Exploration Geochemistry
Chapter Outline
around/in proximity to/away from the source. The dispersing metallic values are less than those in the deposit, but significantly higher than the background values found in enclosing country rocks. These elements occur in traces and are found in soils, rocks, and groundwater. The degree of concentration of specific elements logarithmically diminishes away from the mineralization until it reaches a background value. Geochemical sampling can outline two types of dispersion halos: primary halo and secondary halo.
Primary dispersion halos refer to a geochemical envelope, which is an expression of alteration and zoning conditions surrounding metalliferous deposits. Formation of the primary halo is synchronous to mineralization as a result of moving hydrothermal fluid in rocks. It is essen- tially identical to the geochemistry of unweathered rocks and minerals, irrespective to how and where the orebody itself was formed. The halos are either enriched or depleted in several elements due to the introduction or redistribution related to ore-forming phenomena. The shape and size of the halo is exceptionally variable due to the diverse mobility characteristics of elements in solution and microstructures in rocks. The extremely porous or exten- sively fractured rocks develop widespread primary halos (Fig. 5.1A).
Secondary dispersion halosare the dispersed remnants of mineralization caused long after deposit formation by surface processes of chemical and physical weathering and redistribution of primary patterns. A secondary halo can be recognized in samples taken from soils, rocks, stream sediments, plants, groundwater, and volatile matter at a distance of meters to kilometers from the deposit.
Chemical weathering involves elemental breakdown of rocks and minerals in abundance of water, oxygen, and carbon dioxide. It can move considerable distances from the source. Multicolored laterite and gossan above mineral deposits at Broken Hill, Australia, and Sukinda (Fig. 5.5), Rajpura-Dariba (Figs. 5.10 and 5.11), Khetri (Fig. 5.12), and Malanjkhand in India are unique examples of chemical weathering. Physical weathering implies disintegration of rocks and minerals with little or no chemical and mineral- ogical changes. It may involve long-distance transportation from the source. These minerals are chemically resistant, namely, oxides (cassiterite, rutile, magnetite, chromite) and native forms of gold, platinum, and diamond. Favorable locations aretilland glacialdeposit environments.
5.1.2 Pathfinder Elements
Pathfinder or indicator elements are characteristic pa- rameters in geochemical prospecting. The elements are relatively mobile due to physicochemical conditions of the solutions in which they are found, or may be in a volatile
state (gaseous). They occur invariably in close association with the primary minerals being sought. These elements are easily identifiable as a broader halo around the deposit, or detected easily by simpler and inexpensive analytical methods. The pathfinder elements play a significant role in locating concealed deposits due to these special properties.
The condition of the pathfinder necessitates that the ele- ment(s) used must occur in primary association with the element(s) being sought, for example, copper, nickel, and chromium as pathfinders for platinum-palladium group of deposits (Bushveld chromium-platinum-group element [PGE] deposit, South Africa, Sudbury nickel-PGE deposit, Canada, and Nausahi chromium-PGE deposit, India), zinc for lead-silver-zinc deposits, Rajasthan, and scheelite in Kolar goldfield, Karnataka, India. The elements can also be derived from their parents by radioactive decay, such as use of radon as a pathfinder for uranium deposits. Some com- mon pathfinder elements are listed inTable 5.1.
FIGURE 5.1 A typical illustration of (A) concealed copper deposit with primary dispersion halos, (B) geochemical profile of soil samples viewing background, threshold values, and anomalous zone, and (C) histogram of Cu values showing most frequently occurring samples.
5.1.3 Background and Threshold Value
Backgroundvalues are characterized by a normal range of concentration of elements in regional perspective rather than localized mineral occurrences. It is significant to establish the background value of the study area so that the anomalies due to economic mineral accumulations, if any, can be identified. A large number of samples comprised of rocks, soils, sediments, groundwater, and volatile matter are analyzed for multiple elements separately for each area before exploration begins. Each type of sampled material should be treated separately. The values may vary signifi- cantly between samples. Frequency distribution is usually positively skewed. The arithmetic average (mean) is evidently biased by a few scattered high values. The most frequently occurring values (modes) tend to lie within a relatively restricted range, and are considered to be the normal abundance or background for that particular element of the area (Fig. 5.1C).
Threshold value is defined as the probable upper or lower limit of background value (Fig. 5.1B) at some sta- tistically precise confidence level. Any sample that exceeds this threshold is considered as possibly anomalous and belongs to a separate population. It may vary for each element, each rock type, different types of samples, and in each area (Fig. 5.1A).
