Johnson Camp, Arizona

Johnson Camp has been the site of three independent surveys carried out by the authors of this chapter. The area lies in southeastern Arizona, in the Basin and Range province of the western United States. 

In order to confirm that the sulphur gases were derived from the mineralisation, bags of molecular sieve adsorbent were lowered into the casings of drill holes which intersected the ore, and left for nine weeks to equilibrate. The analysis of these molecular sieves showed that sulphur compounds had been adsorbed. The results are summarised in Table 8-IV. These data demonstrate the advantages of a technique which integrates the gaseous flux over an extended period, as tests showed that no sulphur gases were detected after one week of burial and only trace quantities after two weeks. 

A different approach was used by Lovell (1979) and Lovell et al. (1980) who used the soils themselves as sulphur-gas adsorbents. The field programme consisted of three soil sampling traverses with a total of 185 sites (Fig. 8-17). The sample interval was generally 30 m, but where traverses A-A and C-C overlay Zone I, the interval was narrowed to 10 m 

The samples were analysed for total volatile sulphur gases and the results for the surface microlayer are presented in Figs. 8-18 to 8-20. The data are presented as peak areas from the strip-chart recorder of the analytical unit, as calibration of the response of a pulse of gas containing varying proportions of different species is not possible. On traverse A-A it is clear that there is a distinct pattern of three zones of higher sulphur values. There is a discontinuous series of anomalous values at the southwestern end of the traverse, which begin over the suboutcrop of Zone I and extend into the area underlain by the downdip extension of the mineralisation to a point where the depth to mineralisation is of the order of 80-90 m. This is followed by a series of generally low values until the samples are 

The samples collected from deeper within the soil profile gave only a comparatively poor expression of the mineralisation (Fig. 8-21). Subsequently, Oakes and Hale (1987) resampled the surface microlayer and speciated the desorbed sulphur gases. Anomalous 

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Surface microlayer samples have shown sulphur-gas anomaly patterns that are more closely related to known mineralisation than sulphur-gas patterns from deeper soil samples (Lovell, 1979). At Johnson Camp, Arizona, mineralisation is best expressed by sulphur compounds from the surface microlayer, while sulphur compounds from 0-5 cm reflect the same mineralisation to a lesser extent, and sulphur compounds from 30-40 cm show the least expression of the mineralisation. A comparison of concentrations of COS, CS2, and SO2 degassed from soils collected at depths of 0.5-2 cm and 30-40 cm at the same sites near Casa Grande, Arizona, showed almost identical patterns of sulphur-gas concentrations over a 150 km (58 square miles) area. Average concentrations of COS, CS2 and SO2 were slightly higher in the shallow samples than in the deeper samples (Hinkle, unpublished data, 1981). These data indicate that, at least in arid areas, surficial soil and microlayer samples are superior to deeper augered samples. 

Fig. 8-13. Ore zones, numbers of sampled drill holes and soil air sampling traverse locations at Johnson Camp, Arizona, for survey performed by Hinkle and Kantor (1978).

Sulphur compounds detected in drill holes at Johnson Camp, Arizona... 

Fig. 8-18. Cross section along Lovell s traverse A-A at Johnson Camp, Arizona, showing concentrations of total sulphur and organics in the surface microlayer (from Lovell, 1979).

Fig. 8-22. Cross section along Hinkle s traverse A-A at Johnson Camp, Arizona, showing concentrations of COS, CSi, CO2 in soil and CO2, He in soil air means are average concentrations of gases in the area (reproduced with permission from Hinkle, 1986, J. Geophys. Res., 91 12,359-12,365, copyright by the American Geophysical Union).

Fig. 14-10. Oxygen and carbon dioxide in soil air over pyrometasomatic mineralisation at Johnson Camp, Arizona (from Lovell et al., 1983).

Hinkle, M.E. and Kantor, J.A., 1978. Collection and analysis of soil gases emanating from buried sulphide mineralisation, Johnson Camp area, Cochise County, Arizona. J. Geochem. Explor., 9 209-216. 

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