56 



UNITED STATES MINERAL RESOURCES 



or associated with felsic volcanic and related in- 

 trusive rocks including dacite, andesite, and basalt 

 or their altered equivalents. A few are hosted in 

 metamorphic rocks including quartzite and schist. 



The orogenic belts are mostly post-Precambrian 

 sedimentary volcanic and metamorphic rocks which 

 have been structurally deformed and intruded by 

 great masses of granitic rock. Arsenical gold de- 

 posits in orogenic belts occur in a variety of sedi- 

 mentary and volcanic rocks and are associated with 

 felsic and intermediate intrusive rocks. 



Arsenical gold deposits commonly contain native 

 gold in quartz and (or) sulfide minerals. Arsenic 

 occurs only in arsenopyrite in most deposits, but in 

 some deposits lollingite also occurs. Arsenopyrite is 

 commonly accompanied by pyrite and (or) pyrrho- 

 tite, which together may be as much as ten times 

 as abundant as arsenopyrite. Other sulfide minerals 

 that may be present include chalcopyrite, galena, 

 tetrahedrite, and sphalerite. Scheelite and wolfram- 

 ite occur rarely in some deposits. Sulfarsenide min- 

 erals are known but are extremely rare in arsenical 

 gold deposits. 



Gangue minerals, except for quartz, which is gen- 

 erally present, differ from deposit to deposit and 

 include ankerite, albite, and tourmaline. 



Most deposits occur as replacement veins in or 

 around rock that has been fractured or sheared by 

 faulting. Some deposits are simple in form and 

 occur as single persistent veins that are planar or 

 more commonly curvilinear and of even thickness, 

 whereas others are complex and occur as swarms of 

 lenticular veins, which extend short distances. De- 

 posits which were initially simple in form but which 

 underwent subsequent deformation occur in con- 

 torted pipelike or ribbonlike bodies, reflecting the 

 complexity and intensity of deformation. Sulfide 

 minerals in some deposits are evenly disseminated 

 through the vein, whereas in others they are re- 

 stricted to lenses or pods. 



Arsenical gold deposits characteristically are 

 structurally controlled and commonly occur on or 

 along fractured or sheared rock. Some, although 

 obviously emplaced on fault-related structures, are 

 more numerous where the structures transect a 

 particular rock type such as schist, suggesting that 

 the mineralizing process was sensitive to subtle dif- 

 ferences in rock chemistry. 



The arsenic content of these deposits diifers 

 markedly; most deposits within an area or district 

 may contain but a fraction of a percent arsenic, 

 whereas others within the same area or district may 

 contain several percent arsenic. In some deposits 

 the arsenic content is remarkably consistent, and 



the arsenic occurs in a fixed ratio with gold ; in 

 others, the arsenic content diminishes or increases 

 from place to place disproportionately to gold. 

 Arsenical gold deposits may contain as much as 20 

 percent arsenic, but the average content is no more 

 than 1 percent. 



Arsenical gold deposits range from hundreds of 

 tons to several hundred thousands of tons of ore. 

 The Jardine mine, Montana, between 1879 and 1942, 

 produced more than 7 million tons of ore from which 

 9 million pounds of arsenic was obtained, although 

 this production was for only 8 years of the mine's 

 life. The average grade is estimated to be less than 

 one-half of 1 percent. 



ARSENIC SULFIDE AND ARSENIC SULFIDE 

 GOLD DEPOSITS 



Arsenic sulfide and arsenic sulfide gold deposits 

 occur almost exclusively in orogenic belts and com- 

 monly are associated with Tertiary intrusive igneous 

 or volcanic rocks. Arsenic and mercury sulfide min- 

 erals occur together in some deposits. Elsewhere the 

 association is less intimate; deposits of arsenic, 

 mercury, and antimony occur separately but display 

 a regional zonal pattern which suggests a common 

 genesis. Arsenic sulfide minerals are also abundant 

 in some gold deposits. 



Host rocks include intermediate and felsic intru- 

 sive and volcanic rocks, volcanic sedimentary rocks 

 such as tufi'aceous sandstone, and limestone and 

 noncarbonate clastic rocks or their metamorphic 

 equivalents. 



Arsenic sulfide deposits are characterized by real- 

 gar and orpiment, almost to the total exclusion of 

 other arsenic minerals. Some small vein deposits 

 are composed entirely of one or the other of these 

 minerals. Composite deposits of arsenic sulfide min- 

 erals and mercury contain stibnite, cinnabar, meta- 

 cinnabar, calomel, and native mercury. Arsenic 

 sulfide gold deposits have a more complicated min- 

 eralogy; in addition to realgar and orpiment, they 

 contain pyrite, pyrrhotite, marcasite, arsenopyrite, 

 chalcopyrite, scheelite, molybdenite, sphalerite, cin- 

 nabar, stibnite, native arsenic, and native gold. The 

 gangue minerals include quartz, fluorite, barite, 

 alunite, sericite, and leverrierite. 



Many arsenic sulfide deposits occur as veins that 

 fill fractures and may be tens to hundreds of feet 

 long but which are commonly narrow, generally a 

 few inches to 1 foot thick. Other deposits occur in 

 breccia zones which extend hundreds of feet along 

 strike and dip and are as much as several feet 

 thick. In these deposits, the arsenic sulfide minerals 



