SERPENTINE 



47 



feldspathic; the younger part contains a larger pro- 

 portion of lithic graywacke, considerable greenstone 

 and chert, and some limestone and conglomerate. 

 These differences are believed to reflect an increase in 

 orogenic and igneous activity during the deposition 

 of the group. Probably the older sediments were de- 

 posited in deeper water and farther from their source. 

 The oolitic character of some of the limestone in the 

 younger part, together with the abundance of shale 

 flakes and the presence of conglomerate, indicates that 

 some of the geosyncline was shallow, at least at times, 

 during the accumulation of the younger part of the 

 Franciscan group. 



SERPENTINE 



Serpentine occupies less than 10 percent of the New 

 Almaden district; however, because some of it is hy- 

 drothermally altered to silica-carbonate rock which 

 is the host for all the rich quicksilver ore bodies it 

 merits special attention. Of particular importance is 

 the consideration of the character of the ultramafic 

 material when it was intruded, for this influences the 

 shape and nature of the walls of the intrusive masses 

 along which the ore bodies were locallized. The un- 

 usual opportunity to study the serpentine bodies and 

 their contacts afforded by the perfect exposures in the 

 mine workings has led us to conclude that all the 

 masses were intruded as serpentine, rather than formed 

 in place by hydration of an ultramafic igneous rock. 



Distribution 



The serpentine of the district is exposed along linear 

 zones that trend eastward or southeastward, nearly 

 paralleling the structures of the rocks of the Fran- 

 ciscan group. Disregarding a small exposure in the 

 extreme northeast corner of the district, the northern- 

 most zone lies along the northern slope of the Santa 

 Teresa Hills and is most prominent in the large mass 

 of serpentine forming Tulare Hill, on the east edge 

 of the district. The next zone to the south extends 

 eastward from the vicinity of the Guadalupe and 

 Senator mines to the east boundary of the district. 

 It cannot be traced continuously, for about midway 

 in its course it is covered by the alluvium in the broad 

 valley of Alamitos Creek; the eastern end, however, 

 is especially thick and well exposed in the Santa 

 Teresa Hills. The next zone to the south branches 

 from the last in the vicinity of the Senator mine and 

 extends southeast to the Enriquita mine, east through 

 the New Almaden mine area, and southeast from Mine 

 Hill across Fern Peak. Beyond Fern Peak it swings 

 eastward along the north side of Longwall Canyon, 

 becoming more broken and irregular close to Llagas 

 Canyon. Part of this zone seemingly crosses Llagas 



Canyon and extends southeast at least as far as Uvas 

 Canyon, east of the mapped area ; another part swings 

 northward around an arc and then continues north- 

 west to the prominent serpentine hill at the mouth of 

 Almaden Canyon. The next main zone of serpentine 

 bodies to the south extends from Los Gatos Creek, on 

 the west edge of the district, to the upper part of 

 Llagas Canyon. It is less continuous than the other 

 zones, for near El Sombroso there are gaps of nearly 

 a mile in which no serpentine was found, although a 

 more thorough search in this heavily wooded area 

 might reveal additional small bodies. The southern- 

 most zone of serpentine extends northwestward from 

 the upper part of one of the tributaries of Almaden 

 Canyon and crosses the main divide of the Sierra Azul 

 just south of Mount Umunhum. 



The serpentine masses vary widely in size and 

 shape. The largest mass in the district extends along 

 the Santa Teresa Hills, where it has an exposed length 

 of 41/2 miles and an average width of about half a 

 mile. Several others exceed 1 mile in length, whereas 

 the smaller bodies range downward in size to isolated 

 pods only a few feet long. The smallest masses can 

 best be observed in the mine workings, where one may 

 see sill-like apophyses and pods, in many places less 

 than 1 foot thick, bordering the larger sills. The out- 

 crop patterns of the serpentine masses vary according 

 to their geologic structure. Some of the masses are 

 sill-like bodies conformable with the enclosing rocks of 

 the Franciscan group; others occupy fault zones and 

 are unconformable. The conformable bodies are tabu- 

 lar and generally tilted with the enclosing rocks, giv- 

 ing rise to irregular outcrop patterns, whereas the 

 bodies lying along faults are generally vertical or 

 very steep and give rise to more linear patterns. Ex- 

 amples of the conformable bodies are seen on Mine 

 Hill and in the area to the east and southeast, whereas 

 good examples of the fault-controlled bodies are found 

 along the more southerly zone that extends eastward 

 from Los Gatos Creek and lies north and east of El 

 Sombroso. 



Megascopic features 



Two varieties of serpentine having different struc- 

 ture and texture are common in the district. They 

 grade into each other, but since most exposures can 

 readily be classed as of one or the other variety, each 

 merits a separate description. One of them, here 

 termed "sheared serpentine," is intensely sheared, fo- 

 liate, and shiny; it ranges in color from white through 

 light green to a moderately deep green in fresh ex- 

 posures. It forms very few extensive bodies, but is a 

 marginal phase of many of the larger masses. The 

 other, termed "blocky serpentine," contains massive 



