SILICA-CARBONATE ROCK 



61 



FIODRB 43. Photomicrograph of altered Berpentlne showing early stage 

 of development of silica-carbonate rock in which magnesite (dark) has 

 replaced the chrysotile veinlets and a little of the fibrous antigorlte 

 bordering them. 



PIOCEE 44. Photomicrograph of magneslte replacement of thick veins 

 of chrysotile ; note typical fibrous aspect of the carbonate thus 

 formed. 



is the replacement of the remaining serpentine by 

 quartz. The earliest quartz fills in between the car- 

 bonates as an aggregate of minute grains with rounded 

 outlines, but in a more advanced stage of alteration 

 the quartz recrystallizes into larger crystals showing 

 many straight sharp crystal faces. (See fig. 45.) 

 Where alteration has been very intense, as in some of 

 the ore bodies, quartz locally replaces the carbonate, 

 and both late quartz and dolomite fill small fractures 

 in the silica-carbonate rock. The magnetite of the 

 serpentine disappears at an early stage, the iron doubt- 

 less being incorporated into the ferroan magnesite; 



but chromite or picotite generally remains unaltered. 

 The alteration of sheared serpentine to silica-car- 

 bonate rock probably is a very similar process, al- 

 though the selectivity of the replacement is not so 

 marked (fig. 46). As in the unsheared serpentine, 

 replacement by carbonate takes place first along frac- 

 tures, but here the more open fractures are generally 

 those that have resulted from shearing, rather than 

 from the original replacement of the olivine by ser- 



- 





FIGURE 45. Photomicrograph of silica-carbonate rock derived from 

 serpentine that was derived from dunite. Coarse veins are magnesite 

 (M) ; remainder is largely quartz (Q) with finely divided dark iron 

 oxides. Note the meshwork typical of replacement of olivine by ser- 

 pentine minerals is now outlined by the iron oxides in the quartz. 



pentine minerals. Again the carbonate replacement is 

 followed, and also partly overlapped, by replacement 

 by quartz, the earliest quartz being very fine grained 

 and the latest coarse. 



Chemical composition 



Chemical analyses of some silica-carbonate rocks, 

 and of parts of the unaltered serpentine bodies from 

 which they were derived, are given in tables 11 and 12. 

 These show the silica-carbonate rock to be composed 

 chiefly of about 35 percent silica and 60 percent mag- 

 nesium carbonate, with several percent of picotite or 

 chromite. The analyzed sample of silica-carbonate 

 rock is typical of that in which most of the ores have 

 been formed and represents the most common variety 

 in the district. Some of the rock, of a kind best seen 

 west of the Guadalupe mine and everywhere barren, 

 would show on analysis a much larger percentage of 

 silica and less magnesite. Where carbonate is espe- 

 cially abundant an appreciable part of it may be do- 

 lomite rather than magnesite, as such occurrences of 



