44 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



tribution to the problem is a noteworthy discussion by 

 Taliaferro (1943b, p. 159-182), who concluded that 

 the rocks owed their origin to selective and non- 

 selective additive metamorphism along the contacts of 

 basic and ultrabasic intrusives, being formed only in 

 those places where the quantity of volatiles and solu- 

 tions escaping from the magma during its emplace- 

 ment was large. Primarily because of the areal dis- 

 tribution of the metamorphic rocks in the New Al- 

 maden district, we do not believe that the origin of 

 all the various metamorphic rocks can be ascribed to 

 contact metamorphism caused by the ultramafic in- 

 trusive rocks. 



The problem of the origin of these rocks is two- 

 fold: first, what were the original rocks, and, sec- 

 ond, under what conditions were those rocks reconsti- 

 tuted? Geologists who have studied these rocks in 

 recent years seem to agree that the more widespread 

 and abundant types were formed by metamorphism 

 of the various rocks in the Franciscan group, although 

 it has been suggested that the chlorite-actinolite and 

 chlorite-actinolite-talc rocks may have been derived 

 from serpentine. In the New Almaden district the 

 field relations and textures of many of the rocks indi- 

 cate their derivation from rocks of the Franciscan 

 group ; and, it seems likely, though it cannot quite be 

 proved, that they were all derived from these rocks. 



The hornblende schists and amphibolites can be 

 traced directly to tuffaceous greenstones; they occur 

 in patches along a belt of tuff beds, which for reasons 

 unknown are especially susceptible to metamorphism. 

 Some of the glaucophane schists contain relict textures 

 identical with those of the lavas of the Franciscan 

 group, and others show the incipient development 

 of glaucophane in normal-appearing graywacke. Still 

 others, to judge from their mineral content and 

 general appearance, seem to have been derived 

 from cherts. The crocidolite-bearing rocks, also, 

 are believed to be derived from normal cherts of the 

 Franciscan group, for it was possible to collect speci- 

 mens showing the alteration through small stages. 

 The quartz-muscovite schists grade into feldspathic 

 graywacke, from which they were probably derived. 

 The actinolite-chlorite rocks, found only as polished 

 nodules along the borders of a few serpentine bodies, 

 may be fragments dragged up by the serpentine, or 

 they may be altered mafic rocks; but it seems un- 

 likely that they were derived from serpentine, which 

 nowhere shows a marginal metamorphosed zone con- 

 taining any actinolite or chlorite. 



Evidence as to the metamorphic process might be 

 gained from the areal distribution of the rocks and 

 from their texture, mineralogy, and chemical compo- 



sition. Of these, only the distribution and mineral- 

 ogy are well known for the rocks of the New Almaden 

 district, but a general idea of the bulk chemical com- 

 position can be obtained from the mineralogic com- 

 position. In regard to texture the preponderance of 

 oriented platy and linear minerals in the metamorphir 

 rocks is significant. 



All the larger masses of hornblende-bearing meta- 

 morphic rocks are restricted to the central part of a 

 favorable tuff bed on Mine Hill and to another tuff 

 bed, or possibly a faulted segment of the same bed, near 

 the Guadalupe mine. Most of the masses border ser- 

 pentine sills; but in a few places along the same fa- 

 vorable bed small masses of hornblende rocks are 

 found at a considerable distance from any known 

 serpentine mass. Conversely, serpentine borders the 

 bed in a few places without the development of any 

 hornblende-bearing metamorphic rocks. Elsewhere in 

 the district there are many other serpentine bodies 

 that border tuffaceous greenstones, yet they have pro- 

 duced no apparent metamorphism. A particularly 

 good example of the latter type can be seen along the 

 Day tunnel of the New Almaden mine south of the 

 New Ardilla stope (pi. 4) where a bed of unmeta- 

 morphosed greenstone is included between two ser- 

 pentine sills, each of which is several hundred feet 

 thick. 



The textures of the hornblende-bearing metamorphic 

 rocks shed little light on the problem of their origin. 

 They are generally somewhat schistose, but they show 

 little linear orientation. Foliation is developed in 

 some, but most are apparently nonfoliate. Where 

 field relations are clear, the foliation appears to be 

 parallel to the bedding, and probably it was caused 

 by differences in the chemical composition of indi- 

 vidual beds. 



The hornblende-bearing rocks probably resulted 

 from the recrystallization of the tuff beds under rela- 

 tively low pressures, owing chiefly to their depth of 

 burial. Without chemical analyses it is not possible 

 to know whether the metamorphic rocks differ chemi- 

 cally from the tuffs from which they were derived, 

 and although they all have mineral assemblages that 

 could conceivably result from simple recrystallization, 

 volatile fluid escaping from nearby serpentine sills 

 may have helped to cause the recrystallization. The 

 general lack of metamorphic effects along these sills. 

 however, casts considerable doubt upon the ability of 

 the serpentine sills to effect metamorphism; yet the 

 general association of hornblende rocks with serpen- 

 tine suggests that in certain places, probably where 

 the composition or the water content of a tuff bed 

 was particularly favorable, the serpentine may have 



