170 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



gray, lacks the conglomerate boulders, and apparently 

 was derived from the Deep Gulch drainage area. The 

 cinnabar nuggets were concentrated in the lower few 

 feet of this gravel and were most abundant just above 

 the bedrock. 



The cinnabar nuggets are of exceptional size and 

 purity. The largest one recovered was nearly as large 

 as a man's head, nuggets with a maximum diameter 

 of 3 inches were not uncommon, and probably the 

 majority were between V 2 an d IVfe inches in diameter. 

 Very little of the cinnabar would be classed as "fines." 

 Nearly all the nuggets were at least subrounded, and 

 most were well rounded, although they tended to be 

 somewhat flat. The content of cinnabar was amaz- 

 ingly high. Many of the nuggets appeared to con- 

 tain more than 90 percent cinnabar, and their average 

 content, as determined by retort recovery, was about 

 75 percent cinnabar. 



The original source of the cinnabar must have been 

 the outcrops near the summit of Mine Hill. The 

 apparent reason for the concentration of the nuggets 

 at the junction of the 2 canyons is the difference in 

 the carrying capacity between the stream flowing 

 down Deep Gulch, with a gradient of 1,000 feet per 

 mile, and that in Almaden Canyon, with a gradient 

 of only 50 feet per mile. The marked absence of 

 "fines" in the deposit indicates that the fines were 

 washed on down Almaden Canyon, and minute grains 

 of cinnabar can be panned from the stream gravels at 

 least as far downstream as the junction of Guadalupe 

 and Alamitos Creeks, about 7 miles below the placer 

 deposit. 



The gravel deposit was mined by dragline, which 

 loaded the productive gravels directly into trucks. 

 These moved the material a few hundred yards to a 

 washing plant consisting of a 4-foot by 24-foot trom- 

 mel, a jig, and a sorting belt. The muddy gravel was 

 slowly fed into the trommel, where it was washed by 

 a copious spray of water. The washed material more 

 than an inch in size was fed onto a sorting belt, from 

 which a few men, usually two, picked the bright-red 

 nuggets. The finer material was passed through a jig 

 that recovered the pieces down to about one-eighth of 

 an inch in diameter. The nuggets and finer concen- 

 trates were treated in a D-retort, and because of the 

 high grade of the ore it was found necessary to add 

 about 60 pounds of lime to each charge to oxidize the 

 large amount of sulfur. 



The placer deposit appears to have been mined 

 nearly to exhaustion up Deep Gulch, though other 

 small pockets of nuggets are known to remain in 

 places farther up the gulch where there is a sharp 

 change in gradient. The extent of the deposit down- 



stream cannot be determined accurately. The operator 

 stopped mining in that direction as soon as the work 

 failed to be profitable, but he believed that he had then 

 lost the main channel. As the size of the nuggets had 

 not materially decreased down to the point where he 

 quit, either the channel was wider there than up- 

 stream, so that the nuggets were less concentrated, or 

 else the gradient of the bedrock was slightly steeper, 

 so that many nuggets travelled beyond that point. 

 In either case, there can be little doubt that the stream 

 gravels in Almaden Canyon below Deep Gulch still 

 contain a large amount of alluvial cinnabar. The 

 nuggets cannot be expected to be as concentrated as 

 at the mouth of Deep Gulch, but if local concentra- 

 tions, due to changes of gradient, or to a bend in the 

 stream as at the point of emergence of the canyon 

 onto the valley floor can be found they might be 

 mined at a profit during times of high quicksilver 

 prices. 



FUTURE PRODUCTION 



The outlook for additional production of quicksilver 

 from the New Almaden district must be considered to 

 be poor if predictions are based on the present show- 

 ings or the record of diminishing production. Rut 

 from a geologic viewpoint, the outlook is promising. 

 Whether or not the district is again brought up to a 

 high level of production will depend upon several 

 factors, among which are the enterprise of the opera- 

 tors of the mines and the capital available for explo- 

 ration, the price of quicksilver, and the validity of 

 our conclusion that the district yet contains undiscov- 

 ered ore bodies. 



Continuation of the practice of the past 40 years 

 whereby attempts are made to take ore from the 

 mines without new exploration, and especially with- 

 out well-planned exploration on a sound geologic ba- 

 sis, may be expected to yield only a very small amount 

 of quicksilver, except possibly during short periods 

 when the price of quicksilver is abnormally high. 

 Conversely, if the mines are developed by a well- 

 planned exploration program enough new ore bodies 

 may be found to bring the mines bark to a state of 

 uninterrupted operation and to maintain production 

 at a moderately high level for many years. 



The operation of the mines, of course, depends di- 

 rectly on the price of quicksilver as compared with 

 operating costs. The price of quicksilver had fluctu- 

 ated about 300 percent in the 10 years that preceded 

 the preparation of this report, and when the first 

 draft of this report was written I I'.MS) it was nt the 

 low level of about $70 per flask. If price index is 

 taken into account, this is nearly the equivalent of the 

 lowest price obtained during any part of the 100 



