40 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEX DISTRICT, CALIFORNIA 



chlorite, as shown in figure 30. Chlorite also occurs 

 in irregularly sheared veins and as abundant sharply 

 bounded pseudomorphs. probably after epidote. 

 Sphene is very abundant, amounting to more than 

 4 percent of some specimens. Quartz occurs only in 

 veins. 



The relative proportions of the component minerals 

 vary considerably, but hornblende is the most abun- 

 dant. It makes up more than 95 percent of some 

 specimens, and none of the other minerals makes up 

 more than a third of any specimen examined. 



GLAUCOPHANE ROCKS 



Metamorphic rocks containing glaucophane are 

 somewhat more abundant in the district than those 

 containing nonsodic amphiboles. They are also more 

 widespread, and in a general way the area in which 

 they crop out surrounds the area containing the horn- 

 blende rock. Most of the outcrops of the glaucophane 

 rocks lie in the eastern half of the district in two 

 zones that run parallel to the strike of the surround- 

 ing rocks, but scattered outcrops occur outside these 

 zones, as shown in figure 28. The northern zone 

 trends westward through the east -central part of the 

 Santa Teresa Hills where the glaucophane rocks crop 

 out in an area of graywacke as prominent isolated 

 knobby boulders generally not exceeding 15 feet in 

 diameter. The southern zone trends northwestward 

 from near the junction of Berrocal and Almaden Can- 

 yons for a distance of more than 3 miles, and although 

 glaucophane rocks within this zone are exposed only 

 in patches, the lack of outcrops of other rocks neces- 



Fir.mr. 30. Photomicrograph of garnet-hornblende gnelm. (iarnet 

 (O), hornblemle (H). and chlorite (C). Note chlorite replacing 

 borderi of garnet irarpbyroblnsts iinlii-atlni; t-nini- r.-trmrnulr meta- 

 morpblim. 



sitated mapping pa its of it as continuous bodies of 

 glaucophane rock. In this /.one. a little less than -2 

 miles from the eastern edge of the district, an espe- 

 cially well-developed glancophane-lawsonite schist 

 forms a readily accessible isolated knob a few hun- 

 dred feet in diameter adjacent to the road in Llagas 

 Canyon. 



Megascopic features 



The glaucophane-bearing rocks are characterized by 

 a irray-blue color imparted to them by the soda am- 

 phibole glaucophane. and by wetting the rock this 

 color can be considerably emphasized. These rocks 

 are mostly foliated, with alternate bands rich in glau- 

 cophane or lawsonite, but in some specimens there is 

 no orderly segregation of the minerals. In small part 

 they are true schist:-, but much of the rock that is 

 foliated shows little tendency to split parallel to the 

 foliation. Some of the nonfoliated rock contains relict 

 diabasic textures indicating derivation from malic ig- 

 neous rocks, but most of the glaucopliane rocks are 

 so completely ivcr\>talli/ed that they show no relict 

 textures of any kind. (See fig. 31.) Locally, por 

 phyroblasts of albite are developed in the -chist-. 



Microscopic. rent u n 



The minerals found in the thin section- examined 

 include glaucophane. crossite. lawsonite. albite. mus- 

 covite, chromian muscovite or fuchsite, chlorite, stilp- 

 nomelane (Hutton, 1948, p. 1373-1371). garnet, quart/.. 

 calcite, rutile, sphene. leucoxene, and magnetite. The 

 form and arrangement of the constituent mineral 

 grains varies with the structure of the rock. Glauco- 

 phane in the schists is commonly in long prismatic 

 crystals without terminal faces, whereas in some of 

 the more massive rocks it tends to form stubby Mib- 

 hedral crystals with frayed borders. In the schists 

 most of the prismatic crystals lie with their > axes in 

 the plane of the schistosity and also show a linear 

 orientation, but in the more massive rocks they have no 

 obvious preferred orientation. The glaucophane prisms 

 are rarely more than 2 mm long, but in some specimens 

 their apparent length is increased by cross frac- 

 tures filled with later sutured quartz. In the coarser 

 grained nonfoliated rocks mottling and /.on ing shown 

 liv differences in the color of single glaneophane erv- 

 lals is common, and overgrowths of small needles in 

 optical continuity are not unusual. Crossite was found 

 in a single section as an overgrowth on glaucophane. 

 Lawsonite occurs in eqnant anhedral crystals as much 

 as 2 mm in diameter, but more commonly forms ag 

 irivirate- of minute crystals or subhedral tablet- les- 

 than 1 mm in diameter. Several sections contain inter- 

 irniwths of lawsonite and quart/ simulating graphic 



