30 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT. CALIFORNIA 



normal sea temperature giving up their excess silica, 

 which would probably settle as silica gel. Thus, a re- 

 action between sea water and erupting lava in deep 

 water offers a possible mechanism for both introducing 

 silica into sea water and getting it out again in the 

 form of silica gel. The outpourings of mafic lava that 

 accompanied deposition of the Franciscan sediments 

 were chiefly submarine, and reaction between magma 

 and sea water would be expected. Violent submarine 

 eruptions giving rise to the glassy fragmentnl green- 

 stones probably provided optimum conditions for re- 

 action between the magma and sea water, and even 

 the submarine flows with pillow structure would pro- 

 vide large areas of contact between magma and water. 

 That a reaction between hot basalt and sea water yield- 

 ing silicic acid can take place was demonstrated by 

 Van Hise and Leith (1911, p. 515-516, 525), but the 

 writers are not aware of controlled experiments which 

 would indicate the quantity of silica that might be 

 released by this reaction at elevated pressures. How- 

 ever, if the lavas before reaction contained about 50 

 percent silica and a little less than 1 percent of it was 

 lost by reaction with the water, this would supply 

 sufficient silica to form all the chert in the district. 



Although the exact quantity of silica taken up by 

 the sea water is unknown, it is reasonable to assume 

 that the water when cooled to sea temperatures would 

 be supersaturated and would yield silica gel. This 

 would flocculate and settle by gravity to form a mass 

 with a density greater than the adjacent sea water, and 

 such a mass might be expected to flow as a density 

 current if it formed on even a very low angle slope. 

 This flowage into basins may explain why some chert 

 masses apparently occur a considerable distance from 

 volcanic rocks. 



The process of collection of the silica, expulsion of 

 the water and iron-rich compounds, and the develop- 

 ment of rhythmic bedding has been commented on by 

 many geologists who have studied rhythmically bedded 

 cherts in various parts of the world. No theory has 

 met with wide acceptance. Davis treats the problem 

 of the rhythmic bedding in the cherts of the Francis- 

 can group exhaustively and gives the results of several 

 experiments in which he obtained rhythmic separation 

 of silica gel from clay (Davis, 1918, p. 386-402). As 

 a result of his experiments and careful field observa- 

 tions Davis concluded that the bedding in these cherts 

 owed its origin to colloidal segregation, but it is not 

 clear whether he believed the entire sequence of beds 

 in a lense of chert was formed by segregation of a 

 single mass of gel or from several superposed masses. 

 Although the peculiar lenticular character of the beds 

 in the rhythmically bedded sequences were duplicated 

 in part by Davis' experiments, he was unable to form 



more than a few such layers, whereas in nature se- 

 quences involving hundreds of beds are not unusual. 

 Taliaferro believes each layer was deposited separately 

 and solidified before the deposition of subsequent lay- 

 ers, and as evidence for this he cites the occurrence 

 of chips of chert in the partings between layers (Talia- 

 ferro, 1943b, p. 149). The unusual botryoidal cherts 

 of the New Almaden area are believed to be most 

 easily explained as having been built up by rapid con- 

 solidation of silica gel oozing from a submarine ori- 

 fice, and they also suggest rapid solidification of the 

 silica gel. 



VOLCANIC ROCKS (GREENSTONES) 



The varied mafic volcanic rocks interbedded with 

 the sedimentary rocks of the Franciscan group in the 

 New Almaden district are believed to be fairly repre- 

 sentative of the volcanic rocks found in the group 

 throughout the California Coast Ranges, although 

 they do not include quite all the varieties that have 

 been reported. Because they are so chloritized and 

 otherwise altered that precise field classification is 

 impossible, they have been grouped under the general 

 term of "greenstone," in accordance with the usual 

 practice of geologists of the Coast Ranges. They crop 

 out over An area of about 20 square miles, or one-third 

 of the part of the New Almaden district that is under- 

 lain by rocks of the Franciscan group. They form 

 bodies that are lenticular but well defined, and these 

 bodies are mapped as cartographic and stratigraphic 

 units. (See pi. 1.) 



The greenstones are apparently all derived from ex- 

 trusive rocks, and many, if not all, are of submarine 

 origin. They differ widely in grain size and texture: 

 the coarsest are of ophitic or diabasic texture, others 

 are variolitic or pilotaxitic, and still others are pyro- 

 clastic breccias or tuffs. In spite of textural differ- 

 ences all contain nearly the same mineral assemblage, 

 a fact suggesting similarity in the chemical composi- 

 tion of their parent magmas. The typical primary 

 minerals are sodic plagioclaae and subcalcic or titanian 

 augite, but a few of the lavas contain a little olivine. 

 In addition, material formed by alteration of malic 

 glass (or tachylite) is found in the diabasic rocks and 

 is very abundant in the finer grained volcanic n>ck>; 

 in part of the district altered mafic glass is the major 

 constituent of accumulations of tuff and breccia more 

 than 500 feet thick. 



The alteration of the volcanic rocks to greenstones 

 has been widespread, and varied in both kind and de- 

 gree. In much of the greenstone deuteric alteration 

 has formed minerals of the epidote group from pyn>\ 

 enes, antigorite from the olivine, and a sanssuritic ag- 

 gregate from the plagioclase. In some of it the origi- 



