[dresser] serpentine SERIES OF QUEBEC. 11 



Origin of Serpentine and Asbestos (Chrysotile) 



The derivation of serpentine and asbestos from peridotite may be 

 regarded as one process, differing only in the degree of alteration pro- 

 duced. It has been recently and thoroughly discussed by Graham,^ 

 who shows, in the case of the Black Lake-Thetford area, that the 

 hydration has been effected by hydro-thermal silicious waters. The 

 source of such waters has been a subject of some discrepancy of opinion 

 amongst writers on the subject in the past, a difference which may 

 now disappear. 



It has long been recognized that granite is usually found in, or 

 near, the richer asbestos ground of Thetford, Black Lake and Danville. 

 Accordingly it has been claimed, on one hand, that the asbestos is due 

 to the introduction into the peridotite of acid water by intrusions of 

 granite. On the other hand the richness of the asbestos ground, if 

 dependent on the presence of granite, does not appear to be propor- 

 tionate to the amount of granite present, and the total amount of 

 granite is relatively very small. Consequently, others have looked 

 for the main cause in magmatic waters accompanying the intrusion of 

 the peridotite itself. Granting the differentiation in situ of granite 

 and peridotite from the same magma, these differences no longer have 

 reason to exist. Granite thus becomes an indication, rather than a 

 cause, of the presence of acid waters in the magmatic residue needed 

 to produce serpentine and asbestos. 



Place in the Series: As the least dense and, theoretically, the 

 latest member of the series to solidify, granite might be expected to 

 occupy a place in the upper portion of the series amongst the other 

 acid differentiates. But it does not occur in this position. Instead, it is 

 essentially an accompanying feature of the peridotite, which forms 

 the lowest part of the series, and, moreover, is distributed in it without 

 any order that has yet been discerned. The position of the granite is, 

 therefore, a somewhat anomalous one, if it is regarded as a direct 

 differentiate from the original magma. Considered as a derivative 

 from an ultra-basic phase of the magma, which cooled as peridotite, 

 however, the granite occurs in the position and within the limits to 

 be expected of it. 



This implies some degree of differentiation within the magma 

 after intrusion but while it was still in a fluid state, an assumption 

 that seems to find support in the prevailing and very constant 

 arrangement of the principal rocks of the series in stratiform order in 

 sills, and in annular order in stocks. Also there is variation within 



Ï Economic Geology, Vol. XII, No. 2, 1917. 



