124 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



heat lost to the walls is large enough to counteract 

 the lowering of the boiling point due to decreased 

 pressures at higher levels. Because of the low heat 

 conductivity of rocks, the rate of flow would have to 

 be exceedingly slow to satisfy this condition. 



Some idea of the quantity of water required to de- 

 posit the cinnabar, and hence tlu> rate of flow, may be 

 obtained by considering the quantity of mercury de- 

 posited, the length of time available for its deposition, 

 and the quantity of mercury in the ore- forming solu- 

 tion. The minimum quantity deposited is fairly well 

 known, the time can probably be estimated within 

 reasonable limits, but much uncertainty exists regard- 

 ing the concentration of the ore solution. A theoretical 

 concentration can be calculated by assuming a solution 

 with pH and sulfur content comparable to thermal 

 spring water and saturated with HgS. A slightly 

 alkaline solution containing 0.01 mole dissolved sulfur 

 at about 150C could contain 2 X 10- g Hg per liter, 

 according to the data given by Krauskopf (1951, 

 p. 501-504). For this solution to deposit mercury 

 equal to that recovered at New Almaden (3 X 10 10 g) 

 in a period equal to all Pliocene time (11 million yr) 

 requires a rate of flow of about 1 million gpm (gal- 

 lons per minute). This rate is doubtless excessive, 

 but it suggests a very open system, which in turn re- 

 quires that the water at a depth of 2,600 feet be much 

 cooler than 280C or else extensive boiling should have 

 occurred at higher levels. In contrast to the theoreti- 

 cal data, unduplicated analyses 9 of waters closely 

 associated with mercury deposits, collected at Skaggs 

 Springs, Steamboat Springs, Elgin Spring, and a drill 

 hole at Sulphur Bank, show mercury concentrations 

 of about 1 X 10- 1 g per liter. If this concentration is 

 used in the calculation, the flow amounts to only 10 

 gpm. This is so slow that cooling by wallrock con- 

 duction probably is large enough to prevent the rising 

 water from boiling even though it is everywhere near 

 its boiling point. Until better data on the solubility 

 of HgS in natural waters is available, we must con- 

 clude that the maximum temperature may have been 

 280C, although calculations based on the solubility 

 of HgS in slightly alkaline solutions suggests that 

 this may be too high by 100 or more. 



An approximation of the minimum temperature 

 that may have existed at a depth of 2,600 feet can be 

 obtained by considering the temperature gradient. 

 The gradient was doubtless somewhat greater than 

 the present gradient in the Coast Ranges of Califor- 

 nia, and we may safely assume it was no less than 

 1.5C per 100 feet. Assuming an average surface 



Unpublished anal?*?* made by Buckman Laboratories, Inc., Tenn. 



temperature of 25C, this would give a minimal tem- 

 perature of 64C at a depth of 2,600 feet. 



If all the ores formed at the same temperature, it 

 would lie Let ween 04 and 280C, but temperatures 

 of deposition were surely greater at depth than at the 

 surface. Considering the data available at New Alma- 

 den, it appears that Lindgren's estimate of 50C to 

 200C is reasonable, but the upper limit may be a 

 little too high. 



RELATION TO INTRUSIVE ROCKS 



The space relations between the ores and the intru- 

 sive rocks in the district have led to several widely 

 held misconceptions. Because of the general proxim- 

 ity of quicksilver ores to serpentine, not only in this 

 district but elsewhere in the California Coast Hang.-s. 

 it was widely believed at one time that there was some 

 genetic relation between the serpentine and the ore- 

 forming fluid. Becker (1888, p. 117-138), believed 

 that the serpentine was not igneous, but was formed 

 by metamorphism of rocks of the Franciscan group 

 a belief that compelled him to ascribe some other ori- 

 gin to the ore fluid. After it became established that 

 the serpentine bodies are of magmatic origin, the idea 

 that the ore fluid was related to the ultramafic magma 

 again sprang up. Now, once again, after repeated 

 demonstration in many parts of the Coast Ranges that 

 the serpentine is much older than the ore bodies, this 

 conception has largely been abandoned. 



The reported occurrences of dikes of basalt or dio- 

 rite in quicksilver districts in the Coast Ranges lias 

 been cited as evidence of the derivation of the ore- 

 forming fluid from mafic magma. Schuette (1931, 

 p. 411) stated, referring to the New Almaden mine, 

 "Thus a deep-seated magma is indicated as the origi- 

 nal source and this view is strengthened by * * the 

 occurrence of diorite dikes in the mine." The only 

 rock in the New Almaden mine that might be termed 

 diorite is some of the greenstone, and as this is clearly 

 a part of the Franciscan group of Mesozoie age, the 

 ore-forming fluids of late Tertiary age cannot be in 

 any way related to it. 



The relation between the felsic igneous rocks ex- 

 posed on the hill north of the Senator mine and the 

 ore-forming fluid was emphasized by Becker. He be 

 lieved that this pyroclastic bed. interlayered with 

 sedimentary rocks of late Miocene age. was an intru- 

 sive dike occupying a lissure and genetically related 

 to the ores, for he states. "Only one occurrence of 

 rhyolite is known in I lie whole area This is a 



dike a.t New Almaden (Becker, 1888, p. 156), and fur- 

 ther, "This lissure was probably formed at the time 

 of the rhyolite eruption, to which I also ascribe the 

 genesis of the ores i Becker, isss. p. 468). As the ores 



