Page 42 - Geochemical Remote Sensing of The Sub-Surface

Geoelectrochemistry and stream dispersion 19
C
C,"~,, T

X

..... B

Fig. 2-1. Schematic distribution of metal concentration C in: (A) lithogeochemical dispersion
halos; and (B) stream halos; hl,h2,h3- different depths of the ore body (reproduced with
permission from Putikov, 1993).

transformed into sorbed forms. Then through the process of diagenesis of the oxides and
hydroxides, the metals penetrate into crystalline lattices and may partly replace Fe and
Mn in the crystalline structure, possibly forming new minerals (Antropova, 1975).
Mobile and weakly-confined forms of metals make up only a minor part, less than
2%, of the total content of heavy metals in rocks and their weathering products.
However, it is these mobile forms of metals that can migrate for significant distances
from sources, and thereby convey information about deep ore bodies and oil and gas
reservoirs. Weakly-confined forms have direct and steady equilibrium with mobile
forms, and thus to a certain degree acquire the same property. Moderately-confined
forms also share this property, but only to a small extent.
Conventional geochemical exploration rests largely upon the determination of so-
called total concentrations of metals, which include a high proportion of strongly-
confined forms. The concentrations of metals found at surface by these methods are
highly dependent upon the depth of the source. The amplitude of anomalies, Cmax,
decreases and their width, b, increases with the depth, h, of the source (Fig. 2-1A). This
relation determines the shallow effective prospecting depth of conventional geochemical
methods, which is limited to sources less than 15 m deep (Solovov, 1985). On the other
hand, investigation of the mobile and weakly confined forms of metals has disclosed a
new type of dispersion halo, called the stream (or jet) halo (Ryss et al., 1987b). The main
features of the jet halo (Fig. 2-1B) are as follows: