1916] TJmpleby : Ore on the Limestone Side of Garnet Zones 27 



stone surrounded by granitic rocks comprising the main part of 

 Patagonia range. 1 The sedimentary beds dip and strike diversely in 

 different places, but the dip is prevailingly westward at angles of 

 30 to 90 degrees. The deposits are tabular replacements in limestone 

 both remote from and near to the igneous contact "and have not been 

 observed in any instance to extend ever so slightly into the surround- 

 ing granite." The common gangue is garnet, with which the metallic 

 minerals, chalcopyrite, sphalerite and to a minor extent galena, are 

 intimately associated. It is particularly noteworthy, however, that 

 "the ore-body accompanying a contact garnet zone is always on the 

 inside of the zone, that is, between the main body of the garnet ledge 

 and the limestone, and not between the garnet ledge and the granite." 2 



Crosby 3 states further that "obviously enough, the ores, as we 

 now have them, are due to a metasomatie impregnation and replace- 

 ment of the limestone against and in the massive garnet ledge. ' ' 



The deposits of Washington Camp, therefore, clearly exemplify 

 the formation of ore on the limestone side of garnet zones. It is per- 

 haps worthy of note that here there has been no endomorphism, a 

 feature in which these deposits are different from several of the others 

 which are described. 



Silverbell, Arizona. — In the Silverbell district is a series of lime- 

 stone blocks of Paleozoic age completely surrounded by alaskite, 

 alaskite porphyry, biotite granite, andesite and quartz porphyry, in- 

 truded in the order named. 4 The intrusion of the alaskite porphyry 

 and the biotite granite was followed by intense sericitization and 

 silicification of these rocks and by the development of great masses 

 of garnet, quartz and wollastonite in the adjacent limestone. "Follow- 

 ing close upon these solutions came metal-bearing magmatic waters, 

 which impregnated porphyry, granite and alaskite with cupriferous 

 pyrite, and deposited in the garnet zones chalcopyrite and copper- 

 bearing pyrite that make important bodies of contact metamorphic 

 ores." Veins containing lead-silver ores also occur. Endomorphism 

 is represented by a widespread silicification of the intrusions and a 

 local development of garnet and epidote of igneous-rock derivation. 

 The common section from the boundary of the intrusive rock to the 

 limestone is "three to ten feet thick, of solid garnet and quartz, with 



i Crosby, W. O., The Limestone-granite contact-deposits of Washington 

 Camp, Arizona, Trans. Am. Inst. Min. Engrs., vol. 36, pp. 626-646, 1906. 

 Crosby, W. O., op. cit., p. 632. 

 « Op. cit., p. 641. 



4 Stewart, C. A., The geology and ore-deposits of the Silverbell Mining 

 District, Arizona, Trans. Am. Inst. Min. Engrs., vol. 43, pp. 240-290, 1912. 



