GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



The faults in the district are nearly parallel in strike to the 

 rocks of the Franciscan group, but dip more steeply. The 

 larger displacements are believed to be dominantly strike slip, 

 the block southwest of a fault having moved northwestward, 

 but the direction and amount of displacement can be directly 

 determined for only a few of the faults. Along the largest 

 faults there are wide shear zones formed before the deposi- 

 tion of the late Upper Cretaceous rocks : so the first movement 

 occurred In early Upper Cretaceous time. Other faults were 

 active after the deposition of the upper Miocene rocks, and a 

 minor amount of faulting took place after the deposition of the 

 Pliocene and Pleistocene gravels. Many of the younger faults 

 followed older shear zones and indicate recurrent movement 

 along these zones. 



The most pronounced structural break is the Ben Trovato 

 shear zone, which trends west-northwestward, parallel to the 

 nearby San Andreas fault, through the central part of the dis- 

 trict. It extends beyond the limits of the district, and in places 

 It has a width of more than 4,000 feet. Its apparent horizon- 

 tal offset Is about 10 miles. A second shear zone, which Is 

 partly covered by younger rocks and partly obliterated by in- 

 trusive bodies of serpentine, diverges from the Ben Trovato 

 shear zone east of Los Gatos and extends eastward across the 

 district. It has been followed by the post-Miocene Shannon 

 fault. Many of the other faults diverge from the shear zones 

 at small angles, and they are believed to have less offset. 



Steep fractures, trending north to northeast, traverse only 

 the silica-carbonate rock, but these are of special Interest be- 

 cause they bear a close relation to many of the ore bodies. 

 These fractures generally extend Into the silica-carbonate rock 

 from the margins of the intrusive bodies for only a few score 

 feet, and are widest close to the contact with the rocks of the 

 Franciscan group. The fact that they have a uniform trend, re- 

 gardless of the attitude of the silica-carbonate rock or of the 

 serpentine sill from which It formed, Indicates that they origi- 

 nated In response to a regional force, and therefore they may 

 be regarded as tension fractures. 



Quicksilver ore In place was recognized for the first time 

 within the present confines of the United States on Mine Hill 

 In 1845, but the bright red cinnabar that cropped out there 

 had earlier attracted the attention of Indians and Mexican 

 settlers. The subsequent development of the New Almaden 

 mine, largely In Mine Hill, had resulted by the end of 1948 in 

 a production of 1,046,198 flasks of quicksilver, which sold for 

 $50,000,000. The ores of the Guadalupe mine, whose outcrops 

 were found a little later, have yielded about 112,600 flasks of 

 quicksilver, placing this mine sixth In rank among California 

 producers. The other mines In the district were also first de- 

 veloped many years ago, but although they have been Inter- 

 mittently active, they have made only a comparatively small 

 production. Since 1890 the production from the district has 

 been small as compared with Its earlier output. 



The mineralogy of the quicksilver ore bodies is simple. The 

 only ore mineral of much economic Importance Is cinnabar, 

 although locally native mercury Impregnates and enriches the 

 ores. Accompanying sulfldes, present In only small amounts. 

 Include pyrlte, stlbnlte, chalcopyrlte, sphalerite, galena, and 

 bornlte. The gangue minerals Introduced by the mineralizing 

 solutions are dominantly quartz and dolomite with some hy- 

 drocarbons, but a few other minerals occur here and there. 



Most of the quicksilver ore Is of primary origin, although 

 one alluvial deposit containing nuggets of cinnabar ore has 

 been mined. The typical primary ore bodies were composite; 



the cinnabar in them was deposited In part by replacement of 

 silica-carbonate rock along steep northeastward-trending frac- 

 tures, and in part by filling of the open spaces provided by the 

 fractures. The replacement extended only a few inches out- 

 ward from these fractures, but within this limit It was so 

 complete that commonly more than 50 percent of the replaced 

 rock was cinnabar. In many of the ore bodies the steep frac- 

 tures occurred in swarms, so closely spaced that much of the 

 Intervening silica-carbonate rock was converted to rich ore. 

 In most places the fractures were filled with quartz and 

 dolomite containing very little cinnabar, but In some places 

 the vein filling was sufficiently mineralized to form ore even 

 where the walls were not mineralized. 



The ore bodies that have been mined were large and excep- 

 tionally rich. The largest was about 200 feet wide and 15 feet 

 thick, and extended down the dip for about 1,500 feet. The 

 ore furnaced in the first 15 years of mining at the New Alma- 

 den mine contained more than 20 percent quicksilver, but to 

 obtain this amazingly high grade the ores were cobbed and 

 hand sorted. In the course of time the grade steadily declined 

 to less than 0.5 percent, owing to the utilization of lower grade 

 ores and less careful mining and sorting. The remarkable 

 richness of much of the ore, however, is well indicated by the 

 fact that the average grade of all the ore furnaced in the 

 hundred years during which the New Almaden mine was pro- 

 ductive Is only a little less than 4 percent quicksilver, or about 

 a flask of quicksilver per ton. 



The ore bodies In the district are not distributed at random ; 

 nearly all are restricted to certain rocks and certain struc- 

 tural environments. A consideration of these lithologic and 

 structural factors together with the geology of the area Indi- 

 cates that some places in which there is a reasonable hope of 

 finding ore remain unexplored. Nearly all the ore bodies were 

 formed in silica-carbonate rock, although this rock occupies 

 only a very small part of the district. Furthermore, only the 

 silica-carbonate lying close to the contact with the rocks of the 

 Franciscan group is particularly favorable for ore deposition, 

 and most of the ore bodies were richest within a few feet of 

 this contact. The distribution of the ore bodies along the 

 contacts apparently was influenced by two other structural 

 factors, whose importance varied with the steepness of the 

 contacts. Where the contacts were steep, swarms of cross 

 fractures took a dominant part in localizing ore bodies, but 

 where the contacts were Inclined at less than 45, the shape 

 of the contact Itself was of equal or greater Importance ; along 

 such contacts the ore bodies tended to form at the crests of 

 domes or plunging anticlines. 



The quicksilver ore Is believed to have been deposited during 

 the Pliocene epoch by hydrothermal solutions rising from a 

 deep-seated source. These solutions followed fractures which 

 were best developed In the silica-carbonate rock near contacts 

 with rocks of the Franciscan group. Deposition of the cinna- 

 bar took place through a vertical Interval extending from near 

 the surface to a depth of about 2,600 feet, and In a tempera- 

 ture range believed to have been from 50* to 150 8 C. The rich- 

 est ore bodies were localized along gently dipping contacts, 

 where the solutions spread out and stagnated under a capping 

 of relatively impervious sheared rocks of the Franciscan group ; 

 but along steep contacts replacement by solutions flowing 

 through fractures took place even where structural traps were 

 absent. Although most of the ore bodies are In the silica- 

 carbonate rock formed along the top sides of serpentine sllla. 



