ORIGINAL ARTICLES. 



Differentiation and Ore-deposits. 



By R. H. Rastall, M.A., F.G.S., University Lecturer in Economic 

 Geology, Cambridge. 



A T the present time most mining geologists appear to be in general 

 -^ agreement on the principle tliat primary ore-deposits are derived 

 from igneous magmas. Furthermore, it is now almost universally 

 recognized that the segregation and concentration of the metals 

 and their compounds into payable ore-deposits is but part of a 

 larger and more fundamental problem, namely, the differentiation 

 of the igneous rocks, a problem which has exercised the ingenuity 

 of petrologists for many years j^ast and is still by no means solved. 

 Unfortunately most of the speculations and theories put forward in 

 explanation of these phenomena are in the main founded on facts 

 and inferences derived from the study of the silicates ; comparatively 

 few petrologists have taken into account the behaviour of the oxidic 

 and sulphidic compounds of the useful metals, which are commonly 

 regarded in the light of rare and accidental constituents of the 

 magma, rather than as having any particular bearing on the point 

 at issue. The most notable exception to this general statement is 

 Vogt,^ whose work on slags and ore-deposits is of an epoch-making 

 character from the theoretical point of view, as well as of immense 

 practical importance in smelting and blast-furnace practice. How- 

 ever, within the last few years great attention has been devoted by 

 mining geologists to the subject of the genesis of ore-deposits, 

 mainly because of its bearing on the question of persistence in depth 

 or the replacement at lower levels of one ore by another, such as is 

 known to occur in certain cases. It may be suggested, therefore, that 

 the time has now come when it may be possible to attempt to com- 

 bine the facts hitherto discovered along different lines of research 

 into a coherent whole. 



We have stated that most mining geologists are in agreement as 

 to the derivation of primary ores from magmas, but there is still 

 a good deal of difference of opinion as to the nature of the processes 

 by which this derivation or segregation is brought about. The most 

 unfortunate feature of the whole problem is that investigation by 

 experiment is almost, if not quite, impossible ; it is unlikely that 

 we shall ever be able to realize in our laboratories the temperatures 

 and pressures that must prevail at even small dej)ths in the earth's 

 crust, small, that is, by comparison with the radius, though large 

 when expressed in feet or miles. It is necessary, therefore, to fall 

 back on indirect evidence and arguments from analogy, and it must 

 be admitted frankly that extrapolation is always a dangerous 

 process. 



^ Studier over Slagger, Stockholm, 1884 ; Die Silikatschvielzlosungen, 

 Kristiania, 1903-4. 