5.1.4 Orientation Survey
The success of exploration geochemistry depends on the appropriate detection of pathfinder elements in primary and secondary environments. In practice, a first round orientation surveyis conducted in every new area to draw a detailed work plan that adequately distinguishes anoma- lies from background values. The important parameters to consider in combination are:
1. Host rock environment.
2. Identify criteria that influence dispersion.
3. Possible local contamination.
4. Effect of topography.
5. Best sample medium.
6. Optimum sample interval.
7. Depth of soil sample.
8. Size fraction.
9. Analysis for group of elements.
10. Anomaly enhancement.
11. Analytical techniques for establishing the background and threshold value.
In the reconnaissance phase, regional geological knowledge, the possible presence of economic mineral association, and previous experience elsewhere will be guiding factors to plan an orientation survey. The activities are focused around probable targets with better knowledge of geochemical characteristics of the area for detailed exploration. The procedure should be simplified andfinite.
The orientation survey is always justified for any new area.
It will optimize the sampling program and increase efficiency of interpretation with higher confidence. In turn, it saves considerable time and money in long run.
Anomaly enhancement techniques are commonly practiced during orientation surveys with weak anomalies, particularly for deep-seated mineralization. Value enhance- ment can be obtained by physical, chemical, and statistical means. The physical methods are enrichment of metallic, magnetic, and heavy minerals by panning, magnetic, and heavy media separation, respectively. The chemical methods include selective leaching of iron and manganese oxides in the host environment. The statistical means are anomalous to background and trace elements ratio and the additive and multiplicative halo concept. This technique highlights insignificant values of interest in locating concealed deposits.
5.1.5 Regional-, District-, and Local-Scale Geochemistry
Geochemical province is defined as a relatively large segment of area in which the chemical composition of the bedrocks is significantly different from the surroundings. It is manifested by a certain suite of rocks relatively enriched TABLE 5.1 Common Pathfinder Elements in
Geochemical Exploration Type of Deposits
Pathfinder Element(s) Gold-silver vein type, gold-silver-
copper-cobalt-zinc and complex sulfide ores
As
WeBeeZneMoeCuePb skarns, SneBo veins
B Copper-zinc-lead-silver and complex sulfide deposits
Hg, Zn BaeAg vein deposit, porphyry
copper
Mn Wolframite-tin contact metamorphic deposits
Mo Porphyry copper, barium-silver
deposits
Mn, Mo, Au, Te Platinum-palladium in mafic/
ultramafic rocks
CueNieCrePdeCo
Uranium (all types) Rn, Cu, Bi, As
Uranium sedimentary type Se, V, Mo Sulfide deposits of all types SO4
in certain specific elements, e.g., Southern Australia and southeastern Tanzania are favorable for locating copper, chromium, nickel, and PGE, Aravalli Mountain province for base metals, and Chattish Garh and Goa for iron ore, India. Similarly,metallogenic provincerepresents a large area characterized by an unusual abundance of particular types of metals in country rocks, e.g., the copper-producing area of Peru Chile, Singhbhum and Khetri (India), lead- zinc-silver producing areas of Mt. Isa (Australia), Sullivan province (Canada), and Zawar, Dariba, Rampura- Agucha (India), tin in northwestern Europe, and Bastar in India. There is no definite or unique sample interval.
Traditionally, low-density stream sediment surveys of one sample per 50e200 or 5e20 km2will be adequate for se- lection of a province depending on regional geological knowledge and terrain. The analysis is performed for 16 to 25 elements.
Mineral districtis defined by the presence of known characteristic mineralization such as chromium minerali- zation in Jajpur-Keonjhar district, India. Stream sediment and limited soil and rock chip samples at 3e6 or even 1e2 km2 grid intervals are followed during the pro- specting stage depending on geological heterogeneity and signatures.
Local-scale geochemistry is aimed at outlining the location and broad extent of mineralization by detailed stream sediment sample at intervals of 30e300 m. Rock and soil geochemistry can be exercised in the absence of inadequate drainage systems. Once the target is identified, more closely spaced traverses at 100e300 m apart are sampled for soils and rocks at an interval 10e50 m across.
The interpretation is further upgraded and précised by addition of pitting and deep trenching in a close grid pattern. The target is now ready for drill testing.
The mission and extent of geochemical exploration is generalized by progressively diminishing the size of the search area in which an economic deposit may exist. Ac- tivities continue from grassroots reconnaissance to detailed sampling until a target is defined that can be tested by drilling. The sequential program demands further detail and expensive techniques with a sole objective of maximum probability of discovery at the lowest possible time and cost.