Analytical methods must satisfy optimum cost, best accu- racy, and precision to create an unbiased database for resource estimation acceptable by international major in- vestment stakeholders. The standard methods in order of applicability, affordability, and precision are as follows.
7.5.1 Atomic Absorption Spectrometry
Atomic absorption spectrometry (AAS) is a widely accepted, rapid, precise, and commonly used method for quantitative determination of a large number of samples for multielemental analysis. About 5 mg of sample is treated with 5 mL of aqua regia and digested over a hot plate. The solution is aspirated into an air-acetylene or N2O-acetylene flame for complete atomization of test metals. Absorbance of a characteristic radiation of desired metal is measured for computation of elemental concentration. Approximately 30 elements can be determined from one solution, one after another, by AAS using specific hollow cathode lamps with background correction.
FIGURE 7.34 Sample size reductions by a laboratory-scale disc, ball, or rod pulverizer.
FIGURE 7.33 Sample grinding by a laboratory-scale small roll crusher.
FIGURE 7.35 Sample quantity reductions by coning-and-quartering principles.
7.5.2 X-ray FluorescencedPortable XRF
The XRF technique uses high-energy X-ray photons from an X-ray generation analyzer to excite secondary fluores- cence characteristic X-rays from samples. The characteristic line spectra emitted by the different elements of sample are detected in an analyzer. The intensity of each line is pro- portional to the concentration of individual elements.
The portable XRFor handheld XRF is a lightweight rapid analytical instrument (Fig. 7.38). It is used as a nondestructive test to detect elemental/chemical analysis ranging between Mg and U. Typical applications are detection of minerals, including precious metals in
exploration and mining projects, alloy in industrial loca- tions and scrap markets, environmental remediation, and lead paint identification. The portable XRF unit has revolutionized the sampling of soil and stream sediment.
The instrument/method is handy as part of core and rock chip logging, and with an additional bonus of mineral identification.
7.5.3 Inductively Coupled Plasma-Atomic Emission Spectrometry
Inductively coupled plasma-atomic emission spectrom- etry (ICP-AES) works on an optical emission method excited by inductively coupled plasma. It is a promising emission technique that has been successfully used as a powerful tool for fast multielemental analysis since 1975.
The flame for this technique consists of incandescent plasma of argon heated inductively by radiofrequency en- ergy at 4e50 MHz and 2e5 kW. The energy is transferred to a stream of argon through an induction coil, whereby temperatures up to 100 K are attained.
Sample solutions are forced through a capillary tube, nebulizer system, and spray chamber to a relatively cool central hole of the plasma torus. The spray chamber reduces the particle size of the aerosol to an ideal 10mm. The sample atomizes and ionizes. The radiation from plasma enters through a single slit, and is then dispersed by a concave reflection grating. The light from each exit slit is directed to fall on the cathode of a photomultiplier tube, one for each spectral line isolated. The light falling on the photomultiplier gives an output that is integrated on a
FIGURE 7.37 Sample quantity reductions by a Jones refill splitter.
FIGURE 7.36 Sample quantity reductions for chromite production grade at the mine head by coning-and-quartering practices. Quarters II and IV will be mixed for further reduction.
FIGURE 7.38 Handheld portable XRF analyzer.From Gandhi, S.M., Sarkar, B.C., 2016, Essentials of Mineral Exploration and Evaluation, Elsevier Publication, 422 p.
capacitor; the resulting voltages are proportional to the concentration of the elements in the sample. Multichannel instruments are capable of measuring the intensities of emission lines of up to 60 elements simultaneously.
7.5.4 Instrumental Neutron Activation Analysis
The instrumental neutron activation analysistechnique utilizes high-energy neutrons for irradiation of a sample to produce gamma radiation that can be analyzed for the detection of elements. The method is suitable for the detection of trace elements and rare earth elements with a high level of accuracy.
7.5.5 Scanning/Transmission Electron Microscope
Detailed knowledge of the physical nature of the surfaces of solids is significant in geology, chemistry, and material science. Finer surface information at considerably higher resolution is obtained by scanning electron microprobe.
The surface of a solid sample is swept in a raster pattern with a finely focused beam of electrons in the scanning electron microscopetechnique to obtain a precision image.
