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.
5. Stream sediment survey.
6. Hydrogeochemical survey.
7. Vegetation survey:
a. Geobotany;
b. Biogeochemical.
8. Geozoological/homogeochemical survey.
9. Atmogeochemical (vapor survey).
10. Electrogeochemical survey.
11. Radiogenic isotope geochemistry.
12. Heavy mineral survey.
13. Polymetallic polynodule survey.
14. Hydrocarbon geochemical survey.
5.4.1 Pedogeochemical (Soil Survey)
Pedogeochemical survey is also known as soil survey.
The soil is the unconsolidated weathering product. It generally lies on or close to its source of formation such as residual soil. It may be transported over large distances forming alluvial soil. The soil survey is widely used in geochemical exploration and often yields successful results.
The anomalous enrichment of indicator elements from an underlying mineralization source is likely to occur due to secondary dispersion in the overlying soil, weathered pro- file, and groundwater during the process of weathering and leaching (Cameron et al., 2004). The dispersion of elements spreads outward forming a larger exploration target than the actual size of the orebody.
The soils display layering of individual characteristic horizons differing in mineralogy and trace element composition. Therefore sampling of different horizons will present different results. The soil profile can be classified in three broad groups such as A, B, and C horizons (Fig. 5.3).
Horizon“A” is composed of humus-charged top soil with minerals. Zone “B” represents accumulation of clays enriched with trace elements. Horizon “C” consists of bedrock fragments in various stages of degradation (C1), and gradually changes to hard parent rock (C2). The proportion
of metallic elements in“B”horizon is generally higher than in“A”zone. The anomalous behavior of“C”zone is similar to the parent bedrock. Therefore samples from “B” layer enriched with trace elements are most preferred during soil survey.
Soil samples from residual or transported material play a significant role during the reconnaissance survey (refer to Fig. 7.20). They can provide a quick idea about the pres- ence or absence of target metals in the environment. Soil geochemistry as a successful exploration tool was demon- strated in the discovery and development of the Kalkaroo CueAueMo deposit, South Australia, and large PGE mineralization at the Ural Mountains, Russia. Soil sampling had extensively been used for locating base metal deposits in Khetri, Pur-Banera, Zawar, Rajpura-Dariba, and Agucha (Rajasthan), Malanjkhand (Madhya Pradesh), Singhbhum copper-uranium belt (Jharkhand), and Sukinda chromium- nickel deposit (Orissa), India.
FIGURE 5.2 Data interpretation of soil and pit geochemical samples by contouring copper values to identify the best target for drill testing.
FIGURE 5.3 Diagrammatic presentation of soil horizons, relative verti- cal and lateral spread of secondary dispersion halos, and associated metal contents.
5.4.2 Consolidated Weathered Cover
The weathering of cover sequence has undergone various types of chemical fractionation over millions of years due to the paleoclimatic setup of the region. The consequential resistant residual weathering component of rocks and soils consolidates to form landscape geochemistry. The weath- ered cover can broadly be classified into four types depending on their composition and type of weathering, and can guide in mineral search.
5.4.2.1 Calcrete
Calcreteis the weathered crust in arid and semiarid regions and is represented by a mixture of sand and silt cemented by calcite, dolomite, gypsum, halite, and ferric oxide. The process is simulated mainly by near-surface groundwater and vertical/lateral concentration of minerals like uranium, vanadium, potassium, calcium, magnesium, and base and noble metals. The economic calcrete-type uranium deposits occur typically in Australia, Namibia, South Africa, Botswana, China, and at the desert/semidesert terrains of Jodhpur and Bikaner, western Rajasthan (vast drainage area of Luni and extinct Saraswati river basin), India. The calcrete-type weathering indicates the presence of carbon- ates nearby that may be metalliferrous as in the case of Rajpura-Dariba base metal deposit in Rajasthan, India.
This is also formed near ultramafic intrusions where car- bonate nodules are common in soil along river banks like Subarnarekha, east of Singhbhum copper belt, India, indicating the presence of CueNieCoePGE.
5.4.2.2 Silcrete
Silcrete is a surface crust of residual weathering where sand and silt are cemented by silica. This is formed in semiarid regions simulated by stable groundwater conditions. The silcrete is commonly found in association with gossan over the copper deposits of Khetri region, Rajasthan.
