16 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



aggregates of chlorite, sericite, and perhaps other 

 minerals. A few detrital grains of augite, epidote, 

 clinozoisite, or zoisite are generally present, and some 

 varieties contain several percent of biotite or musco- 

 vite or both. Other detrital minerals occurring in 

 minor quantity are chlorite, sphene, zircon, garnet, 

 ilmenite, leucoxene ( ? ) , and magnetite. Tourmaline 

 and hornblende, reported from other localities by 

 Taliaferro (1943b, p. 135), were not found. 



The quartz grains, which make up from 10 to 35 

 percent of the graywackes, are commonly angular, but 

 some are subangular. A few well-rounded grains were 

 observed, as was a single grain showing a definite 

 euhedral shape. The quartz is clear, generally con- 

 tains liquid and gas-filled cavities, and rarely includes 

 needles of zircon. Many of the grains show undula- 

 tory extinction, and some are composites of several 

 crystal units separated by sutured boundaries. 



Feldspar grains generally account for from one-third 

 to two-thirds of the monomineralic grains, and feld- 

 spars are also common constituents of the rock frag- 

 ments. The monomineralic grains are generally 

 angular or subangular, and H surprisingly small pro- 

 portion show straight edges that coincide with the 

 cleavage direction. In some sections many of the 

 feldspar grains appear to be euhedral crystals modi- 

 fied only to the extent of having slightly rounded 

 corners. The feldspar is generally cloudy enough to 

 be easily distinguished from the clear quartz but fresh 

 enough to show sharp lamellae in the grains that are 

 twinned. Much of the feldspar, however, is un- 

 twinned, and this, together with the incipient altera- 

 tion, makes the determination of the relative abun- 

 dance of different kinds of feldspar difficult. To 

 ascertain the ratio of potash feldspar to plagioclase 

 10 thin sections were etched with hydrofluoric acid 

 and stained with sodium cobalt initrite solution, which 

 differentially stains the potash feldspars a bright yel- 

 low. Two of these thin sections contained a few 

 grains of potash feldspar, but in the others none was 

 found; this admittedly inadequate sample suggests 

 that orthoclase is a minor constituent of only some of 

 the graywacke. The relative amounts of albite, oligo- 

 clase, and andesine have only been approximated by 

 comparing indices of the feldspars with the index of 

 balsam along the edges of thin sections, which is a 

 rather unsatisfactory method lx>cause many of the 

 feldspars are so clouded with alteration products that 

 a reliable comparison cannot In- made. This method 

 leads to the tentative conclusion that albite is by far 

 the most common plagioclase. oligoclase is generally 

 present, and andesine is rare. Features that might be 

 suggestive of a granitic source for the feldspars, such 



as graphic intergrowths, myrmekite, or microcline, 

 were not found. 



Rock fragments may make up from less than 10 

 percent to more than 75 percent of the clasts in the 

 graywacke. Mo.-t of the fragments are mafic lavas 

 or greenstones, similar in texture and mineral content 

 to the massive greenstones of the Franciscan group, 

 and a single thin section will generally include several 

 varieties of greenstone fragments. Some of them 

 consist of completely altered mafic glass, others con- 

 tain albite and scattered relicts of pyroxene in an 

 altered fine-grained or glassy groundmass, and still 

 others are composed largely of plagioclase with only 

 a little altered groundmass. Less common are frag- 

 ments of shale, phyllite, and chert showing varying 

 degrees of recrystallization. Shale fragments are gen- 

 erally tabular and in%-ariably bent around adjacent 

 grains of quartz and feldspar, whereas the fragments 

 of mafic volcanic rock have rounded and irregular 

 shapes due to distortion of the original grains by 

 flowage to form a better fit with adjacent grains. 



The term "matrix" as applied to gniywarke needs 

 to be defined before it can be discussed because in 

 many of these rocks there is no clear break in grain 

 size between the coarsest and finest material. The 

 matrix, however, is generally con-idcn-d. as it will be 

 here, to include the material between grains that is 

 itself so fine grained as to be only partly determinable 

 under high magnification (about 0.002 mm in diame- 

 ter) and the somewhat coarser material that is recry-- 

 tallized from this fine-grained paste. The quantity of 

 matrix in the Franciscan graywacke varies from an 

 amount large enough to provide a groundmass in 

 which the other grains are clearly isolated to a thin, 

 scarcely discernible film l>etween closely packed grains. 

 Much, and perhaps all, of the matrix is recrystallized. 

 It contains quartz, sericite. chlorite and probably also 

 albite and actinolite. In some varieties it replaces the 

 margins of feldspar grains or rock fragments giving 

 these a fuzzy outline. In addition to the normal ma- 

 trix in some varieties there are small areas in which 

 the matrix is replaced by calcite. 



< MI-MI irnl features 



The few chemical analyses that have been made of 

 f_n:iywackes of the Franciscan group are shown in 

 table 1, and for comparison we have included an aver- 

 age of 30 graywacke analyst's from other localities 

 and an average of 40 granodiorite analyses. The new 

 analysis given in column 2 is of a specimen collected 

 from the center of a 10-foot boulder that had Keen 

 blasted apart, and it appear- to be entirely fre-h. A 

 photograph of this rock is shown in figure 3. 



