82 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEX DISTRICT, CALIFORNIA 



position during movement of the faults. Most of the 

 serpentine occupying a fault zone appears to have 

 been emplaced before the deposition of the post-Fran- 

 ciscan rocks, but, locally, small masses have been 

 squeezed into rocks that are clearly post-Franciscan. 

 Structures involving rocks younger than the Fran- 

 ciscan group include both faults and folds, and their 

 positions and trends are governed largely by the struc- 

 tures previously developed in the rocks of the Francis- 

 can group. Most of the folds are fairly open and 

 strike nearly east. The most important of the faults 

 following older structures are the Shannon fault, 

 which separates the northern third of the district from 

 the remainder, and a braid of more discontinuous 

 faults, displacing Miocene rocks, along the southwest- 

 ern border of the Ben Trovato shear zone. Compara- 

 tively young normal faults strike northwest across the 

 displaced gravels of the Santa Clara formation in the 

 northwest corner of the district. 



MAJOR STRUCTURAL BLOCKS 



For description, the New Almaden district may be 

 divided into three major blocks separated by major 

 faults as shown in figure 57, and each of these ex- 

 hibits a different structural pattern (fig. 58). The 

 northernmost, called the Santa Teresa block, has an 

 indefinite northern limit covered by the alluvium of 

 the Santa Clara Valley, but is bounded on the south 

 by the Shannon fault, which trends a little south of 

 east across the entire district. The middle, or Los 

 Capitancillos block, lies south of the Shannon fault, 

 and is separated from the southern, or El Sombroso 

 block, by the Ben Trovato shear zone, which diverges 

 from the Shannon fault in the Blossom Hill area and 

 extends southeastward to the southeast corner of the 

 district. 



SANTA TERESA BLOCK 



The northern, or Santa Teresa, block contains struc- 

 tures that strike generally eastward, but in its western 

 part it is cut by faults that swing northwestward from 

 the Shannon fault. The rocks exposed in this block 

 range in age from the Upper Jurassic and Cretaceous 

 rocks of the Franciscan group to the Recent alluvium 

 filling the Santa Clara Valley. The alluvium, which 

 itself is not deformed, covers the older structures in 

 a large part of the block. The Pliocene and Pleisto- 

 cene gravels of the Santa Clara formation exposed in 

 the northwest corner of the district have been slightly 

 deformed and are cut by normal faults that produced 

 only minor topographic scarps and irregularities in 

 the drainage pattern. The formations of Miocene, 

 Eocene, and latest (?) Cretaceous age have been thrown 

 into open eastward-trending folds, which are cut by 



eastward-trending faults. The basement rocks of the 

 Franciscan group, which are largely graywacke in 

 this block, are poorly exposed, but the few attitude 

 that could be obtained indicate that they are plicated 

 into folds with a similar strike. The rocks of the 

 Franciscan group also are traversed by two eastward- 

 trending shear zones intruded by thick masses of ser- 

 pentine. 



LO8 CAPITANCILLOS BLOCK 



The middle, or Los Capitancillos, block consists 

 chiefly of folded interbeclded sedimentary rocks and 

 greenstones of the Franciscan group intruded by ser- 

 pentine. The major anticline of the district follows 

 the Los Capitancillos Ridge, but it is cut obliquely by 

 several widely spaced faults which diverge northweM 

 ward from the Ben Trovato shear zone. ( See fig. .">". i 

 All these faults on which one can determine the dis- 

 placement have a large strike-slip component, the west 

 side moving northward. The westernmost fault, named 

 the Enriquita fault; extends from the Ben Trovato 

 shear zone to a zone of serpentine bodies believed to 

 have been intruded along an old fault zone that has 

 been followed fairly closely by the younger Shannon 

 fault. The other faults are roughly parallel to the 

 Enriquita; but as their courses are marked only in 

 part by serpentine intrusions, they are harder to fol- 

 low. The Almaden fault, for example, diverges south- 

 ward from the precursor of the Shannon fault 7,000 

 feet east of the Enriquita fault. Its course near its 

 northern end is indicated by offsets of the sedimentary 

 rocks and greenstones of the Franciscan group, but 

 on reaching the New Almaden mine area it become- 

 more nearly parallel to the strike of the older rocks 

 and cannot be traced on the surface. It was, however. 

 apparently reached by some of the underground work- 

 ings of the New Almaden mine. Along its southward 

 projection are intrusive bodies in line with it that may 

 mark its continuation or be in shear zones parallel 

 to it. Similarly in the area south and west of the apex 

 of Mine Hill, the north-northwesterly alinement of 

 bodies of serpentine and silica-carbonate rock suggest 

 they were squeezed into, or dragged along, fractures 

 parallel to the Enriquita and Almaden faults, but this 

 cannot be demonstrated by offsets of the surround 

 rocks. The easternmost parallel fault in this block, 

 termed the "Calero fault," likewise probably diverges 

 from the Shannon fault, though the point of junction 

 is covered with alluvium. From the Calero fault 

 branch two other faults, which are more nearly paral- 

 lel to the Ben Trovato shear /.one than the other 

 oblique faults are. The Franciscan strata in the fault 

 bounded areas west of the Calero fault are variously 

 deformed, but before the development of the oblique 