5.4.2.3 Ferricrete
Ferricrete (ferruginous concrete) is a hard erosion- resistant layer of sedimentary rock (conglomerate/
breccia) cemented by iron oxides derived from the oxidation of percolating solutions of iron salts. The cementing materials are usually transported from distances and form a surface cover. The common types are alluvial and colluvial iron oxyhydroxide and manganocrete de- posits. The ferricrete deposits occur around the Gulf and Atlantic coasts of the United States. Ferricrete is widely used in South Africa and in Orissa and Gujarat in India to create local roads in rural areas.
5.4.2.4 Laterite
Lateriteis a surficial/near-surficial consolidated product of humid tropical weathering/oxidation and supergene enrichment as a result of physical and chemical processes on felsic/mafic/ultramafic/clay rocks. It is composed of goethite, hematite, kaolin, quartz, bauxite, nickel, and other clay minerals. The color is red or brown to chocolate at the top showing a hollow, vesicular, botryoidal structure.
It changes progressively from a nodular iron oxide-rich zone at the top to a structureless clay-rich red-yellow limonite transition and finally merges with partially altered (saprolite) to unaltered bedrocks (Fig. 5.4). Laterite causes the concentration of metals/ore deposits of Nie CueCoePGE, Au, Al, Fe, and Mn by the action of a paleoclimatic environment, and occurs within the regolith (the layer of soft/fragile/loose, heterogeneous superficial material covering solid rock).
The deeply weathered profiles are widespread on con- tinental landmasses between latitudes 35N and 35S.
FIGURE 5.4 An ideal lateritic profile showing the formation of nickel- copper-cobalt-platinum group metals formed by the supergene enrich- ment process of physical and chemical weathering in humid tropical countries. Nickel-copper-cobalt metal enrichment is in the highest order in the saprolite/garnierite/serpentinite horizon and gradually diminishes to the up- and downside.
The key controlling factors for lateritic formation are the presence of host rocks, a stable paleoclimate receiving more than 1000 mm annual precipitation, average cold month temperatures between 15 and 27C, warm month temperatures between 22 and 31C (Robert et al., 2012), deeply weathered profiles, widespread establishment on continental landmasses, topographically moderate relief andflat enough to prevent erosion, long periods of tectonic stability, tectonic activity, excess leaching of chemically weathering products, climate change that can cause surfi- cial erosion, and essentially a prevailing humid tropical to temperate climate. The basic dominant extractable minerals/metals are:
1. Aluminous laterite (bauxite): large bauxite reserves include Weipa mine, Queensland, Australia, Guinea, Vietnam, Jamaica, and East Coast, India.
2. Ferruginous laterite (iron ore): Capanema deposit, Minas Gerais, Brazil, and Goa, India.
3. Nickeliferous laterite (nickel ore): eastern coast of Brazil, Madagascar, Indonesia, Papua New Guinea, Goro nickel, New Caledonia, Nonoc, the Philippines, Mutrin Murrin, Western Australia, Çaldag deposit, Western Turkey, and Sukinda, India (Fig. 5.5).
4. Chromiferous laterite (chromite ore): Sukinda, India.
5. Manganiferous laterite (manganese ore), Uttara Kannada, India.
5.4.2.5 Gossans
Gossansare the signposts that point to what lies beneath the surface. Gossans are exceedingly ferruginous rock, which is the product of oxidation by weathering and leaching of sulfide mineralization. The colors significantly depend on mineralogical composition of iron hydroxides and oxides phases, and vary between red (hematite), yellow
(jarosite), brown, and black (galena) with stains of azure blue, malachite green, and peacock (copper). The texture can be brecciated, cleavaged, banded, diamond mesh, triangular, cellular, contour, sponge, and colloform with box work of primary sulfides. The texture assumes a hon- eycomb pattern (box work) of various colors that exist in capping as sulfide grains oxidize and residual limonite remains in cavities. The characteristic relic textures and colors resulting from the weathering of certain primary sulfide minerals like sphalerite, galena, and chalcopyrite will be specifically diagnostic. Identification can be corroborated by microscopic observation and chemical analysis. Field observations make it easy to detect gossan in prospecting areas with good outcropping conditions. The study of color aerial photographs and satellite images is of much use to focus on certain dark reddish bodies, which have to be checked in thefield. The depth of gossan may extend from a few centimeters to hundreds of meters.
The weathering of sphalerite above massive primary sulfide deposits usually depicts a yellow-brown color with coarse cellular box work and sponge structure (Fig. 5.6).