The beam is swept across a surface in a straight line (the X- direction), then returns to its starting position and shifts downward (the Y-direction) by standard increments. The process is repeated until a desired area of the surface has been scanned. A signal is received above the surface (the Z- direction) during the scanning process and stored in a computer for conversion to thefinal image.
Atransmission electron microscopeis used either in biological applications for ultrahigh-resolution trans- mission electron photomicroscope studies of thin slices of cell and tissue material, or in metallurgical studies, including investigation of defect structures in alloy mate- rial. Subject to some limitations, equivalent studies are carried out on geological samples. However, samples must be prepared as thin foils. The quantitative measurements of photomicrograph data can be derived in two ways: (1) crystal structure data from the pattern resulting from elec- tron diffraction within the sample, and (2) compositional data from characteristic X-ray emissions excited by the electron beam as it is transmitted through the foil.
7.5.6 Electron Microprobe and Secondary Ion Mass Spectrometer
Theelectron microprobeprovides a wealth of information about the physical and chemical nature of surfaces. With the electron microprobe method, X-ray emission is
stimulated on the surface of the sample by a narrow focused beam of electrons; the resulting X-ray emission is detected and analyzed with either a wavelength or energy dispersive spectrometer.
The secondary ion mass spectrometer (SIMS) has proven useful for determining both the atomic and molec- ular composition of solid surfaces. SIMS is based upon bombarding the surface of the sample with a beam of 5e20 KeV ions, such as Arþ, CSþ, N2þ, and O2þ. The ion beam is formed in an ion gun in which gaseous atoms or molecules are ionized by an electron impact source. The positive ions are then accelerated by applying a high DC potential. The impact of these primary ions causes the surface layer of atoms of the sample to be tripped off, largely as neutral atoms. A small fraction, however, forms as positive (or negative) secondary ions that are drawn into a spectrometer for mass analysis.
7.5.7 Fire Assaying
Fire assaying is quantitative determination in which a metal or metals are separated from impurities by fusion processes and weighed to determine the amount present in the original sample. Thefire assay method is most accurate, but totally destructive for determination of precious metals.
It is a critical cupellation step by refining in metallurgy, where ore/precious metals/alloys are treated in fire under extremely high temperatures, and have controlled opera- tions to separate noble metals like gold, silver, platinum, and palladium from base metals like copper, lead, zinc, arsenic, antimony, and bismuth present in ore. If performed on ore materials using fusion followed by cupellation separation, detection may be in parts per billion/million.
Fire assaying can be applied for proving the existence of precious metals, especially as a prerequisite concentrating step prior to neutron activation analysis or ICP-MS analysis.
7.5.8 Carbon Dating
Carbon dating is a standard method for determining the age of an object containing organic material (plant or ani- mal) by using properties of radiocarbon (14Cda radioiso- tope of carbon). 14C is continuously being created in the atmosphere by the interaction between cosmic rays and atmospheric nitrogen. The resulting 14C combines with atmospheric oxygen to form radioactive CO2. 14C in- tegrates into plants by photosynthesis and subsequently imparts to animals through nourishment. The process stops exchanging carbon in its environment after demise of the organism, and the contained14C endures radioactive decay.
Amounts of14C decay can be measured to predict when the organism died with a range of accuracy. The best samples
for 14C age dating include wood used as wall supports, baskets, ladders, platforms, water channels (Figs. 4.3, 4.5e4.7), and animal bones from ancient mining/smelting sites.
Examples can be cited from ancient mining history at Kolar (129090 to 1500115) and Hutti (194570) gold mine (Agarwal and Margabandu, 1975e1976) and Zawar (2410100) and Rajpura-Dariba (3040150) zinc-lead mines, (Willies, 1987), India.
7.5.9 Choice of Analysis
The primary objective is to identify techniques that are widely used for the unbiased reporting of elements, and examine the analytical potential of individual techniques.
1. AAS, XRF, and portable XRF are most widely used, and acceptable for determination of major elements.
2. Since 1984, ICP-AES has become better established for trace and rare earths, but is costly. SEM, TEM, and electron microprobes are excellent, efficient, and accu- rate for surface- and structure-related compositional analysis. Instruments are expensive, research oriented, and may not be within the reach of everyone.
3. Fire assaying is a link between (2) and (3). 14C age dating is used to establish exploration/mining/smelting heritage.