DIABASE OF THE COBALT DISTRICT, ONTARIO 



REGINALD E. HORE 



Here referred to is that portion of the District of Nipissing from 

 Lake Temagami north to the Hudson Bay watershed, an area about 

 eighty miles square. A general description of the region has recently^ 

 been given by the writer, and some of the geological features are out- 

 lined in the following table. 



In this area are many recently discovered deposits of native silver, 

 the most important being at Cobalt. All the deposits are in pre- 

 Cambrian rocks, most of the valuable ones in Huronian conglomerate, 

 and a few in Keewatin greenstones and Keweenawan diabase. In 

 all cases the deposits are closely associated with masses of diabase, 

 and are probably genetically connected with them. 



In the following description of the diabase, mention is also made 

 of minor intrusions and fissure-fillings, it being suggested that all or 

 part may have originated in the diabase magma and are an expression 

 of the phenomena which occurred on cooling. 



The diabase masses. — In almost every township there are outcrops 

 of quartz-diabase. Many of the exposed masses are very irregular 

 in outline, while others show decidedly elongated forms with the 

 longer dimension generally north and south. In several instances 

 the diabase conforms to, and apparently has had its shape determined 

 by, the bedding planes in intruded shales; but some similar masses 

 show in places more stocklike characters, intruding the shales at 

 high angles and forming schistose and slaty contact zones. In rocks 

 other than shales the diabase shows less pronounced sheetlike forms, 

 and appears rather as small stocks and dikes. 



General character of the diabase. — The greater part of the diabase 

 masses is of gray to dark-gray color, of medium grain, and with 

 ophitic texture. The specific gravity is about 3.00. The rock is 

 composed chiefly of gray or greenish sodi-calcic feldspar and dull- 



I R. E. Hore, "Silver Fields of Nipissing," Toronto meeting, Canadian Mining 

 Institute, 1910. 



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