;62 



AU rURE 



[August 8, 1907 



If Dickson's fncts be right, ihe Sudbury ores are neces- 

 sarily aqueous and not igneous in origin. 



(3) Scandinavian Iron Ores. — The other important 

 mining field of which the ores are claimed as of igneous 

 origin is Swedish Lapland. Its ores are rich and the ore 

 bodies colossal. One mine, Kirunavaara, yielded more 

 than one and a half million tons of ore in iqo6, and 

 according to a recent agreement with the .Swedish Govern- 

 ment the annual output of ore from that mine may be 

 raised to three million tons by 1913. 



The chief mining fields of Lapland, although situated 

 to the north of the .Arctic Circle, have long been known ; 

 for some of them contain veins of copper which were 

 worked, for example, at Svappavaara in the seventeenth 

 century. The iron ores, however, could not be used until 

 a railway had been laid through the swamps of Lapland 

 to carry the ores cheaply to the coast. In 1S62 an ill- 

 fated English company began a railway to the Gellivara 

 mines, and thirty years later this was completed across 

 Scandinavia, from the head of the Gulf of Bothnia at 

 Lulea to an ice-free port at Narvik, on the Norwegian 

 coast. 



This railway, the most northern in the world, passes 

 the two great mining fields of Gellivara and Kiruna. The 

 mining field of Kiruna is the larger and at present of the 

 greater geological interest, as its structure is simpler and 

 its rocks less altered. 



The ore body at Kiruna outcrops along the crest of a 

 ridge two miles long, and it is continued beneath Lake 

 Luossajarvi to the smaller but still immense ore body of 

 Luossavaara. At Kiruna the ore rises to the height of 

 816 feet above the surface of the lake, and it varies in 

 thickness from 30 to 500 feet, with an average thickness 

 of about 230 feet. According to the report by Prof. 

 Walfrid Petersson,' submitted this year to the Swedish 

 Parliament, Kirunavaara contains 200 million tons of ore 

 above lake-level, and Luossavaara another 22i million tons. 

 The ore is high-grade. According to Lundbohm 60 per 

 cent, of the trial pits showed a yield varying from 67 to 

 71 per cent, of iron, and 21 per cent, of them showed a 

 yield of from 60 to 67 per cent, of iron. The average of 

 nineteen analyses published in Prof. Petersson 's recent 

 report gives the contents of iron as 64-15 per cent. Unlike 

 the Taberg and Routivaara ores, the percentage of 

 titanium is very low ; thus in nineteen analyses given by 

 Petersson the average of titanic acid is only 023 per cent., 

 and it varies in the specimens from 0-04 to o-S per cent. 



The ore lies between two series of acid rocks, which 

 have been very differently interpreted, but will no doubt 

 be fully explained bv the researches now in progress under 

 the direction of Mr. Lundbohm. The rocks " were first 

 called halleflinta, as by Fredholm, and regarded as of 

 sedimentary origin. They are now accepted as an igneous 

 series, associated with some conglomerates, slates, and 

 quartzites. The ore body itself is bounded on both sides 

 by porphyrites, of which that on the lower or western 

 side is more basic than that overlying the ore to the east. 

 The basic western porphyrite is in contact with a soda- 

 augite syenite of which the relations are still uncertain. 

 Interbedded with the overlying eastern porphvrite are rocks 

 that appear to be volcanic tuffs, and both in the tuffs and 

 in the upper porphyrite are fragments of the Kiruna ore. 



Three main theories of the genesis of the Kiruna ores 

 have been proposed. Their sedimentary origin was urged 

 on the ground that they occur regularlv interstratlfied in a 

 series of altered sediments, and that the ores, therefore, 

 are also sedlinenlarv. This view may be promptly dis- 

 missed, since the adjacent rocks are igneous. 



The second theory has been advanced independentiv bv 

 Prof, de Launav and Dr. Helgc Backstrom : according 

 to them the pcrphvrites above and below the iron ores are 

 lava flows, and the ore was a superficial formation de- 

 posited in an interval between the volcanic eruptions. 

 .According to de Launay the iron was raised to the surface 

 as emanations of iron chloride and iron sulphide ; the 

 iron -was deposited as oxide, and most of it subsequentlv 

 reduced to magnetite during the mctamorphism of the 

 district. 



The third theory— that the ores are of d 

 1 Bihittig till RUd. Prol.. 1907, i 



ft Igneous 

 Afd, , i)4 Haft, Nc. 107, pp. 



