60 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADKN DISTRICT. CALIFORNIA 



fragments. Such materi.il, as seen in shallow adit> or 

 opencuts, is a porous, locally brittle ocher, not easily 

 recognized. Siliceous skeletons of hast it e pseudo- 

 morphs or vaguely lenticular textures are discernible 

 in places, and nearly everywhere the ocher will yield 

 chromite or picotite if panned. Where the rock is 

 somewhat more siliceous, it crops out as white to 

 brown rounded boulders having pitted surfaces, due 

 to the carbonate having been removed and a frame- 

 work of silica left behind. As might be expected, the 

 blocky siliceous variety of silica-carbonate rock forms 

 prominent knobs or ledges. 



Silica-carbonate rock, derived from blocky serpen- 

 tine is not abundant, but excellent exposures of it can 

 been seen in the Day tunnel of the New Almaden mine, 

 about 2,000 feet from its portal. Here the walls are 

 unevenly encrusted with various white secondary salts 

 in a manner that brings out the structure of the rock. 

 Where the walls are not too heavily coated, one can 

 see every small detail of the sheared matrix and un- 

 sheared blocks nearly as well as in the best exposures 

 of unaltered blocky serpentine, even though the ser- 

 pentine is completely converted to a hard variety of 

 silica-carbonate rock. The part derived from the 

 sheared matrix is similar in all respects to the silica- 

 carbonate rock previously described, but that derived 

 from the unsheared blocks retains a porphyritic ap- 

 pearance resulting from the selective replacement of 

 the bastitic pseudomorphs occurring in the serpentine. 

 In some of the blocks, however, the serpentine de- 

 rived from olivine is largely replaced by fine-grained 

 gray magnesite, and the accompanying bastite pseudo- 



FIOCRI 42. Pollihed iurfaoe on nillca-carbonate rock derived from 

 IlKhtly iheared ierpentlne. The light areas on the left are pieudo- 

 morphi after rr.vKtal* of pyroxene. 



morphs are replaced by more coarsely crystalline white 

 carbonate enclosing a bright -green platy mineral, 

 probably a chlorite. In such rock the pseudoporphy- 

 ritic texture is very pronounced because of the color 

 contrast between the grayish "groundniass" and the 

 pale apple-green "phenocrysts" (tig. 4-2). A few 

 blocks of fine-grained silica-carbonate rock that has 

 replaced serpentine derived from dunite were found in 

 the same general part of the Day tunnel, and in 

 these it was possible to distinguish a mesh texture 

 inherited from the serpentine. Such examples of 

 silica-carbonate rocks derived from massive serpentine 

 are unusual and of small extent. They are of special 

 interest, however, because of what they reveal about 

 the process through which the silica-carbonate rock 

 was formed. It replaced the serpentine with so little 

 volume change that no effects due to expansion or 

 contraction are apparent. 



Microscopic features 



Thin sections that show all stages of alteration from 

 serpentine to silica-carbonate rock were examined. 

 Those sections showing the least replacement contain 

 in addition to the serpentine minerals only a little 

 carbonate, whereas sections that show more advanced 

 alteration contain quart/, as well as carbonate and lit- 

 tle or no serpentine. Although the alteration does 

 not everywhere take place in the same way. the fol- 

 lowing statements will describe the general process as 

 inferred from a study of the entire suite of sections. 

 To show most clearly how some of the serpentine min- 

 erals are replaced before others, the rock derived from 

 the unsheared serpentine is described fust, even though 

 it is not the most common variety. 



The alteration of unsheared serpentine begins with 

 the crystallization of magnesite in the mesh of anti- 

 gorite and chrysotile that has replaced olivine. This 

 carbonate first forms a network of irregular veinlets. 

 which have ragged edges because the crystals grow 

 outward from cracks as replacement of the serpentine 

 advances. (See fig. 43.) The bastitic pseudomorphs 

 are not generally attacked at this stage, but in places 

 the carbonate fills narrow sharply bordered cracks that 

 traverse them. The larger veins of chrysotile, on the 

 other hand, are especially susceptible to replacement 

 by a carbonate that retains a fibrous aspect even 

 though it is actually in rather eqnant crystals. (See 

 fig. 44.) Further alteration of the serpentine results 

 in rosettes of magnesite in the serpophite cores of the 

 mesh and in the bastite pM-mlomorphs. and at the 

 same time the meshwork of carbonate becomes better 

 defined and coarser. With continued alteration more 

 carbonate may be added, but a mure striking change 



