ORE DEPOSITS 



115 



EXPLANATION 



1-//65 

 Intrusive contact, 

 showing eleva- 

 tion and dip 

 Dashed where pro- 

 jected; scalloped 

 on side of intru- 

 sive rock 



Si'tca-carbonate 

 rock below 

 contact 



Slope 



> 



Chevrons point 

 down incline at 

 5-foot vertical 

 intervals 



Rocks of the 

 Franciscan 

 group above 

 contact 



Workings and geology by U. S. 

 Geological Survey, 1945 



FIGURE 82. Map showing the localization of the Machine slope ore 

 body of the New Almaden mine beneath a small dome or "blister" on 

 the upper contact of an intrusive sill. The shape of the intrusive 

 contact Is indicated by structural contours drawn on its surface. 



more widely separated from the main intrusive mass, 

 they were mined in individual stopes. As the apophy- 

 ses are generally thin, they were in many places min- 

 eralized across their entire thickness, and the result- 

 ant ore bodies lay both above and below alta walls. 

 In the extreme case they formed in small pods of 

 silica-carbonate rock only a few feet long, isolated 

 from the main intrusive body by the shearing that 

 accompanied the intrusion. These thin ore bodies in 

 apophyses were common in the San Francisco area of 

 the New Almaden mine; a particularly good example 

 was the ore body mined in the Warren stopes (fig. 

 85). 



Ore bodies lying above alta 



Several ore bodies in the New Almaden district lie 

 close to the lower contact of serpentine masses and 

 above, rather than below, the alta. Although this 

 type of occurrence was recognized in the early days 

 of mining, it seems to have been almost disregarded 

 in the subsequent development of the New Almaden 



mine, and most of the ore bodies so situated were 

 found accidentally in workings driven for other pur- 

 poses. The ore bodies above alta are similar in most 

 respects to those beneath alta : in both cases the ore 

 is richest and most abundant close to the contact, hilo 

 fractures are important in localizing the ore, and the 

 same structures that result in the localization of ore 

 beneath alta are effective even where the relative posi- 

 tion of the rocks is reversed. 



An ore body localized above a dome-shaped contact 

 was mined in the southeastern part of La Ventura 

 stope of the New Almaden mine (fig. 86). Ore bod- 

 ies found along the crest of an inclined arch struc- 

 ture include those in the Curasco and the New Ardilla 

 stopes, although in mining the latter, the trend of the 

 hilos rather than the apex of the arch appears to have 

 been followed upward. (See figs. 87, 79.) Other ore 

 bodies found in silica-carbonate rock above alta in the 

 New Almaden mine include those taken from the Far 

 West, Santa Rosa, and El Collegio stopes (fig. 88), 

 from the San Pedro, America, and 400 level San Fran- 

 cisco stopes, and from the small stope near the portal 

 of the Juan Vega tunnel. The Mason stope in the 

 Guadalupe mine and the ore bodies in the No. 2 vein 

 of the Senator mine are similarly situated. 



Ore bodies not near alta 



A few of the ore bodies found in the district appear 

 to be in the middle of carbonatized serpentine sills 

 and to bear no relation to any known contact. The 

 ore body mined in the Moreno and Water stopes of 

 the central part of the Guadalupe mine appears to be 

 surrounded by silica-carbonate rock and to owe its 

 localization entirely to the abundance of hilos. (See 

 fig. 89.) Another such ore body may have been the 

 one mined from the main stopes of the Enriquita mine, 

 although here the geology is not well known and the 

 ore may possibly have been localized along a thin sep- 

 tum of alta separating two parallel sills of serpentine. 



Ore bodies In rocks of the Franciscan group 



Only a few small ore bodies have been found in 

 rocks of the Franciscan group. These include the 

 ore bodies of the San Mateo mine, which consist of 

 veinlets and irregular replacement bodies of cinnabar 

 in altered graywacke, and those of the upper Cora 

 Blanca and Los Angeles workings of the New Al- 

 maden mine, which consist of veinlets in altered tuff. 

 Because of their low grade, these ore bodies have 

 mostly been mined in highly irregular workings by 

 selective gouging of individual veins or swarms of 

 closely spaced veinlets. (See fig. 90.) 



