Section IV, 1922 [169] Trans. R.S.C. 



Secondary Processes in Some Pre-Cambrian Orebodies. 



By R. C. Wallace, M.A., Ph.D., D.Sc, F.G.S., F.R.S.C. 



(Read May Meeting, 1922) 



The accumulated results of investigations in the Pre-Cambrian 

 areas of Central Canada have demonstrated that glacial erosion in 

 Pleistocene times has been fairly complete, and that, where any 

 considerable secondary changes are found in exposed Pre-Cambrian 

 rocks or orebodies, conclusive evidence must be adduced to support 

 any theory of pre-glacial weathering. It will be recalled that Van 

 Hise suggested that the limited occurrences of haematite orebodies 

 in Canada, in contrast to the extensive deposits in Minnesota and 

 Wisconsin, might be explained by the greater ice erosion in Canadian 

 territory.^ Secondary changes in surface exposures are in general 

 of small importance in Canadian Pre-Cambrian territory southeast of 

 Hudson's Bay with which the writer is familiar. Where weathering 

 is pronounced, therefore, it is of some interest to investigate the 

 factors which have led to the rapid changes, both in oxidation and 

 reduction, which have taken place since glacial times — ^if, indeed, the 

 changes are all to be relegated to the post-glacial period. 



Sulphide Bodies in Northwestern Manitoba 



The mineralogy and mode of origin of the sulphide deposits in 

 the Athapapuskow district of Northern Manitoba have been some- 

 what fully discussed elsewhere,^ and for the purposes of this paper 

 it is necessary to make only the briefest reference to the primary 

 mineralization. In the Mandy and Flin-Flon orebodies, which have 

 been formed by replacement processes in sheared and faulted zones, 

 pyrite, chalcopyrite, sphalerite, and galena are the important primary 

 minerals, and the order of deposition was, in general, the order given 

 in the above list. In several of the other sulphide occurrences in the 

 same district, pyrrhotite is found associated with pyrite and chalco- 

 pyrite, more rarely with sphalerite or galena. The sulphides are 



lU.S. Geol. Surv. XII, Ann. Rep., Pt. Ill, p. 411. 



^Bruce: Amisk Athapapuskow Lake District. Memoir 105, G.S.C. 

 Econ. Geol. 15, 1920, p. 386. 



Hanson: Econ. Geol. 15, 1920, p. 574. 



Wallace: Mining and Mineral Prospects in Northern Manitoba. Northern 

 Manitoba Commission Bulletin. Bull. Can. Min. Inst., Feb., 1921, p. 106. 



