is older than the overlying quartz 
porphyry, while at the same time it is younger than the underlying syenite 
porphyries. The underlying porphyries show characteristic evidence of a hydro- 
chemical or pneumatolytic action, which has left no similar traces on the overlying 
porphyries, and must therefore have occurred in the interval during which the ore 
was formed. 
From these facts we may presume that the magnetite-apatite sheet was formed 
by the voleanic activity which produced the overlying as well as the underlying 
rocks, and that the hydrochemical or pneumatolytic transformation of the under- 
lying porphyries was effected by the same agents which brought the iron and 
phosphorus up to the surface of the earth. 
The Ekstrémsberg ore-field, situated about twenty miles west of Kirunavara, 
ranks as the third of the great Lapland iron mountains; the probable quantity of 
ore has been estimated by the author at one hundred million tons. The ore is partly 
magnetite, partly hematite, with 63 per cent. Fe, 1°25 per cent. P, on the average. 
The surrounding rock is a quartz porphyry, but with potash instead of soda as 
the dominant alkali, the same variety occurring on both sides of the ore sheet. 
In the ore-field of Mertainen, situated eighteen miles south-east of Kirunavara, the 
ore is magnetite without apatite, but the quantities of rich ore present are compara- 
tively small, the principal part of the ore-field being occupied by ore breccias 
or mixtures of porphyritic and ore material. The original rock of the district is 
a syenite porphyry. In the ore-field this is penetrated by veins of magnetite with 
biotite, hornblende, and titanite. These minerals, and especially the magnetite, 
also occur, filling amygdules, This penetration of the original rock by magnetite 
in those places where it was most favourable for penetration, resulted in the 
formation of the ore breccias, in which the ore, so to say, has eaten away most of 
the porphyritic material. An intense transformation of the porphyry has gone 
on hand-in-hand with this infiltration of the ore material. Of the original pheno- 
erysts of the dark minerals not even pseudomorphs are found; the biotite and 
hornblende present are of secondary formation. The plagioclase is very often 
partly transformed into biotite or titanite, but still more often into scapolite, which 
latter transformation is of very great interest when the transformation of plagio- 
clase into scapolite along the apatite veins occurring in gabbros is remembered. 
That the iron ore of Mertainen, occurring to a great extent as veins and 
ore breccias and accompanied by such transformations of the surrounding rock, 
should be of pneumatolytic origin, seems very probable. But the author thinks 
this theory must be adopted also for the great masses of Kirunavara-Luossavara 
and Ekstrémsberg, closely connected as they are with voleanic rocks. Hence these 
iron-ore deposits have probably got their material from below through volcanic 
emanations belonging to the last phase of the volcanic activity (or to an interval 
in the activity, as in Kirunavara), emanations of iron, phosphorus, or titanium 
compounds, essentially chlorides and fluorides, in the form of gases or superheated 
solutions, which on reaching the surface regions were decomposed by the water 
and the silicates with which they came in contact. 
This theory may appear rash, especially when applied to such enormous masses 
as the Kirunavara-Luossavara magnetite-apatite sheet, but it seems to the author 
to be only an extension of the generally accepted theory of the origin of the con- 
tact deposits. In both cases the material has come with the eruptive rock from 
