MANGANESE IN GOLD DEPOSITS 37 



in depth, though highly suggestive, and especially so when gold 

 and silver are shown separately, are supplemented by studies of the 

 paragenesis and by physiographic studies, in order that the approxi- 

 mate rate of erosion of the lode at various places may be known. 

 In the absence of such knowledge, it is generally impossible to 

 tell the genesis of a particular sample of ore from a mine. When all 

 the data are assembled, however, greater confidence may be placed 

 in the conclusion, since all the factors in the problem are intimately 

 related. 



4. Criteria for the recognition of secondary enrichment. — I shall 

 not attempt to review all the criteria for the recognition of second- 

 ary enrichment. They involve practically all available data relat- 

 ing to the geology and physiography of a region, as well as the 

 observed characteristics of its ore deposits. But each group of 

 deposits may be studied with certain general criteria in view. 

 Among these are: (1) the vertical distribution of the richer por- 

 tions of the lode with respect to the present surface and to the 

 level of ground-water; (2) the mineralogy of the richer and poorer 

 portions of the deposit, and the character and vertical distribution 

 of the component minerals; (3) the paragenesis, or the structural 

 relations shown by the earlier ore and that which has been intro- 

 duced subsequently. 



In applying these principles, it should be remembered that 

 circulation is generally controlled by post-mineral fracturing; 

 that the changes depend upon climate and rapidity of erosion, and 

 are affected by regional changes of climate, etc. Although the 

 mineralogy of the ore is a useful aid, there are many minerals 

 which are precipitated from cold solution and also from ascend- 

 ing hot solutions, and there are many others, the genesis of which 

 is uncertain. Of the minerals formed in the zone of secondary 

 sulphide enrichment, few, if any, are known positively to form 

 under such conditions only. There are some, however, such as 

 chalcocite and covellite, which nearly everywhere are clearly of 

 secondary origin. Ruby silver is frequently, but not always, 

 secondary. Other minerals, such as chalcopyrite, bornite, argen- 

 tite, etc., have no definite indicative value unless their occurrence 

 suggests that they are later than the primary ore. Where minerals, 



