Page 154 - Geochemical Remote Sensing of The Sub-Surface

Carbon dioxide dispersion halos around mineral deposits 131

surfaces of soil particles. These two forms of CO2 (especially the former) therefore give
a good indication at surface of buried and blind ore deposits.
Some contribution to these patterns may come from CO2 released by post-
mineralisation fracturing of fluid inclusions that trapped CO2 during metallogenesis. A
third source is the release of CO2 from carbonates as a result of sulphide oxidation.
Where mineralisation is deeply buried and below the water table, however, any
contribution from this process seems likely to be small.

Factors affecting carbon dioxide anomalies

In wet soils, background levels of CO2 are increased due to the hydrolysis of
carbonates. A traverse over mineralisation at Zhanglian was sampled once when the soil
was wet and again in early spring when the soil was relatively drier. Sampling of wet
soils produces a number of false anomalies of CO2, whilst the dry soils produced a
satisfactory low contrast CO2 anomaly over the mineralisation only. Therefore sampling
along river banks or in soils of varying moisture content should be avoided in order to
obtain clear anomaly definition.
Thin soils underlain by limestone can also be the source of false anomalies. On a
traverse across a molybdenum deposit at Tongshan, Jiangsu province, which is covered
by 0.3-1.0 m of eluvium, CO2 anomalies were found not only above the ore body but
also on a nearby ridge. Here soils are particularly thin and poorly developed, and the
underlying limestone creates an environment in which the pH can be as high as 8.5,
allowing carbonate enrichment which in turn produces a false anomaly of CO2.
The natural organic carbon content of the soil can affect the amount of CO2 detected,
and evidence from one traverse suggests that proximity to the exhaust residues of road
traffic could also be a source of CO2 in soil. The ratio of CO2 to organic carbon can be
used to resolve this problem.

CONCLUSIONS

Carbon dioxide, probably introduced at the time of emplacement of mineralisation,
occurs in a form that is easily released at 100~ as halo around and above many mineral
deposits. Along soil traverses over nine mineral deposits in China, determination of this
CO2 yields good-contrast anomalies, even when the mineralisation is deeply buried.
There is also an elevated CO2 expression in soils over faults in mineralised areas.
In general, over those mineral deposits in China where both methods have been
tested, there is considerable agreement between CO2 patterns and thermally-released Hg
patterns (Chapter 13). The continuity and intensity of CO2 anomalies tends to be a little
poorer. On the other hand, in some cases CO2 anomalies are present over mineralisation
where thermally-released Hg anomalies are absent.