36 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEX DISTRICT, CALIFORNIA 



FIOURE 26. PolUhed surface on breccia composed chiefly of fragments of altered tachyllte. Color zoning within 

 Individual fragments U typical of the altered mafic glass in the district ; dark horseshoe-shaped band is due 

 to weathering. 



riched with sodium, and that all of them in the New 

 Almaden area were so enriched. The analysis given 

 in column 1 is of the least altered diabnsic greenstone 

 found in the district. In thin section the rock can be 

 seen to contain completely fresh augite, plagioclase 

 that is somewhat clouded by alteration products but 

 still fresh enough to give good measurements of ex- 

 tinction angles, and a groundmass that was originally 

 glass but is now completely ehloritized. The plagio- 

 clase, which appears to be primary, is andesine (An 35 ). 

 If olivine was ever present, it was in small amount 

 and is now replaced by chlorite. A few thin veins of 

 quartz and chlorite cut the rock, and a few small 

 patches of calcite are also present. The analysis con- 

 tains such a moderate amount of sodium and such a 

 large amount of calcium that one would expect the 

 plagioclase to be inure calcic if the normal process of 

 i-rystalli/.atiim had been followed. 



The analysis given in column 2 is of the altered 

 tachylitic tuff shown in figure - J7. It consists chiefly 

 of chlorite derived from the original glass, but it also 



contains a few crystals of plagioclase and augite. The 

 plagioclase is albite (An 2 ), and although it is com- 

 pletely unaltered, some grains appear to have reerys- 

 tallized. The effect of metamorphism that has taken 

 place since the glass reacted with sea water cannot be 

 determined, and the present composition could, of 

 course, be very different from that of the chilled glass 

 originally deposited. If there has been no postilepo 

 sitional loss of sodium, the present low value for 

 sodium and the low sodium-calcium ratio are indeed 

 surprising. 



These two analyses from the New Almaden district 

 are inadequate to -how anything other than the prob- 

 lem involved in trying to learn from chemical anal\-.- 

 the original chemical character of the now altered vol- 

 canic rocks. These two rock* are not rich in sodium 

 in spite of the soda-rich character of their plagioclase, 

 and they are not chemically comparable to either nor- 

 mal diabase or spilite. The three oilier analyses of 

 greenstones of the Franciscan group given in columns 

 3-5, table *, show a higher sodium content, but in 



