Page 146 - Geochemical Remote Sensing of The Sub-Surface

Geochemical Remote Sensing of the Subsurface
Edited by M. Hale
Handbook of Exploration Geochemistry, Vol. 7 (G.J.S. Govett, Editor)
9 Elsevier Science B.V. All rights reserved 123
Chapter 4

CARBON DIOXIDE DISPERSION HALOS AROUND MINERAL DEPOSITS

M. ZHANG

INTRODUCTION

The carbonate gangue associated with many types of mineral deposits is evidence
that large quantities of CO2 often accompany the emplacement of mineralisation.
Detection in soil of traces of the dispersion pattern of this CO2 is postulated to produce
anomalies indicative of concealed mineralisation. Therefore a method for determining
this CO2 was developed. It has been tested in China over various types of mineralisation
covered by different thicknesses of soil in the provinces of Gansu, Shandong, Anhui and
Jiangsu and in the region of Shanghai.

METHOD

Soil samples are normally taken at intervals of 20 m along traverses. Over known
mineralisation or zones of structural control, the interval has been reduced to 10 m in the
case histories described here, whereas far from mineralisation the interval has been
increased to 30-40 m.
Two methods of sample preparation have been tested. In the first, the sample is
placed in a plastic bottle immediately and the bottle is sealed. The contents are allowed
to dry for 40-80 days prior to sieving. This method minimises contamination, but sample
transportation is inconvenient and samples may still be damp after the period allowed for
drying, making sieving difficult. In the second method, samples are air dried soon after
they are collected, then sieved through a 80-120 mesh sieve. In orientation studies the
first method gave slightly sharper anomalies than the second, but the difference was
insufficient to warrant the additional inconvenience. Thus the air-drying method has
been used is the case histories described below.
Analysis involves boiling the sample in distilled water to release CO2 into solution
from carbonates that break down at temperatures up to 100~ Dissolved CO2 is then
determined by titration using a colour indicator and photoelectric device to control the
termination of titration.