1916] Umpleby: Ore on the Limestone Side of Garnet Zones 31 



into the intrusive mass, but they are also most pertinent to the present 

 discussion. In figures 3 and 4 the occurrence of ore on the limestone 

 side of the lime-silicate zone is evident. In figure 5 the central lime- 

 stone has been changed to ore, but its former extent into the area now 

 ore is clearly shown by the relations at the top and bottom of the 

 figure. In figure 2 the ore occurs in the central part of a lime-silicate 

 mass which in the plane of the section is surrounded by the intrusive 

 rock. 



Dolores Mine, Mexico. — The Dolores Mine, near Matehuala, S. L. P., 

 Mexico, is situated in an area of Mesozoic limestone, overlain by shale, 

 which forms a great half-dome down faulted on the east. 12 Cutting 

 the sedimentary rocks in the central part of the field are two masses 

 of quartz monzonite. Both of the intrusives have caused noteworthy 

 metamorphism, but about the northern, or Cabriza body, contact rock 

 is especially abundant and widespread. The contacts of the intrusive 

 masses are very irregular and much of the lime-silicate rock has formed 

 at the expense of quartz monzonite, although perhaps most of it is 

 of limestone derivation. The metamorphic minerals include pyroxene, 

 garnet, vesuvianite, wollastonite, fluorite, quartz, and the metallic 

 sulphides, chalcopyrite, pyrite, arsenopyrite, pyrrhotite, blende and 

 galena. Cupriferous pyrite is the most important ore mineral. Careful 

 study has revealed a definite sequence in mineral formation as 

 follows: 13 1, aluminous pyroxenes; 2, aluminous garnet (grossularite) 

 and vesuvianite; 3, wollastonite; 4, lime-iron pyroxene (hedenbergite) ; 

 5, lime-iron garnet (andradite) ; 6, fluorite, quartz and metallic sul- 

 phides (and actinolite and hornblende) ; 7, metallic sulphides (and 

 quartz and fluorite) ; 8, calcite. As the caleite is quite certainly 

 residual from the limestone, it is clear that in the Dolores deposit the 

 metallic sulphides were among the last metasomatic minerals developed 

 and formed later than the lime-silicates. 



The paper above cited does not describe in detail the spaeial 

 relationship of the ore bodies to the limestone and monzonite, although 

 it is stated that they "occur largely at the very contact, frequently 

 with limestone as one wall." 14 In the paper by Prescott, 15 however, 

 sketches of the ore chimney at Dolores Mine, reproduced herein as 

 figures 6 to 8, are pertinent. Figures 7 and 8 are clearly illustrative 



12 Spurr, J. E., Garry, G. H., and Fenner, C. N., Study of a contact meta- 

 morphic ore deposit: The Dolores Mine, at Matehuala, S. L. P., Mexico, Econ. 

 Geol., vol. 7, pp. 444-484, 1912. 



13 Ibid., p. 444. 



14 Ibid., p. 446. 



i ■> Op. cit., p. 65. 



