of the ore at the time of its deposition, we would 

 have to estimate how much erosion there has been 

 since then, and it will be hard to make satisfactory 

 estimates of that until the late Tertiary and Quater- 

 nary history of the southern San Francisco Bay re- 

 gion has been more carefully worked out. The depth 

 of erosion since middle-Pliocene time we estimate on 

 the basis of what is known of the late history of the 

 San Francisco Bay area to be between a few hundred 

 and a thousand feet, and to assume an intermediate 

 value of 600 feet, which is small as compared to the 

 total depth of deposition, will not introduce a very 

 serious error. We do not know, indeed, how close to 

 this surface the highest ores were deposited, but the 

 occurrence of detrital cinnabar in (late Pliocene or 

 early Pleistocene) gravels that are nearly as old as the 

 ores (post-Miocene) indicates that the cinnabar ores 

 extended nearly to the surface and may have reached 

 it. Therefore we may conclude that the quicksilver 

 ores in the New Almaden district were deposited 

 through a vertical range extending at least from within 

 a few hundred feet of the surface down to a depth of 

 2,600 feet below the surface. 



The question naturally arises why the ores did not 

 form at still greater depths whether it was because 

 of unfavorable structures or because the temperatures 

 and pressures were too great. The question cannot be 

 answered with certainty, because the surveyors' records 

 show little of the character or occurrence of the ore in 

 the North and South Randol ore bodies, which were 

 in the deepest part of the New Almaden mine. Be- 

 neath the North Randol ore bodies the structural con- 

 ditions for deposition along the ore-localizing contact 

 were less favorable than where the ores of those bodies 

 were formed, for there the contact is steep and locally 

 even overturned; but beneath the South Randol ore 

 body there appears to be no special change in the atti- 

 tude of the contact. The shell of silica-carbonate rock 

 persisted with about the same thickness to the deepest 

 levels, but the ore bodies apparently became thinner 

 with increased depth. It therefore seems probable, as 

 both the North and South Randol ore bodies died out 

 at about the same level, that the temperature-pressure 

 limits for cinnabar deposition had been reached. It is 

 noteworthy that the ore bodies of the three largest 

 mines in the district all died out downward at about 

 the same level in the Senator mine at 500 feet below 

 sea level, in the Guadalupe mine at 400 feet, and in 

 the New Almaden mine at 300 feet below sea level. 



PRESSURE AND TEMPERATURE OF ORE DEPOSITION 



The range of temperature and pressure through 

 which the ores were deposited can be estimated ap- 



ORE DEPOSITS J23 



proximately if certain assumptions that seem to be 

 justified by the available facts are made. These are 



1. The ores were deposited through a vertical range 



extending from near the original surface to 2,600 

 feet below it. 



2. The ores were deposited from water solutions which 



were nowhere above their boiling point for the 

 prevailing pressure. 



3. The channel ways for these solutions were suffi- 



ciently open for the system to be considered as 

 one under hydrostatic rather than lithostatic 

 pressure. 



The evidence for the first of these assumptions has 

 been given. The evidence for the second is twofold. 

 The fact that the ores were formed by replacement, 

 a process that entailed the removal of large amounts 

 of material as well as the deposition of cinnabar, indi- 

 cates the presence of a liquid phase; and replacement 

 by cinnabar just beneath the alta in structural domes 

 indicates the absence of a vapor phase, which, if pres- 

 ent, would fill the structural highs and keep out the 

 ore-deposting liquid. The validity of the third as- 

 sumption that of an open hydrostatic system is 

 questionable, but it is indicated by the openness of 

 the channel ways and the quantity of cinnabar depos- 

 ited, which in turn seems to require the availability of 

 large amounts of ore-forming solution. 



If these assumptions are correct, the maximum pres- 

 sure possible would be the hydrostatic pressure of a 

 column of fluid, largely water, 2,600 feet high. The 

 pressure at its base would be a little below 1,000 

 pounds to the square inch. 



The temperature range in which the ores were de- 

 posited cannot be directly determined from any of the 

 contained minerals, but it was doubtless low as com- 

 pared with the temperature of deposition of most 

 hypogene ores or magmatic solutions. Lindgren has 

 estimated the temperature of formation of quicksilver 

 deposits as between 50 and 200C. (1933, p. 212). 



The maximum temperature allowed by the above 

 assumptions would be the boiling point of water un- 

 der the pressure equivalent of a 2,600-foot column of 

 water, or about 280 C. Although the inclusion of dis- 

 solved salts and gases would modify this temperature 

 somewhat, they tend to compensate each other and 

 probably would produce no significant change. This 

 provides a maximum temperature for a static system, 

 but it may be considerably greater than the tempera- 

 ture in the flowing system believed to have deposited 

 the New Almaden ores. For boiling not to occur at 

 higher levels if the water at depth is near its boiling 

 point, the water must move upward so slowly that 



