172 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



stripped from the Santa Mariana body of silica- 

 carbonate rock (pi. 3 and fig. 92), though its average 

 grade is not precisely known. The other bodies of 

 silica-carbonate rock exposed at the surface in the 

 New Almaden mine area are thought to be either too 

 lean, too small, or too unfavorably situated to supply 

 submarginal ore that might be mined by power shovel. 

 Underground in the New Almaden mine submargi- 

 nal ore could doubtless be obtained in several places, 

 but the old workings are so badly caved that more or 

 less extensive development work would be required to 

 reach them. In the area south of the Almaden shaft 

 "low-grade ore" was reported to have been found on 

 the 500 and 600 levels in 1890, when the average grade 

 of ore being furnaced was about 2 percent quicksilver, 

 and this ore was never mined. The Randol stopes 

 below the 1000 level became inaccessible at a time 

 when 1-percent ore was considered economic, and 

 doubtless these extensive stopes contain a large ton- 

 nage of ore containing several pounds of quicksilver 

 per ton. The stopes in the rich Velasco area have 

 been inaccessible for such a long time that not even 

 rumors of what was left behind can now be heard. 

 This area may contain a sizable quantity of submargi- 

 nal ore, but it appears to offer even greater promise 

 of containing a small amount of rich ore that should 

 prove suitable for retort operation. 



HIGH-GRADE ORE 



The search for high-grade ore in the New Almaden 

 district can be directed toward finding extensions of 

 known ore bodies or satellite bodies lying near known 

 bodies and on the same structures; or search may be 

 directed to the finding of wholly new ore bodies in 

 hitherto unexplored places that satisfy the conditions 

 described on pages 108-112, as having been effective in 

 localizing the known ore bodies. For reasons now to 

 be explained, the chances of finding rich ore by fol- 

 lowing either of the first two methods seem to be less 

 favorable than the chances of finding new ore bodies 

 not closely related to those now known. 



EXTENSIONS OF KNOWN ORE BODIES 



In general, the ore bodies of the district have been 

 so sharply limited and so thoroughly exploited that 

 the possibility of finding extensions of known ore 

 bodies is rather remote. Some exceptions, however, 

 occur in a few places where the trend of an ore body 

 was not apparent, or where the ore became inacces- 

 sible before it could be completely mined out. An 

 example of unperceived change of trend may be af- 

 forded by the New Ardilla stope of the New Almaden 

 mine (fig. 79 and pi. 4) in which the extension of an 



ore body was looked for unsuccessfully along the in- 

 tersection of a contact with a group of hilos; this ore 

 body may change trend slightly to follow a roll in the 

 contact that strikes more easterly than do the hilos. 

 An example of the second condition, that of known 

 ore left unmined because it suddenly became inacces- 

 sible, is provided by the America mine, where an ore 

 body that was reported to extend downward below 

 the 600 level was left unmined when the America shaft 

 caved. 



SATELLITE ORE BODIES 



The chances of finding satellite ore bodies near 

 others that have already been mined out are not par- 

 ticularly good, because most of the contacts thought to 

 be favorable for the localization of ore in the mines 

 have been explored beyond the limits of the known 

 ore bodies. It was a general practice, however, where 

 the contacts were not followed by exploratory drifts. 

 to look for new ore bodies by following along the in- 

 tersection of a contact and hilos, and many new ore 

 bodies were found in this way in the central stope 

 area in the New Almaden mine. But there was little 

 exploration along the contacts at right angles to the 

 trend of the hilos, which raises the question whether 

 some satellite ore bodies lying parallel to those that 

 have been mined may not have been missed. At any 

 rate, the amount of exploration at right angles to the 

 hilos in this area is far less than would seem justified 

 by its fabulous concentration of cinnabar. In the 

 Guadalupe mine, the importance of the hilos north- 

 west of the New Guadalupe Inclined shaft apparently 

 was not realized, for they have not been followed 

 downward as far as they should have been. Also, in 

 the San Mateo mine, where there is no silica-carbonate 

 rock and no well-defined contacts or veins to guide 

 exploration, the downward extent of the ore zone does 

 not seem to have been thoroughly prospected. 



POSSIBLE NEW ORE BODIES 



The possibilities for finding isolated new ore bodies 

 in places of favorable environment appear to be best 

 in the area already explored by the workings of the 

 New Almaden mine. This is due partly to the fact 

 that all three dimensions of the structures involved 

 are here most fully understood, and partly to the fact 

 that ore-bearing solutions are known to have passed 

 through the area, so that any places favorable for ore 

 deposition have probably been mineralized. It may 

 seem paradoxical to believe that the area most ex- 

 plored is also the one in which favorable places re- 

 main unexplored, but this belief seems at least partly 

 justified by the fact that large potentially ore-bearing 

 structures were neglected because two beliefs were 



