96 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



tions. These supplement the comprehensive history of 

 the New Almaden mines given on pages 176-195. 



QUICKSILVER ORE BODIES 



In the New Almaden district the mineralized area 

 likely to contain ore bodies is relatively small, and 

 during the past hundred years all the surficial expo- 

 sures that appear to be mineralized have doubtless 

 been thoroughly examined. Nevertheless, it is be- 

 lieved that the area still contains undiscovered sub- 

 surface ore bodies, which must be found by either 

 actual mining or drilling. The prime purpose of this 

 report is to call attention to the places where the 

 chances for finding hidden ore bodies are best, in or- 

 der that any additional expenditure of money and 

 energy may have the best opportunity to revitalize 

 the district and add to the meager domestic produc- 

 tion of quicksilver. 



To arrive at conclusions as to whether or not a dis- 

 trict may contain hidden ore bodies, and, if so, where 

 they are most likely to be found, and whether they 

 can be expected to be worth the exploration cost nec- 

 essary to find them, an ore geologist must pursue sev- 

 eral lines of inquiry. In all of these, however, his 

 most reliable guide is the ore bodies already found 

 in the district under consideration or in similar dis- 

 tricts. He studies the character of the previously dis- 

 covered ore bodies to learn from their mineralogy and 

 textures how they were deposited and to accumulate 

 data leading to an understanding of the conditions 

 that prevailed when they were formed. He also stud- 

 ies the geologic environment, or structural control, of 

 the known ore bodies to learn what places were most 

 favorable for the deposition of ore, and then prepares 

 and studies maps of the entire district in order to 

 locate other places where similar environmental con- 

 ditions prevail. Such observations, however, are not 

 sufficient in themselves to allow him to predict that 

 areas with similar rocks and structures will contain 

 ore bodies; he has also to consider several other more 

 elusive problems. Such problems are what medium 

 transported the ore metal, what pathways did it fol- 

 low, what was the temperature and pressure raiige 

 over which it deposited the ore minerals, and at what 

 depth were the ores deposited. The solution of these 

 problems requires some knowledge of the time of 

 mineralization. Finally, the geologist must collect 

 information on the size and grade of the ore bodies 

 already mined, in order to present data that will be 

 helpful in judging whether the search for undiscov- 

 ered ore bodies is justified by the expectable value of 

 the ore that might be found. 



The following pages, together with the maps, will 



present as much of this information as is required to 

 understand why we believe that certain areas may 

 contain undiscovered subsurface ore bodies. Factual 

 data pertaining to the ore bodies are presented first, 

 interpretations follow, and the conclusions based on 

 I hoe data are given on pages I"n-l7t'>. The factual data 

 are presented in the following order: (1) Descrip- 

 tion of minerals, including ore minerals and others 

 deposited with them; (2) the distribution of these 

 minerals in the rock to form ore bodies, including size 

 and grade of the ore bodies: and (.'$) the distribution 

 of the ore bodies in the mining district, including a 

 discussion of their lithologic and structural environ- 

 ment. 



MINERALOGY 



The mineralogy of the quicksilver ore deposits is 

 comparatively simple. The only quicksilver-bearing 

 mineral of real economic importance is cinnabar, 

 though native mercury is found locally, and a little 

 metacinnabar and tiemannite are said to have been 

 found. Other sulfides that are closely related to tin- 

 ore deposits include pyrite, stibnite. rhulropyrite, 

 sphalerite, galena, and bornite, but of these only py- 

 rite is common or widespread. Arsenopyrite. which 

 was reported in 1854 (Blake, 1854, p. 438), has not 

 been observed since and probably was erroneously 

 identified. No marcasite has been found. The com- 

 mon nonmetallic gangue minerals, other than those 

 present in the silica-carbonate rock formed before the 

 quicksilver mineralization, are dolomite and quart/. 

 Calcite is rare and possibly nowhere gentle-ally re- 

 lated to the ore mineralization; chalcedony and opal 

 are also uncommon. Single occurrences of apophyl- 

 lite, gyrolite, pilinite, and barite have been noted. 

 Hydrocarbons, both tars and oils, accompany the ores 

 in many places, and in some place.- they appear to 

 have been deposited contemporaneously with them. 

 Secondary minerals deserving no further description. 

 are various iron and manganese oxides, jarosite, epso- 

 mite, aragonite, and zaratite. 



ORB MINERALS 

 Cinnabar (llgS) 



Cinnabar, the red mercuric sulfide, occurs most abun- 

 dantly as fine-grained to microcrystalliiie aggregate* 

 replacing silica carbonate rock, and it is less common 

 in pore spaces in other kinds of rocks, and as open 

 space fillings in veins. Well-formed crystals of cin- 

 nabar are unusually rare in the New Almaden district, 

 though small ones have been found in nearly all the 

 mines. Isolated equant crystals from 1 to _' mm in 

 diameter have been formed in some places on the 

 walls of fractures and in small vug>: these commonly 

 have rhombohedrons and pinacoid as their dominant 



