34 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEX DISTRICT, CALIFORNIA 



which has affected their other physical properties, such 

 as hardness and manner of fracturing. Their colors, 

 which seem to reflect those of the rocks with which 

 they are associated, range from dark to light green or 

 brownish green in the chloritixed tuffs associated with 

 massive greenstone to lighter huffs or reddish or pur- 

 plish browns in the tuffs associated with sediments. 

 Those tuffs that have been hydrothermally altered and 

 contain abundant clays, such as many exposed in the 

 workings of the New Almaden mine, are light buff, 

 light gray, or locally nearly white in color. AVhere 

 the textures are least obliterated the tuffs are marked 

 by very fine banding, which is due in some places to 

 different concentrations of light and dark material. 

 and elsewhere to differing degrees of oxidation of their 

 iron oxides or to differing sizes of their minute angu- 

 lar clastic grains. (See fig. 24.) In the coarser tuff 

 breccias the textures are more obvious, with poorly to 

 moderately well-sorted angular fragments, generally 

 less than an inch in diameter, embedded in a matrix 

 of fine-grained clastic material. 



Microscopic features 



As seen in thin section the original pyroclastic na- 

 ture of most specimens is masked by alteration, but in 

 some sections layering and vague clastic texture are 

 discernible. The original minerals identified in a few 

 of the fresher rocks are broken tablets of plagioclase, 

 subhedral clinopyroxene, magnetite, and a little quartz; 

 where the texture is coarse, fragments can be distin- 

 guished (fig. 25). Material that has replaced mafic 

 glass is present in some, but no curved shards, such as 

 are commonly found in tuffs, were recognized. The 

 commonest alteration products include chlorite, quartz, 

 calcite, celadonite, and clays. The earliest of these is 

 the chlorite, which is derived mainly from the ferro- 

 magnesian minerals and glass but also to some extent 

 replaces the feldspars. Quartz, which has been the 

 next mineral to form in some of the altered tuffs, oc- 

 curs as granular aggregates replacing the rock and as 

 veinlets. Later calcite. as replacements and veins, is 

 common. Near the ore bodies of Mine Hill hydrother- 

 mal solutions extensively altered the tuffs to clay min- 

 erals before the introduction of most of the quartz 

 and calcite. 



TACHYLJTIC TUFTS AND BRECCIAS 



Tachylitic rocks, both tuffs and breccias, composed 

 almost entirely of altered basaltic glass, have not been 

 reported from other areas underlain by rocks of the 

 Franciscan group so far as we know, but in the New 

 Almaden district I hey underlie extensive areas and 

 make up a considerable part, of the greenstone of the 

 district. They occur in two principal areas: one is a 



wide band that extends eastward from Los Gatos 

 Creek to the Ouadalupe mine, the oilier is a narrower 

 band containing scattered bodies and extending along 

 the north side of LoDgwall Canyon. The rocks are 

 particularly well exposed both along Los (iatos Creek, 

 south of Los (iatos. and near the middle fork of Long- 

 wall Canyon, in the southeastern part of the district. 

 The fresher tacliylitic rocks are readily recognized 

 from their unusual textures, dark- to light green color, 

 and the serpent iuelike appearance of their fragments. 

 The fine-grained varieties, especially where weathered, 

 closely resemble graywacke: but they can be distin- 

 guished from it by their general lack of quartz, their 

 slippery feel, and their relict textures. 



*, 



1 inch 



FIGURE 24. Polished sections of drill cores showing two of the more 

 common kinds of tiiffnn'ous ^riNMisioiu 1 in the Frnnciscan group. 

 Core on left shows cream -colored tuff bleached by h.v<lrotliTuial o- 

 lutlons Interlayeri'd with black shale. Right core shown layers of 

 rl. purple, and green tuft* containing a few larger fragments Iden- 

 tifiable as altered mafic glass. 



