84 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



faults they were probably folded into a single anti- 

 cline with a very steep southwest limb and a more 

 gently inclined northeast limb. Subsequent deforma- 

 tion, largely caused by adjustment to movement along 

 the various faults, has superimposed local plications 

 upon the broad anticlinal structure, and in parts of 

 the block there was extensive crushing, intricate fold- 

 ing, and rock flowage. 



The tapering west end of the wedge-shaped Los 

 Capitancillos block contains the Guadalupe and Sen- 

 ator mines, and local details of the geology are dis- 

 cussed in the descriptions of these mines. In general, 

 the rocks in this area lie along the steep southwest- 

 dipping southwest limb of the anticline, and have 

 merely been compressed and steepened by later de- 

 formation. 



The part of the Los Capitancillos block which lies 

 between the Enriquita fault and the Almaden fault 

 and its southward projection contains the New Alma- 

 den mine, together with several smaller mines. The 

 geology adjacent to and in these mines is discussed in 

 detail in the mine descriptions. Viewed broadly, the 

 structure of this part is an anticline striking west- 

 northwest, cut obliquely by faults that strike north- 

 west and on which the dominant movement is strike 

 slip. The middle part of the anticline contains many 

 relatively thin sill-like bodies of serpentine, along the 

 altered borders of which most of the ore bodies were 

 found. The southwest limb of the anticline merges 

 into the Ben Trovato shear zone, and it is probably 

 cut, particularly on the southwest slope of Mine Hill, 

 by several faults that diverge from the shear zone 

 parallel to the Enriquita and Almaden faults. But 

 as any such faults have been at least in part obliter- 

 ated by serpentine and also subsequently deformed, 

 their positions are so uncertain that no attempt has 

 been made to show them on the accompanying maps. 

 The northeast limb of the anticline is more regular, 

 and is well marked by the outcrop of a thick sequence 

 of predominantly tuffaceous greenstones extending 

 from near the Almaden Dam to the junction of the 

 Enriquita and Shannon faults. 



The area lying between the Almaden fault and the 

 southern branch of the Calero fault contains beds that 

 formerly had relatively low dips to the north, but these 

 beds have subsequently been so crumpled that the out- 

 crop patterns of even the larger rock masses cannot be 

 resolved into any conventional pattern of folds and 

 faults. In this area particularly, rock flowage appears 

 to have been an important factor in the deformation. 



The rocks in the next, sliver to the east are more 

 orderly, and they are folded into recognizable anti- 

 clines and synclines of northwesterly trend. The 



wedge-shaped area northeast of the Calero fault con- 

 tains seemingly massive greenstone in its southern 

 part, but along its northern border is an area of gray- 

 wacke, chert, and limestone that is much crushed and 

 broken. The sliver lying north of the northern split 

 from the Calero fault is similarly sheared close to the 

 fault, and near its west end the alinement of masses 

 of serpentine and silica-carbonate rock parallel to the 

 Calero fault suggests these are in parallel fractures. 

 In general, however, the block retains such coherence 

 that a relatively thin bed of greenstone tuff can be 

 traced throughout the exposed length of the sliver. 

 In all the smaller units that make up the Los Capi- 

 tancillos block, there are sizable masses of serpentine. 

 Most of these are tabular bodies parallel to the bed- 

 ding of the country rock and are sills, but also in- 

 cluded are a few that have been described as lying 

 along faults and were probably squeezed into their 

 present position. 



EL HOMBHOHO BLOCK 



The southern, or El Sombroso, block, which extends 

 from the Ben Trovato shear zone southward beyond 

 the limits of the district, consists mainly of Francis- 

 can rocks; but bordering the shear zone on the north 

 are infaulted wedges of Miocene rocks, and along the 

 south-central border of the district it includes the 

 rocks of Late Cretaceous age in the Sierra Azul. The 

 older rocks of the block generally strike west-north- 

 west and dip to the north, though in several areas 

 folds and minor irregularities result in their having 

 other attitudes. Viewed broadly, however, the rocks 

 show neither the extreme plication and crumpling nor 

 the flowage that is characteristic of the middle, or Los 

 Capitancillos, block. This more massive El Sombroso 

 block is cut into elongate slivers by several major 

 strike-slip faults, most of which trend west-northwest, 

 nearly parallel to the strike of the Franciscan rocks; 

 the Berrocal fault zone, however, diverges from the 

 Ben Trovato shear zone in the central part of the dis- 

 trict and extends southeastward for several miles, cut- 

 ting a silver from the east end of the block. 



The northernmost of the major west-northwest strik- 

 ing faults, termed the "Limekiln fault." extends from 

 Los Gatos Canyon on the west to the Berrocal and 

 Ben Trovato shear zones in the central part of the 

 district. The Franciscan group to the north of the 

 Limekiln fault consists of a thick sequence of ma^ 

 feldspathic graywacke that dips northward and is 

 overlain by tachylitic breccia and luff, which in turn 

 is overlain by a thin bed of foramini feral limestone. 

 The area is surprisingly regular in structure: road 

 cuts and other good exposures exhibit minor shears 



