54 



UNITED STATES MINERAL RESOURCES 



during mineralization. The evins may be hundreds 

 to thousands of feet long, as much as several tens 

 of feet thick, and widely spaced, or they may be 

 relatively shoi-t, thin, and closely spaced. The veins 

 are commonly curvilinear and from place to place 

 change in strike and dip; some veins split to form 

 one or more additional veins. 



The massive deposits occur in lenticular bodies 

 measuring hundreds to thousands of feet in width 

 and length and as much as 200 feet thick ; they con- 

 form to controlling structures such as bedding and 

 to the contacts between rock units. The vein depos- 

 its are structurally controlled, although most occur 

 in granitic rock. They are in multiple sets of small 

 faults or fractures, some of which may have been 

 guided along a preexisting structural grain. Pre- 

 ferred mineralization of one set of fractures or 

 faults over another may be due to availability of 

 openings during the mineralization process. 



Some massive bodies measuring hundreds of feet 

 in length and width occur along or associated with 

 major faults. The Cerro de Pasco deposit of Peru is 

 located along a thrust fault of regional extent, but 

 the significance of the fault as a controlling factor 

 is unknown. The upper plate is mineralized and 

 encloses manto-type sulfide deposits. 



Enargite-bearing copper-zinc-lead deposits are 

 among the largest ore deposits known. The deposit 

 at Butte, Mont. (Meyer and others, 1968, p. 1395- 

 1403), from 1880 to 1964 produced 318,234,680 

 pounds of arsenic compounds, although arsenic was 

 not recovered during all this period. The tenor of 

 mined copper over this period was about 21/2 per- 

 cent ; that of arsenic is estimated to have been about 

 0.1 percent. Some of the other deposits of this type 

 are, or will be when fully developed, comparable in 

 size and tenor. 



Other important deposits are Chuquicamata, Chile 

 (Taylor, 1935, p. 473-484), Cerro de Pasco (Mc- 

 Laughlin and others, 1935, p. 517-527) and Moro- 

 cocha, Peru (Graton and others, 1935, p. 528-544), 

 and Lepanto, Republic of the Philippines (Kinkel 

 and others, 1956). 



Enargite-bearing vein deposits are generally 

 thought to be hydrothermal in origin and to have 

 been derived from adjacent granitic plutons. 



The massive deposits were long considered to be 

 sedimentary-syngenetic veins or lodes that were 

 deposited by ascending solutions or by hydrothermal 

 replacement of crushed or sheared zones. More re- 

 cently, they have been considered to be of volcanic- 

 syngenetic origin and to have been produced by 

 base-metal-rich sulfide emanations emitted during 

 submarine volcanism (Anderson, 1969). 



ARSENICAL PYRITIC COPPER DEPOSITS 



Arsenical pyritic copper deposits occur in Pre- 

 cambrian terranes associated with metasedimentary 

 and metavolcanic rocks, in Precambrian magmatic 

 granite, and in orogenic belts with carbonate rock. 

 Host rocks include rhyolite or tuffaceous schist, bio- 

 tite schist, chlorite or chlorite-sericite schist, anda- 

 lusite, sericite schist, conglomerate, calcareous brec- 

 cia, and limestone. 



The metallic minerals include pyrite, chalcopyrite, 

 pyrrhotite, arsenopyrite, sphalerite, galena, pyrar- 

 gyrite, tetrahedrite or tennantite, and native gold; 

 gangue minerals include quartz, tourmaline, dolo- 

 mite, ankerite, and sericite. Bismuth minerals are 

 present in some deposits. Arsenopyrite and tennant- 

 ite are the only arsenic-bearing minerals; arseno- 

 pyrite occurs sparingly or abundantly and is either 

 evenly distributed or concentrated in nodular 

 masses ; tennantite, although more evenly distributed 

 in deposits in which it is present, is very minor. 



Arsenical pyritic copper deposits occur in massive 

 tabular or lenticular bodies as much as several 

 hundreds of feet long and at least 100 feet thick, 

 or in pipelike masses several hundred feet in diam- 

 eter and thousands of feet in vertical extent. Some 

 deposits of this type seem to be controlled by steeply 

 dipping shear zones or drag folds. They also occur 

 consistently at or near the contacts of rock units. 

 Deposits in foliated rocks follow the grain of the 

 schistosity, a feature preserved in the ore; frac- 

 tured or brecciated textures that were formed dur- 

 ing mineralization are also displayed in the ore. 



The size of the deposits ranges from a few hun- 

 dreds of tons to millions of tons. The Boliden mine 

 of Sweden, now exhausted, produced 8,341,550 

 metric tons of ore which contained 556,000 metric 

 tons (620,000 short tons) of arsenic, an average of 

 6.91 percent, in addition to 1.42 percent copper, 15.2 

 grams per metric ton of gold, and 49 grams per 

 metric ton of silver. Other deposits of this type 

 which contained more copper (5-6 percent) con- 

 tained only a fraction of 1 percent arsenic. 



In addition to the Boliden deposit (Grip and 

 Wirstam, 1970), examples of this type include the 

 Rammelsberg deposits in the Federal Republic of 

 Germany (Lindgren and Irving, 1911), the United 

 Verde mine in the Jerome area of Arizona (Lind- 

 gren, 1926, p. 32-35, 61-78 ; Anderson and Creasey, 

 1958, p. 92), and several deposits on the island of 

 Honshu, Japan (Kanehara, 1935, p. 688). 



Arsenical pyritic copper deposits have been con- 

 sidered epigenetic and to have formed by replace- 

 ment; those in schistose rock, for example, display 

 relic foliation inherited from the host. The miner- 



