Ch. 29] REFERENCES 535 



water content of the kaolinic matter in the shale, also were influential 

 in localizing ore deposition in this particular situation. 



Pyritic Replacements 



A large proportion of the pyritic deposits, such as the fahlband type, 

 occur in rocks that have been moderately metamorphosed and recrys- 

 tallized and are of siliceous-aluminous rather than carbonate facies. 

 It is my impression (on the basis of no very extensive knowledge) that 

 the fine-grained members of a series often seem more favorable than 

 the coarse-grained types. Whether, if true, this is due to grain size, 

 bed composition, or structural incompetence and amenability to shear- 

 ing is not definitely known and, so far as I am aware, has not been in- 

 vestigated seriously. The apparent preference for siliceous-aluminous 

 rocks, in contrast to the pronounced tendency of lead-zinc ores to favor 

 limestone, likewise is unexplained. A diversion of some of the research 

 activity centered on limestone replacement into this field or, preferably, 

 a comparable expansion of interest into this phase of mineral deposi- 

 tion seems quite desirable. 



CONCLUSION 



In conclusion, it may be remarked that this discussion has been 

 limited, by the premises, to replacement problems as related to the 

 original or acquired characteristics of sedimentary hosts. Most of 

 what has been said would apply equally well to deposits in igneous 

 rocks of layered character, such as tuffs or lava flows. The zinc- 

 copper deposits of Buchans, Newfoundland, the pyritic copper-gold- 

 zinc deposits in the layered lavas, etc., of the Noranda district, Quebec, 

 and the copper deposits of the amygdaloid lavas of Michigan are good 

 examples. For all these bedded epigenetic deposits, igneous and sedi- 

 mentary, many useful empirical facts have been accumulated; some 

 general principles of wide application are apparent; but acceptable ex- 

 planations of many important features and principles of replacement 

 are still to be formulated. 



REFERENCES 



Bain, G. W. (1936). Mechanics of metasomatism: Econ. GeoL, vol. 31, pp. 505- 



526. 

 Bateman, Alan M. (1942). Economic mineral deposits: John Wiley & Sons, New 



York, pp. 217-243 (Residual processes). 



