August 8, 1907J 



NA TURE 



361 



Ore deposits present so many perplexing features that 

 deep-sealed igneous agencies were naturally invoked to 

 explain them, and some of the most thorough-going 

 champions of the igneous origin of ores make claims that 

 remind us of the eighteenth-century Plutonists. The 

 question is to some extent a matter of terms. Many of 

 the ores which \ ogt, for example, describes as of igneous 

 origin he attributes, not to the direct consolidation of 

 material from a molten state, but to eruptive after-actions 

 due to the hot solutions and heated gases given off from 

 cooling igneous rocks. Igneous rocks probably play a 

 notable part in the genesis of most primary ore deposits ; 

 for the entrance of the hot ore-bearing solutions is 

 rendered possible by the heat of the igneous intrusions, 

 as Prof. Kemp has well shown in his paper on " The 

 R6le of Igneous Rocks in the Formation of Metallic 

 Veins." Prof. Kemp limits the term "igneous" to 

 materials formed by the direct consolidation of molten 

 material ; and this decision seems to me to be most 

 convenient. For example, the quartzite that is so often 

 found beneath a bed of basalt is due to hot alkaline water 

 from the lava cementing the loose grains of sand ; the 

 process is an eruptive afier-action, but it would be unusual 

 to call such a quartzite an igneous rock. 



(i) Igneous Ores. — That there are ores which are the 

 products of direct igneous origin is now almost universally 

 admitted. The mineral magnetite is a most valuable 

 source of iron, and it is a constituent of most basic 

 igneous rocks. If iron were a high-priced metal, such as 

 tin or copper, of which ores containing one or three per 

 cent, are profitably worked, then basalt would be an ore 

 of igneous origin. L nder present commercial conditions, 

 however, basalt cannot be regarded as an iron ore. But 

 if the magnetite in a basic rock had been segregated into 

 clots or masses large enough and pure enough to pay 

 for mining, then they would be iron ores formed by 

 igneous action. There are cases of such segregations large 

 enough to be mined. The most famous is Taberg, a 

 mountain in SmSland, near the southern end of Lake 

 Wetter, in Sweden. It is a locality of historic interest; 

 a view of it, as a mountain of iron, was published by 

 Peter Ascanius ' in the Philosophical Transactions in 1755, 

 and Sefstrbm discovered the element vanadium in its ore 

 in 1830. 



Taberg consists of an intrusive mass of rock composed 

 of magnetite, olivine, iabradorite, and pyroxene. Many 

 theories of its formation have been advanced. The view 

 generally adopted is that of Tornebohm, who described 

 the rock as a variety of hyperite in which there has been 

 a central segregation of magnetite to such an extent that 

 some of it contains 31 per cent, of iron. Tornebohm 

 claims to have traced a gradual passage from normal 

 hyperite to a variety poor in felspar, then to one without 

 felspar, and finally to a granular intergrowth of magne- 

 tite and olivine. This Taberg ore was mined and smelted 

 for iron in the eighteenth century, when transport was 

 more costly and commercial competition less keen than it 

 is to-day. The ore has been worked at intervals as late 

 as 1870 ; and as the hill is estimated to contain too million 

 tons of ore above the level of the adjacent railway, it is 

 not surprising that efforts are being again made to utilise 

 the deposit, in spite of its low grade and high percentage 

 of titanium. The Taberg rock has almost reached the 

 line which divides magnetite-bearing rocks from useful 

 iron ores. Its igneous origin, however, has not been 

 universally accepted. The theory has been rejected bv so 

 eminent an authority as Posepny. according to whom the 

 ore occurs in solid veins as well as in grains ; and he 

 holds that, like other Scandinavian iron ores, it was due 

 to secondary deposition. During a visit to the mountain, 

 I failed to see any secondary veins, except of insignificant 

 value. The microscopic sections of the ore show that it 

 is a granular aggregate of olivine, generallv with Iabra- 

 dorite and pyroxene. Hence I have no hesitation in 

 accepting the view of the Swedish geologists and regard 

 Taberg as a magmatic segregation. Posepnv - has in this 

 case carried his Neptunist thegry of the genesis of ores 

 too far. 



^ vol. xlix. pp. 3'v-34, p'. ii. 



2 F. Posepny. "The Genesis of Ore Deposits," Tians. Amer, Inst. Min. 

 Eng., 1893, p. 323. 



