Vol. 4] 



Knopf. — Foothill Copper Belt. 



423 



quartz porphyry has been breeciated, and more rarely, converted 

 to a sericitie quartz porphyry schist. The width of this shear 

 zone is about 30 feet, though within this belt there are horses 

 of unschistified rock. The lode strikes N. 75° W., dip 62° S. 

 The hanging wall is quartz porphyry and is fairly regular; the 

 foot is the meta-andesite, is soft and rotten, and more irregular. 



The ore is solid ehalcopyrite, massively schistose and often 

 roughly banded with ruby zinc. Calcite and some galena are 

 occasionally noted. The unschistified lenses of rock noted in the 

 croppings form horses of second-class ore, 6 inches to 2 feet in 

 diameter and several feet long. They are more poorly mineral- 

 ized, and contain a larger proportion of iron pyrites. Where 

 cubes of pyrite have developed in the vicinity of the calcite 

 amygdnles of the hornblende andesite, they fail to replace the 

 carbonate, but remain imperfect and have grown around it. 



RESUME. 



The Foothill copper belt lies west of the main gold-produc- 

 ing region, and includes a series of copper deposits occupying 

 the lower foothills of the range. Great stress cannot be laid on 

 the unity of this belt as a metallographic province, inasmuch as 

 two typical occurrences, notably Pine Hill and Valley View, were 

 long worked as gold mines, until deeper exploration revealed the 

 unsuspected presence of copper. Along this belt are also the 

 gold mines of Ophir, described by Lindgren.'* Copperopolis and 

 Napoleon are held to be on separate branches of the copper belt. 

 Between them is a great thickness of black clay slates, upon which 

 is located the Royal, a large gold-quartz mine (120 stamps). 



The copper deposits are, hoAvever, all closely associated with 

 the meta-andesites, and belong to the general type of replace- 

 ment deposits along shear zones. This gives a certain geologic 

 unity to the entire belt. The replacement has been equally thor- 

 ough in very diverse rocks, producing the large ore bodies of Cop- 

 peropolis in chlorite schists, and the extensive, and more massive, 

 ore bodies of Campo Seco in quartz porphyry schists. 



" ; W. Lindgren, 14th Ann. Eept., U. S. G. S. 



University of California, 

 May, 1906. 



