Geochemical Methods in Mining Exploration
1 Introduction
Exploration geochemistry employs chemical analyses of natural materials to locate and
characterize mineral deposits. By identifying geochemical anomalies (concentrations of
elements that deviate from their normal background levels) these methods guide the discovery
of economically significant resources. Studies are conducted in two main stages:
reconnaissance surveys and detailed surveys.
2 Geochemical Exploration Methods
2.1 Soil Geochemistry
Soil geochemistry is a fundamental technique in mineral exploration. Soil profiles typically
comprise three horizons:
- Horizon A : Rich in organic material and unsuitable for exploration due to leaching.
- Horizon B : Transitional layer, enriched in metals like cobalt and zinc adsorbed by clays and
oxides.
- Horizon C : Composed of weathered bedrock, closest to the source of mineralization.
Sampling is conducted at horizons B or C, as they are more reliable for detecting anomalies.
Common tools include hand augers, sieves, and sample bags for collecting fine-grained
materials.
Figure 1 : Tools for soil sampling, including augers and sieves.
2.2 Sediment Geochemistry
Sediment-based methods utilize materials transported by water, focusing on stream sediments
and heavy minerals.
- Stream Sediments: Fine sediments from waterways reflect upstream geochemical
conditions. These include clay fractions enriched in secondary oxides and organic matter.
- Heavy Minerals: Dense, weather-resistant minerals are concentrated in specific trap sites
like meanders or channel junctions.
Figure 2 : Left- Heavy minerals concentration; Right- Trap sites in stream channels where
sediments are deposited.
2.3 Selective Geochemical Extraction
Selective geochemical extraction methods, such Mobil Metal Ion (MMI) or Ionic Geochemistry,
target labile (loosely bound) metal ions in soils. Metal ions from deep mineral deposits migrate
vertically and are weakly adsorbed onto soil particles. Selective reagents dissolve these ions,
providing a direct indicator of buried mineralization.
Figure 3 : Schematic showing ion migration pathways and MMI extraction zones
(Mann et al., 1997)
2.4 Lithogeochemistry
Lithogeochemistry involves analyzing the complete chemical composition of rocks. It provides
insights into rock classification, tectonic settings, and alteration processes. Samples are
carefully collected to avoid weathered surfaces.
Figure 4 : Rock sampling and drill core analysis.
2.5 Hydrogeochemistry
Groundwater often dissolves and transports trace elements from mineralized zones, making it
a valuable medium for exploration in regions with poor surface exposure.
- Sampling: Water samples are collected in plastic bottles, acidified with HNO₃, and analyzed
using spectrometry.
Figure 5 : Diagram illustrating ion migration in groundwater (Gandhi et Sarkar, 2016)
Figure 6 : Water sampling Tools and measurements in situ.
2.6 Atmogeochemistry
Gases are a potentially interesting medium to sample, as they can diffuse through thick layers
of terrain. Several gases have been used, with mercury being the most successful. Mercury is
the only metallic element that forms a vapor at room temperature and is widely present in
sulfide deposits, particularly base metal deposits associated with volcanism. Radon, generated
by the decay of uranium, has also been used with some success. More recently, the enrichment
of carbon dioxide and depletion of oxygen caused by the weathering of sulfide deposits have
been tested, particularly in the western United States (Lovell and Reid, 1989).
2.7 Biogeochemistry and Geobotany
Vegetation is used in two ways in exploration geochemistry. First, the presence, absence, or
condition of a particular plant or species can indicate the presence of mineralization or a
particular rock type; this is called geobotany. Second, the elemental content of a particular
plant has been measured, which is known as biogeochemistry. Biogeochemistry has been used
more widely than geobotany and has found particular application in the forested regions of
northern Canada and Siberia, where surface sampling is difficult.
Figure 7 : Vegetation sampling (Alexandre, 2021)
2.8 Electrogeochemistry
Deeply buried sulfide deposits act like giant batteries, and the constant measurement of
specific conductivity can help delineate a target. Electrogeochemistry depends on the
movement of ions in an electric field. In a mineral body, electrochemical dissolution occurs over
a vast area of electroactivity. According to Xianrong et al. (2008), the process leads to a high
concentration of metal ions in the dispersion halos surrounding a mineralized body. Under the
effect of the geodetic electric field, metal ions that do not react or are not absorbed eventually
rise into the soils and regoliths near the surface and form ionic anomalies in dynamic
equilibrium.
