[TYRRELL] PRE-CAMBRIAN GOLDFIELDS OF CENTRAL CANADA 115 



varieties predominate. In those sections in which basic plagioclase 

 becomes abundant, the orthoclase disappears, so that the rock passes 

 out of the granite-rhyolite family into the diorite-andesite group of 

 rocks, and is, therefore, not properly quartz porphyry but quartz 

 porphyrite." 



"Throughout considerable areas the porphyry and greenstone 

 appear to be very much intermingled, so much so as to form a pseudo- 

 conglomerate, and it is possible that some of this rock may be simply 

 the interior, coarser portion of acidic volcanic flows. In a few places, 

 however, well-defined dykes of the porphyry intrude the greenstone, 

 and a similar relationship holds for the rhyolite and aplite." 



Abitibi. Forty miles north of Larder Lake gold is found in similar 

 quartz veins on the shores of Lake Abitibi. W. G. Miller^ gives the 

 following description of these veins. 



"The half-dozen deposits examined occur in rocks of Keewatin 

 age. These rocks here consist essentially of green schists, which are 

 cut by dykes of fine-grained granite or porphyry, varying in width 

 from a few inches to fifteen feet or more. They have been shattered, 

 narrow cracks running across them èharacteristically transversely 

 from wall to wall. These cracks are filled with quartz, and there are 

 also at times lenses and irregular masses of quartz replacing the dyke 

 material or enclosed between it and the wall rock. Fragments of the 

 dykes are frequently cemented by the quartz, forming a breccia." 



Harricanaw. In the Province of Quebec, eighty miles east of Lar- 

 der Lake, gold has been found in quartz veins with pyrite on the shore 

 of a beautiful body of water known as Kienawisik Lake, which dis- 

 charges its waters northward by the Llarricanaw river. Mr. Bancroft,^ 

 in describing the district, says: 



"The gold occurs in quartz veins, all of which contain much tour- 

 maline, very little calcite, pyrite, and a little chalcopyrite. One of 

 the veins also carries a little galena and zinc blende." 



In describing one particular vein, he says, "The marginal portions 

 of the vein are, in places, very nearly pure tourmaline. For a few 

 feet the enclosing granodiorite has been brecciated in small fragments, 

 which are now distributed through a matrix of quartz and tourmaline, 

 the latter mineral predominating." 



The rocks underlying the country and associated with the gold 

 veins are, amygdaloidal basalts and greenstones of Keewatin age, 



1 Willet G. Miller. "Lake Abitibi Gold Deposits." 16th Rep. Ont. Bur. Min. 

 1907, pp. 219-220. 



^ ]. A. Bancroft. "Report on the Geology and Natural Resources of an Area 

 embracing the Head Waters of the Harricanaw River." Report of Min. Operations, 

 Quebec, 1912, pp. 199-236. 



