702 



UNITED STATES MINERAL RESOURCES 



zinc minerals, especially to hemimorphite, but it is 

 not an approved mineral name. Franklinite, wil- 

 lemite, and zincite are the ore minerals of the 

 renowned unique zinc deposits at Franklin Furnace 

 and Sterling Hill, N. J. These minerals are uncom- 

 mon elsewhere in the world; however, willemite is 

 the chief zinc mineral in the recently discovered 



sizable deposits of Beltana and Aroona in the Flin- 

 ders Range of South Australia; and willemite, 

 zincite, hemimorphite, and hydrozincite constitute 

 the ore minerals in the extensive deposit at Vazante 

 in Minas Gerais, Brazil. 



Some characteristics of the common ore minerals 

 of zinc are given in table 152. 



Table 152. — Composition, zinc content, and selected physical characteristics of the common ore minerals of zinc 



Zinc 

 Mineral Composition content Specific Crysta 



(percent) gravity Hardness systen 



Sphalerite 



Wurtzite 



Zincite 



Franklinite 



Smithsonite 



Hydrozincite 



Willemite 



Hemimorphite 



Much zinc ore contains economically significant 

 amounts of lead, copper, silver, and gold. Where 

 present in the primary ores, the lead occurs as 

 galena (PbS) and the copper as chalcopyrite 

 (CuFeSo) . The silver is in solid solution in the ga- 

 lena or as exsolved traces of complex silver sulfides ; 

 more rarely, it accompanies the sphalerite. The gold 

 is generally native, although it may also occur with 

 silver in telluride minerals or in solid solution in 

 some of the iron sulfides such as pyrite. Such min- 

 eralogically complex ores are typical of deposits 

 formed in many areas of the Western United States 

 during episodes of Tertiary and Cretaceous volcan- 

 ism. They contrast markedly with other major zinc 

 deposits, such as most of those of the Tennessee 

 region, where the only sulfides are sphalerite and 

 local lesser concentrations of pyrite, and where, in 

 some deposits, barite or fluorite or both are signifi- 

 cant coproducts or byproducts. 



TYPES OF DEPOSITS 



Zinc generally occurs with lead in many diverse 

 geologic environments. The deposits have such dif- 

 ferent habitats and origins that they have been 

 classified according to many different schemes. 

 Within any mineralized district, area, or region 

 there is generally more than one type of deposit, 

 and it is not always clear where one type ends and 

 another begins. Consequently, for the purpose of 

 this report, zinc deposits, together with their asso- 

 ciated lead and other elements, are classified by 

 broad geologic environments as: (1) contact- 

 metamorphic deposits; (2) irregular replacement 

 deposits and associated fissure fillings; (3) vein 

 deposits ; (4) stratabound deposits in metamorphic 

 rocks; (5) stratabound deposits in cai'bonate rocks 



(Mississippi Valley- or Alpine-type deposits) ; (6) 

 stratiform deposits ; and (7) deposits formed by 

 supergene enrichment or laterization. 



Ore deposits or mines containing or producing 

 significant quantities of zinc, as the chief commodity, 

 as a major coproduct, or as an important byproduct, 

 are frequently classified by the associated metals, 

 thus: (1) zinc; (2) zinc-lead or lead-zinc, depend- 

 ing upon which metal is dominant ; (3) zinc-copper 

 or copper-zinc ; and (4) base metal, if all three 

 metals are of about equal importance. If silver is 

 also an important coproduct or byproduct, it too is 

 added to the class designation. 



CONTACT-METAMORPHIC DEPOSITS 



Contact-metamorphic deposits, also known as con- 

 tact-metasomatic or pyrometasomatic deposits, are 

 those contained in metamorphosed sedimentary 

 rocks adjacent to igneous intrusives. They are found 

 chiefly in carbonate rocks that have been altered 

 metasomatically to assemblages of silicated rocks 

 commonly referred to as skarn, tactite, or garnet 

 rock. Chalcopyrite, pyrite, pyrrhotite, sphalerite, and 

 molybdenite are common ore minerals. The sphalerite 

 is generally iron rich. The ore may be in tabular 

 bodies parallel to bedding in the host rock, in irregu- 

 lar or pipelike bodies roughly conformable to the 

 igneous contact, or in veins that transgress the 

 metamorphosed host rock. Small irregular replace- 

 ment deposits may occur in unaltered limestone 

 some distance from the intrusives. Although many 

 deposits of this type have been found in the United 

 States, most are small and only a few have been 

 significant producers. Examples include many of the 

 zinc deposits in the Central district of New Mexico 

 (Hernon and Jones, 1968) and those of the Darwin 



