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UNITED STATES MINERAL RESOURCES 



is a close similarity in mineralogy between large- 

 scale alteration zones extending outward from 

 porphyry deposits and small-scale alteration zones 

 successively enveloping sulfide veins. 



Ore pipes are rodlike in form as contrasted to 

 tabular veins. They may form in fractured or brec- 

 ciated rock at the intersections of faults or by ex- 

 plosive release of gases derived from intrusive 

 magmas. These latter are called breccia pipes be- 

 cause of the broken rock associated with them. At 

 many places copper deposits in breccia pipes show 

 a close spatial and genetic relation to mineralized 

 porphyry intrusions as at Cananea, Mexico (Ve- 

 lasco, 1966) or a close geographic relation to por- 

 phyry copper deposits as in the porphyry copper 

 belt in Chile (Sillitoe and Sawkins, 1971). 



Replacement deposits form in reactive host rocks 

 near the intrusive contacts or along mineral veins. 

 These hosts may be sedimentary rocks such as lime- 

 stone, dolomite, or calcareous sandstone or even 

 diabase sills as at Ray, Ariz. (Metz and Rose, 1966). 

 Replacement deposits tend to be tabular in form 

 and may be roughly concordant with bedding. 

 Though high in grade at some mines, when these 

 deposits are removed as part of a large low-grade 

 ore body they are generally regarded as part of 

 the porphyry copper ore body, as at the Mission 

 and Pima mines in Arizona. Mining of porphyry 

 ore at Bingham, Utah, encroaches steadily on chal- 

 copyrite replacement bodies in limestone beds to the 

 west, and the distinction between ore in porphyry 

 and ore in wallrocks becomes academic. 



Favorable host rocks for replacement deposits 

 may have continuity over large areas, and the 

 recognition of their stratigraphic position is of great 

 importance in the search for new deposits. In Ari- 

 zona, increasing attention is being paid to details 

 of local stratigraphy in attempts to locate hidden 

 replacement deposits (Eyde, 1972). 



Vein, pipe, and replacement ores have a more 

 varied mineralogy than the porphyry deposits. Chal- 

 copyrite and bornite are the most common hypogene 

 minerals. Chalcocite and covellite may be present as 

 hypogene minerals in addition to forming secondary 

 enrichment blankets. At Butte, Mont. (Meyer and 

 others, 1968), main-stage mineralization gave rise 

 to veins of four distinct types: veins of the deep- 

 level chalcopyrite zone containing chalcopyrite, 

 bornite, and tennantite; veins containing chalcocite- 

 enargite in the central zone; bornite-chalcocite- 

 chalcopyrite-enargite veins in the intermediate zone ; 

 and carbonate veins containing rhodochrosite, spha- 

 lerite, galena, and rhodonite in the peripheral zone. 

 At the Magma mine in Arizona (Hammer and 



Peterson, 1968) vein ores contain chalcopyrite, 

 bornite, enargite, tennantite, chalcocite, digenite, 

 and sphalerite. Enargite predominates in the lower 

 levels and gives way upward to tennantite, but 

 sphalerite predominates in the upper levels. Digenite 

 is present only in the lower levels. In the limestone 

 replacement ores, chalcopyrite is predominant. 



Mineral zoning such as is exhibited at Butte and 

 Magma is present to some extent in most vein and 

 replacement deposits. In addition to this a larger 

 scale mineral zoning has been noted around most 

 mineral deposits related to felsic intrusive rocks. 

 Lowell and Guilbert (1970) in their tabulation of 

 27 porphyry copper deposits described some periph- 

 eral mineralization in all the deposits and arcuate 

 clusters of small mines and prospects in 23 of the 

 deposits. Most of these peripheral veins contain 

 gold and silver values along with pyrite, chalco- 

 pyrite, sphalerite, specularite, enargite, famatinite, 

 tetrahedrite, barite, and manganese and vanadium 

 minerals. Recognition of a cluster of small veins as 

 part of an outer zone surrounding a buried ore de- 

 posit may be an important step in the discovery of 

 new copper resources. 



Vein and replacement deposits are difficult to 

 characterize in terms of tonnage and grade. If the 

 Butte district is considered as one deposit — and no 

 criteria seem to exist as a basis on which to sub- 

 divide it into smaller deposits — the amount of con- 

 tained copper metal is enormous, rivaling that of 

 the largest Andean porphyry deposits. In the pe- 

 riod 1880 to 1964 the Butte mines produced more 

 than 8 million tons of copper (Meyer and others, 

 1968). In Arizona, major vein and replacement de- 

 posits fall into a field (in fig. 22) with an average 

 copper content of 350,000 tons of copper each. 



In general, copper resources in vein, pipe, and 

 replacement deposits are small in comparison to 

 those of the closely related porphyry copper depos- 

 its, and discoveries of new deposits similar to Butte 

 or Magma have declined sharply in the last few 

 decades. The relatively small size of the ore bodies 

 and alteration envelopes makes these deposits diffi- 

 cult targets in areas of deep overburden. It is likely, 

 however, that vein, pipe, and replacement ore bodies 

 will be discovered incidentally in the search for 

 porphyry deposits. 



STRATA-BOUND DEPOSITS IN SEDIMENTARY ROCKS 



Strata-bound deposits in sedimentary rocks in- 

 clude some of the world's largest sources of copper. 

 The sedimentary deposits of Zaire and Zambia alone 

 make up 15 percent (table 39) of the world's identi- 

 fied resources, and enormous deposits are known in 



