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sericitization, carbonatization, silicification, and pyritization; hydro- 

 thermal alterations are less extensive than near the vein deposits of 

 the Middle Tertiary age. At Sylvanite, orthoclase (not adularia) is found 

 in small veins more or less closely related to pegmatites, but which are 

 notwithstanding far from the normal pegmatite. 



The Santa Rita (Chino) and Burro Mountain deposits also were 

 probably formed in the first concentration at the time of the intrusion 

 of the early Tertiary porphyries. These disseminated copper ores are 

 greatly concentrated by oxidizing surface waters and resemble in many 

 respects the "copper porphyries" of Arizona, Utah, and Nevada. 



Replacement deposits in limestone, not contact metamorphic deposits, 

 form an important group which is likewise in close genetic relation to 

 the early Tertiary intrusions. At Lake Valley the eroded ore deposits 

 are covered by andesite. Strongly indicating deposition by ascending 

 magmatic solutions, these deposits have been found in eight of the dis- 

 tricts below beds of shale. In six other districts they are fissure veins. 

 Silver is generally the most important metal; lead is almost always 

 present; gold is absent, barite is rare. 



The gangue is siliceous, with one or more carbonates. Other 

 minerals are fluorite, wulfenite, vanadite, zinc blend, pyrite, chalcopy- 

 rite, argentite, cerargyrite, silver, limonite, and pyrolusite. No heavy 

 silicates are found in the limestone along these veins, but silica or jas- 

 peroid has been developed. 



Veins of gold and silver ores connected with volcanic rocks of Middle 

 Tertiary or later age are developed in ten mining districts. They are 

 contained in rhyolites, its tuffs and breccias, or in andesites, which have 

 latitic transitions. Some of these deposits are older than early Quater- 

 nary basalts. Base metals and sulphides are not prominent in these 

 veins, lead and zinc are rare, though copper is present in considerable 

 amounts in several districts. The gangue is quartz, which may be 

 accompanied by calcite, fluorite, and barite. Adularia is present in 

 two districts. Pyrite and chalcopyrite are common; bornite is 

 probably primary. Other minerals are zinc blend, galena, chalcocite, 

 telluride, tetrahedrite, cerargyrite, etc. Hydrothermal alteration is wide- 

 spread. These veins are believed to have been deposited by hot waters 

 very near the present surface at the time of deposition. Possibly the 

 waters, such as those which have been analyzed from hot springs at 

 Ojo Caliente, are solutions of the same genesis and character. The 

 discussion of the relation of the deposits to waters of this character is 

 an exceedingly suggestive and valuable section of the paper. 



