ZINC ORES OF THE EDWARDS DISTRICT 3 1 



hydrous silicates, a material modification of the hypothesis might be 

 advanced. This would leave the sulphides as deep-seated and 

 moderately high-temperature minerals, as they are here regarded, 

 but would interpret the talc and serpentine as much later and rela- 

 tively superficial in origin, the product of cold superficial waters or 

 even of weathering. 



The question as to the conditions under which talc and serpentine 

 are formed is one whose thorough discussion would far transcend 

 the limits of this paper. On the one hand, it is argued that they 

 are products of weathering; on the other, that they are due to the 

 action of magmatic waters on magnesian silicates. As thus baldly 

 stated, both views are rather extreme, but the writer, as may be 

 gathered from the foregoing pages, is strongly in sympathy with 

 the latter. It seems highly probable that the formation of talc 

 and serpentine is a process that goes on below, rather than in, the 

 " belt of weathering " within which latter these two minerals, while 

 relatively resistant, are by no means end products, but tend to 

 break down into simpler compounds. There is no doubt that talc 

 and serpentine are formed below the belt of weathering by both 

 magmatic and meteoric waters. When once formed, the minerals 

 are stable and may be regarded as end products, so far as these 

 conditions are concerned. 



Many years ago the writer took the ground^ that the extensive 

 talc deposits of the Edwards district were formed by deep circu- 

 lation although, with less knowledge of the geology of the region 

 than we now have, appeal was not specifically made to magmatic 

 waters. With the addition of the latter factor, this hypothesis as 

 to the origin of the talc still holds good, and as the zinc ores occur 

 in the talc belt and under practically identical geological conditions, 

 it has direct bearing upon the present case. 



After what has already been said it is, perhaps, hardly necessary 

 to state that the hypothesis here ofi^ered for the genesis of the ores 

 regards the granite magmas as the source, not only of the heated 

 vapors and solutions that served as transporting agents for the ores, 

 but, also, of the actual materials of the ores themselves, both the 

 sulphur and the metals. During the cooling of the magmas, the 

 gases and solutions were given off, carrying the sulphur and metals 

 and depositing them in the limestone by replacement. The ores are 

 of magmatic derivation, marking a late stage in the history of the 



^ Smyth, C. H. jr, The Genesis of the Talc Deposits of St Lawrence 

 County, N. Y., School of Mines Quarterly, 1896, i7:333-4i- 



