ARSENIC 



55 



alizing solutions were thought to be hypothermal, 

 although no deposit could be traced with certainty 

 to a magmatic source. More recently, the supra- 

 crustal host rocks appear to be the source of the 

 metals ; the metals, mobilized by deep-level granitiza- 

 tion, moved ahead of a migmatite front to accrete 

 in favorable host structures or strata (Grip, 1960, 

 p. 3-14). A volcanic-syngenetic origin also has been 

 suggested for deposits of this type. 



NATIVE SILVER AND NICKEL-COBALT 

 ARSENIDE DEPOSITS 



Deposits containing native silver and arsenides 

 of nickel and cobalt are numerous in Precambrian 

 shield areas and in orogenic belts. In shield areas 

 the deposits are associated with belts of volcanic 

 and related intrusive rocks and with sedimentary 

 rocks derived from volcanic rocks, whereas in oro- 

 genic belts the deposits appear to be associated with 

 small igneous bodies that were intruded into a 

 variety of sedimentary rocks. The host rocks are 

 diabase, mafic and intermediate lavas, conglomerate, 

 graywacke, quartzite, and metamorphosed and gra- 

 nitized sedimentary rocks. 



The major metallic minerals in these deposits are 

 native silver, argentite, chalcopyrite, galena, pyrite, 

 pyrrhotite, sphalerite, tetrahedrite, and native bis- 

 muth, accompanied by lesser amounts of proustite, 

 pentlandite, polybasite, stephanite, and stromeyerite. 

 Collectively important is the suite of arsenide, sul- 

 fide, sulfarsenide, and antimonide minerals which 

 include smaltite-chloanthite (CoAsz-NiAsa), domey- 

 kite (CusAs), safflorite (CoAsa), rammelsbergite 

 (NiAsa), skutterudite (CoAss), gersdorfite (NiAsS), 

 cobaltite (CoAsS), glaucodot ( [Co,Fe]AsS), nicco- 

 lite (NiAs), dyscrasite (AgaSs), and breithauptite 

 (NiSb), as well as lollingite and arsenopyrite. Ura- 

 ninite is present in a few of the deposits. 



Secondary minerals include the hydrous arsenates 

 annabergite (NigAsoOg-SHaO), erythrite (CoaAsoOs 

 •8H2O), and scorodite, as well as limonite and man- 

 ganese oxide. The gangue minerals differ from de- 

 posit to deposit and include mainly calcite, dolomite, 

 and quartz and lesser amounts of rhodochrosite, 

 barite, and fiuorite. 



Native silver and nickel-cobalt arsenide deposits 

 commonly occur in braided vein systems in which 

 the individual veins in general range in thickness 

 from a fraction of an inch to a foot ; a very few are 

 as thick as 8 feet. The veins are commonly relatively 

 short, extending only a few hundred feet along 

 strike and dip, but some have been traced for 

 thousands of feet. 



Some deposits in shield areas occur along exten- 



sive faults or fault-related fractures, but more com- 

 monly they are localized by more subtle features, 

 such as fractures that developed parallel to the con- 

 tacts of sills and other intruded rock bodies and 

 along unconformities underlying locally deposited 

 favorable host rocks. In orogenic belts native silver 

 and nickel-cobalt arsenide deposits have formed at 

 such sites as intersection of fracture veins with 

 flat crushed zones in gneiss or in preferred host 

 rocks such as slate. 



Native silver and nickel-cobalt arsenide deposits 

 range widely in size and tenor; individual veins 

 have produced tens of thousands of tons of ore. 

 Arsenic constitutes a significant part of the ore; 

 carload lots of smelter-run material have run as 

 high as 42 percent arsenic, but the average runs 

 about 2-3 percent. 



Important producing areas include the Cobalt 

 and Thunder Bay districts of Ontario, Canada 

 (Lang and others, 1970, p. 210-215), the Annaberg 

 and Schneeberg areas of the German Democratic 

 Republic (Lindgren, 1933, p. 602-603), the Andreas- 

 berg area in the Harz Mountains of the Federal 

 Republic of Germany (Lindgren, 1933, p. 601-602), 

 and the Kongsberg district, Norway (Vokes, 1960, 

 p. 14-16). 



Native silver and nickel-cobalt arsenide deposits 

 originally were considered to be derived from dia- 

 base sills, either through differentiation and vein 

 injection, or by diffusion and metasomatic fixation. 

 It was later found, however, that some deposits are 

 located where diabase sills are absent and that 

 where deposits are located near sills, the sills were 

 completely crystallized prior to vein formation. Re- 

 cently, the restricted lateral and vertical extent of 

 the veins has been interpreted to indicate that the 

 metals were derived from the rocks either cut by the 

 veins or in close proximity to the veins, rather than 

 from a magmatic source. The only rocks sufficiently 

 rich in copper, nickel, and cobalt to supply the metals 

 are greenstone and black pyritic rock interbeded 

 with greenstone. It is theorized that these metals and 

 other relatively rare metals, including arsenic, mi- 

 grated by diffusion through a static water medium 

 to favorable structural or stratigraphic depositional 

 sites (Lang and others, 1970). 



ARSENICAL GOLD DEPOSITS 



Arsenical gold deposits occur in the broad shield 

 and lesser areas of Precambrian rock and in oro- 

 genic belts. 



The lithologic characteristics of the Precambrian 

 terranes differ in detail but are alike in general. 

 Precambrian arsenical gold deposits are mostly in 



