42 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



is scattered all through the boulder, it is coarsest and 

 most abundant in a layer that follows the periphery. 

 Where the tablets are abundant, the rock superficially 

 resembles a coarse diabase. (See fig. 32.) 



PIOUBI 32. Polished surface on chlorlte-lawaonlte rock showing de- 

 ceptive pseudodlabaslc texture resulting from random orientation of 

 euhedral tablets of lawsonlte. 



Thin sections of various parts of the boulder show 

 it to be composed almost entirely of variable propor- 

 tions of lawsonite and chlorite, with the latter gener- 

 ally conspicuous. Most of the chlorite is of a nearly 

 isotropic variety, but another variety showing anom- 

 alous gray-green interference colors locally forms 

 coarse decussate groups. The lawsonite occurs as 

 sharply bounded unoriented tabular crystals with 

 large (001) and narrow (110) faces, but it also ap- 

 pears in thin sections as broken fragments lying along 

 fractures that are healed with chlorite. Leucoxene, 

 occurring as fine dust, is the only other mineral rec- 

 ognized. 



QUARTZ-MUSCOVITE SCHISTS 



Quartz-muscovite schists, both with and without 

 sodic plagioclase, were found on Mine Hill, where 

 they are closely associated with hornblende-bearing 

 metamorphic rocks. Except for their schistosity and 

 shiny muscovite-covered parting planes, they resemble 

 the sheared feldspathic graywacke of the Franciscan 

 group, from which they doubtless were derived. In 

 thin section the quart/, which is the dominant mineral . 

 is invariably seen to be sutured, to have undulatory 

 extinction, and to contain many liquid inclusions. 

 Albite occurs in one wet ion as clastic grains with over- 

 growths of the same mineral in optical continuity. The 

 muscovite forms tabular crystals largely concentrated 

 along the parting planes. Each thin section of quart 7.- 



mica schist that was examined also contains several 

 percent of euhedral crystals of garnet, although in 

 some sections they are largely replaced by chlorite. 

 A little chlorite is also found with the muscovite 

 along the parting planes, but it does not exceed 5 per- 

 cent in any of the sections examined. 



QOARTZ-ACTINOLITE SCHIST 



Quartz-actinolite schist containing nearly equal 

 amounts of fine-grained sutured quartz and minute 

 needles of actinolite, together with a little chlorite 

 and leucoxene, is interbedded with graywacke of the 

 Franciscan group in the upper part of Longwall Can- 

 yon. It is an unusual rock in the district, and prob- 

 ably was derived from an impure tuff. 



ACTINOLirE-CHLORTTE ROCKS 



Small nodular masses of semioriented closely packed 

 actinolite prisms with interstitial chlorite are found 

 along the margins of serpentine bodies in a few places 

 in the Santa Teresa Hills; similar rocks occur in areas 

 of hornblende schists along the road a little less than 

 a mile south of the junction of Guadalupe and Los 

 Capitancillos Canyons. Such nodular masses are com- 

 mon elsewhere in the Franciscan metamorphic rocks of 

 the California Coast Ranges, but they are exceedingly 

 rare in the New Almaden district. Taliaferro (1043b. 

 p. 181) suggests that they have been formed from 

 serpentine by an endomorphic reaction. 



METACHEBT 



Most of the chert in the district shows no obvious 

 metamorphic change, but in places we found scattered 

 boulders of varicolored "metachert" displaying un- 

 usual minerals or textures. Some of the changes that 

 have produced these metacherts probably took place at 

 low temperature, during the transformation of a silica 

 gel to a hard siliceous rock; other changes, resulting 

 in the formation of minerals that indicate higher tem- 

 peratures or pressures, are tnie metamorphic chai 

 Because of the inherent difficulty of separating the 

 rocks showing diagenetic changes from those which 

 are truly metamorphosed, all these unusual varieties 

 of chert are herein considered together. 



"Orbicular jaspers" 6 and yellow cherts with a mi 

 crospherulitic texture are fairly common as isolated 

 boulders. Most of these consist of massive chert, and 

 they are believed to result directly from the radial 

 growth of chalcedony (length-slow variety termed 

 "lutecite") during the crystalli/ation of a silica gel. 

 An unusual orbicular jasper rock, shown in figure :;:'.. 



"Orbicular jasper" Is the term widely used for luch rocks by ama- 

 teur mineral collectors and lapidary enthusiasts. 



