170 ALLEN: ORE DEPOSITION 



GEOCHEMISTRY. — Studies in ore deposition with special 

 reference to the sulphides of iron. E. T. Allen, Geophysical 

 Laboratory. Communicated by A. L. Day. 



General Principles. — The genesis of ores is essentially a chemi- 

 cal problem. It involves in each individual case the formation 

 of one or more minerals. Barring those rare instances in which 

 the geologist may be able to observe a mineral in the actual proc- 

 ess of formation, the conditions of formation have to be inferred 

 from the data obtained by field observation, supplemented by 

 what chemical information is available. At the present time our 

 chemical knowledge of minerals generally embraces something 

 of their behavior toward certain reagents, and often little, if any, 

 more. Accurate data on the genesis of most of them is still 

 wanting. This will have to be accumulated by actually forming 

 the minerals in the laboratory and studying the conditions in 

 systematic fashion. The questions of first importance in min- 

 eral synthesis concern, of course, the condition of temperature 

 and pressure and the composition of the systems in which the 

 minerals have formed. 



By laboratory study, certain limits to these conditions may be 

 set, outside of which the mineral can not exist, though in general 

 the limits will be wide. Thus if we learn that a certain mineral 

 can not exist above 450°, we know only that the mineral in nature 

 must have formed somewhere between this point and the lowest 

 temperature which prevails in the earth's crust. It is quite 

 probable that we shall be able in many cases to narrow down these 

 temperature limits by the study of paragenesis. Thus if we 

 could determine the temperature limits of two different min- 

 erals which, on account of their intergrowth or otherwise, could 

 be shown to have formed at the same time, it would be certain 

 in that particular instance that the minerals formed inside the 

 temperature range which is common to both. Thus far, we have 

 been able to set these temperature limits for comparatively few 

 minerals, chiefly those which are capable of existing in more 

 than one crystal form. Here the temperature is often the most 

 important condition determining which crystal form shall appear. 