NO. 



1 97 1, VOL. 76] 



origin — has been maintained by Lofstrand, lliigbom, and j 

 Stutzer ; according to them the ores are segregations of ■ 

 magnetite from the acid igneous rocks in which they 1 

 occur. The segregation theory has been opposed, amongst 

 others, by de Launay and V'ogt. Thus, de Launay inain- 

 tains that the segregation would have been impossible in 

 such fluid lavas as the Kiruna porphyrites, and is improb- 

 able, since there is no transition between the ore and the 

 barren rock. 



The segregation theory has serious difliculties, and is 

 faced by several obvious improbabilities. The ore occurs 

 as a band nearly forty times as long as it is broad. It 

 has the aspect, therefore, of a bed or a lode. The ore 

 has not the granular, crystalline structure of an igneous 

 rock like the hyperlte of Taberg, but the aspect of a 

 material deposited from solution or formed metasomatic- 

 ally. It is almost free from titanium, the undesirable 

 constituent so abundant in the ores of Taberg and 

 Routivaara. 



The igneous theory cannot, however, be lightly dis- 

 missed, as it is supported by the high authority of Prof. 

 Hogbom, and therefore demands careful consideration. 



It has been advanced in two main forms, the one 

 considering the ore to have been deposited at the time 

 when the igneous rocks are consolidating, the other con- 

 sidering it was deposited at a later period. .According to 

 Prof. Hogbom, the ore was syngenetic, being a true 

 magmatie segregation from a syenite. But, according to 

 Dr. Stutzer (100*1), the segregation was later than the 

 consolidation of the syenite. He describes the lode as an 

 intrusive banded dyke, of which the chief constituents 

 are magnetite and apatite; and the injection of this dyke 

 pneumatolytically affected the rocks beside it, producing 

 an intermediate zone, impregnated with ore, which he 

 compares to contact deposits.' 



In spite of the high authority of Prof. Hogbom, I am 

 bound to con-fess that the Kiruna ores do not impress me 

 as of igneous formation. Their bed-like form, microscopic 

 structure, and poverty In titanium are features in which 

 they difTer from those admittedly due to direct magmatie 

 .segregation. The microscopic sections that I have ex- 

 amined suggest that both the magnetite and apatite were 

 deposited from solution and later than the consolidation 

 of the underlylna porphyrite, which the ore in part re- 

 places. .An examination of the field evidence supports the. 

 conclusions of de Launay and Biickstrom as to the ore 

 being a bedded deposit overlying a lava flow, but enlarged 

 by ■secondary deposition. 



V. Futitre Suf^f^Jy of Iron Ores. 

 This conclusion is perhaps economically disappointing. 

 The possible existence of such vast segregations of Iron 

 In the acid Igneous rocks has an important economic 

 bearing. There is only too good reason to fear that the 

 chief iron ores are comparatively limited in depth ; for 

 most of them have been formed by water containing oxygen 

 and carbonic acid in solution, which has percolated down- 

 ward from the surface. Ores thus formed are therefore 

 restricted to the comparatively limited depths to which 

 water can carry down these gases. On the theory, how- 

 ever, that these ores are primary segregations from deep- 

 seated igneous rocks there need be no limit to their depth. 

 Thev would rather tend to increase in size downward, 

 while maintaining, or even improving, in the richness of 

 their metallic contents. For these bodies may be regarded 

 as fragments of the metallic barysphere which have broken 

 away from it and revolve around it like satellites float- 

 ing in the rocky crust. On this conception these ore bodies 

 would be of as great interest to the student of the earth's 

 structure, as their existence would be reassuring to the 

 Ironmaster, haunted as he is by constant predictions of an 

 iron famine at no distant date. It is no doubt true that 

 many of the richest, most accessible, most cheaply mined, 

 and most easily smelted iron ores have been exhausted. 



J In a later paper, nf which nnlv a short ahsfrnct hai been issued. T)-. 

 .Stutzer. however, explains ihat " the Intrusion of the ore dyke was at rela- 

 tively the same time as the formation of the syenite, and that the ores were 

 formed by maematic separa'lons /'« .Ki'fi/. or as nereerinatinff maematic 

 separations (maematic veins and be-^ded streams^" He adds that " pneu- 

 matolysts plays no inconsiderable rt^/r in the formation of these veins.'" Pr. 

 Ptutzer's position may be summarised as recnrdincr 'he ores as collected by 

 segregation, but deposited in the:r present position by erupti^-e after-actions. 



