706 



UNITED STATES MINERAL RESOURCES 



Beneath the gossan, an enriched zinc deposit was 

 formed by the deposition of willemite and other 

 secondary zinc minerals ; some galena and sphalerite 

 were replaced by supergene chalcocite. Continued 

 weathering further enriched the zinc, with the for- 

 mation of hemimorphite and hydrozincite on a 

 major scale. Further alteration followed, with the 

 consequent formation of zincite and additional 

 hydrozincite. 



The latter stage of enrichment probably began at 

 some time during the Tertiary, and it continues 

 today with additional concentration being effected 

 by eluvial processes. The foi-mation of the zincite 

 and hydrozincite in the last stage strongly suggests 

 the desilication process so typical of the last stage 

 of laterization in the formation of the bauxite and 

 iron and manganese oxide ores in the tropics. It 

 is now an easy step of speculation to concur with 

 the hypothesis of Rastall (supported by Palache, 

 1935, p. 23-24) that the Franklin Furnace-Sterling 

 Hill oxide-silicate ore in New Jersey is merely a 

 metamorphosed ancient iron-manganese-zinc laterite. 



Other high-grade oxidized ore deposits found in 

 the tropics are at Broken Hill in Zambia, one of the 

 larger zinc deposits in the world; and one recently 

 discovered and being developed in the Pa Daeng 

 Mountains of Thailand. The Thai deposit is reported 

 to contain about 31/3 million tons of ore containing 

 35 percent zinc fonned by the " * * * secondary 

 replacement [of] limestone, sandstone, and shale by 

 smithsonite, calamite [hemimorphite], hydrozincite, 

 and zinc-bearing clays" (Eng. and Mining Jour., 

 1972). The previously mentioned high-grade wil- 

 lemite replacement deposits in Cambrian dolomite 

 in the Beltana district of South Australia are also 

 considered tentatively as having been formed by 

 Tertiary supergene enrichment; although no asso- 

 ciation with primary sulfides has yet been discov- 

 ered, Muller and Donovan (1971, p. 233) reported 

 1-2 percent zinc in weakly mineralized limestone 

 with some of the zinc in solid solution in dolomite. 

 Also, " * * * Tertiary lateritic material, which 

 overlies the limestone in places * * * appears to be 

 capable of adsorbing up to 2 percent Zn." 



RESOURCES 



Table 153 lists currently estimated zinc resources 

 of the world, subdivided into recoverable and sub- 

 economic and into identified and undiscovered; it 

 also compares these classes of resources of the 

 United States with those of the rest of the world. 



Erickson (Crustal abundance chapter) indicates 

 that the zinc content of the earth's crust is 2,250 

 X 10'- metric tons, of which 1,220 X 10'- metric 



Table 153. — Identified and undiscovered zinc resources of the 

 United States and the world (estimated in millions of metric 

 tons) 



Identified Undiscovered 

 resources ^ resources Total 



United States 



Recoverable MS 60 105 



Subeconomic ' 75 230 305 



Total 120 290 410 



Rest of the world 



Recoverable H90 285 475 



Subeconomic n,200 3,000 4,200 



Total 1,390 3,285 4,675 



Total world 



Recoverable ^235 345 580 



Subeconomic "1,275 3,230 4,505 



Total 1,510 3,575 5,085 



> IDENTIFIED RESOURCES: Specific, identified mineral deposits that 

 may or may not be evaluated as to extent and grade, and whose 

 contained minerals may or may not be profitably recoverable with 

 existing technology and economic conditions. 



PRESERVES: Identified deposits from which minerals can be extracted 

 profitably with existing technology and under present economic 

 conditions. 



3 CONDITIONAL RESOURCES: Specific, identified mineral deposits 

 whose contained minerals are not profitably recoverable with exist- 

 ing technology and economic conditions. 



tons is in the total continental crust of the world, 

 of which 73 X 10'- metric tons is in the continental 

 crust beneath the United States. Within a depth of 

 1 kilometer (approximately 3,000 ft) the continental 

 crust contains 34 X 10'- metric tons of zinc, of 

 which 2 X 10'- metric tons is within the United 

 States. Erickson further estimates that the world 

 reserve (recoverable identified resources) potential 

 is 3,400 million metric tons, of which 198 million 

 is in the United States. These latter tonnages are 

 for ores averaging 4-percent zinc and, for the most 

 part, presumably of the conventional sulfide type 

 currently being exploited. Although the figures pre- 

 sented are highly speculative, they represent an 

 expectable order of magnitude. 



Comparison of the data in table 153 with those of 

 Erickson suggests that neither the United States 

 nor the world as a whole has yet reached its jw- 

 tential for "high-grade ores." The total of esti- 

 mated identified and undiscovered recoverable zinc 

 resources of the United States, 105 million tons, is 

 about one-half of Erickson's potential 200 X 10'' 

 metric tons, whereas those of the world are esti- 

 mated at only about one-seventh of his world po- 

 tential of 3,400 X lO'' tons. 



Most of the newly discovered recoverable zinc 

 resources found in the last few years have been 

 largely in deposits of two types, the stratabound 

 deposits in metamorphic rocks (massive sulfide ores) 

 and the stratabound ores in carbonate rocks (Mis- 



