Ch. 30] ANALYTICAL PROCEDURES 547 



concentrations of some material derived from a bedrock ore is followed 

 upstream to the source. Following the precedent set by alluvial pros- 

 pectors, Sergeev (1946) has described a sampling pattern for stream 

 waters wherein samples are collected in groups of three near the mouth 

 of each tributary to the main stream. One sample is taken from the 

 main stream above the junction, one sample downstream from the junc- 

 tion, and one from the tributary itself. Such data may be plotted 

 either on graphs or directly on a map (Sokoloff, 1948; Huff, 1948) to 

 show the source of any unusual amount of metal present in soluble 

 form. 



Sampling of ground water is limited, of course, by the distribution 

 of wells and springs where samples can be collected, and it constitutes 

 a special problem that must be separately evaluated for each field 

 project. 



Work by the U. S. Geological Survey in the southwestern Wisconsin 

 zinc district has shown that zinc ore below the permanent water table 

 does not discharge sufficient metal into the ground water to give an 

 indication of its presence. When the ore is at or above the water table, 

 however, the associated ground water has a significant high zinc con- 

 tent, of the order of 0.4 part per million, as compared with a back- 

 ground of less than 0.1 part per million. To date, the most successful 

 results with water sampling have been obtained with zinc, though 

 experiments with sulphate show some promise (Vogt, 1939) , and under 

 some conditions copper in stream water appears to be indicative of 

 copper ore (Vogt and Rosenqvist, 1942). Studies by the U. S. Geo- 

 logical Survey, however, indicate that the copper content of natural 

 water cannot generally be used as a prospecting guide. 



Natural water is outstandingly susceptible to contamination. It is 

 extremely difficult, and many times impossible, to obtain water from 

 a well that is not hopelessly contaminated with zinc and copper from 

 the plumbing. Stream water, except in wilderness areas, may be 

 seriously contaminated from metals in sewage and rubbish heaps, dis- 

 charge from factories, mine dumps, and abandoned mines. Spring 

 water, however, is commonly free of artificial contamination. 



ANALYTICAL PROCEDURES 



Most geochemical methods of mineral exploration depend on chemi- 

 cal analysis of soil, vegetation, or water for traces of ore metals. Be- 

 cause of the large volume of samples that must be tested to obtain a 

 working picture of the dispersion patterns of metals, the analytical 

 methods should be both rapid and inexpensive. In addition to these 