The primary mineral sphalerite (ZnS) often changes to
FIGURE 5.5 The older ultramafics have been extensively weathered to form laterite (yellow and red limonite) at the top of Sukinda intrusion. The nickel-bearing laterite has complex metallurgy for economic recovery and is stacked separately for future technology update.
FIGURE 5.6 The yellow-brown color, cellular box work, and sponge structure of a unique gossans formation above Rajpura-Dariba zinc-lead- copper-silver deposit indicate the presence of sphalerite underneath.
willemite (Zn2SiO4). Gossans of multisulfide deposits are often associated with typical contour box work from silver- rich tetrahedrite (copper-stibnite) and tennantite (copper- arsenic) minerals.
The dark chocolate color with cleavages, crust, radiate structures, and cubic diamond mesh cellular box work (Fig. 5.7) in gossan indicate the presence of galena and tetrahedrite as primary mineralization of sulfide deposit at depth (Bateman, 1962). Primary galena (PbS) often changes to anglesite (PbSO4), cerussite (PbCO3), pyromorphite (PbS(PO4)3Cl), and mimetite (PbS(AsO4)3Cl).
The chalcopyrite often changes into native copper, melaconite (CuO), azurite (Cu3(CO3)2(OH)2), and mala- chite (CuCO3Cu(OH)3). Dark shades of peacock blue, green, and black colors (Fig. 5.8) and triangular cellular structures are easily recognizable and associated with primary copper sulfide at depth.
The massive sulfide deposits contain typically large quantities of iron sulfides and carbonates (pyrite, pyrrhotite, marcasite, siderite, and ankerite) that oxidize and produce an exceptional acidic environment in the above-ground water table, and induce the formation of characteristic gossan.
Geochemical studies of gossan have been success- fully used in the exploration of MoeNieCu at Malanjkhand copper deposit, India, which shows com- plete alteration and enrichment profile (Fig. 5.9) to form a typical textbook gossan. The thin oxidized cap is represented by limonite with stains of malachite, azurite, and native copper. This is followed by a zone of secondary sulfide enrichment in the central and southern parts of the orebody with predominance of covellite, bornite, chalcocite, and chalcopyrite. This is the most copper-enriched horizon of the deposit. The secondary enrichment grades into primary orebody with gradual decrease in secondary minerals with predominance of chalcopyrite and pyrite.
More significant examples can be cited from the unique world-class textbook gossans formation at Rajpura-Dariba ZnePbeCueAg deposit (Figs. 5.10 and 5.11) and Cu deposits of Khetri belt (Fig. 5.12) (India), ZnePbeAg deposit, Broken Hill (Australia), Ashanti CueAu deposit (Ghana), Zn deposit at Togo (western Africa), Rouez gold deposit (France), Hassai CueZneAu deposit (Sudan), and Al Hazar CueAu deposit (Saudi Arabia).
All facies of the gossan have to be sampled because they may correspond to different types of primary mineral- ization. The weight of the sample is about a few kilograms scooped from the surface depending upon homogeneity and chips sampling in a grid pattern for consolidated mass.
Multielemental analysis is appropriate during the recon- naissance stage. True gossan can be difficult to distinguish from ironstone and other iron oxide accumulations such as laterite.
5.4.3 Lithogeochemical Survey
Lithogeochemical survey (rock survey) is useful during regional work to recognize a promising geochemical province and favorable host rocks. The most epigenetic and syngenetic mineral deposits show primary dispersion around the mineralization by the presence of anomalous high-value target elements. Lithogeochemistry aims at identifying primary dispersion, other diagnostic geochem- ical features, and trace element association, which are different from country rocks. Some granite possesses above average contents of Mn, Mo, Au, and Te indicating a potential for hosting porphyry copper deposits (Malanjkhand, India). The rocks associated with tin deposits in the Tasman Geosyncline contain 3e10 times
FIGURE 5.8 The dark shades of multicolor peacock blue, green, brown, and black, and the tiny box work of a unique gossans formation above Rajpura-Dariba deposit indicate the presence of primary chalcopyrite, tetrahedrite, and tennantite ore deposit.
FIGURE 5.7 The dark chocolate color, crust radiate structure, and dia- mond mesh cellular box work of a unique gossans formation above Rajpura-Dariba deposit indicate the presence of galena and tetrahedrite underneath.
tin value than do country rocks. The sedimentary exhalative-type zinc-lead-silver deposits often show a primary pyrite halo.
The lithogeochemical survey is based on analysis of unweathered rock/individual mineral(s). Sampling on a uniform grid across a geological terrain includes several rock types from fresh outcrops, wall rocks, and drill cores. The rock chip consists of five to six small fragments that individually or collectively represent a sample. A sample weighing w1 kg will be adequate.
