56 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



completely crystallized as enstatite and olivine, and 

 water remaining could only be in the vapor phase. 

 It would, therefore, seem impossible for enough water 

 to remain in the pore spare of the rock, even if it con- 

 sisted of loosely parked crystals, to produce more than 

 incipient serpentinization. whereas the New Almaden 

 rooks are thoroughly serpent in i/ed. If the water were 

 slowly added from deeper seated parts of the magma 

 chamber, one might expect the serpentinization to be 

 more complete; but one would also expect it to be 

 most complete in structural traps or in places where 

 the rock was excessively sheared. No such distribu- 

 tion of more and less serpentinized rock is apparent, 

 however, in this district; all the serpentine bodies, re- 

 gardless of size or degree of sharing, are serpentinized 

 to about the same degree. 



Younger intrusive rocks of appropriate age are lack- 

 ing and cannot be depended upon as the source for 

 the water. Assimilation of water from the surround- 

 ing sediments, which are notable for their lack of 

 porosity and permeability, seems also unlikely, espe- 

 cially in sufficient quantity to serpentinize masses hun- 

 dreds of feet thick. Moreover, unless the water were 

 charged with silica, its reaction with olivine would 

 form brucite as well as serpentine, not serpentine alone 

 (Bowen and Tuttle, 1049. p. 452). 



Many phenomena seem to forbid our invoking ex- 

 pansion by hydration to account for the internal shear- 

 ing in the masses. As has been pointed out, the blocks 

 in the serpentine bodies are massive and unsheared, and 

 show pseudomorphic textures after peridot ite with ap- 

 parently unrotated remnants of olivine crystals, which 

 would seem scarcely possible if expansion were effec- 

 tive. To obtain serpentine from peridotite this pseu- 

 domorphism, of course, requires a loss of material, 

 principally magnesia and some silica, and the proi-e~> 

 is therefore one of replacement rather than simply 

 hydration. Inasmuch as the matrix of sheared serpen- 

 tine has exactly the same chemical composition as the 

 blocks it encloses, it becomes unreasonable to suppose 

 that the matrix owes its origin to a simple hydra! ion 

 process involving expansion, rather than to the same 

 replacement process that formed the serpentine of the 

 blocks. Other objections to the proposed expansion 

 in place are found in the lack of local outward bulges 

 along the contacts, and in the complete absence of any 

 veinlets of serpentine minerals in the bordering wall- 

 rock, such as might be expected to result from the 

 squeezing out of any residual liquid by the expansion 

 process. 



Intrusion of low-temperature serpentine magma has 

 been shown by Bowen and Tuttle < l!l!>. p. d:'.! to be 

 impossible. It was also discarded by us because it 



failed to explain the internal structures of the serpen- 

 tine bodies. 



The theory of origin that best tits our observations 

 assumes that the serpentine masses were plastically 

 injected as serpentine. This allows two possibilities. 

 The material may have begun its upward migration 

 as a crystal mush resulting from the cooling of a 

 magma, and have been serpent inized during intrusion 

 by loss of temperature concurrent with hydration by 

 absorption of water from the surrounding rocks. Or, 

 as appears more likely, the serpentine masses may rep 

 resent plastic injections from a deeper seated mass. 

 which had the composition of peridotite but had al- 

 ready been serpentinized. 



The writers visualize the serpentine as having 

 formed from solid peridotite at an unknown depth, 

 largely by a process of replacement involving tin 

 cape of large amounts of magnesia and some silica 

 into the walls of the surrounding chamber. Subse- 

 quently the serpentine mass was broken up and 

 squeezed plastically into its present positions. An 

 exceedingly small amount of the serpent ini/.at ion prob 

 ably did take place, however, after the breccia! ion of 

 the rock; this is indicated by the peripheral develop- 

 ment of serpentine minerals in small fractures around 

 the margins of the blocks, generally -it right angles 

 to their margins but locally parallel to them. 



This concept is believed to explain adequately the 

 hydration of all the serpentine, regardless of the size 

 of the body in which it occurs or of position within 

 the body. It explains the textures of the unsheared 

 blocks, which could only have formed in a solidified 

 rock, and it further permits the differentiation band- 

 ing seen in some of the blocks. This concept als" 

 plains the general lack of either thermal or hydro 

 thermal alteration alonjr the walls of the intrusive 

 bodies, for the temperature must have been less than 

 500C, above which serpentine cannot exist. It will 

 account for the blocky structure of the inner parts 

 of the larger bodies, the more sheared condition of 

 their margins, and the thorough shearing of the 

 smaller bodies. 



That :-erpentine is capable, of plastic intrusion is 

 shown bv the known examples of "cold intrusions" of 

 serpentine into some of the vounger rocks of the ('oast 

 Hanges, by its ability to form extensive slides on sur 

 faces with a low angle of slope, and by the way the 

 floors and walls of some, mine workings in serpentine 

 even at relatively shallow depths tend to move inward. 

 The degree of plasticity required for the intrusion of 

 the serpentine masses, particularly the small ton- 

 and apophyses bordering some of the larger ma 

 is perhaps the chief objection to ibis concept, and is 



