708 



UNITED STATES MINERAL RESOURCES 



mated at a total of about 50 million tons of con- 

 tained metal. 



As mentioned previously, deposits fonned by 

 laterization may have a high resource potential, 

 although this type of deposit has not been sought 

 specifically with the persistence given to the search 

 for massive-sulfide and Mississippi Valley-type ores. 

 More likely the so-called zincian laterites will be 

 found in tropical regions, although ancient tropical 

 regimes might also be sought among older geo- 

 logical terranes. Geochemical search techniques 

 would be as appropriate for ores of this type as 

 for the more conventional sulfide types (Muller and 

 Donovan, 1971). 



Some potential resources await the development 

 of unconventional mining and beneficiation tech- 

 niques. Such resources include metal-bearing muds 

 (Degens and Ross, 1969) and manganese nodules 

 (Mero, 1962, 1965) on the sea floor. Muds in the 

 Atlantis II Deep of the Red Sea contain an estimated 

 2.9 million metric tons of zinc deposited from hot 

 brines (Bischoff and Manheim, 1969). The more than 

 IV2 trillion tons of manganese nodules in the Pacific 

 Basin, that have an average content of 0.05 percent 

 zinc and 0.1 percent lead, are expected to contain 

 about 34 and II/2 billion tons of zinc and lead. 



Evaporite deposits are another potential but un- 

 conventional source of zinc. Weber (1964) reported 

 several tenths percent zinc in samples of dolomite 

 associated with Devonian and Triassic evaporite 

 sequences in Alberta and elsewhere in western 

 Canada. Davidson (1965), in discussing the associa- 

 tion of stratabound sulfide deposits with evaporite 

 sequences, cited the occurrence of a 21/2-foot zone 

 that has a content of 0.2 percent Zn in the modem 

 evaporites of Lake Eyre in South Australia. 



PROSPECTING TECHNIQUES 



As with other metallic minerals, four main tech- 

 niques are used in the search for new deposits of 

 zinc: geological, geochemical, geophysical, and 

 direct physical. 



The geological method consists primarily of plot- 

 ting on a base map the distribution and character 

 of all rocks in the area studied. Geologic structures 

 such as bedding, foliation, faults, joints, and other 

 features also are plotted, in addition to the boundar- 

 ies between formations. Indications of rock altera- 

 tion other than weathering, especially signs of min- 

 eralization, are also mapped. Discoveries of new ore 

 bodies or extensions of old ones determined by geo- 

 logic investigation are commonplace. (See Kiilsgaard, 

 1965.) 



Geochemistry is a relatively new exploration tool 

 that has become widely used in recent decades. Sam- 

 ples of soil, rock, and stream sediment are taken 

 systematically and analyzed for zinc and other trace 

 elements by colorimetry, spectrography, atomic- 

 absorption spectrometry, or other analytical tech- 

 nique. Geochemistry is particularly useful in 

 eliminating areas of low potential and selecting 

 favorable targets in areas of anomalously high metal 

 concentration. (See Hawkes and Webb, 1962). 



Geophysical techniques depend upon contrasts 

 between ore bodies and their surrounding rocks in 

 certain physical properties such as magnetic, elec- 

 trical, or gravimetric properties, or in response to 

 seismic waves. Most physical properties of zinc 

 minerals do not contrast sharply with those of their 

 host rocks ; thus, geophysical methods are useful in 

 the search for zinc only to the extent that other 

 minerals or special conditions are present in the ore 

 body which do respond to the instruments employed. 

 For example, magnetite and pyrrhotite, if sufficiently 

 abundant, give a magnetic response, whereas pyrite, 

 chalcopyrite, galena, and other sulfides commonly 

 associated with sphalerite respond to self-potential, 

 induced-polarization, resistivity, electromagnetic, 

 and audiofrequency magnetic methods. Massive sul- 

 fide ore bodies also may be detected by gravimetry. 

 Commonly, two or more geophysical methods are 

 employed simultaneously, and airborne instruments 

 have proved to be outstandingly useful. (See Dobrin, 

 1960; Parasnis, 1966; Soc. Explor. Geophysicists, 

 1966, 1967). 



Although the geological, geochemical, and geo- 

 physical techniques have been discussed separately, 

 they are most productive when used together. The 

 geology is fundamental, but geochemical and geo- 

 physical methods have proved so useful as to become 

 standard in most comprehensive exploration pro- 

 grams. A common procedure is to perfoiTn the geo- 

 logical and geochemical investigations first, and then 

 the geophysical technique on selected targets. In 

 vast drift-covered areas, as in Canada, airborne geo- 

 physical investigations often precede the geological 

 and geochemical. Ultimately, however, a discovery 

 is made only by direct physical means such as drill- 

 ing, trenching, test-pitting, stripping, sinking a 

 shaft, or driving an adit. The other three techniques 

 are but a preclude to this most expensive final ex- 

 ploration step and are useful only to the extent that 

 they reduce costs by picking the most favorable 

 targets. In general, a direct physical method should 

 be used as soon as a favorable target is indicated by 

 one or more of the three less direct techniques. (See 

 McConnel, 1965.) 



