ARSENIC 



57 



form a part of the cement around the breccia frag- 

 ments. 



Arsenic sulfide gold deposits occur in vein systems 

 that comprise numerous ore shoots. Such veins mea- 

 sure thousands of feet in length and several hundred 

 feet along the dip, and range in thickness from a few 

 feet to 200 feet. Lenses of realgar and stibnite as 

 much as 20 feet long and 4 feet thick occur in some 

 of the ore shoots. 



Both arsenic sulfide and arsenic sulfide gold de- 

 posits are controlled by such structural features as 

 fractures, shear zones, or other fault-related struc- 

 tures. The deposits occur as fissure fillings and large- 

 scale replacements. In some deposits or in the deeper 

 parts of some deposits, replacement was nonselec- 

 tive and all strata transected by the veins were 

 affected, whereas elsewhere replacement was selec- 

 tive. Some deposits in volcanic terranes may have 

 been concentrated along the contact of rocks of 

 different permeability. 



Arsenic sulfide deposits are small and range from 

 those which have produced a few tens of tons of ore 

 to those which have produced a few thousands of 

 tons. Some of the ore contains as much as 20 percent 

 arsenic but averages only a few percent. Arsenic sul- 

 fide gold deposits have yielded tens of thousands and 

 hundreds of thousands of tons of ore; arsenic in 

 these deposits was recovered as a byproduct and con- 

 stituted a small fraction of the ore, averaging not 

 more than 1 percent. 



Examples of arsenic sulfide deposits include sev- 

 eral in the State of Washington (Shedd, 1924, p. 

 51-55). The occurrence of arsenic sulfide minerals 

 with mercury has been reported in Oregon (Brown 

 and Waters, 1951, p. 234) and Alaska (Berg and 

 Cobb, 1967, p. 89-94). Arsenic sulfide gold deposits 

 are best exemplified in Nevada, where several de- 

 posits of this type have yielded byproduct arsenic 

 (Hewitt, 1968, p. 872-873 ; Nolan, 1935, p. Ill, 151- 

 156). 



Arsenic sulfide deposits are generally considered 

 to have come from sources deep within the crust 

 but to have been emplaced fairly close to the surface 

 at low temperatures. Their occurrence near centers 

 of volcanic activity reflects the common source of 

 mineralizing solutions and flow rocks, both of which 

 are assumed to be differentiates of the same magma 

 (Lindgren, 1933, p. 455). 



ARSENICAL TIN DEPOSITS 



Arsenical tin deposits occur mostly in orogenic 

 belts, but they occur sparsely in Precambrian ter- 

 ranes. The deposits are closely associated with gra- 

 nitic stocks or batholiths, are commonly emplaced 



in such bodies or close to their peripheries, and are 

 most numerous near the cupolas or summit areas 

 of the granitic bodies. 



Arsenical tin deposits also are emplaced in a 

 variety of sedimentary rocks (sandstone, shale, and 

 limestone), in metamorphic rocks (schist, gneiss, 

 and slate), and in older granitic rocks. No one host 

 rock seems to be more favorable than another. 



The ore minerals commonly are cassiterite, native 

 bismuth, bismuthinite, arsenopyrite, and wolframite 

 and lesser amounts of chalcopyrite, pyrite, pyrrho- 

 tite, galena, and sphalerite. Hematite, magnetite, 

 and ilmenite occur rarely. The gangue minerals are 

 quartz, which is most abundant, fluorite, lepidolite, 

 topaz, tourmaline, apatite, axinite, and, rarely, beryl 

 or orthoclase. Cassiterite is the principal ore min- 

 eral. Arsenopyrite is the only arsenic mineral in 

 this type of deposit. 



Arsenical tin deposits occur in veins and pipes. 

 The veins occur in simple planar or undulous form. 

 Some occur in braided systems; others are very ir- 

 regular and occur in stockworks. They may be as 

 much as 3 miles long, as much as 3,000 feet in ver- 

 tical extent, and a few inches to several tens of 

 feet thick. They formed by filling of open fissures 

 and replacement of wallrock. 



The pipes are roughly circular in plan, range 

 from several tens of feet to more than 100 feet in 

 diameter, and are several hundreds of feet in ver- 

 tical extent. The ore in some pipes is disseminated, 

 and the walls are indefinite; in others, the contact 

 of ore and wallrock is well defined. In some pipes, 

 arsenopyrite occurs in segregated masses. 



Vein deposits of arsenical tin occur in fissures 

 which formed in the chilled shell of congealing gra- 

 nitic plutons and extend into the adjoining country 

 rock. Faulting took place along some of the fissures 

 during mineralization, with the result that composite 

 veins developed. 



Some pipe deposits developed along shear zones or 

 other structures, but other deposits do not appear 

 to be located on preexisting structures. It is theor- 

 ized they were emplaced explosively in the manner 

 of diatremes and were initiated at points of incipi- 

 ent Assuring during a period of extraordinarily 

 high pressure. 



The size of arsenical tin deposits ranges from 

 hundreds of tons to hundreds of thousands of tons. 

 The early mined veins, lodes, and pipes were of 

 relatively high tenor, averaging several percent, but 

 because of their depletion, larger but lower tenor 

 deposits are now being worked. The arsenic content 

 of the deposits ranges from a fraction of 1 percent 



