242 THE ROYAL SOCIETY OF CANADA 



sent the ore bodies associated with the basic intrusives that preceded the 

 intrusion of the Algoman granite and followed the deposition of the 

 Timiskamian sediments. There is proof that many important ore 

 deposits have been removed by erosion, and it seems not unlikely 

 that the rocks of certain epochs, not now productive, contained deposits 

 which have disappeared through the removal of vast thicknesses of 

 material. 



Since the authors have given elsewhere an explanation of the 

 nomenclature employed in the table, it is not necessary to deal in this 

 paper with the names applied to the various subdivisions of the pre- 

 Cambrian.^ 



Certain metals may occur in economic quantities in more epochs 

 than the table shows that they do, the authors not having been able 

 to determine definitely the age relations of some deposits. 



Age Classification of Ontario Ore Deposits. 



Keweenawan — Epoch, following basic intrusions, of (a) Silver, cobalt, nickel and 

 arsenic at Cobalt and elsewhere, (b) Nickel and copper at Sudbury, and copper 

 elsewhere. Certain gold deposits, not now productive, appear to belong to 

 this epoch. 



Animikean — Epoch of deposition of "iron formation" as a chemical precipitate. 



Algoman — Epoch, following granite intrusions, of gold at Porcupine and at many 

 other localities, and of auriferous mispickel. Deposits of galena, zinc blende, 

 fluorite and other minerals also appear to have been derived from the granites, 

 but some of them were not formed till post pre-Cambrian time. Preceding the 

 intrusion of the Algoman granites, basic intrusives, that appear to be of post- 

 Timiskamian age, gave rise to nickel and titaniferous and non-titaniferous 

 magnetite deposits and chromite. 



Timiskamian — Epoch of minor deposition of "iron formation" as a chemical pre- 

 cipitate. 



Laurentian — Granite intrusions probably gave rise to ore deposits which have been 

 removed by excessive erosion as is known to be the case with deposits of later 

 origin. 



Loganian : 



Grenville — Epoch of deposition of extensive "iron formation" as a chemical 



precipitate among other sediments. 

 Keewatin — Composed largely of basic volcanic rocks. 



Sequence of Intrusion and Metal Deposition. 



The table brings out an interesting alternation of intrusion and sedi- 

 mentation, and the importance of the igneous rocks in the formation 

 of ore deposits. It will be seen that there are broadly five great epochs 

 of igneous activity, basic and acid rocks alternating; viz., (1) Keewatin, 

 basic; (2) Laurentian, acidic; (3) pre- Algoman, basic; (4) Algoman, 

 acidic; (5) Keweenawan basic, passing in places into a considerable 



1 Ont. Bur. Mines, Vol. XXII, Part 2, pp. 123 et seq., and 

 Geol. Soc. Am., 1914, abstract. 



