52 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



in figure 39; however, because many small unreplaced 

 granules of olivine are evenly distributed throughout 

 the rock, it is possible to deduce with a fair degree of 

 certainty the character of the original clunite. Most 

 of the olivine grains were anhedral, although some 

 had a few crystal faces; the largest grains were more 

 than 3 mm long, but the average length was about 

 1 mm. In shape they ranged from cquant grains to 

 slightly embayed prisms with a length four times their 

 width. The resultant texture was xenomorphic gran- 

 ular, like that normally found in fresh dunites. The 

 only original mineral other than olivine is chromite 

 or picotite, which occurs as subhedral grains about 

 0.1 mm in diameter. 



The first step in the process of serpentinization is 

 fracturing, which is closely followed by replacement 

 of the olivine by fibrous antigorite and the filling of 

 cracks with chrysotile. The fractures are variously 

 controlled ; a few are parallel to cleavage within single 

 olivine grains, others radiate from chromite grains, 

 and still others extend continuously through several 

 differently oriented olivine grains. The fractures all 

 belong to a single system, for they terminate and bend 

 at their junctions. The widest fractures are also the 

 longest and straightest, and in section these form a 

 polygonal pattern, each polygon of which may be fur- 

 ther divided into small polygons by smaller and less 

 continuous fractures. The resultant fracture pattern 

 is the basis for the familiar mesh structure developed 

 from olivine. The olivine bordering these fractures 

 is replaced by a wave of fibrous antigorite growing 

 from the cracks toward the olivine, and simultaneously 

 the fractures are filled with a chrysotile of low bire- 

 fringence, which generally encloses magnetite dust. 

 When the chrysotile has filled the narrow fractures 

 it replaces the fibrous antigorite previously formed 

 along the walls. 



The development of these two serpentine minerals 

 probably results from a single reaction, so that the 

 two are nearly contemporaneous. Nowhere was either 

 mineral found without its companion, and the two 

 occur in a fairly constant ratio of about three parts 

 of fibrous antigorite to one of chrysot ile. The reaction 

 apparently stops when the fibers of antigorite attain 

 a length of about 0.03 mm, unless, as happens in many 

 places, additional fractures open parallel to the origi- 

 nal one are filled with more chrysotile. If the process 

 of serpentinization is stopped :it this point, the partly 

 serpent in i/.ed dunite will contain many residual sub- 

 angular fragments of olivine, like "eyes." in the mesh 

 of serpentine minerals. The olivine fragments are 

 fresh, and their contacts with the fibrous antigorite 

 are sharp. The several remnants from each olivine 



crystal extinguish exactly together, which indicates 

 that they have not rotated during serpentinization and 

 suggests that the process has been strictly one of re- 

 placement involving no significant expansion. With 

 increasing serpent inization the remaining olivine is 

 replaced by serpophite. which has a slightly higher 

 index of refraction and much lower birefringence than 

 the fibrous antigoriie: and invariably the original con- 

 tact between antigorite and olivine is preserved as the 

 contact between antigorite and serpophite. At the 

 same time the magnetite dust is collected into larger 

 isolated crystals or strings of crystals along the wider 

 veinlets. The borders of chromite grains become fu/./y. 

 and in reflected light the grains arc seen to be coated 

 with magnetite. At the same stage in the alteration 

 process magnetite also replaces the chromite along 

 fractures. 



The serpentine of the district is mostly derived 

 from a harzburgite, a rock containing olivine and 

 orthopyroxene, but some of it is derived from Iherzo- 

 lite. which contains these minerals and also clino- 

 pyroxene. The distribution of the harzburgite and 

 Iherzolite is unknown, for the two pyroxenes cannot 

 readily be distinguished in the field; both rocks, how- 

 ever, are known to occur in a single body of serpen- 

 tine. Although the pyroxenes in most of the serpen- 

 tine have been replaced by bast it ic minerals, the 

 amount of pyroxene originally present in an unsheared 

 serpentine is easily estimated from the proportion of 

 bastite pseudomorphs. In most of the serpentine this 

 proportion is between 10 and 25 percent, but the 

 dunite previously mentioned contains no pyroxene, and 

 certain bands and segregations in the ultramafic rocks 

 contain more than 85 percent of pyroxene replaced by 

 bastite. 



No systematic variation in degree of sorpentiniza- 

 tion, either from the margins of a mass inward or 

 from the surface downward, was found. The process 

 of serpentinization of the typical peridotites, includ- 

 ing both harzburgites and Iherzolites, can be fairly 

 well traced by studying various thin sections of rock 

 collected throughout the district, if they are properly 

 arranged in increasing order of serpentinization. a- 

 was done in this study; it should be emphasized, how- 

 ever, that the result is a synthesis rather than a re- 

 port of the. changes effected in a single body of 

 peridot ite. 



Of the least serpent ini/.ed peridot ite studied in thin 

 section, a little less than half consisted of primary 

 minerals, including olivine. enstatite, and a elinopyrox- 

 ene that is probably augite. (See fig. 40.) The olivine 

 originally occurred as rounded anhedral grains, where- 

 as the pyroxene- generally show some crystal faces 



