ARSENIC 



53 



stituting about 2 ppm (parts per million) of igne- 

 ous rocks (Onishi and Sandell, 1955) but 60 ppm 

 or more of magmatic sulfide minerals (Rankama and 

 Sahama, 1950, p. 739). Geochemically, arsenic is 

 similar to antimony and bismuth. All three elements 

 combine with sulfur, selenium, and tellurium. They 

 form sulfosalts, arsenides, and antimonides with 

 various heavy metals and show a preference for 

 combining with copper, iron, nickel, and cobalt. 



Some sulfide ores contain varying amounts of 

 arsenic, antimony, and bismuth. Research on the 

 mineralogy of these elements is incomplete; their 

 presence has been established only in galena. 



Arsenic occurs sparingly in the early and main 

 stages of magmatic crystallization. Early magnetite 

 and hematite contain little arsenic, and although the 

 early magmatic sulfide minerals contain relatively 

 large amounts, the late magmatic sulfides are 

 strongly enriched. Arsenic first occurs as a distinct 

 mineral (arsenopyrite) during the pegmatitic stage, 

 but in most minerals it develops in the pyrometaso- 

 matic and hydrothermal stages of magmatism. 



Arsenic tends to precipitate in extremely fine 

 grained sedimentary materials that are made up of 

 chemically undecomposed comminuted rock and 

 insoluble rock residues derived through hydrolytic 

 decomposition. It is especially concentrated in sedi- 

 ments that contain relatively large quantities of 

 iron and manganese oxides, principally by adsorp- 

 tion on ferric hydroxide. Whereas as little as 4 ppm 

 arsenic is found in bottom sediments of the Atlantic 

 Ocean, as much as 65-650 ppm is found in ancient 

 sedimentary iron ores. It is driven from iron-rich 

 sediments through metamorphic processes. Arsenic 

 is also precipitated with pyrite that was formed in 

 carbonaceous oozes. More than 3,000 ppm of arsenic 

 is reported in sedimentary pyrite. 



Arsenic forms the anion AsO„ which is iso- 

 morphous with the anion POi; consequently, arsen- 

 ates substitute for phosphates and are present in 

 both marine organisms and phosphate deposits. A 

 small part of the arsenic in marine environments, 

 however, escapes precipitation and remains in solu- 

 tion, possibly as an arsenite. 



MINERALS 



Arsenic occurs in more than two dozen minerals, 

 but only a few are commonly found in ore deposits. 

 The primary arsenic minerals are arsenopyrite 

 (FeAsS), lollingite (FeAs,), smaltite (CoAso), 

 chloanthite (NiAs,), niccohte (NiAs), tennantite 

 (CUvAssSr), enargite (Cu.AsSJ, and proustite 

 (Ag:,AsS,). 



The supergene arsenic minerals are arsenolite 



(AsoOa). mimetite (PbCl-Pb.AssOis), olivenite 



(CusAs^Os-CuCOH),), realgar (AsS), orpiment 



(AS2O3), scorodite (FeAsOi -21120), and pearceite 

 (Ag,AsSe). 



TYPES OF DEPOSITS 



No ore deposit is currently being mined solely 

 for arsenic, although in the past, a few deposits high 

 in arsenic were so mined for brief periods. Arsenic 

 occurs in various types of metalliferous deposits 

 which are classed in their approximate order of 

 importance on the basis of estimated past produc- 

 tion. They are as follows : 



1. Enargite-bearing copper-zinc-lead deposits 



2. Arsenical pyritic copper deposits 



3. Native silver and nickel-cobalt arsenide deposits 



4. Arsenical gold deposits 



5. Arsenic sulfide and arsenic sulfide gold deposits 



6. Arsenical tin deposits 



7. Other deposits 



ENARGITE-BEARIXG COPPER-ZINC-LEAD DEPOSITS 



Enargite-bearing copper-zinc-lead deposits occur 

 almost exclusively in orogenic belts in areas char- 

 acterized either by granitic bodies or by volcanic 

 rocks including vent deposits and mafic dikes and 

 sills. Host rocks, in addition to granitic and vol- 

 canic rocks, include metasedimentary rocks of clastic 

 origin and rarely carbonate rocks. 



Enargite-bearing copper-zinc-lead deposits have 

 complex mineralogies. This is due in part to repeti- 

 tive episodes of dissimilar mineralization, in part to 

 supergene alteration, and in part to the location of 

 some deposits in extremely arid climates where 

 otherwise soluble minerals are stable. 



The more important minerals characteristic of 

 the ores are enargite-famatinite, tetrahedrite, and 

 pyrite, with quartz gangue; enargite-famatinite 

 with native bismuth, argentian galena-sphalerite, 

 tetrahedrite, pyrargyrite, and stephanite, with 

 pyrite gangue; pyrite, enargite, tennantite, bornite, 

 chalcocite, sphalerite, and a little chalcopyrite and 

 covellite ; and pyrite, chalcopyrite, sphalerite, galena, 

 and lesser amounts of tetrahedrite and enargite. 

 Supergene chalcocite is important in those deposits 

 where near-surface copper was oxidized, leached, 

 and redeposited at depth. In the oxidized zones of 

 some deposits, native silver is important. 



Enargite is the only important arsenic-bearing 

 mineral; arsenolite and scorodite are present in the 

 oxidized zone of some deposits but are sparse. 



The deposits occur in veins and in massive blanket 

 or lensoidal bodies. Vein deposits occur in sets of 

 fractures or faults ; some of the faults were active 