Primary halos play a significant role in discovering deep- seated hidden deposits, particularly having a supporting structure.
The key guiding factors in a lithogeochemical survey are:
1. Large concentration area with target elements w10 times higher than background values.
2. Additive halo technique by adding all anomalous content of a group of indicator elements.
3. Multiplicative halo concept by ratio between the product of all economic elements and the product of impurity elements.
4. Vertical zoning of metal distribution.
5. Linear productivity, which is the product of width of anomaly with % content of economic element.
6. Anomaly ratio, which is the ratio of anomalous to back- ground value.
FIGURE 5.9 A generalized profile of gossans formation over massive copper sulfide deposits conceptualized after Malanjkhand copper deposit, India.
FIGURE 5.10 A unique world-class gossans hill above the massive silver-rich zinc-lead-copper deposit at Rajpura-Dariba. In 1977 the site was designated as a“National Geological Monument”by the government of India.
FIGURE 5.11 Part of the unique gossans collapsed in the eastern part of the hill during active underground mining operation leaving fresh expo- sure.Photo January 2010.
5.4.4 Drift or Till Geochemical Survey
Drift prospecting is a broad term for sediments created, transported, and deposited under the influence of the moving ice of glaciers particularly in steep mountain terrain. The various sizes of rock fragments travel longer distances to form drift sequences. The size and shape of mineralized boulders along with stream sediments reveal the extent of transportation and to trace back the source of the parent deposit a few kilometers away at higher elevation. The deposits are classified as glaciofluvial gravels and sands, silts and clays, and till or moraine. Till is a favored sample medium for locating mineral deposits in glaciated terrain. The basal till sequence is studied for the presence of mineralized clasts, heavy minerals, and relative abundance of major, minor, and trace elements to assess potentiality. The sample density, depth, and method should be selected according to the needs of the explo- ration program. The concentration of oxidized ore min- erals and their product of decomposition can be detected from fine fractions in the surface till. Portable reverse circulation drills are used for collection of till samples at depth and to determine the vertical and lateral variation in till geochemistry. The survey also traces the detrital dispersal of bedrock mineralization to the primary source.
The activity components include collection of basal till sample, determination of ice flow history, and data interpretation.
Till geochemical exploration is extensively conducted in Canada (Thompson NieCuePGE and Bell copper deposit), North America (Eagle Bay CueMoeAueAg deposit), Glenlyon and Carmacks CueZnePbeAgeAu deposits of Yukon-Tanana terrain, and Finland. Gorubathan massive multimetal deposit of Himalaya at Darjeeling dis- trict, India, was discovered by the presence of float- mineralized boulders in the downstream. The parent body was located a few kilometers uphill.
5.4.5 Stream Sediment Survey
The geochemical survey based on the chemical analysis of samples of an active stream sediment from drainage courses have long been used as an exploration tool. The composi- tion of the stream sediment samples reflects the bedrock geology of the catchment area, overburden cover profile, and any contained metalliferous mineralization. The stream sediment survey is most widely practiced in all reconnais- sance, prospecting, and detailed surveys of drainage basins.
Many minerals, particularly sulfides, are unstable in stream/
weathering environments, and greatly disperse as a result of oxidation and other chemical reactions. The greater dispersion means greater ability to discover an orebody from a greater distance.
The process motivates secondary dispersion of both ore and indicator elements. The elements move in solid and solution form to further distances within a drainage basin.
The stream sediments usually comprise clastic and hydro- morphic components that include clays, detrital fine- grained rock and mineral particles, inorganic colloids, organic matter, and iron and manganese coatings on clasts.
The mobility of different elements will vary significantly.
The detrital grains enriched in ore and indicator elements will be deposited downstream. The samples may lead to reach the mineralization target location following the
“path”of increasing values upstream.
The samples represent the best possible composite of weathered and primary rocks of upstream catchment areas.
The unconsolidated materials are in a state of mechanical transportation by streams, springs, and creeks. The initial sample density during reconnaissance survey will be as wide as sample 1 in 200 sq. km block size. The same will be planned as close as few meters during prospecting stage, following the course of natural stream. The initial location of the sample collection point upstream should be at least 50 m away from roads, habitations, and active and closed
FIGURE 5.12 Gossans are very common features as a surface exploration guide at Khetri copper belt, India. The dimension is small compared to Rajpura-Draiba belt.