Page 148 - Geochemical Remote Sensing of The Sub-Surface

Carbon dioxide dispersion halos around mineral deposits 125

traverse reveal broadly similar CO2 patterns (Fig. 4-3). In the centre of the traverse, over
the mineral deposit, soils from 30 and 60 em exhibit anomalies reaching 400 ppm CO2,
whilst anomalies in soils from 100 cm are rather weaker. All of these anomalies stand
out clearly against the low background in the southern part of the traverse, where the
overburden is eluvium. However, they have, at best, poor contrast relative to the higher
background in the northem part of the traverse, where the overburden is alluvium.
The Qixiashan lead-zinc deposit is a hydrothermal lead-zinc mineralisation in a
cataclastic fault zone in limestone. The ore body is 300 m below surface, and the
overburden comprises 30 m of Quatemary alluvium. Soil samples were collected along a
traverse over the mineralisation in three successive years and analysed for CO2 (Fig. 4-
4). The samples from Year 1 yielded the highest COz concentrations and broadest
anomaly, but contrast is poor. In Year 2 the anomaly is about 200 ppm CO2 and is best
developed around a fault that cuts the mineralisation at depth. Anomaly contrast is put at
4.2. Samples were taken from a depth of 80 cm in Year 3, compared to 30 cm in earlier
years; the resulting CO2 pattern, however, is closely similar to that found in Year 2.
Analysis of bore hole samples for CO2, Hg, Pb, Zn, Cu and Ag showed that the
Qixiashan deposit is vertically and horizontally zoned (Fig. 4-5). The highest COz
concentrations are found close to the richest ore. The CO2 halo extends upward on the
hangingwall side of the deposit and spreads laterally. This halo is much better developed
above the mineralisation than those of the base and precious metals; only the Hg halo
extends as far upward, and it is also rather wider.

Fig. 4-2. Relation between carbon dioxide in soil and iron mineralisation at Wang-wang,
Shandong province, China.