64 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEX DISTRICT. CALIFORNIA 



and 4, based on percentage times the average specific 

 gravity of the rocks, show the relative amounts of the 

 various oxides present in a unit volume of the average 

 rocks. Column 5 shows the gains and losses of the 

 oxides involved in the change from serpentine to 

 silica-carbonate rock, and the same is presented graph- 

 ically in figure 47. It is readily apparent that the 

 principal changes are loss of water and a gain of car- 

 bon dioxide, but there is also some loss of magnesia. 

 Other minor changes indicated are a loss of aluminum 

 and ferric iron and a gain of silica and ferrous iron, 



although these changes are so small that they might 

 be due to sampling errors or an insufficient number 

 of analyses. Some calcium also has been added, but 

 this was largely introduced in late veinlets of dolo- 

 mite. The calcium, therefore, represents an addition, 

 rather than an essential part of the reactions involved 

 in the conversion of serpentine to silica-carbonate 

 rock. 



This conversion of serpentine to magnesite and 

 quartz can be represented by the following equation 

 (after Turner, F. J., 1948, p. 135): 



H 4 Mg 3 Si 2 O 9 +3CO 2 - 

 (276 g; llOcc) 



SMgCO, + 

 (252g;84cc) 



2SiO 2 +2H 2 O 

 (120 g; 44.5 cc) 



If the process were strictly one of replacement, however, 

 the quantities indicated on the right side of the equation 

 would be too large, and it is necessary to assume that 



silica or magnesium, or both, have gone off in solution. 

 As the foregoing chemical analyses surest the loss of 

 magnesium, the equation may be revised as follows: 



H 4 Mg 3 Si 2 O 9 



(276 g; 110 cc) 



2.34MgC0 3 



2SiO 2 



(198.6 g; 65.5 cc) (120 g; 44.5 cc) 



2H 2 Q-fQ.66MgCO 3 

 Goes off in solution 



Using the values given and the amount of magnesium 

 carbonate in the average silica-carbonate rock (column 

 2, table 13), one obtains a theoretical amount of 34 

 percent silica if only magnesium is lost and there is no 

 volume change. This agrees so closely with the 

 analytical value of 33.77 percent that there can be 

 little doubt that the process of replacement of serpentine 

 by magnesite and quartz also involved a loss of 

 magnesium. 



That the solutions responsible for the conversion of 

 serpentine to silica-carbonate rock are not genetically 

 related to the serpentine itself is indicated by two lines 

 of evidence (1) the distribution of the silica-carbonate 

 rock relative to the serpentine bodies in the district 

 and (2) the age relations of the two. The geologic 

 map of the district (pi. 1) shows that the silica-car- 

 bonate rock is virtually confined to two of the serpen- 

 tine zones, in both of which it is abundant ; the map 

 shows also that some of the largest areas of serpentine 

 are not accompanied by silica-carbonate rock. If the 

 rock were formed by solutions that had their source 

 in the serpentine or a related magma body, one might 

 expect it to have a more widespread and regular dis- 

 tribution. Furthermore, the serpentine was largely 

 intruded in the Late Cretaceous, whereas the silica- 

 carbonate rock was not formed until after the middle 

 Miocene. 



The determination of the age of the silica-carbonate 

 rock is based in part on the abundance of pebbles and 

 boulders of serpentine, without accompanying silica 

 carbonate rock, in a conglomerate of middle Miocene 



age exposed in a road cut about 5,000 feet southwest 

 of the summit of Mine Hill. Supporting evidence is 

 afforded by a small contorted mass of unbroken si lira - 

 carbonate rock enclosed by Miocene shale, exposed in 

 a roadcut about 5,000 feet west of the bend in Guada- 

 lupe Canyon below the Guadalupe mine. This mass of 

 silica-carbonate rock is believed to have formed after 

 the injection of serpentine into the shales, for it is 

 hard to imagine how so brittle a rock could have been 

 so extremely contorted without being shattered. The 

 contortion probably took place in the serpentine be- 

 fore it was altered. 



The upper limit to the age of the silica-carbonate 

 rock can be placed only relatively to the quicksilver 

 ores, which are believed to be Pliocene in age. Be- 

 tween the formation of the silica-carbonate rock and 

 the formation of the ores contained in it the rock was 

 extensively fractured, but whether or not this resulted 

 from forces applied during a long-extended period of 

 hydrothermal alteration, responsible for the formation 

 of the host rock and in its later stages the ores, could 

 not be determined. 



OTHER UPPER CRETACEOUS ROCKS 



Rocks apparently younger than the Franciscan 

 group, but also of Late Cretaceous age, crop out in 

 two widely separated parts of the district. Hera use 

 they differ in lithology and degree of deformation. 

 they were mapped as separate cartographic units and 

 are believed to have been deposited at different linn-: 

 one of these is exposed only in the Sierra A /.til in the 

 south-central part of the district : the oilier is exposed 