NO. 197 1, VOL. 76] 



At Routivaara, in Swedish Lapland, there is a still 

 larger mass of magnetite, which is claimed, in accordance 

 with the descriptions of Petersson and Sjogren, to be due 

 to segregation from the magma of the surrounding gabbro. 

 This mass of magnetite is of colossal size, but it is of no 

 present economic value, owing to its high percentage of 

 titanium and its remote position. 



An igneous origin is claimed by Prof. Hogbom for some 

 small masses of titaniferous magnetite in the island of 

 Alno, opposite Sundsvall, on the eastern coast of Sweden. 

 This case is of interest, as the surrounding rock is not 

 basic : it is a nepheline syenite, containing only 2 per 

 cent, of magnetite, which, however, has been concentrated 

 in places, until some specimens (according to an analysis 

 quoted by Prof. Hogbom) contain as much as 64 per cent, 

 of magnetite, 9 per cent, of ferrous oxide, and 12 per cent, 

 of titanic oxide. 



The Alno magnetites, again, are of no practical value, 

 as they are too low in grade and too refractory in nature. 

 I understand that about 500 tons of the material have been 

 smelted, but with unprofitable results, and the rest of the 

 material quarried has been left on the shore. We may 

 therefore accept the iron-bearing masses of Alno and 

 Routivaara, as well as that at Taberg, as due to mag- 

 matic segregation, without having conceded much as to 

 the igneous formation of ores. The process in this case 

 has formed rocks, rich in titaniferous magnetite, from 

 which iron could be obtained, but rocks which no iron- 

 master is at present willing to buy as iron ore. Whether 

 a basic igneous rock is to be regarded as an iron ore, or 

 as only useful for road metal, depends on cost of treat- 

 ment. The definition of the term " ore " is very elastic. 

 Petrographers speak of the minute grains of magnetite 

 or chromite in a rock as its ores ; but that is a special 

 use of the term " ore." Usually ore means a material 

 which can be profitably worked as a source of metals 

 under existing or practicable industrial conditions.' 

 .According to this definition, the Swedish deposits of 

 titaniferous magnetite are at present doubtfully within the 

 category of iron ores. 



The famous iron mines of Middle Sweden at Danne- 

 morra, Norrberg, Griingesberg, and Persberg occur under 

 different geological conditions ; they work lenticles or bands 

 of ores in metamorphic rocks, of which some are altered 

 sediments ; and the view has therefore been held by 

 de Launay and \'ogt that the ores also are altered 

 sediments. 



That ores are formed by igneous segregation of sufficient 

 size and purity to be of economic importance is a theory 

 which rests on two chief cases — the nickel ores of Sudbury 

 in Canada and the iron ores of Swedish Lapland. 



(2) The Sudbury Nickel Ores. — The nickel ores of Sud- 

 bury are the most important historically. They have been 

 repeatedly claimed as of direct igneous origin by Bell 

 (1S91), von Foullon (1892), \'ogt (1893), Barlow (1903), and 

 bv other geologists ; and his view was advocated before 

 the .Association at the Johannesburg meeting by Prof. 

 Coleman. The theory was stoutly opposed by Posepny in 

 1893, and Prof. Beck in 1901 described some of the 

 brecciated ore, and showed that its metallic minerals are 

 sharply separated from the barren rock. He held that 

 such ore must have been formed, not only after the con- 

 solidation of the rock, but even after or during its sub- 

 sequent metamorphisni. The views of Posepny and Beck 

 seem to have been established by additional microscopic 

 study of the ores by C. VV. Dickson (1903). He has 

 shown that the sulphides are separated from the barren 

 rock by sharp boundaries, and without any indication of 

 a passage between them ; that the fragments of ore in 

 the rock have short corners, whereas, had thev grown 

 in a molten magma, the angles would have been rounded, 

 and the faces corroded. Most of the ore, moreover, occurs 

 as a cement filling interspaces between broken fragments 

 of barren rock and along planes of shearing. The Sud- 

 bury ores, therefore, appear to have been deposited from 

 solution during or after the brecciation of the rocks in 

 which they occur, and long after their first consolidation. 



1 The Oxford Dictionar>' adopts a still more restricted definition : accord- 

 ing to it an ore is " a native mineral containing a precious or useful metal 

 in such quantity and in such chemical combination as to make its extraction 

 profitable." 



