ZONES OF OXIDATION AND ENRICHMENT 181 



tion of masses of oxide carbonate sulphates, chloride or bromide ores, 

 which are enrichments whose origin is more clearly apparent and about 

 which there is no doubt. The latter deposits belong to the zone of oxida- 

 tion proper and are the commonly accepted products of superficial altera- 

 tions of ore deposits. Their mineral character is in sharp contrast to that 

 of the sulphides, sulpharsenites, and sulphantimonides which form the 

 subject of this paper. 



Zone of Oxidation 



The upper parts of ore deposits are generally more or less changed by 

 atmospheric agencies and rarely show the same mineralogic and phys- 

 ical features that prevail in the lower part of the deposit. If the original 

 deposit holds much pyrite the outcrop is often a mass of impure limonite, 

 the "iron hat" or "gossan' 1 of the mines. Lower down the altered, iron- 

 stained vein matter contains the carbonates, oxides, and other ores re- 

 sulting from oxidation. This uppermost part of the vein, which is often 

 the most remunerative to work, especially in gold veins, is here desig- 

 nated the zone of oxidation, as oxidation has been complete in it. This 

 zone practically corresponds to that of superficial alteration of most 

 writers, though, as will be shown, it is really only the upper part of the 

 portion traversed by downward seeping surface waters. 



Zone of Enrichment* 



Beneath the zone of oxidation and between it and the unaltered pri- 

 mary vein matter lies the zone of enrichment. Its upper limit is usually 

 very sharply defined, and may be told by contrast with the rusty color 

 of the overlying deposit, though there is no definite plane, but usually 

 an extremely irregular boundary. The lower limit of the zone of enrich- 

 ment is also very irregular, though in some cases it is sharply defined. 

 Generally, owing to a fracturing of the original vein matter, the second- 

 ary sulphides occur in cracks and crevices extending down into the pri- 

 mary ore, even as far as many hundred feet, especially along fault planes 

 which have become channels for descending waters. 



Zone of primary Sulphides 



The lowest part of the vein, below the permanent ground water level, 

 consists of the unaltered sulphides which compose the original ore of the 

 vein. This part constitutes the zone of primary sulphide ore. 



* This term has been already used by Kemp in discussing the chalcocite bodies of the Butte 

 (Montana) copper veins. See Ore Deposits of the United States, New York, 1890, p. 163. 



