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771A statistical method of determining the threshold concentration in geochemical exploration is described. Examples are presented of its use in prospecting surveys at Parakao, Northland; Shotover River, Moke Creek, and Waitahuna, Otago; and the Longwood Range, Southland.

fractory metals and alloys are only incompletely attacked by the HF-dichromate combination. The function of the phosphoric acid is not fully understood. In the case of tungsten and molybdenum, the formation of phosphotungstic or phosphomolybdic acid can be conjectured as aiding the dissolution of the sample. Ferric salts may also be complexed by phosphoric acid. No mechanism is apparent to account for the role of phosphoric acid in dissolving nickel, cobalt, niobium, or tantalum. It was also found that phosphoric acid prevents the formation of CrF3 which is otherwise only slightly soluble in hydrofluoric acid.Other Acid Combinations. In the case of ch omium base alloys, it was noted that a more rapid decomposition of the sample can be achieved if part of the hydrofluoric acid is replaced by 2.5M hydrochloric acid.Effect of Nitrate Ion in Proposed Procedure. Because in the proposed method the nitrate ion must compete with a large excess of dichromate ions, it was conjectured that nitrates would be less harmful than in methods using unstable oxidants. See Table I.The data in Table I confirm that in the proposed procedure only a fraction of the nitrate is reduced. RESULTSThe suggested method was applied to the determination of nitrogen in the four samples (3), four Air Force samples (4), and several other samples for which data on nitrogen were available. In some instances, where nitrogen data were not available, both the dichromate and the brómate methods were used. The results presented in Table II indicate that the proposed method is suited for the determination of trace amounts of nitrogen in a variety of refractory metals and alloys. Steel samples were included only to prove the applicability of the methods. Most steel samples can be dissolved in hydrochloric acid without the addition of an oxidant.Although not within the stated scope of this paper, it should be mentioned that the proposed method is equally applicable to samples containing large amounts of nitrogen. Most nitrides are readily attacked by the HF-H3P04-K2Cr207 combination. It was found that the method is particularly suited for the determination of the N/U ratio in uranium nitrides containing various metal additives.

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Results of geochemical prospecting surveys for copper in the Moke 'Creek area, the Shotover district, and the Skippers Creek area of west Otago are presented. Copper in soils was found to be contained almost entirely within the finest fractions, and to be associated mainly with clastic chlorite. Anomalies in soils indicate that the cupriferous sulphide laminae of the Moke Creek Copper Lode extend beneath the soil at least 500 ft southward from the most southerly prospect pits, but the lack of anomalies to the north indicates that the copper lode does not extend much beyond Moke Creek. The source rocks of a cupriferous boulder found in the Shotover River may be a greenschist band in Ironstone Creek, with which are associated soil and water anomalies. Additional anomalies were found during a reconnaissance hydrochemical survey of the upper Shotover drainage system. The absence of strong geochemical anomalies in the Skippers Creek area suggests that cupriferous schist at Skippers Point does not extend into that area.