Peti'oUxjif tiiid <7<'(>lo<ji/ of (Jiweiisioirji D'uslrici . 127 



piiiiiarv ((uartz <j:iaiiis of the I'ouk, and are tliei'el'ore also primary. 



Conijiarisou of (Jueeiti^toini DIoiite iritlt the MortiiiKj Star Dior/tc, 

 U'liod'.-: I'oinf. — The eheiiiioal analyses of these rueks show elose 

 siniilai-ities. From the relative pereentage of the alkalies, we should 

 expect a greater j)i-opoi'tion of oi'thoelase in tlie Queenstown diorite 

 than in the Morning Star roek, and microscopieal investigation 

 •eonlirnis this I)elief. Tlie high percentage of water in both analyses 

 is expressed hy the relative abundance of chlorite after horn])lende. 

 The higher jiereentages of ferrous oxide and titanium dioxide in 

 tlie Moi-ning Star I'ock indicate relatively more hornblende and 

 ilmenite, than in the (^»ueenstown lock. 



Mineralogieally tlie rocks closely reseml)le one another. Especially 

 • characteristic is the occurrence of brown hornblende in each of 

 them. Zoned felspars are not usually piesent in the Morning Star 

 ■ dioi-ite. and epidote is uncommon. Micropegmatite, which is 

 •characteristic of the Wood's Point rock, is only present in i)etrma- 

 titic veins at Queen stown. 



C. — Alterotion of fhf Diorite. 



Three types of alteration are recognisable. 



I. A regional propylitic alteration during, or immediately after, 

 the consolidation of the ]-ock, at relatively high temperature and 

 pressure, resulting in the formation of chlorite, epidote, leucoxene, 

 -carbonate, pyrrhotite, and a little sericite. 



II. Hydrothermal alteration near the vein Avails at mucli lower 

 ■temperature and pressure, resulting in extensive cpriVitisation and 

 ■carbonation, and in the disappearance of the femic minerals, 

 hornblende, chlorite, and pyrrhotite^ and their replacement by 

 muscovite, carbonates, and pyrites. 



III. Surface decomposition by present-day vadose solutions. 

 Kaolinisatiou is i-eferable to the action of these solutions. 



I. On most mining fields where ore deposits occur in igneous 

 rocks, a regional propylitic alteration of the country rock can be 

 recognised in addition to the hydrothei^mal alteration near . the 

 veins, but differences of opinion exist as to the cause of this altera- 

 tion, Propylitisationi of the andesites at Tonopah, Nevada, has 

 been ascribed by Spurr2 to the vein-forming waters filtei-ing thi-ough 



1 Propylitisation is here used in the sense defined by Vogt, " Genesis of Ore deposits," 1901, and 

 liindgren, "Mineral Deposits," p. 446, 1913, and not in the restricted sense of other writers. 



2 Geolo<,'y of the Tonopah Mining District, Nevada, U.S.G.S., P.P., No. 42, 1905. 



