112 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



Serpentine 



N 



i 



EXPLANATION 



Hilo. or narrow quartz- 

 dolomite vein, showing 

 d.p 



Dotted #ftere projected 



Vertical hilo 



1027ft 



Intrusive contact, showing 

 dip and elevation 



Set! 'oped on side of intrusive 

 rock 



Gradational contact 



50 FEET 



Rocks of the Fran- 

 ciscan group, 

 chiefly tuffa- 

 ceous green- 

 stone 



Stope 



1111 ' El 



DATUM IS MEAN SEA LEVEL Top of winze 



Workings and geology by 

 U S. Geological Survey 



FIGURE 79. Map of the New Ardllla stope of the New Almaden mine 

 showing the localization of ore along a swarm of hllos cutting silica- 

 carbonate rock formed on the lower side of a serpentine (111. 



that in some other parts of the mines where hilos are 

 very abundant (fig. 80), no ore has been found, and 

 that in some rich ore bodies, such as those of the 

 Cora Blanca workings of the New Almaden mine, 

 hilos were apparently scarce. 



Proximity to contact 



Most of the ore bodies that have been found in 

 silica-carbonate rock extended either to the contact 

 with the rocks of the Franciscan group or to within 

 a few feet of it, and the ore was generally richest near 

 the contact. From the contact the ore may extend 

 into the silica-carbonate rock for only a fraction of a 

 foot or for more than 20 feet, but as the ore gener- 

 ally grades almost imperceptibly into barren rock, its 

 limit is economic. As is shown on some of the maps 

 accompanying this report, the ore occupies in sonic 

 places the full thickness of the silica-carbonate rock, 



but in most places it is not nearly so thick. The 

 close relation between ore and contact prevails nearly 

 everywhere, regardless of whether the contact is nearly 

 flat or steeply inclined, and regardless of whether the 

 silica-carbonate rock lies above or below the rocks of 

 the Franciscan group. Examples of ore bodies lying 

 along both upper and lower margins of the carbon - 

 atized serpentine sills are described in following sec- 

 tions of this report. 



Shape or the contact 



Many of the ore bodies in the silica-carbonate rock 

 were apparently localized along parts of the contact 

 that are so shaped as to retard and concentrate the 

 ore-forming solutions rising from greater depths. 

 Shapes that have been effective include domes, ter- 

 races, and rooflike structures with either flat or rather 

 steeply plunging ridge lines. Structural controls ap- 

 pear to be most effective on contacts with dips of less 

 than 45. Along steeper contacts, as in the North 

 Randol workings of the New Almaden mine and in 

 the Senator mine, their influence is overshadowed by 

 that of swarms of fractures. 



In addition to these major structures of the intru- 

 sive contact, other smaller structures locally assume 

 importance in affecting the size of ore bodies formed. 

 Some of the serpentine bodies are bordered by many 

 thin apophyses, which branch at low angles from the 

 main intrusive and are separated from it by septa of 

 Franciscan rocks only a few feet thick. These thin 

 tongues of serpentine increase the ore possibilities in 

 a block of ground in two ways. First, within a nar- 

 row zone along the border of the main intrusive body 

 they may increase the area of the contacts along 

 which ore can form, and thus increase the thickness of 

 minable ore. Second, where they branch and taper 

 upward from the main sill, their enclosed extremities 

 form natural traps for rising ore solutions. 



The structures described as aiding in the localiza- 

 tion of ore bodies are not all equally important, and 

 some, especially those due to branching apophyses. are 

 difficult to predict in advance of mining. Neverthe- 

 less, a mine operator who pays attention to geologic 

 structure has a far better chance of finding ore. and 

 of staying with it when he has found it. than one who 

 does not. Some examples will now be given of ore 

 bodies found in various structural environment <. 



EXAMPLES OF ORE CONTROL 

 Ore bodies beneath alta 



Most of the ore bodies in the New Almaden district 

 lie in silica-carbonate rock close beneath a "capping" 

 of alta. Ore bodies so situated include ta) all those 

 in the New Almaden mine that lie on the upper mar- 



