[TYRRELL] PRE-CAMBRIAN GOLDFIELDS OF CENTRAL CANADA 103 



"On examining a thin section of this latter rock, it was found to be 

 a porphyry, whose phenocrysts instead of being quartz are plagioclase 

 feldspar in distinct individual crystals and twin crystals which exhibit 

 beautiful zonal extinction. From the extinction angle of this plagio- 

 clase, it appears to be from the albite end of the series and probably 

 indicates that the first part to separate out was andesine or oligoclase, 

 while the latter part is albite. The groundmass is extremely fine- 

 grained and is made up principally of feldspar in which are a small 

 number of inclusions of chlorite, and a few larger crystals of hornblende 

 altering to chlorite." 



At the Bully Boy Mine. "The vein accompanies a porphyrite 

 cutting altered trap." 



"The light-coloured rock accompanying the vein shows on the 

 freshly broken surface phenocrysts which have a conchoidal fracture 

 and in the field would be taken for quartz ; consequently it was assumed 

 that the rock was quartz-porphyry. On examining a thin section of 

 this rock under the microscope, it was found that instead of being a 

 quartz-porphyry, the rock is a porphyrite having approximately the 

 composition of andésite. The phenocrysts proved to be plagioclase, 

 very similar in composition to that found at the Combined mine. 

 Apparently the feldspar is albite or oligoclase." 



At the Regina Mine the vein cuts the contact of the Keewatin 

 greenstones and a later granite. 



Manitou District. A little farther east, in the Manitou district, 

 sedimentary rocks, of later age than the Keewatin, begin to make their 

 appearance, and at the Victory Mine the country rock is a sedimentary 

 quartzite, associated with a fine-grained light green calc-phyllite, with 

 occasional bands of magnetite. Gold is present in narrow veins of 

 quartz of later age than the quartzite. 



The locality is a short distance west of a band of diabase-porphy- 

 rite or altered quartz-porphyry, but whether these are older or newer 

 than the quartzites was not determined. 



Farther east, at the Foley Mine on Shoal Lake, the gold-bearing 

 quartz veins occur in a granite of post-Keewatin age close to an intru- 

 sion of lamprophyre, which is stated by Dr. A. C. Lawson to be slightly 

 younger than the conglomerate of his Seine River series here included 

 in the Temiskamian, and therefore the lamprophyre would be approxi- 

 mately of Algoman age. No direct connection has been traced be- 

 tween the quartz veins and the lamprophyre, but the absence of such 

 veins around the border of the granite batholith where lamprophyre 

 is also absent would indicate the probable connection between the 

 two. It would seem likely, therefore, that the veins at the Foley Mine 

 are also of Algoman age. 



